Okay, i've heard many sides to the backpressure issue. At first I had thought that insufficient backpressure would cause lower engine compression. On some accounts, backpressure is a bad thing.

What is the truth? I would like this settled in my mind once and for all.

 

Search for Five7kid's explanation. Back pressure is never good.

Even Popular HRTV had a short explanation of it, you want zero backpressure.

Don't listen other wise.


People confuse it with tuning the pulse of the exhaust.

 

Ah, that cleared things up alot.

Damn, I wish I could undo what I did to my exhaust today.. sigh. Homemade exhaust is a no no unless you know what your doing

 

yea but if u had no back pressure woulndt the exhaust valves burn up?

 

No, that's not true. The improved scavenging in a free-flowing exhaust actually makes for cooler exhaust valves because the heat and pressure are reduced.

 

I assume you were referring to f-crazy's reply.

 

Backpressure = bad
Scavenging/Proper velocity = good

Pipes that are too small = bad
Pipes that are too big = also bad

Valves burning: I've only head this happens on either NO manifold at all, or maybe open shorties. Its just a heat extraction thing, nothing to do with backpressure as far as I understand.

 

Back pressure takes away top end horse power. Too large a exhaust can possibly take away the scavenging ability of your exhaust system thus a 305 with 3 inch true duals isnt nessesarily gonna be the right choice. Back pressure is not required.

I'm thinking that all the countless hours of testing by edelbrock, flowmaster, hooker, SLP ect... was for a reason. Unless your running nitros or are super or turbo charged I doubt runnning anything more than a set of quality headers and a hooker or flowmaster cat back system would make much if any diffrence. but it does make a diffrence to have one of those systems compared to stock manifolds and stock exhaust. It not only sounds off, it helps make more power.

 

so me running around new york with an open exhaust wont do anything but **** people off ...well thats good to hear cuz i dont have the money for an exhaust so i dont know how id get money to replace valves..but i gotta look out for people running out of there apartments chasing me down he block cuz i set off every car alarm down the street lol

 

what would happen if you put a high speed fan in the exaust system that would basicly suck all the exaust out of the heads? somthing that would put the cylinders into a vacume at TDC and create even more of a vacume when the piston pulls back down. any one have any thoughts on this kinda idea?

 

id think that would be more of a resriction then anything. Proly hurt more than it would help.

 

how do you figure it'd be a resriction? with a strong and fast fan that would suck the exaust gases out i'd think it would improve the flow, wouldn't it?

 

I guess it would just depend on how you did the fan. It would have to be a pretty high powered fan. Also if was possible to put the motor for the fan somewhere else besides behind it then it might work.

 

LMAO, sounds like that Tornado rip off.

Exhaust gases already pulsate rather quickly, that fan would have to spin pretty fast, I would think, and that would just put more of a strain on the alternator pulley , thus eliminating any benefits.

 

good luck finding the power to spin up that fan... let alone the hot exhaust gases will destroy it :lala:

 

now ive read a few places that if we could come up with somthing to actually suck the exhaust out of the cylinder during overlap (ahem! scavenging!) we would negate the need for super-long duraiton exhaust valve openings and also clean out the combustion chamber for the fresh charge...(scavenging) which should be forced in via-supercharger. heh, sounds like it would be one hell of a ride. say, did somone say the word "scavenging"? I could have sworn...

your right it would never work.
theres no such thing as "scavenging". nope. nada.

 

If it would actually work, somone would have probably done it 30 years ago.

 

it does work and they have been doing it for 30 years. its called scavenging and almost all cams have it ground in....

 

What length exhaust would be too short for proper scavenging? I assume dumping before the rear tires isn't too short. If you put exhaust length into google and a few motorcycle specific length calculators come up, but nothing for V8s.

When you put a crossover into the exhaust, you find the hot spot and put it there. Would there be any significance to that point for overall length as well?

 

hehe

hey mark you think I should bring my thread back up???? *insert evil grin*

 

simple answer


backpressure makes it hard for the exhuast to get out of the exhuast system therefor you have a bunch of burning hot exhuast gasses just sitting around transfering heat into whatever is around them

that is bad

get rid of the backperssure and exhuast gas is outta there so it keeps things cooler

 

I will get back to this when I have more time


right now though it's kinda short

 

As for the exhaust fan idea, remember that the volume of exhaust gas produced is way larger the volume of air breathed in through the intake. The internal combustion engine relies on the expansion of gases to produce energy. Therefore, a fan to evacuate exhaust gases from the headers/manifolds/what have you, would have to move many times the volume of air moved by a forced induction device on the same engine. Think of it as a 'reverse' turbo that removes exhaust gas from rather than stuff air into the engine, only several times larger. Hell, if you had such a working setup, you could probably use the exhaust gases for added propulsion . I dunno I'm just rambling here but it really doesn't seem worthwhile. Not enough gain for a LOT of work.

 

ummm not so fast junior....the volume of exhaust gasses is actually quite a bit less than that of the intake charge. Because of the combustion event a portion of the volume of the gases are used up (changed to energy) in the form of heat mostly and sound energy. think about it.....thats why exhaust valves and ports are so much smaller than the intake valves and ports. they dont need to flow the same volume of gases.

 

383backinblack, I think you're off there. I'm no physicist but I do know this much that any internal combustion engine, be it piston or Wankel, uses the rapid expansion of gases due to the heat of combustion to push the piston down/spin the rotor. Heat or sound energy alone can't do that. Since expanding gases push the piston down, it is only logical that the volume of exhaust gases is larger than the intake charge that produced those gases.

I stand firm by this unless you can present some proof to the contrary. I agree exhaust vlaves and ports and valves are relatively smaller than intake valves and ports but in no way does that prove that the volume of exhaust gases is actually smaller than the intake charge that produced it.

 

The reason the exhaust valves/ports are smaller than the intakes, is b/c the exhuast has a piston to push it out. Without a power adder, the intake has nothing to push the incoming air in, except atmospheric pressure. Now, here is my take on backpressure. When I was younger, I was told that you take the pipe, w/o mufflers, straight off the headers, and paint it. Run the car for awhile. Look to see where the paint is not burned off, and cut the pipe and install the mufflers. That was all you needed. Well, if that holds true, it probably wouldn't work on a 3gen. On to present times. I ran my car for awhile with open headers. It ran good. It would rev to the moon, and get there real quick. It did lack torque, but the fact it would rev so fast, and blow the tires off, I didn't care. My friends 2000 Z-28 would take me out of the hole, until I got mine wound up, and kept it there, then I could go around him. Anyway, I have exhaust now, and the torque is awesome. Hole shots are all mine, but this cost the topend pull that I had. On the street, I didn't care. If it were a track car, I would. On the street, I want my torque. I don't want to rev the pee out of the engine to move at my pace. I would venture to say it is personal preference. I would almost guarantee there is a certain amount, whether it is none, or a lot, of backpressure that every engine needs to run at its best. I would think all the millions of dollars spent on NASCAR, and drag racing for R&D, has produced some numbers to go by.

 

For the ideal length Question... You'll see adjustable length header turndowns for sale in jegs and summit of course there only for open exhaust which doesn't do a whole lot for you unless you've got some power adders or are being restricted quite a bit with you current exhuast. I'd guess that the lenght of pipe would have some effect, just not sure, its the same as with shorty headers vs long tubes. I'd guess that the "Ideal" length would be pretty short though as in only a few extra feet of length if that.

As for an exhaust fan it wouldn't work. Exhaust is pushed out of the cylinder by a nicely sized piston, and typically doesn't need a whole lot more help. I think I've heard of hooking electrical fans up through the intake but they were complete scams, at the speed that they would actually help the engine out they couldn't supply enough air and actually became a restriction.

 

any backpressure is BAD

there is a difference between backpressure and scavaging

 

I've heard the same method described to find the ideal place to put an H or X pipe.

 

<b>there is a difference between backpressure and scavaging</b>

Finnally the truth.
ZERO backpressure will ALWAYS give you the BIGGEST horsepower numbers. Always. 4 cylinder, 6, or 8. dont matter. always.

now, having a CERTAIN amount of "scavenging" is what we need to make the proper "torque curve" on the street. without "scavenging" our VE% drops so low it just wont make any cylinder pressure. and the reason is simple, piston or no piston, exhaust gasses are stubborn as heIl. at higher rpms we have the cam's "overlap" to help purge them. but at lower rpms, the air just isnt moving fast enough. not to mention we dont get that much air to begin with... and the effect is multiplied by "big" cams. a big cam with super open exhaust will make almost NO low end torque. the engine will sputter and backfire, wont make any power. putting a somwhat restrictive exhaust on a "big" cammed engine somwhat "bandaids" the problem, but then you have a bottle neck up top, which defeats the purpose of having a big cam in the first place.

you cant win. big cams and street cars dont mix. we know this. but what people cant seem to figure out is big exhaust and street cars dont mix either.

Now when i say big, I'm not referring to that 3" mandrel bent catback. thats not big. thats perfectly acceptable on a 305 even, provided a decent muffler is used. When i say big, im referring to your open headers, or your duel 4" dumps. thats big. try that on the street somtime with a big cam... and street gears with street stall. uh uh.

we want scavenging on the street, plain and simple. the only way to get scavenging at low rpms is to create a certain amount of backpressure, unfortunatelly. we dont need much though, just enough to "help" the exhaust get out. like putting your finger over the end of a hose, adding a little backpressure helps get it all out.

 

and back to the fact that exhaust gasses are larger than intake charges...

Conservation of mass. You can't make it or destroy it, but you sure can make it bigger

 

Why do you say that overlap purges exhaust gasses from the cylinder?

I think your analogy is flawed.

 

OK..... here's the deal.

rx7speed is right (along with a few others) that backpressure is BAD.

The only way scavenging is effective in an engine is due from valve overlap. If there isn't any overlap, forget it. Your common street cam doesn't have any overlap. It takes over 220*/dur.@ .050" on a 110* LSA before any overlap takes place. Over 224* @ .050" on 112 LSA, and 228* @ .050" with 114* LSA. And even if you have a cam with overlap, it's irrelevant unless the cam timing (intake centerline) is set to use that overlap to scavenge the cylinder.

Too large of an exhaust is not good BECAUSE exhaust gases are HOT. If they flow into a pipe that is huge, they cool faster. The cooler they are, the slower they move. That's also why exhaust valves are smaller. The hot exhaust is easier to move, therefore requiring less area at the same rate of flow.

The guy that mentioned the "paint the pipes" trick works. The reason; you find the area where the exhaust becomes coolest (or at least cool enough to not melt paint). That's where you want to get rid of the exhaust. The more cool, the more dense. The more dense, the slower it moves. So get it out ASAP!!!!.

There are also 'pulse waves'. This is a little more technical than a street car will ever need, but the theory is to keep any lost pressure from flowing back up the pipe into the exhaust port of the cylinder head. This would cause the exhaust to cool even faster, which would (as mentioned) cause it to flow at a slower rate. This is also in relation to RPM. The higher the RPM, the less time for negative pressure to flow back up into the exhaust pipe. So the only thing it benefits is low RPM torque.

Then there's a "pressure wave terminator box." You can attach a large "empty" muffler (at the same spot as mentioned earlier with the paint trick) and it becomes the point where the exhaust "dumps". Then you form a tapered pipe from that 'box' into another pipe that flows into a muffler. This gives you all the benefits of open exhaust, while still keeping it "quiet", and you lose no flow at all. The only downfall is the box has to be as large in displacement as the engine (actually 8 x the CID of one cylinder for you V6 guys ) A 350 CID box!!!!! BTW, That's only .2 cu/ft.

Don't believe me? Read "How to Build Max Performance Chevy Small Blocks On a Budget" by David Vizard. He mentions all these things in that book. There are all kinds of formulas to play with if any of you love punching numbers.

 

Scavenging has to do with exhaust gas velocity. Momentum of moving exhaust pulses help to pull the gasses out of the engine. Since momentum is mass x velocity, high velocity is good. This is where the confusion comes in. High velocity comes from smaller exhaust tubing. So, assuming smaller tubes could support the required flow, they are good. However, small tubes can't flow the same volume as easily as larger tubes (frictional losses, etc). So, we need to be careful not to confuse velocity with backpressure. Yes, small tubes create both. Velocity is good, but backpressure is bad. Actually, I'd say backpressure is worse than velocity is good, if I have to pick sides. Ideally, you'd want the smallest tubing that will support the flow that you need. Not oversized (reduced velocity/momentum) and not undersized (can't flow enough).

 

I know that, and you know that, but I just wanted to make sure that Kingtal0n knows that. He's getting cylinder scavenging mixed up with header/manifold scavenging.

 

<b>Why do you say that overlap purges exhaust gasses from the cylinder? </b>

everyone, even somone stupid like me, knows that overlap is the primary reason the exhaust gasses get "purged" right after the exhuaust stroke. its why we HAVE overlap, and different LSA's. the incoming air from the intake valve, rushes in the forces the rest of the exhaust gasses out. its called "overlap" and "purging exhaust gasses" as you call it.

<b>I know that, and you know that, but I just wanted to make sure that Kingtal0n knows that. He's getting cylinder scavenging mixed up with header/manifold scavenging.
</b>

they are the same thing, when you look at it from a street point of view. You CAN NOT properly scavenge a cylinder without PROPER header/manifold scavenging, at low RPMS on the street.. they go hand in hand. having a big exhaust and a little cam is bad, like having a little exhaust and a big cam. both of those combos will mess with your low-rpm scavenging, both in the cylinder, and in the exhaust.

<b>I think your analogy is flawed.</b>
it wasnt the best darn thing i could think of, but its the same thing. having a smaller exhaust promotes higher exhaust gas velocities (putting your finger over the hose) and THAT helps scavenging. i think it was a darn good analogy.

<b>Your common street cam doesn't have any overlap.</b>
ALL camshafts have overlap. ALL of them. I dont care what size engine it is. the intake valve ALWAYS opens before the exhaust valve closes. and thats called overlap.

Here, read it all for yourself:
http://compcams.com/catalog/007.html

 

Dude...... you should have stopped while you were ahead.

Now you're in for it. :sillylol:
Well you got one thing right....... in your first sentence.

The intake is rushed IN by the exhaust gases PULLING THEM IN, not the other way around. It says that in the link you just posted. If you're gonna argue, at least try and understand what it is you're arguing about.

Wrong. All you have to have is a free flowing exhaust. As long as there is no restriction in the exhaust, then scavenging will happen. It's just to what degree that headers help.

Putting your finger up your butt doesn't make the poop come out faster!!! So maybe you should get your thumb out of yours.

I was speaking @ .050" , since that's the standard for which duration is measured these days. If you want to talk about advertised duration, then that's useless because different companies advertise at different lifts. Some us .004", some .006", etc. That's why .050" was established. But to say that ALL cams have overlap is wrong. Don't believe everything you read.

 

i once drove my car all summer with open ypipe. no problems with valves. reflecting back it sounded like ***, unless i was pulling maybe 4500+ rpms. but that was with the 305 and 2 yrs ago...

 

Since somebody has referenced Vizard previously, I'll throw in something he wrote about overlap and scavenging...

"When the intake valve is opened early and the exhaust valve is closed late, there is a period of time when both valves are open. This overlap period occurs when the piston is near TDC. Having both valves open at once may not seem like a good idea, however, this technique harnesses the moving mass of exhaust flow as a sort of "vacuum-cleaner" to draw out residual gasses. In fact, this vacuum-cleaner effect is so powerful that it also helps to initiate induction flow. This early induction flow -- prompted by exhaust gas energy -- is called scavenging, and it improves cylinder filling and increases horsepower, especially at high rpm. While excessive valve overlap does reduce low rpm torque, these losses are minimized when the overlap duration is tuned to the application -- about 40 degrees for a street cam to about 85 degrees for a competition profile."

So, assuming we accept that he knows what he is talking about, overlap allows exhaust gas momentum to aid in cylinder filling by drawing intake air/fuel into the cylinder while both valves are open. The reason the exhaust helps the intake would be that exhaust gasses are at a higher energy state. Thus, it loses energy by transferring it to the intake air. I don't see how it would work the other way around.

MikeS

just adding fuel to the fire


and guys, let's keep this as a discussion/debate, nothing more.

 

There's no need to get nasty.

Putting your finger over the end of a hose creates a restriction that increases pressure inside the hose. This increases the velocity of the water leaving the hose, but it decreases the rate of flow. Use a stopwatch and compare how long it takes to fill a bucket with a hose with your finger over the end and how long it takes to fill the same bucket without any restriction on the end of the hose.

Scavenging depends on the compression and rarefaction waves that aren't found in a continuous flow, so a hose of waterisn't a great model for an exhaust system in any case.

 

It was supposed to be comic relief... sorry

I guess I'll take mine outta mine then. <--- There is is!!!!! :sillylol:

 

ok I put bold on that statement b/c backpressuer doen't create scavaging
it hurts it
you can have great scavaging with almost nil backpressure if you set the exhuast up right


backpressure will hurt your low and high rpm torque

it isn't the resistance or "backpressure" that creates this scavaging it is velocity

the faster your exhuast moves out the exhuast pipe the stronger the low pressure zone will be behind it

if you put backpressure in there (a.k.a. a restriction) you are going to SLOW DOWN the exhuast gas AT ALL RPMS

 

marking this so I can edit it later

but I would think even without overlap scavaging would help on a car


if you are not scavaging the exhuast from the cyl then you might not get all the gas out of there

don't just think in terms of direct effect. exhuast pulls intake charge in, you can also think of exhuast gets pulled out more effienctly therefore creating more of a vac inside the cyl then when the intake opens it will suck more intake in

as for pulse waves you can use them for higher rpms range
since it takes less time that is when you make a shorter exhuast...this way it will have less distance to go

I would think a pressure wave box or whater might hurt low end power a bit though due to reduced velocity at the point it hits the open space thereby making the exhuast puaules more or less hit one another

 

isnt the reason for overlap to give more help in reducing pumping loss by letting the exhuast do part of the work in getting the intake charge into the motor

as for the analogy
kinda a different thing there going from a pulse type flow to a steady rate flow.
and with your setup it prolly would hurt scavaging if you put some sort of flapper door in your exhuast to give the same type of feel as your thumb over the hose
yeah might speed the exhuast gas up only during the point it hits that but before that and after that it is going to slow down more then it would without it

 

mr dude sir. hey no need to be impolite here, even if he isn't right
we all make mistakes

and I don't really want to see this thread get all locked up yet

 

thats better

 

whooooa, there goes rx7 on another whoring rampage lol

Ok, for the guy that said that there is no way the exhaust gases are smaller in volume, they absolutely are. if the reason the exhaust valves were smaller was because the piston pushes to get rid of the exhaust gas, the motor would be designed to intentionally work against itself. using this reasoning, exhaust valves woukld be made the biggest they could be in order to reduce restriction on the engine, and this is obviously not the case.

and for this guy,

your a little misinformed on this one junior....its not conservation of mass...its the law of conservation of mass/energy. Mass and energy can neither be created nor destroyed...they can only be changed (and yes they can be changed into each other, although more easily into energy from mass than vice versa) a good amount of the mass of the intake charge is converted into heat, and sound energy...as well as contributing to the power output of the engine.

Also, the expanding gases are what forces the piston down....however, another form of conservation applies here. After the intake charge enters the chamber, it is compressed, and then expands to roughly the same size again once the piston reaches BDC. It provides power by expanding again to its original size. But that does not mean pressure is equal allowing for the smaller mass.

 

It will. There's a difference between cylinder scavenging, and exhaust scavenging.

Exhaust scavenging only happens at the joint in the exhaust system (aka collector). Cylinder scavenging only happens in the cylinder during overlap.


Of course you will. The piston pushes it out, to a degree. The cylinder scavenging is also relative to valve timing. The same way that cylinder pressure is. When the valves open and close in relation to the position of the piston has a greater effect on cylinder scavenging than overlap does.


But that's just it. You don't want any air going back up into the exhaust. It takes way more equipment than you or I will ever see to find this "area". And again, it's all in relation to RPM. By that I mean the DESIRED RPM (aka "powerband").

Quote:

I would think a pressure wave box or whater might hurt low end power a bit though due to reduced velocity at the point it hits the open space thereby making the exhuast puaules more or less hit one another

Well then you need to tell David Vizard that. I doubt he's making this up. I think the guy sleeps on his dynomometer every night.

 

what do you guys think of 350 with full length headers 1 5/8" tubes, with 60" of straight 2.5" pipes with turbo II cheapy muffler at end of both pipes ?

good ?
bad?
ok ?

it sounds good DD

good luck, interesting discussion up in here

 

Do not try this at home.

Everything I mentioned may not have been quoted, but its "stuff" i "read" that i beleive and that makes sense to me. If you think Im making this stuff up to pretend I know what im talking about, your wrong. No, i dont beleive everything I read.
But that was a hypocritical statement, you've already mentioned David Vizard several times. Im not getting nasty, its just that you assume everything YOU have read is correct, while everything I have read, is just stuff im making up. Im sorry if Im misleading anyone, but thats just how I read it. I dont know everything and the original post was... what? I think we all forgot the issue at hand, (the truth about backpressure) and are more concerned with proving each other wrong than actually discovering the REAL truth about backpressure.

and on top of all that, it all comes from theory. this whole scavenging, backpressure thing, is just one big theory. its one of those "we dont know why it works, but it does. so lets call it scavenging since thats what we think it is."
No one, not even the Comp Cams engineers, has ever actually seen a scavenge with their own eyes. you cant hold one in your hand, and you cant taste it and feel it. like atoms, supposedly they are in everything, but have you ever actually seen one? theory. 300 years ago we KNEW the earth was flat. 100 years ago we KNEW the earth was round. today we KNOW the earth is sort of egg shaped. tommarow we will KNOW the earth is....

get my point? theory. maybe it has NOTHING to do with the valve timing. maybe its a little green man that says "ah HA! he opened the exhaust valve too soon! lets rob all his torque..." and BING we now have crappy throttle response.

CamaroDriver we both drive camaros.I drive mine in Florida, miami to be exact. I've played with open headers and the cops have played with my wallet. I've played with normal exhaust and the cops persist. in the end, my fuel economy and headaches have decided that normal muffled exhaust is whats right for me. not some theory about scavenging and exhaust velocity or backpressure.

<b>so lets get back to what we KNOW(laf) rather than what we theorize about stuff </b>

we<b> KNOW</b> street cars dont react well to huge exhaust and open headers. we KNOW stock 305's dont like 4" mandrel bent catbacks. we KNOW blown motors like super free breathing exhaust. we KNOW having the "right" exhaust is what its all about on the street. we KNOW backpressure is bad for HP numbers. we KNOW backpressure is a necessary part of exhaust scavenging. we KNOW exhaust gas velocity is a necessary part exhaust scavenging. we KNOW I could do this all day.

And tommarow, just think of what we will KNOW.

'nuff said.