Chris87TA350

I'm gonna check my clearance over the worst speed bump I know of near my buddy's house this weekend, and I might know of a guy willing to give me a good deal on welding one up. What I wanna know is, is 3" pipes too much for a daily driver or will I be able to deal with it? I'll be using 1 5/8" Hooker shorties attached to Catco cats and $20 Summit 3" mufflers. Will the cats give me enuff backpressure to make this work?

JPrevost

Just thought you might want to know that the Summit 3" mufflers are only 2.5" internal diameter. Their $30 versions which are fully welded are true 3".
Oh yeah, and I've heard from many sources that a single 3" will outflow dual 2.5" pipes. If that's what most people are saying then I would definatly think dual 3" is WAY too much for a daily driver. A decent 350 would want true dual 2.5" almost all the time. Besides, why the dual 3"?

Mark A Shields

Dual 3" is definately too little backpressure for your LG4.

Chris87TA350

Just as a note, it's an LG4 cc quadrajet on a 350. I just think dual 3" is a cool thing to have especially later on down the road when I build a 383 or something. What I was thinking was installing removable plates near the tail pipes so I could restrict flow for more backpressure (like in a Borla system). Would this work? That way, I'd still have true dual 3s for the track.

afgun

"Too little backpressure" is a myth. Too much backpressure is a bad thing, but too little is not. Just tune the car after you make the exhaust change.

Mark A Shields

Umm, no it's not a myth.

rx7speed

backpressure is a myth
think about it
you have something in the way of you exhaust and not letting it move free in turn it makes your motor work harder to push the exhaust out which causes pumping loss at ALL rpms, and since your motor is going to have to work to push the exhaust out that means the exhaust has a lower velocity which in turn is not going to create a vacuums in the exhaust system to help with scavanging so you are not going to pull out as much of the exhaust with a free flowing system


what you are confusing is backpressure with velocity
the reason why ppl might go to a smaller pip is not to make backpressure higher but to make the velocity of your exhaust faster.

backpressure will do nothing more then slow the stuff down
kinda like me standing in your way when you are trying to walk.
just doesn't work well

Chris87TA350

If the goal is to make the exhaust gases move as fast as possible, then I see my dumb restrictor plate idea is just a blockage in flow and won't help my low end torque, but with dual 3" attached to 1 5/8" headers (as opposed to 1.75") won't these small diameter pipes speed up exhaust flow enough where it counts right at the exhaust valve? In this case, the 3" piping to the back of the car may slow it down somewhat, but shouldn't hurt the scavenging. Isn't this the best of both worlds?

rx7speed

the further away the object is the less of an effect it will have on the performance of a car

what you can do is run the headers as you said and pump them into dual 2 1/2 inc pipes and then either right before or right after (not sure on fit under your car) the muffler step it up to the 3 inch just to make sure you do not have any velocity problems.

that way you put the big pipes a little bit away from the motor so there will not be that much of a loss in flow
also a byproduct that "might" occur though I doubt it since it is a ways away is where the exhaust tries to flow back into the motor.

Mark A Shields

You can have too little backpressure. Not to say it's a total bad thing though. You end up trading some low end torque for some high end HP.

rx7speed

you can NOT have too little amount of backpressure

the less the better
low backpressure will create low end torque as well as high rpm power

if you have something that will prove me wrong I would please like to see so. again just think of the meaning of word. from just the way the word is alone it is stating that you are putting pressure against the flow of the exhaust. how is putting pressure that your motor has to overcome to give you more low end torque
ideal would be a vac inside your exhaust ( that would have no pressure at all ) and with that it would help reduce pumping loss from the motor trying to force all your exhaust gas out. and also would help suck out a lot more of your exhaust which would create a high volumetric efficiency.
when you induce backpressure into the exhaust you are doing nothing more then making everything fight.
problems that come from backpressure is lower VE, pumping loss, high exhaust temps, high motor temps, more stress on your motor (small as it might be) and I am sure there are other things
just remember backpressure does not create a high velocity in the exhaust, and there is a difference between backpressure and velocity.
though by putting huge pipes on your car that is not really going to help you out either, it is not due to lack of backpressure though. what will go on if you put a large pipe on the car without the flow to back it up then the exhaust gas doesn't really have the walls of the pipe holding it into a controlled flow pattern instead the gas will flow around and have eddies in the system and try to turn back itself. if you get the right size pipes for your car you will reduce as much backpressure as low as you can but also keep the exhaust in check by not letting it just wander around but keep it all flowing in the right direction
and if backpressure was something that helped performance wouldn't ppl try to put something to create backpressure inside there intake also? instead ppl try to open up the intake, port it out, polish it make everything as smooth as they can to reduce the force the motor has to give to pull the air in. same thing on the exhaust side.

sorry if I am coming of as a ***** not meaning to be that way. just another one of those bad days.

ChevyLuva3

You said it yourself, your engine will run harder to push the exhaust out.... exactly. The slower something moves the more torque it has. Like for a bike with big gears, you're pushing really hard on the pedal and it taking a while to make one revolution, but man do you move when you get that push! But if you switched to a smaller gears, its way easier to push, creating almost no torque, so you could sustain high speeds with out much strain, but you certainly can't kick up much more speed, due to your none existant torque.

Another way to describe it would be trying to open a jar of babyfood or opening a soda bottle (which has a tight, stuck cap on it). Try to open it moving your hand real fast... ya can't open it. Because they're is no torque existing. Now put tension on your hand, and slowly but surely, try to twist the cap off, and while you're doing it, pretend that the "exhaust gases is what is keeping the cap on, and you are the engine trying to get them off, and be able to release full horsepower." (this is usually why max torque is created before max horsepower, because the engine has to overcome stagnant exhaust gases, during that time, you will have lots off torque, when the exhaust gases haved moved and now are creating the scavenging affect, the engine can you use its full power)

I mean even look at tug boats, they can pull anything in the world, due to their huge amounts of torque, but you don't see them moving any faster than a turtle, if ya know what I mean.

OK... I'll pretend you're in my way. I will have to slow down, as push harder to get you to move out of the way, but during that period of moving you away I'm creating massive torque. The second you're out of the away, I would probably launch forward, due to the torque I was creating now has no resistance.

OK, one last analogy pretend ya have a gallon of water in your fridge, but there happens to be very little water in it. However you think there is still a gallon in it. I'm sure you've done this before, you ever lift the water thinking there was a gallon, and there was hardly nothing, and then ya hit the top of the fridge, I'm sure you have. Why did you do that? Cuz you thought that there was a lot of backpressure, so what did you do, you put tension on your arm creating torque. But since there really wasn't any more backpressure, you just dented the fridge.

Alright, so after all that torque... I mean talk... jeez I've been talking bout it too much. This is the conclusion...
BACKPRESSURE IS THE CAUSE TO CREATING TORQUE'S EFFECT.
Thats the only way to overcome backpressure is to increase torque, if you get rid of backpressure, you lose torque. (thats right thats a period, finally)

rx7speed

yes you are using more power from the motor to push the exhaust out
but would you rather use the effort the motor is creating to push your exhaust or turn your crank/flywheel and the likes
you instead of putting that power where it should are redirecting it into a pumping loss by making the motor work to push exhaust out.
pumping loss sux


ok I am going to take the stock 4.10 rear on my car and put a 2.73 on my car since I will now have more torque and make my car go faster right?
you make more torque on the bike with the small gears it is called torque multiplication. why do you think a lot of ppl switch over there 2.73 for something with a higher number? but when you go to the larger gears you are sacrafising that torque multipliaction to gain more distance per less movement you put out. you are not gaining but losing torque in doing that and instead have to put out more torque to get the same effect

a reason why it is easier to move. the torque that you are putting out through your own legs is being multiplied by the gearing so there is not the need to put out more effort.

when you put larger gears into your motor no matter how you look at it you are not going to gain torque. you will have a loss of it through the drivetrain and instead have to put more out at the flywheel to have the same effect.
the smaller gear creates leverage which takes the torque that you put out unassisted and makes it out to be a lot more.

but then what does any of this have to do with the exhaust?


why in the heck would you want the motor to work to push exhaust gas out again I ask that?
where do you want the power to go?
if you want your motor to try to push exhaust gas out rather then put that energy into spinning your crank/flywheel and try to overcome the weight of the car thats fine by me but you are not going to do yourself any favor by doing that.
lets say your motor has 200 tq/ft ok?
wouldn't you want all 200 tq/ft to go to the crank?
instead from what you make it sound like you should put all 200 tq/ft to the exhaust to push it out b/c somehow that is going to create more torque.
you are wasting energy however you look at it when you put that effort into pushing the gas out
also most the time max torque is created before max hp is b/c how many motors do you know of that can rev up to 5252rpms and still have a upward curve on there torque
hp is (torque * rpms)/5252

this one here I will put up again
look at it
you are making such a contradiction to yourself I can't even believe it
the engine has to overcome stagnant exhaust.... guess what you are starting to prove what I have been saying all along true. you are redirecting the power of the motor to the exhaust. how can fighting the exhaust make your motor create more power?
stupid analogy but hey maybe you can understand this one
you(motor) get into a fight with some guy (weight of car,drivetrain blah blah blah) and instead of trying to beat the guy up you go and hit the wall (exhaust gas) in and you think that will make him hurt?
and also how can the exhaust gas move if there is pressure holding it back? it can't at least not as easily if there was no pressure. you dont have the scavanging effect if the gas is slow you need to have that movement and to get that movement you want as little amount of backpressure and as much velocity as you can get instead


yes you would launch forword. that massive amount of torque you were WASTING by trying to push me is now being used on what it SHOULD be used on and that is moving you and not me

since I am not fighting the exhaust also knows as you thing of water I am now able to put that power to better use.
could I have put a dent in the fridge if the thing of water was full? maybe but it woudl be harder b/c I am fighting the weight of the water kinda like you motor trying to fight the weight of the car and then you add in the backpressure from the exhaust.
and wait hey there was no backpressure in this case as it is since you said it there was almost nothing there so where did all that torque come from? according to you since there was nothing there I should have no torque to lift it up.

rx7speed

well ran out of room so on we go

I can see how you came up with the ideas you pointed out but the application in which they are used is wrong. you are thinking of how the torque is used and you are using more of it yes but not creating more

back pressure is not creating the torque effect instead it is making motor USE more of it to expell the gas.
you even said your self that the only way to overcome backpressure is to increase torque. but again the motor is not creating more torque but just taking it away from the crank and instead putting it as a pumping loss of geting rid of the exhaust gas

ChevyLuva3

quote: (from me)

{The slower something moves the more torque it has. Like for a bike with big gears, you're pushing really hard on the pedal and it taking a while to make one revolution,but man do you move when you get that push!}
quote:
---

quote: (from you)

{ok I am going to take the stock 4.10 rear on my car and put a 2.73 on my car since I will now have more torque and make my car go faster right?
you make more torque on the bike with the small gears it is called torque multiplication. why do you think a lot of ppl switch over there 2.73 for something with a higher number? but when you go to the larger gears you are sacrafising that torque multipliaction to gain more distance per less movement you put out. you are not gaining but losing torque in doing that and instead have to put out more torque to get the same effect}
-----------------

...You clearly do not know how to read. I just said a bigger gear will take longer to do one revolution. Which means its moving slower than a smaller gear. So apparently 4.10s will have more torque than 2.73s, cuz 2.73s will turn a full revolution before 4.10s, but apparently you don't like to read, and put words in people's mouths.

quote: (from me)

{But if you switched to a smaller gears, its way easier to push, creating almost no torque}
---

quote: (from you)

{a reason why it is easier to move. the torque that you are putting out through your own legs is being multiplied by the gearing so there is not the need to put out more effort.}
------------------------

...The reason why it's easier is that the back wheel will move more times than you, so you don't need to push as hard.

quote: (from you)

{when you put larger gears into your motor no matter how you look at it you are not going to gain torque}
------------------------

...You will not gain torque but you sure as hell will get torque to its max quicker, creating more torque. With small gears, it will take longer than big gears to get to max torque, bigger the gears, the quicker you'll gain your max torque.

quote: (from you)

{the smaller gear creates leverage which takes the torque that you put out unassisted and makes it out to be a lot more.}
-------------------------
...Leverage my ***, small gears have no leverage, unless you're the type who drives off starting at 3rd gear, oh yeah leverage.

quote: (from you)

{but then what does any of this have to do with the exhaust?}

...Its called an analogy, its used to tell stupid something so simple, in words they would understand, but apparently you can't get past that.

quote: (from me)

{because the engine has to overcome stagnant exhaust gases, during that time, you will have lots off torque, when the exhaust gases haved moved and now are creating the scavenging affect, the engine can you use its full power)}
---------------------

quote: (from you)

{this one here I will put up again
look at it
you are making such a contradiction to yourself I can't even believe it
the engine has to overcome stagnant exhaust.... guess what you are starting to prove what I have been saying all along true. you are redirecting the power of the motor to the exhaust. how can fighting the exhaust make your motor create more power?}
----------------------
...Once again you are confusing power with torque.

quote: (from you)
{stupid analogy but hey maybe you can understand this one
you(motor) get into a fight with some guy (weight of car,drivetrain blah blah blah) and instead of trying to beat the guy up you go and hit the wall (exhaust gas) in and you think that will make him hurt?
and also how can the exhaust gas move if there is pressure holding it back? it can't at least not as easily if there was no pressure. you dont have the scavanging effect if the gas is slow you need to have that movement and to get that movement you want as little amount of backpressure and as much velocity as you can get instead}
-----------------------------
...YOU ARE CONFUSING HORSEPOWER WITH TORQUE! And to use your little fight analogy, I would tense up my arm (creating stored torque) and hit dat fvcker hard. It doesn't matter how fast you move your fist at him, you better have a tensed arm and fist (meaning torque) or you aint gonna do nething to him.

quote: (from me)

{OK... I'll pretend you're in my way. I will have to slow down, as push harder to get you to move out of the way, but during that period of moving you away I'm creating massive torque. The second you're out of the away, I would probably launch forward, due to the torque I was creating now has no resistance.}

quote: (from you)
{yes you would launch forword. that massive amount of torque you were WASTING by trying to push me is now being used on what it SHOULD be used on and that is moving you and not me}
------------------
...Wasting? I was CREATING it. I was moving just fine until you got in my way, I slowed down, built up torque, and knocked you the fvck down.

quote: (from me)

{OK, one last analogy pretend ya have a gallon of water in your fridge, but there happens to be very little water in it. However you think there is still a gallon in it. I'm sure you've done this before, you ever lift the water thinking there was a gallon, and there was hardly nothing, and then ya hit the top of the fridge, I'm sure you have. Why did you do that? Cuz you thought that there was a lot of backpressure, so what did you do, you put tension on your arm creating torque. But since there really wasn't any more backpressure, you just dented the fridge. }

quote: (from you)

{since I am not fighting the exhaust also knows as you thing of water I am now able to put that power to better use.
could I have put a dent in the fridge if the thing of water was full? maybe but it woudl be harder b/c I am fighting the weight of the water kinda like you motor trying to fight the weight of the car and then you add in the backpressure from the exhaust.
and wait hey there was no backpressure in this case as it is since you said it there was almost nothing there so where did all that torque come from? according to you since there was nothing there I should have no torque to lift it up.}
------
... Just another analogy to bring you to the light, unforturnately, you still don't get it. Yes, you had torque to lift it up, because you thought it was heavy, so you tensed up, creating torque, and lifted it, but to your surprise you didn't need all that torque and you dented the fridge.

----------------------------------------------------------------------------------
----------------------------------------------------------------------------------
Well, after all that bandwith, BACKPRESSURE IS NEEDED. Like someone said before, if you free up some back pressure, you will lose low end torque and gain high end horsepower. That is due to the fact that the backpressure is gone, and doesn't need as much torque anymore. If we didn't need backpressure than (during the time of no emissions policies and all that cr@p) we wouldn't need manifolds, or even headers. The air would just come out and have no where to go (according to you no need for backpressure, then why have pipes in the way). BUT thats not so, backpressure creates the SCAVENGING effect. The pipes make the air go through them, and while the air is moving, its also sucking out the new exhaust gases coming out of the exhaust valve. I'm done, if you still don't get it, oh well, whether its a small amount or big amount, backpressure is NEEDED, period.

rx7speed

honest it is very easy to understand
when you have pressure in the exhaust you are diverting power that the motor would use against the crank to spin that which would then overcome all driveline loss as well as the weight of the car into instead using some of that torque on pushing the exhaust gas out

if there was a perfect vac inside the exhaust system it would pull all your exhaust out of the combustion chamber without the motor having to work at to do so and in that case you would gain a lot of torque.

if backpressure really was needed as much as you say it is then ppl would run there car with a 1 inch exhaust with 3 cats and huge *** mufflers

why do you think ppl put headers on?
to reduce the backpressure.

I agree that you use more torque from the motor to push the gas out but that is now wasted energy. again energy not used to push the crank.
do you not think that the motor has enough resistance by trying to move a car around?

Agent_D

i think back pressure is useless, i totally agree with RXspeed, i guess if i woulda had cats and some kind of exhaust restriction on my 14 sec n/a RX-7 that i woulda ran faster, it had a single sided 2.5" pipe with no cats or mufflers, no resriction at all, i noticed a big gain in torque and power with it and my other mods, but i guess were just idiots, also, my buddies Toyota Camry AWD 5spd, we put his exhaust straight out, he makes more power/torque than before, i guess were wrong though. but i dont need 3-5 litres to make power or run quick, 1.3litres is just fine with me. not to start a flame but back pressure is not needed, oh also, back pressure kills gas mileage too

ChevyLuva3

People put headers on to help the engine breath better. But the headers DO take away low end torque, but they give more high end horsepower. Ever see racing headers? They specifically state they are for racing only because of the amount of torque that is taken away, it would not provide enough low end torque for normal street driving.

rx7speed

also here is a something that was posted by one of our great moderators here

https://www.thirdgen.org/messgboard/...ML/005393.html

No, you absolutely cannot have too little backpressure. That is one of the biggest and dumbest fallacies that exist in relation to 4 stroke IC engines. Why? I'll tell you.
Pumping Losses The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at teh exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.

Scavenging We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving.

The same principles apply to the entire exhaust system, from primaries to collectors to pipes to mufflers. I am too tired to explain it all, books have been written on these topics. I have just scratched the surface, but hopefully you all understand a little better why their is no such thing as good backpressure. I know some of this has been a little oversimplified, but it think it gets the message across.
thank you please drive through...ed




or another one about backpressure is not being related to torque posted by five7kid

https://www.thirdgen.org/techbb2/sho...t=backpressure

They are NOT related!

What Apeiron is describing (and you with your PVC analogy) is maintaining gas velocity to produce the scavaging. Having too great a flow area reduces the velocity, reducing the post-pulse "vacuum" effect and therefore cylinder filling for the next cycle, reducing cylinder pressure upon ignition, reducing torque produced, and requiring higher rpms to get gas velocity up to where it does begin to produce the scavaging effect and increase torque, etc. (remember, horsepower is just torque times rotational speed).

In practice, you want to match the primary tube size to the RPM and cylinder volume of your engine (compression ratio has next to nothing to do with it). For example, a high-reving 305 can use as large a primary tube as a low-reving 400, but will seem lacking in torque - as much from the cam timing/duration as from the primary tube size.

Let's stop this talk about "back pressure = torque", and get an understanding of the physics involved. We'll all be much better off for it.



and also for your post that is right above mine the reason they sell them for racing only is yes they do reduce low end torque I agree. but not b/c of lack of backpressure but instead b/c of lack of velocity
when you have the exhaust moving fast it makes a low pressure zone behind it. that it turn will reduce the effort the motor needs to spend on moving the exhaust so instead it can put power to the crank. also it will pull more of your exhuast out ofthe combustion chambers once again I say that.
that is called scavenging effect not backpressure

Agent_D

to help it breath better, hence reducing back pressure, if there is back pressure you think the engine can breath??

Jza

Hey, someone remembered my PVC analogy.

The fact is, you try to dumb it down so a newbie can understand it, and some of the Richard Nogginses tried to make it into a highly technical debate (which is happening again). BTW, I skipped over a lot of the lengthy posts, so if I just repeated something, sorry.

Just to answer the poor guy's question: the pipes are gonna be way too much for your motor, and good luck fitting them down there besides... On a ladder bar or older suspension type car, it'd be easier because you could run a pipe on either side of the driveshaft. But on one side of OUR driveshaft lies the torque arm.

rx7speed

um Mr. D you make things way too easy

but chevyluva3 dude
read this
https://www.thirdgen.org/messgboard/...ML/000453.html
jrr has a good explanation on this

This myth of "backpressure" and good street throttle response is completely backwards. The correlation is spurious at best. "Inertia" is the guilty property where headers are concerned.
The "backpressure" that is seen in a small diameter header tube is caused by the column of gas not wanting to move RIGHT AWAY when the slug of exhaust gas comes out of the port.

In other words, the slug comes out but the gas in the tube is at rest and will compress before it moves. Once the column of gas in the tube starts moving, it will want to keep moving. THIS property of inertia (a mass of gas in motion) is used later in the exhaust cycle to create a relative vacuum and scavenge the combustion chamber. There is a certain RPM range at which the compression and rarefaction waves are ideally suited for scavenging. This is when the header tube is "resonating". Header tubes that are too large will not have sufficient velocity to prevent the rarefaction wave from pulling the exhaust back into the cylinder. Hence, sucky performance on the street.

A properly designed exhaust manifold works just the opposite way. It is designed to offer no reaction at idle and low RPMs. A good exhaust manifold has almost no runner and a large open space for expansion. The slug comes out with no column of gas to push out of the way and lots of room to spread out. The drawback comes in the form of turbulence and lack of a column of moving gas to scavenge as RPMs go up. Somehow, people look at the backpressure problem and jump to a false conclusion about what makes low RPM torque.

The presence of backpressure reveals the deficiencies of either design. Loss of LOW RPM performance, after installing headers, is NOT caused by lack of backpressure! Loss of street performance is caused by the inertia of the gas in the tube and the carb not being jetted properly for headers. Most stock carbs are not calibrated with headers in mind. Again, loss of performance and jumping to a wrong conclusion about backpressure.

Bottom line: The myth of performance and backpressure is completely backwards.



as well as Glenn91L98GTA another one of our great moderator

Backpressure is bad...it is a complete myth that should be exterminated like the plague.
What people are confusing is VELOCITY with backpressure. You need to maintain the right velocity to obtain maximum torque. This is the argument behind too large of headers or too large of an exhaust system. On smaller engines they don't maintain enough velocity to obtain maximum torque at lower engine speeds.

Backpressure is when you have restrictions that do not necessarily maintain optimum velocities. Similar to shoving a potato with holes in your exhaust system. Yes, you have backpressure, but you have no velocity. An extreme example, but old pellet type catalytic convertors or certain mufflers do just that. They increase backpressure, but do nothing for velocity.

1 5/8" headers vs 1 3/4" headers introduce very little backpressure, but the 1 5/8" headers maintain higher velocities. At lower rpms this promotes torque. At higher rpm, the 1 5/8" headers become saturated with exhaust gas due to the volumes they can hold from the cylinder, and produce less power at top rpm. At this point they introduce backpressure and kill performance.

Conclusion, backpressure is BAD!

ChevyLuva3

Hello?! I just said if you got racing headers you can not drive it on the street! Why? Cuz the racing headers took away too much back pressure (and low end torque). Which means now you only have a car for racing! Now what do you do if you want the same car with the racing headers to be able to drive on the STREET? Thats right INCREASE back pressure! So back pressure apparently is NEEDED. Period. Period. Period.

rx7speed

hey nothing else to do this time of night

rx7speed

wrong again bucko
you make velocity higher
not backpressure
there is a BIG difference

Agent_D

some people just dont get it. BACKPRESSURE IS BAD, gets you bad gas mileage and kills your power. like you said earlier, LEARN TO READ, maybe if you go back to the 3rd grade and learned to read again you might be able to understand this. velocity is what makes the torque, NOT back pressure. like is stated in the post RX7speed listed, go stick a potato in your exhaust pipe if you want back pressure, see if you make any power

rx7speed

thanx man I really needed that laugh

ChevyLuva3

Yeah well since you guys seem to want to be smart today. Then tell me why Racing headers take away so much low end Torque that its not streetable?

rx7speed

the reason why a racing header is going to reduce low end torque is b/c when the exhaust leaves the port on the motor it is going to go into a large chamber that is going to create backpressure b/c the current of exhaust will flow back on itself and start eddies.

also with this large pipe system you are not going to maintain the velocity needed b/c of those eddies and reversion of the exhaust

you now lose the scavenging effect that the exhaust has which in turn will cause low end power loss


read a few of the post up above that were made by the mods as well as jss

they will explain it quite well

gettaway305

From a porsche technology consulting page:

Q. Why do engines need backpressure?


A. I’ve always understood that you want to keep the exhaust gases moving, even once you get them out of the cylinder. One thing that will cause the gas to slow down is a sudden decrease in temperature, which is one reason why auto makers don’t try to cool the exhaust manifold. The other thing that can cause a decrease in flow velocity is rapid expansion and turbulence, like going from a small passage (like the exhaust port in the head) to a 6-inch pipe. A smaller-diameter pipe geometry will tend to keep the flow rate up, but it will also lose heat more quickly (less exhaust gas per linear inch of pipe). However, a large pipe will slow the velocity due to expansion. Worse still, the exhaust is constantly cooling from the moment it leaves the cylinder, meaning it’s getting denser and slower. See the problem here? It’s all about compromises. The proper pipe size is going to be influenced by the flow rate (volume rate, which is related to RPM and engine displacement), exhaust velocity (again related to RPM), exhaust temperature (constantly changing, and as high as 2,000&Mac251;F, if I remember right), and undoubtedly an array of other factors. All of this is dependent on the application: is this a street car, a race car, or something in between? Where will the engine spend most of its time? Idle, full throttle, part throttle?

I’ve also heard the thing about back pressure, but I believe that they’re really talking about flow velocities. That is, if you can keep the exhaust gases moving in the exhaust pipe, they will cause a small reduced-pressure area behind the closed exhaust valve, in the exhaust. This happens because the gases have momentum. They move away from the valve, creating a localized reduction in pressure. When the exhaust valve opens, this reduced-pressure zone will help evacuate the burnt gases from the cylinder. Or so the theory goes, as I understand it (two disclaimers there).

The bottom line is that Porsche has probably already considered all this and other factors when sizing the exhaust on the 944. Unless you’re significantly increasing the flow rate, you’re likely to cause more harm than good.




All of this is dependent on the application: is this a street car, a race car, or something in between? Where will the engine spend most of its time? Idle, full throttle, part throttle?


This is where racing headers and a lot of you people being wrong comes into play.

gettaway305

So in answer to the question in the thread, dont go ape **** when picking out new piping and exhaust, keep it fairly conservative, like a 3" to dual 2.5" pipes or something

QwkTrip

iTrader: (22)

Chevyluva…

Erase your brain and start over. RX7Speed is correct. Read his posts and learn.

There are a few fundamental ideas you need to learn.

1. How does air flow? Air flows when there is a pressure differential. It flows from a high pressure region to a low pressure region. Temperature changes play a role here but I don't want to get into that.

2. What is a restriction? A restriction is anything that impedes the flow of air. This includes anything that changes the direction of air flow such as pipe bends, muffler baffles, o2 sensors in the flow path, pipe size changes, unions, joints, etc.

3. What is “backpressure?” It is a concept and a concept ONLY! It is conceptually any form of pressure that impedes the flow of air from the engine cylinder to the tail pipe. Personally, I hate the term backpressure. In all my years of studies in engineering, including 7 courses in fluid dynamics, I never once heard the term backpressure.

Imagine for a moment you’re in the engine cylinder. The piston moves up and pushes the air out of the exhaust valve. This creates a high pressure region at the exhaust valve. Air will flow to a low pressure region. This happens to be the tail pipe. Any restrictions in the exhaust system will impede the flow of air from the exhaust valve to the tail pipe, effectively creating “backpressure” in the exhaust system. This backpressure works against the air flow. ANYTHING YOU DO TO INCREASE BACKPRESSURE WILL DECREASE FLOW RATE and decreased flow rate will reduce your power output. Additionally, the interaction of the air streams from each header tube as they merge into the exhaust pipes has an effect on the air flow.

The purpose of an exhaust system is to purge air from the cylinders. The idea is to maximize air flow from the engine to the tail pipe. Different engines move different quantities of air at different engine speeds. The quantity of air you need to move determines the capacity of the exhaust system (pipe diameter). The time you have available to purge air from the cylinder determines the design of the header tubes. Keep in mind that the time available to purge air from the cylinders is dependent upon the camshaft profile and the engine speed. The smaller the cam the less time the valve is open. The higher the engine speed the less time the valve is open.

A street engine doesn’t move much air. In order to keep the air flow rate high enough to purge the cylinder in time, a small diameter header tube is employed (1-5/8). The rest of the exhaust system is sized appropriately for the volume rate of air that the engine flows. Again, anything you do to increase backpressure will decrease flow rate and decreasing flow rate will reduce your power output. Putting 3” duals on an ordinary engine is complete overkill because the engine doesn’t need that must flow capacity. In fact, due to the dynamics of air flow it’s actually hurting performance. But that’s too complex to describe here. A race engine moves a lot of air and it does it at high rpm. The shear volume rate of air needed to be moved increases the size requirement for the exhaust system. The header tubes will have to be increased in size but it’s still important to keep them small enough to efficiently purge the engine cylinder AT THE RPM THAT THE ENGINE WILL BE USED. Everything has to be sized appropriately. Bigger is not better. Small is not better. Getting it right is better.

QwkTrip

iTrader: (22)

Ditto on the Porche artical. That was posted while I was drafting my response. Good stuff.

I suspect the concept of backpressure is a result of engine builders writting those tech books you can buy at the auto store. They don't really have the education behind them and perpetuated the myth of something called "backpressure." Like I said, I never heard of any such thing. I tried to relate it to flow resistance but that was just for the sake of context for this conversation. I would suggest you all just drop the term from your vocabulary.

rx7speed

qwktrip you have far too much time on your hands

nice write up though


too bad we can not put this one at the top of the page with a sticky or something that way we can try to get rid of the backpressure myth and also there is some good info here from ppl like you and a few of the others that have been talked about

QwkTrip

iTrader: (22)

Never say I'm useless

Mark A Shields

Ok, I'm no auto genius and I'll admit that I was wrong. But one thing, so it's ok for a stock Honda Civic to run 3" duals. His car wouldn't benefit better from having some backpressure.
Also there was a post a while back that listed HP ranges and the pipe size you should get. Then why even have that just slap a 4" pipe on a stock LG4, wouldn't a 3" would out peform it.

QwkTrip

iTrader: (22)

ChevyLuva… I think I figured out exactly where you’re confused.

You are saying that by restricting the exhaust system (creating backpressure ) you will create more torque.

First of all, drop the term backpressure. I don’t want to hear it any more. That in and of itself is your main point of confusion. Anyway, we need to back up for a second and understand torque. Torque is the work that the engine can deliver. As the engine goes from idle to redline it will produce different amounts of torque at various rpms. As you change the configuration of the exhaust system you will change the state of tune of the engine and effect how much torque it makes throughout the rpms. By restricting the exhaust system you take away the ability for the engine to develop torque at higher rpms and will increase torque development at lower rpms. Why? Because the exhaust won’t flow well at higher rpms. Note that if you restrict the exhaust too far you’ll simply reduce torque across the board and have a dog of an engine. Likewise, if you use a larger exhaust system you will encourage torque development at higher rpms and will make less torque at lower rpms. Why? Because the exhaust is tuned to flow well at higher rpms but is inefficient at lower rpms. Same warning applies though. Go too big with the exhaust and you will reduce torque across the board and have a dog of an engine.

What you need to realize is that Hp is a function of torque and rpm, where

Hp = (Torque)(rpm) / 5250

What you see here is that the more torque you can make at higher rpm, the more Hp you’ll develop. So, which is faster… a car with a dinky exhaust system making tons of torque down low or a car with a healthy exhaust system making tons of torque up high? I’ll take the car that makes more Hp.

The other thing you should realize is that for a street setup, a car making a ton of Hp will make a lot more torque down low anyway. The rpm limit on a street engine is so low that whatever you do to make Hp at 5000 rpm is also going to bump up your low end torque. It’s a complete waste of time to purposely build low end torque.

QwkTrip

iTrader: (22)

You're doing the same thing! Stop using the term backpressure. It's a falacy. It's the root of your problem. It doesn't exist. I made it very clear that you have to tune the exhaust system to the engine. This means factoring in flow rate, engine size, power band, and more.

3" duals on a Honda Civic is obviously overkill. There's so much capacity there you might as well dump straight to the atmosphere. You size the exhaust and headers to help scavenge the air from the engine. The only case in which it might be acceptable is if you put out 700+ Hp from that little Civic.

4" exhaust on a 305 is the same problem. It's not appropriate for the state of tune of the engine. Those 4" systems are equivilent to 3" duals and were made for people who are running 10 second quarter miles times. Match the exhaust to the engine for the best performance. the 305 actually has a tough time using a 3" single. A 2.5" single would sacrifice a tiny bit of peak Hp but likely give better overall performance.

rx7speed

now just to throw in a little more fun
what about a turbo?

I am running 3 inch off the turbo into dual 3 inch pipes

I know that is a LOT of exhaust

this is the point I start to get a little iffy
I know that you want high velocity but with a turbo you also get caught with wanting to reduce as much obstucrtion and make max flow a lot higher as you can to help spool the turbo up though I know overkill is easy to do

maybe just not thinking about it as I should and thinking that a turbo setup is going ot make things a whole lot different when it is about the same

I am sure a lot of the same rules apply but how do you tune for something like that?

QwkTrip

iTrader: (22)

I don't know much about turbos. I'm just taking a stab at this. Correct me if I'm wrong...

Turbo is different. They are flowing more air by mass than a naturally aspirated system. Where this really comes in to play is that you have pressurized air entering the turbine that drives the turbo unit. When the air exits the turbine it has greatly expanded and cooled. This expansion very much increases the volume of air you are flowing (mass stays the same). The increased volume requires that you enlarge the exhaust system after the turbo.

The other factor is that you probably make more Hp by creating as great a pressure drop across the turbine as possible than by focusing on exhaust gas removal after the turbine. By putting a honking system after the turbo you create a larger pressure drop across the turbo which will allow you to make more power.

By placing the turbo in the exhaust path and by pressurizing the manifold you've changed a whole lot. Maybe someone else can shed more light on the subject.

ChevyLuva3

I think I also know where I got confused. Sorry QwkTrip, I'm going to have to use the word Backpressure first tho. I considered Backpressure to be the restriction in exhaust, I refer Backpressure to the EXHAUST PIPES. So when I say " you need Backpressure, I meant you need Exhaust pipes that weren't too small or too big." I always considered exhaust pipes was the backpressure. So when I heard someone say "you do not need backpressure", to me, it sounded like they were saying they didn't need exhaust pipes, or get bigger ones. So thats why I thought you do need it. See what I'm saying.

But Rx7Speed has shown me that Backpressure is created from the GASES existing in the exhaust pipes. Not the exhaust pipes. So if the gases slow down, or just sit there, it will create backpressure.

So check this out... we were both saying the same thing just with the wrong words.

I thought Backpressure meant the Exhaust Pipes. I thought that if you get TOO BIG exhaust pipes that would DECREASE backpressure, and give bad power. So thats why I said you need backpressure, meaning you need good sized pipes. (not neccesarily big pipes) So seeing my view, when someone said "you don't need backpressure", it sounded like they meant you need either bigger exhaust pipes or no exhaust pipes.

But Rx7Speed, said Backpressure was the exhaust gases. So getting TOO BIG pipes would INCREASE backpressure, giving bad power. Why? Because the pipe is so big that the SCAVENGING effect won't pull the exhaust gases out. It would just sit there. So when the valve was opened there would be no vacuum and hit stagnant exhaust gases. So from his view, when someone said "you do need backpressure", it sounded like they meant the engine will run better without the scavenging effect and with stagnant gases.

So if you look at both point of views, you can see where we are coming from. So even though we used the wrong terms, or atleast I did. We come up with the same result...
TOO BIG PIPES ARE NOT GOOD, TOO SMALL PIPES ARE NOT GOOD, ONLY PERFECT SIZED PIPES DESIGNED TO WORK WITH YOUR ENGINE WILL WORK.

And for the effect of turbo, the SCAVENGING effect does not apply to either turbo or supercharged engines. Get big pipes, period.

...And since QwkTrip doesn't like the word BACKPRESSURE, lets now refer to it as "a slowing down of the velocity of the exhaust gases, decreasing it's vacuum."

rx7speed

a GREAT explanation on the whole thing can be found at

http://personal.riverusers.com/~yawpower/jantech.html

http://personal.riverusers.com/~yawpower/febtech.html


granted the guy builds rotary motors but you can still apply a lot of the same theory to a piston motor I would think

rx7speed

chevyluva3 it's cool though

you let me kill some time and kinda nice to have a debate

and if nothing else I have put out almost 100 post in the last 24 hours :-D

ChevyLuva3

Yeah, it was a cool debate, learn something new everyday.

JPrevost

I'm just starting out in the Ohio State engineering program so nothing I say is from the books but why aren't you guys seeing the car as a system and not just a piece of the puzzle. I don't about your small analogies, they mean jack when you don't start with the whole system. The system would include performance and that is measured in how fast you can get a car from point a to point b. Now look at the weight of that car, the distance, the power of the engine, the shift points!!!, the exhaust flow, intake flow, air/fuel ratio blah blah blah.
From my personal experience, a v8 has lots of torque, give it the best flowing exhaust system. Notice key word best and not biggest. Also another key word is flowing as in not stop and go with pressure problems near the valve etc. If you have a heavy car and a small engine you don't want a big exhaust system unless you plan on making power way up in the rpm range. If that's the case you also have to look at the whole package which includes gears, shift points, and the cam specs to figure the engine power band. You want the flattest torque curve possible!!! How many times do you think I've said that this year, I don't even want to think about it. Anyways...to get a flat torque curve do you think I should have a HUGE intake and a HUGE exhaust system with a mild cam and a stock torque converter!!! Get my drift? Simply put, you want the best flowing exhaust system for our cars and that has been proven by countless companies making exhaust equipment for racing and street applications. THEY have the engineers, THEY know what they're talking about, THEY get PAID to know what they're talking about. If it was the case that bigger is better then don't you think flowmaster would have 5" mufflers and hooker would be making street headers with 2" primaries.
So I hope this made some sence and you picked up some info because I'm still lost with the whole exhaust valve, gas temp, cam duration system. Maybe I'll learn more about that next quarter.

gettaway305

Doubtful...All that engineering education gets you is the tools to be able to learn what you want on your own

QwkTrip

iTrader: (22)

Actually, before people jump all over this guy I want to back up what he's saying. You will learn nearly squat about cars in engineering unless you take specialized automotive courses. All it does it give a technical knowledge base to pull from and analytical skills. But even at that most the information you'll learn in engineering won't be applicable to cars. You might realize that most people with the faster cars out there are NOT engineers. They are hobbyists.

Don't for a moment assume that the companies selling speed equipment really know what they're doing. They're out to do one thing: make a dollar. They're priority is NOT to give you the best equipment for your car. They're priority is to give you something that you'll buy. Some products suck, some are good, and some are great. Price is usually a good indicator of which is which. You're a bit idealistic in your views.

Chris87TA350

Hmmm, I didn't mean to, but it looks like I sparked a huge debate here. Thanks for the info, guys, I think I really learned alot about exhaust tech! I'm glad you guys worked everything out. I too, agree that Chevyluva and RX7 were almost saying the same thing. Regardless, I get it. I knew 3" duals would be overkill, but I didn't know that they'd actually hurt performance; now I do. I think I'll have the guy weld me up a custom single 3" exhaust with that fully welded Summit muffler and a cutout before it so I can go uncorked on Powertour. Anyways, if anyone thinks this is a bad idea, lemme know; I really trust you guys who know what you're talking about when it comes to fluid dynamics. It's been great reading your replies, fellas. Thanks again!

gettaway305

Thanks! Some people think that after graduating in engineering you can do anything, but most of the skills you learn come from actual job experience. As a junior in engineering i can assure you that the main things taught are physical laws and working in groups.

SSC

Geeze, I wish I knew this was going on in this post! I would have backed up RX7 from the word go. There was a post about a year ago similar situation and we hammered it into the ground final result was backed by a couple of Moderators. Backpressure is not required by any engine to operate! Less is better. Headers reduce backpressure and if one reads the gains from installing headers (the promo from manufacture) they say, "reduce backpressure and can reduce 60ft times by one full second!" That falls into the torque category. Removing C/converters increases power by reducing backpressure. The story of power loss from removing converters comes from cars with O2 sensors behind the cat. The owner didn't realize the O2 sensor would be sniffing rich and that sends lean down to the ecm due to the increase in flow through the engine, hence reducing power by running lean. Modern machines gotta love the stupid things.
Anyone who thinks more backpressure is better or removing the cat from their call will reduce performance hasn't got a clue.
I don't mean to flame anyone but the gospel must be spoken.

Pastor Clint
SSC

Edit: Forgot to mention, 3in would work great but 2.5 is standard for 90% of cars running duals. I have a truck and the camaro running 12's (thats non correct also@4800ft)on 2.5in pipes so dont shy away from a lesser pipe that would make installation easyer and cheaper.