my cat was plugged, peices plugged the rusted stock muffler up. i poked out the cat and put it back in, cut off the muffler, feels alot slower now, is it the lack of back pressure or just all in my head?
it doesnt sound as loud as i thought it would either.
what is a good cheap muffler to put in there? its a 305 s/e bird...1984

 

Engines do need SOME back pressure, but I would think the pipes would provide enough, to make it better than a plugged system was.


I ran a gutted cat, and hacked off my muffler, running a new piece of pipe straight out the passenger side. Didn't feel like a big loss of power. The existing pipes were enough to keep it quiet enough, for the cops not to bother me. Granted, it was a bone-stock 92 RS (K&N doesn't really count as a mod, to me), but it still was basically an open exhaust, but wasn't TOO loud.

 

I agree some back pressure is needed for low end torque.A Dynomax Super turbo is a good choice for flow and sound.In test they actually outflow Flowmaster below 3500 RPM where most of us drive.

 

lies all lies. backpressure isn't your friend. it resist the flow of the gas and makes your engine work harder to expell the gas creating a lack of scavenging.

you could have a few other problems here though. best thing to do is replace the cat with a straight pipe. that gutted cat doesn't flow as good as a straight pipe. also changes in exhuast flow and scavenging can cause problems with lean/rich running till your comptuer adjust itself. pipe length can cause differences in scavenging also.

 

I agree, the engine needs some back pressure, It was design to operate with some back pressure. changing the configuration may hamper performance to a degree.

 

the engine doesn't need or want any backpressure. please read the sticky at the top as well as a thread that I posted in "3 inch duals for a daily driver"

backpressure is a resintace to flow which make the engine waste energy doing something it shouldn't do and hampers scavenging.

 

friend cut off exhaust b4 the cat day he got this car ALOT FASTER!!!!! took stock manifolds off, put headers 2 1/2 duals till trans, then ran 3" to an 80 with dual 3" tails, seemed better but not hold on to your hat Fred diff i cant say on mine, i went from bone stock to 100% custom in one step

 

We don't need to read stickies. We already know that back pressure enhances the scavenging effect....when done correctly.

 

huh? I'm sorry but that is what scavenging is. low pressure zone at the exhaust valve which creates a higher pressure differential helping to suck the exhuast out of the cylinder. how is having backpressure (high pressure zone) going to help to create a higher pressure differential?

backpressure doesn't help scavenging but hurts it.

 

If a pressure wave exits too soon, there will be no scavenging effect. Hence, backmpressure can increase the scavenging effect. Too much, it hurts, too little, same thing. Not really HURTS, but isn't as effective, as it could have been.

 

that would be more an effect of tuning the length of the runners/pipes for sound waves. backpressure isn't really a help/hurt on that one.
backpressure itself does hurt though. it reduces the pressure differential resulting in less of a pull effect on the exhuast.

 

Backpressure is bad. Always. Period.

 

well also the new sound of it can trick you into thinking it seems differant too.it actually sounds pretty beefy and not too loud. i love it, but want a nice muffler on it cus its too loud for me at 3grand

 

It doesn't seem to matter how many times this is said, the issue still comes up

 

ya know, i'm no scientist....but i do have an electronic exhaust cut out....i have stock manifolds (i dont know if it would perform better with headers or not) and when i open the dump valve...i dont notice any real difference, my head doesnt get thrown back, i dont smoke the tires while fish-tailing down the road and i dont go 0-60 in a 2 seconds.....but when i close it, i dont feel like im in a diesel chevette, going 0-60 in a couple days, i would have to say that this is all about what accessories you have on your car and such


ps, i forgot to mention, i took out the cat and replaced it with the electronic cut-out so i am running from a y-pipe and also a full exhaust

 

I would bump timing up to 13deg to get more performance

 

Some of you "back pressure" ***** are confusing two different phenomenon. Back pressure bottlenecks are not good, but consistent moderate back pressure that results in velocity is good. You can't have one without the other. Anyone who goes on endlessly telling everyone that back pressure is bad and harmful is right and wrong at the same time. You have to maintain some back pressure to maintain velocity in the pipe. When there is no back pressure, the exhaust velocity falls off, scavenging is destroyed and the gasses move lazily through the system. It will run OK in some part of the RPM band, but overall a major power loss results.

So, for you "back pressure is bad always" guys, you are showing that you don't really know how an exhaust system works, you're basically fixated on only a small part of a very complex equation. Try pulling off all exhaust system components at the heads and see how it runs. If you're right, this will result in maximum power everywhere in the power band. If I'm right, it will only run well in an extremely narrow part of the power band, and will otherwise loose power and run poorly.

TA

 

No, you're confusing cause and effect.

Back pressure doesn't "result in velocity". Back pressure is just an unavoidable and undesirable consequence of any practical exhaust system intended to take advantage of other desirable effects which are completely unrelated to backpressure. Back pressure can be minimized, but not eliminated.

Saying that back pressure will make more power is like saying that paying more income tax will give you a higher salary.

Back pressure is never good. Back pressure is always bad. There will always be people who don't understand this, just like there will always be back pressure itself.

 

Uh, it must be really important to you to be right, but it is you, grasshopper, that does not understand. How can something be cause and effect, yet be unrelated? Your response contradicts itself.

You also say that you won't make more money by paying higher taxes, but the two are related, aren't they? Again, you're trying to over-simplify a complex equation, but if they were not related, why is there a book of equations from our beloved IRS that shows the relationship? Again, your response reveals a tendency to want a simple yes or no answer to an essay question.

If you have experience with header and/or exhaust system design, you will find that certain diameter tubes are just too big for a given engine system, and loose power. WHY?

You will also find that cutting off megaphones after the headers in incremental lengths will increase, then begin to decrease power. Why?

If your statement was right, all race vehicles of any type would run no exhaust system at all after the heads. That is the closest to no back pressure you can get. But nobody does that, NOBODY. Why?

Smokey Yunik recommends putting a "venturi" (bottleneck) in the exhaust system. Why?

Because your equations are oversimplified and thus give you the WRONG CONCLUSION.

Well, no wonder why every time this subject is posted that it results in an "issue". It's because you feel that if a lot is good, and more is even better, that way too much must be ideal!

Sorry to tee off on you guys, but you are being very heavy-handed with an opinion that contradicts what I learned from interviewing Darryl Bassani & Todd O'Neal of Bassani Exhaust and George Kryssing of Kooks Custom headers, who's feedback represent about 50 years of exhaust design experience. You can read the story : Exit Strategy, Performance exhaust parameters" in Sept issue (#9) of Fastest Street Car magazine, LS-Power sectin, pgs 10-12.

TA

 

i have a solution...............how about someone hacks up their exhaust to figure this out....anyone? i call not it with my car

 

I already did, went from a stock to a Mufflex 4" exhaust on my GTA, adjusted the FPR, installed a cold air intake, and not only didn't see any power gains, I actually lost power across most of the power band (until the Hooker Headers went on). All this was done before and after on a chassis dyno, so when I had the "guru's of exhaust" answering my questions, I put that one to all of them, and they all gave me the same answer, tubes are too big, they do not provide enough back pressure to keep velocity up, so scavenging and exhaust energy are adversely effected, thus no additional power or a power loss.

They also universally recommended that I go with a smaller diameter system, and would see a power increase. To his credit, Denny @ Mufflex told me the same thing when I called to order, 4" systems are for big-inch power adder race only engines, but I didn't listen, I wanted the big swingin' 4" system. I'm satisfied that I was wrong and learned my lesson the hard way. You can jockey around all the terms you like, but in the cold light of day, you need velocity, which requires back pressure. Period.

Any of you "back pressure is bad" guys in the market for an almost new 4" mufflex system? It'll make your car a ROCKET!!

TA

 

Like the Bowflex commercial..."Size matters."

Bigger is not always better!

 

With my car, 88 350 tpi, I cut the muffler off so I could replace the fuel pump. I still had the cat on there. Driving down to the muffler shop, it couldn't accelerate properly, would hesitate, and wouldn't even get goin faster than 30 mph. It would cut out and hesitate bad. It was such a difference. Once I was there and he welded the muffler on it was night and day. All the power came back, no hesitation, and ran like a champ! I don't know why but it was not even close to running without a muffler.

I did gut the cat once but I noticed a more low end howl but also lost some low end torque. It didn't feel worth the gut. If the cat breaks apart just put a magnaflow high flow version and a good magnaflow muffler will give you that grunt that we all like. I had a flowmaster and now I'm ready to move on to a magnaflow.

Rabi C.

 

I'm not arguing for either side, either for back pressure or against back pressure, but I think the general rule is to match your exhaust with your engine and power level. You don't want the exhaust flow to roll through the pipe and cause turbulence to itself, you want it to come out in as straight of a stream as possible. Standard exhaust port on chevy SB heads is 1.5" x 1.5" (correct me if I'm wrong) which means 2.25inch2 of area per port, 9inch2 of area total per side for the exhaust to flow through. This is only if the valves, exhaust timing on camshaft, and flow allow all 9inch2 of that to be used. A 4" diameter pipe has about 12inch2 of surface area, or area to flow through, and that extra 3inch2 is not only wasted space, but causes a less smooth transition as the exhaust pours around it. I don't think it's back pressure that causes the better flow, but just the flow pattern that does. Unfortunately, a proper flow pattern brings more back-pressure than non and thus back-pressure is directly related to proper flow in some minds.

One person cutting up an exhaust system, then switching parts around can not justify an experiment. So many different variables could be present in that. The only just way to determine is to take a large sample (say 10-20 engines) seperate in half, take a base reading of both halves and average it. Then leave one half as the control, and clamp different exaust systems on the other half. Record the differences.
You can argue against my opinion all you want, but it's still my opinion and I didn't declare any of it factual.

This comes to exhaust scavenging. The hot exhaust cools as it goes towards the end of the pipe, contracting. This creates a low pressure zone, and also coming out of the pipe acts as a siphon. Well if the exhaust doesn't make a complete seal around the inside of the pipe (theoretical seal of course) then air from outside is just sucked in to fill void since it's the path of least resistance. This causes even more tumbling.

 

Air flow is like water flow. A 6" flow could flow more, IF it was under more pressure, then a 3" pipe. But AT THE SAME PRESSURE, the 3" will flow more, and flow faster. Hence, why firefighters use a smaller diameter hose. To spray MORE water, FASTER, and FARTHER.

A 6" hose is good for filling a truck tank, but from the hydrant, it is also flowing from a bigger diameter pipe to begin with, and under higher pressure. Ever wonder why those big tanks are so high above ground? The water tanks? GRAVITY is feeding the water to your town, or at least, to substation tanks & pumps.

Correct pipe size (header tube) is a factor of increasing velocity.

If you want to blow out a cake candle, do you press your lips together, to increase velocity, or open your mouth as wide as possible?

 

editing:

 

TA some things for you to think about.

you asked why you don't remove the exhaut manifold as that would remove all backpressure. the reason for that is scavanging. that part has nothing really to do with backpressure.

to have the exhaust gas scavenge the next pulse it needs to create a low pressure zone behind it. for that it needs more or less a sealed envrioment. you remove your exhuast the envrioment is not sealed anymore and the exhuast can not create low pressure zone behind it. the only way this would work is if your engine was spinning fast enough to have an exhaust pulse still in the exhaust port while the valve opened up again to let another pulse go. needless to say your engine would have to be turning VERY VERY fast for this to happen.

usually a venturi is a fairly efficient at producing a low pressure zone without producting much of a restriction. on some cars it might help otehrs it might not I'm not sure if the restriction would be enough to overcome the lower pressure zone or not.

you don't need to maintain backpressure to maintain scavanging.. think of a mechanical driven exhaust pump that puts a vacuum on the exhaust. what would happen with how the exaust would work with scavenging?



just some random quotes here with their sources

https://www.thirdgen.org/forums/exha...t=backpressure


https://www.thirdgen.org/forums/exha...als-daily.html


as far as it goes backpressure is not wanted still. it is a byproduct that we can't control unless you wish to put a scavenging pump on your exhaust that can handle the heat and flow.
air will flor from a area of high pressure to low pressure. any fluid/gas substance will try to flow under that situation. it's how most any pump or fan works. this is why a turbo or supercharger work so well is they create a huge pressure differential an engine is a real fancy airpump. with the intake stroke and that piston moving down it creates a low pressure zone in the cylinder relative to the atmosphere which will then draw air into that cylinder. we know what happens now compress, boom, expansion... blah blah blah not part of our conversation . next part though is that exhaust which is part of the topic. when those gasses go boom the heat up and create a lot of pressure. your exhaust port is an area of low pressure. this creates a pressure difference and the higher that pressure difference the greater the flow if all else is kept the same. your goal with an exhaust is to keep that area right at the port of as low of a pressure as possible. to say that backpressure helps is like saying this. to get a low pressure zone you need to keep the pressure up and put a restriction on the exhaust to build that backpressure so you have a low pressure zone...????? huh????? you want the low pressure zone that is the goal. to reduce the pressure as much as possible. the problem with a large exhaust isn't lack of backpressure but other problems.





eh had more to say but I Forgot



stephen your thing with a 6" vs a 3" hose isn't quite right. a 6" hose and a 3" hose at the same pressure the 6" would flor more water. sure the 3" would shoot it further but it still wouldn't flow as much as the 6" hose. either way though that doesn't quite have a lot to do with exhaust being the two work very different. an exhaust setup is a pulsed system while a hose is setup generally a static pressure with a mass going through the whole thing.

 

That was good reading rx7.

It seems as though a proper understanding of the stage that your engine is in (stock, modified, race) and then matching it up with a proper sized exhaust. It is relative. I comes down to maintaining that balance.

Rabi C.

 

I'm thinking our reference to back pressure, is more of an issue of the venturi effect, which needs a restriction, which could be seen as a back pressure effect, which would be needed, for the venturi effect.

As for my water vs air & 6" vs 3" reference....I am correct. My years of firefighting taught me that much. Not only was I a firefighter, but used to own Southwest Fire & Apparatus, and build Fire & Rescue trucks. I know a bit about flow testing.

 

Lol now we're all just saying the same thing in different levels of detail. However, we've all hit different nails on the head, on the same board (no pun intended).

 

how would a larger diam pipe flow less when both are at the same pressure? to even get the pressures the same you would need to have a higher flow rate.

it's like saying don't go with the 1/2 fuel line go with the 1/4" fuel line it flows more. unless I'm miss understanding you

 

No, we have fundamental differences here. Our point of departure is a very basic misunderstanding of scavenging, and a willingness to take other uninformed peoples' thoughts, musings and opinions as facts. That anyone would state that back pressure or restriction and scavenging are not directly related shows an ignorance of scavenging and exhaust system dynamics. This is certainly forgivable, nobody was born knowing this stuff. But to be so heavy-handed when you obviously don't know your facts is just too much for me to stand by and let happen.

If scavenging were just low pressure pulses in between high pressure pulses, and all back pressure was detrimental, the ideal exhaust system would be completely separate perfectly straight pipes of a similar diameter as the exhaust port going to infinity, and the placement of a collector would not only be irrelevant, but represent a restriction that is not desirable. Call any header designer in the world and propose that a collector or it's placement is irrelevant, and he'll laugh you off the phone. This is because you don't really understand scavenging. Never mind Top fuel engines, because they run at such high blower rates and compression pressures that and scavenging pulses are totally overwhelmed.

The speed of and exhaust pulse is dictated by the energy of the wave, diameter of the tube and temp of the gas, but it requires some form of restriction or back pressure to send an echo wave back up the pipe to the exhaust valve, which collides with the incoming wave of raw intake which would otherwise find it much easier to go out the exhaust valve at valve overlap, which is right next to the intake valve, instead of starting to fill the cylinder. This timed pulse collision effectively "stalls" the exhaust port flow long enough to stop the intake flow going out the open exhaust valve before top dead center. The exhaust flow out the valve is what starts to "pull" the intake charge through the intake valve, so timing of this return pulse is very important, thus diameter of the tube and placement of the collector are also critical in the strength of this phenomenon. No scavenging at all will result in a lame running car, but scavenging at the wrong time will make the car virtually undrivable.

Now that you have a better, but still much simplified understanding of scavenging, you see that your basis for argument, that scavenging is not related to a restriction or back pressure, is fundamentally and completely wrong, you understand better that most of the other people's opinions that you are citing are of lay-persons who also don't understand scavenging either. Nobody was born knowing this stuff, it took 50 years of internal combustion development before it became known, and those who have figured it out and how to use it generally don't run around telling everyone.

If you really want to understand scavenging better, look up 2 stroke engine expansion chamber design. Since 2 strokes have no poppet valves in the ports, they are dependent on using scavenging to run efficiently, and an expansion chamber works like a stepped header on steriods. All the principles are the same, but to see an expansion chamber and how it works will clarify in your mind what scavenging really is and how it works, and that it is completely dependent on "tuned" restrictions or back pressure.

We now return you to your regularly scheduled opinions.

TA

 

You guys are all arguing the same point, with different words. You want velocity and scavenging. Thats acheived with properly sized exhaust. Back-pressure, to me, means pressure against the direction of flow. This, in a 4 stoke engine is not desirable. Scavenging on the other hand is a slightly different phenomanon, and as TA describes above regarding header primary and collector desgin, is important to good exhaust desgin. As TA eluded to, 2 stroke engine design operates with a large amount of overlap based on port timing, and the exhaust is critical, an actual wave of backpressure, generated by the unique (and critical) design of the expansion chamber, is used to force excess intake charge pushed out the exhaust port back into the engine, force effeciency and power. This also applies to 4 stoke applications, but to a lesser degree.

People hack off the exhaust, think they lost "backpressure," and then attribute that they needed it to make better power. What they needed and lost was the velocity and scavenging created by that exhaust. In 2 stroke applications, they also lost that critical return pulse, if you think hacking a 4 stroke exhaust hurts, run a performance 2 stroke application w/o any exhaust, you'll drop the power 50% or more. By optimizing the size and design of the exhaust instead of merely removing it, they may have been able to further enhance there power, especially with headers and x or h pipes, which further combine scavenging effects.

TA and Apeiron, I think your both trying to make the same point.

 

you are start to put words in my mouth that I didn't say. as well as ideas I'm getting at that I'm not.

yes backpressure, restriction, and scavenging are related. it doesn't mean they all go hunky dory with each other though. no an ideal exhaust wouldn't go to infinity since there is frictional losses. the placement of the collector is still relevent as it allows one cylinder to create a low pressure zone for a different cylinder. I never said it was.

as far as overlap there is a reason cams have overlap. it is so the exhaust will help pull the spent gasses out to help with cylinder filling. to have a pulse go back up the exhaust to prevent stop the intake charge from leaving the exhaust would also stop the exhaust from leaving. also this could start to cause problems with reversion of the exhaust which is partly why those huge cammed motors have that nasty lope. if you are trying to "Stall" the exhaust there is a much more simple and easy way to do it. close the port sooner, and eliminate overlap. yes you loose some intake charge out the exhaust sometimes. again necessary evil. we would like that intake charge to stay in the combustion chamber but you generally will get much better cylinder filling with that overlap and little bit of intake leakage as well as a much cleaner intake charge not being dilluted with spent exhaust if you have the overlap there.


I know what a collector is and what it does. it allows scavenging to happen where one cylinder will help scavenge the next. yes it creates a restriction. doesn't mean you want that restriction though as it is unavoidable. BUT the advantage of what you gain far outweighs the restriction. it's like putting a turbo setup on your car sure it's more weight weight is bad so why add the turbo. simple we all know the answer the added power the turbo creates far outweighs the weight it gives. you still don't want that weight though just like you don't want the backpressure, it's just unavoidable at times.


also 2 stroke systems while have some things that are similar are still a much different beast.



also just to clarify that what I'm refering to with backpressure is a restriction in the exhaust system that causes a partial blockage of flow creating a higher static pressure inside the exhaust.



and atc thanks for sending some of that love my way

 

[QUOTE=rx7speed;3471143]you are start to put words in my mouth that I didn't say. as well as ideas I'm getting at that I'm not.

OK, I'll use your words directly:

"yes backpressure, restriction, and scavenging are related. "

but your prevous post says:

"scavanging. that part has nothing really to do with backpressure. "

So you contradict yourself. In a court of law, that usually results in you going to jail.

"as far as overlap there is a reason cams have overlap. it is so the exhaust will help pull the spent gasses out to help with cylinder filling. to have a pulse go back up the exhaust to prevent stop the intake charge from leaving the exhaust would also stop the exhaust from leaving."

This is why a pulse is "timed", and why headers run best at certain RPM's. By nature, a pulse is a momentary phenomenon. The more you argue, the bigger hole you are digging yourself into. You notice Aperion "Back pressure is never good. Back pressure is always bad." has not chimed in for a while. He's the smartest of all of us.

"if you are trying to "Stall" the exhaust there is a much more simple and easy way to do it. close the port sooner, and eliminate overlap."

Holy mackerel!! I've never seen a cam in my life that had no overlap, to eliminate overlap the vehicle would not be able to accelerate and would certainly make no power. I'm starting to feel stupid for spending this time arguing with you.

"I know what a collector is and what it does. it allows scavenging to happen where one cylinder will help scavenge the next. yes it creates a restriction. doesn't mean you want that restriction though as it is unavoidable. BUT the advantage of what you gain far outweighs the restriction."

Read that again, and see for yourself if you can simplify that statement into anything that sounds like "All restriction is always bad" or "there is no advantage or gain from restriction or back pressure". You contradicted yourself again.

"it's like putting a turbo setup on your car sure it's more weight weight is bad so why add the turbo. simple we all know the answer the added power the turbo creates far outweighs the weight it gives. you still don't want that weight though just like you don't want the backpressure, it's just unavoidable at times."

Ok, a turbo creates a massive back pressure in your exhaust system. It also creates a large amount of positive pressure in the intake, which far outweighs the back pressure. By your definition, that back pressure is unavoidable? No, you can remove the turbo, and the back pressure. The back pressure gives a benefit that far outweighs it's detrimental effects. This is my point. If you remove all back pressure from your system, your car will run very badly, but you guys insist on saying "all back pressure is bad always" then say "back pressure is unavoidable to make your car run well". These again contradict themselves. You keep trying to separate back pressure from the positive effects it has, and instead insist these advantages come out of nowhere, are unlrelated, Ok related a little bit, OK, directly related, but back pressure is still really bad, always and forever.

HUH?


"also 2 stroke systems while have some things that are similar are still a much different beast."

Nope, they run on exactly the same principals as a 4 stroke, as do rotarys and diesels. Some of the details are different, but when you understand all of them, they are all birds of a feather, just different breeds. Over and again, what works on one almost always works on the others.


"also just to clarify that what I'm refering to with backpressure is a restriction in the exhaust system that causes a partial blockage of flow creating a higher static pressure inside the exhaust."

Like a turbocharger?

Quote: "All back pressure is always bad."

So, your own words pretty much sunk your canoe in my book. If you haven't gotten my point by now, you never will, but that's just my opinion. I'm not politically correct, and I'm not everyone's cup of tea, but I know what I'm talking about.

I'm going to go make someone else's life miserable for a while.

C-ya

TA

 

alright, well i will put it this way....go to your local drag strip next time there is a big event...ya know, dragsters, funny cars....maybe some monster trucks (they have them near me)....note the ehaust...they are headers, they are about 2 ft long, and they cant even keep the combustion in themselves they are so short....yet that vehicle runs like what, 300+ mph (i actually havent been to one since i was like 10 (11 yrs ago)) with the exception of the monster truck....i think we can all agree that a monster truck cant go 300+ (now lets not start another argument lol) but we can agree that they all put out massive amounts of power..........................

now heres my theory, and i know im not correct but i am not trying to say anyone here is wrong, my theory is that the more the power, the less the backpressure needed, now i know this sounds dumb but the only reason i say this is because these vehicles that put out like 1000 hp have hardly headers, my car that has like MAYBE 300 hp (305 tpi, fully stock but that will change) loses power on the start off when i open the electronic exhaust cut out that replaces the cat....to tell you all the truth, i dont really feel much of a difference whether its open or closed, but i can hear the difference

and to answer 84noname's question from the start........
#1, i personally think that the idea of having no muffler and a gutted cat to make your car slower isnt all in your head, it is a possibility, there could be some unexplainable reason (atleast in my book) as to why a car would run faster with a whole exhaust system than without a muffler
#2, it may not sound as loud because you cut off the muffler (and assuming right at the muffler) so therefore all of the sound is being broadcasted behind you....that ismy opinion
#3, a good cheap muffler.....that depends on what you want...i hear good things about magnaflow, i have a flowmaster on my car right now, but i usually have the exhaust cut out open, but the cheapest muffler would probably be the stock one

that is all i had to say, sorry its such a long post, i have been following this thread for quite some time now and i personally cannot see a reason why back pressure is good, i do not fully understand scavaging, but i can tell you that unless you run with open headers, you probably wont tell the difference...i cant, and mine is opened up right behind the y-pipe

-brian

 

ta I still stand by what I say. backpressure is bad. just going back to that turbo thing. yes it puts a puge restriction in your exhaust. we both agree with that. it also puts more air into your cylinders we both agree there.

so let me ask you this though. where does the power come from with a turbo? the restriction you put in the exhaust or the air it puts into your motor?

it's just like adding that big block. you get more power your car should have m ore power and more weight. does that mean weight is good? no it doesn't but to get that big block it is going to take more weight.

or a spoiler. sure you get more downforece but you get more drag also. we don't want that drag but we do want that downforce.


then listen to my next set of words.sometimes to get the good you have to have some bad. the hope is though that the good will out weigh the bad. this still doesn't mean the bad is wanted but an evil that we have to live with. it is the same with backpressure we don't want it but as has been said many times on this board it is an evil that we have to live with. the backpressure still isn't good.

and I still believe as well that lack of backpressure isn't going to cause these problms you speak of but the lack of something else. most of these things backpressure isn't even a cause of just a byproduct that there just isn't a way around

 

this isn't a cause of backpressure that is giving the scavenging. it's rather lack of a low pressure zone. just as I was saying further down in the same post




"as far as overlap there is a reason cams have overlap. it is so the exhaust will help pull the spent gasses out to help with cylinder filling. to have a pulse go back up the exhaust to prevent stop the intake charge from leaving the exhaust would also stop the exhaust from leaving."

if your talking about accoustic scavenging your right the pulse is timed and is based on the length of the exhaust tubing to create a resonant frequency where a low pressure accoustic wave hits the exhaust when it opens.

they have had motors that run without overlap. the new rx8 engine actually has no overlap if I recall correctly. this is a rotary motor and remember what you said about all engines operating the same.
though for most situations your right no overlap is bad and I'm sure you know I wasn't getting at removing overlap. even more so with what I was saying as the overlap would have to happen if the exhaust was to put any sort of pull on the intake charge since both ports would have to be open for such a thing to happen.

didn't contradict myself there. read above post. yes the restriction is bad. if we could do this without causing the restriction that would be better but at this time this is the easiest way to do it and the good outweighs the bad. that restriction though still doesn't help and if we could eliminate it and yet keep the good effects would be best. at most all we can do with a collector is remove the restriction as much as possible which header manufacts try to do already.

by my def I never said we can just remove the backpressure. I even said "necessary evil" at times. read above post regarding that.
also you can remove the backpressure by removing the turbo and yet still keep the positive pressure on the intake. this can be done by a supercharger but then your trading one problem (backpressure) for another problem (belt drive). I'm sure you agree putting more objects to be belt driven is bad correct? even mroe so something as heavy and spinning as fast as a supercharger right? but again good outweighs the bad. doesn't mean you want the belt driven part still though.


think I even said something along that lines. they do have some things that are the same. some ideas are still quite different though. port timing between a 4 cycle and 2 cycle and rotary is one of them. all are VERY different. and with the scope of this backpressure debate this difference in port timing can cause some problems if you look at them all the same.


yes like a turbocharger


ah come on this is fun

 

Brian, this was a subject TA eluded to in an earlier post.

Because the pressures are so high, creating any kinda of scavenging or return pulse effect would be next to impossible. Topfuel dragsters are running almost on the verge of hydro-lock at full power, the exhaust is being ejected with such force it would be nearly impossible to make a workable return pulse. They aren't shooting for any kind of fuel/power effeciency, just power, so if fuel is being wasted out the exhaust, they'll just cram more in with adt'l intake boost. It is true that in a boosted race application that exhaust becomes less important, because any scavenging losses can be compensated for with more boost. In an normally asperated race application, say like a Nascar or Formula 1 car, exhaust is critical to power production.

 

ATC:
Thank you for restorning my faith in this board. We are in total agreement here.

RX7, where we differ is that you keep wanting to separate one phenomenon as good that is completely dependent on another that you consider bad. The wing for instance. You CANNOT have downforce without drag. Sure it's nice to minimize drag, but apart from incremental efficiency improvements, if you reduce drag, you reduce downforce. Old Formula 1 cars in the late '60's had wings on hinges, and they went flat on the straights, and tilted up on the brakes. After a few broke off and caused huge accidents, they were banned. Even in this case, the flat wing still caused drag, it was never eliminated. So, is the drag bad and the downforce is good? Part of why they "popped up" on the brakes is that they also functioned as and airbrake, so in that case, the drag was very very good! But most importantly, they are vectors of the EXACT SAME Physical property!!! You can't have one without the other, so separating them is impossible, just like scavenging and back pressure. How can one be bad and the other good, when they're components of the same laws of physics?

Again with the weight. The NMCA Pro Stock champion told me that adding weight to his car was what won him the championship, because he was able to put the weight exactly where the car wanted it, and it launched better at every track. You are hung up on "bad vs good", the real world doesn't always line up in neat mathematical rows like that. You will dramatically impede your own learning and progress if you can't look past the "always bad/always good" and see you need to put aside your simple thinking to find how to use a "disadvantage" to your advantage. Like a weight penalty that actually allowed someone to win every race he ran, he would not have been successful if he had just bolted the weight on anywhere and bitched about how bad it was, but instead performed tests to see where he could ad the "bad" weight that benefited his performance instead of hindering it.

TA

 

no offence on atc for this but as far as what he said I have no beef with that. so what is so important about that to restore your faith in this board that just one person seems to be able to ruin?

ok you caught me with the weight one. I wasn't thinking of using the weight as a balast . even more so I know that balast on top fuel cars is needed. or else they will lift the front end and flip the car.
you also caught me with the wing one cause though I knew that one with the drag and though I knew of using that as an air brake.

you are right with there are times where backpressure can be used for good. a turbo is a great example of that. though if you could get the same effects withotu the backpressure the result would be even better yet. as far as backpressure helping with scavenging I have yet to see where causing backpressure in and of itself can help. I'm not talking about doing something that as a side effect will cause backpressure (ie turbo) but rather having backpressure in itself will help at least even more so under N/A 4 stroke situations.

also would you mind please not doing the put downs anymore. I understand our views might be a little different and that's cool it's what makes life fun and I enjoy these little debates but it seems every thread of your is laced with a little put down somewhere.

 

oh, thank you for correcting me atc....some of these posts are so long i jsut cant sit through reading the entire thing lol plus the arguing about if back pressure is bad or good and whats the best pulse and people arguing about the same point of scavenging is starting to hurt my head a little haha
-brian

 

OK, I get your point about knocking off with the put-downs, but some people on this very board, maybe you and maybe not, have been absolutely merciless to anyone who did not agree with "all back pressure is bad always", and as the saying goes, the best defense is a good offense, and I'm as offensive as they come

It's a bit frustrating that you cited sources from this board who obviously took time and effort to write down their thoughts, but simply arranged their observations and figures to agree with what their opinion already was, and completely discarded or overlooked anything that did not move toward that goal, did no real world experiments, talk to any professionals, or have any intention of finding anything but the answer they were looking for. The fact that these "studies" are made into stickies and used as reference in discussion, points out the danger of getting your information from message boards, which can sometimes be the blind leading he blind. Even if we put all of our experience and training in our signatures, who would verify them?

Don't misunderstand, I've gained a lot of useful knowledge from these boards and the huge majority of information is either useful, harmless, or quickly refuted by someone who knows better, but every time this subject has come up, it seems to get squashed by the "back pressure is bad" Gestapo. I've seen and heard professional engine builders and magazine editors who have made much bigger and more elementary mistakes than this topic, and I've made some major blunders myself, but I've seen how members of this very board can behave towards someone who does not agree with this curiously popular misconception, as I believe some people use it to justify very loud exhausts. Just as bigger is not necessarily better, louder is not necessarily more powerful (but it's not a bad way to bet).

Again, to visit the turbocharger scenario, you CAN separate the back pressure or exhaust restriction from the intake boost, by running the compressor off of the engine via a belt or gears, which then becomes a belt driven supercharger instead of a Turbo-supercharger. You must put energy into the system to get energy out, that is a law of physics. Because the exhaust energy used by a Turbocharger is primarily wasted, it is the most efficient way to drive the compressor, but has it's drawbacks. One of the drawbacks is it does not provide enough of an exhaust restriction at low end, so energy sneaks by and you get turbo lag, or solving that, it presents too much of a restriction at high end, strangling intake/exhaust balance and killing top end power . When the piping is the right diameter, the boost is in the right range, and the impeller is the proper size, shape and distance from the exhaust valve, you get VERY strong scavenging with a turbo and it will outperform any other kind of power adder for power per engine unit area, but when the boost increased to make a higher power level, the more lag becomes a problem and the more outrageous the "on boost" power transition becomes, and the more difficult the car becomes to drive. Designs have been tried that incorporate "variable pitch" impellers, but the extreme heat and vibration, the need for super precision balancing of they impellers, and the added weight/cost/complexity of this design have not brought the hoped for results. (yet)

So, from that standpoint, it is desirable to reduce backpressure while increasing energy input into the turbo, to increase efficiency so to speak, but you can never separate one from the other. It is only possible to "steal" x amount of energy from the exhaust (via the restriction of the impeller) and get -x amount back on the intake side, you can't get more energy out of the system than you put into it, you always loose some. But the back pressure is the siphon, that is where energy is taken from one place in the system where it was being wasted and returned in another where it is constructive, as it is with scavenging pulses returning to the exhaust valve. Without a method to pull energy from the exhaust system and re-directed to where you want it, scavenging doesn't get done, turbos don't get done, and the engine runs like baby diaper filler.

I hope this is less offensive and more clear. Back pressure, or whatever you want to call it, can be used to "steal" energy from the exhaust system and put that energy to work doing things that increase power. From that standpoint, all back pressure is not bad, and is in fact needed to make modern engines run well.

TA

 

i completly cut my exahust off and im reading about a 20 hp gain. its headers y-pipe and out the side.

 

Your muffler/cat could have been clogged or messed up. Usually a 20hp gain wouldn't be just because of a muffler or cat. I'm giving you the benefit of the doubt here in that you actually dyno tested it, so I'm not going to say "you're full of BS" or something like that.

 

i was a believer in the all back pressure was bad theory running 3" gutted cat with the "chambered pipe out back, due to alot of hassles we wont get into about my loudness i put a single in single out flowmaster 40 series in place of the "powerstick" since i dont run tips just a dump after the muffler and noticed a good set of the pants feel in power(sounds a ton better too) giving the system a lil backpressure and the "scavenging" effect that flowmasters supposably produce, very very happy with my choice to put a real mufler on, picked mine up brand new 3" from a guy in town for 40 bucks i have seen em on craigslist ranging from 10 bucks to 50 bucks brand new

 

I can see the potential to get 20 HP from dropping the entire stock system and putting on headers and straight dump tubes. However, especially on a stock or near-stock engine, you'll probably get the same increase from the headers, a high-flow cat, and a cat back that dumps under the rear bumper, but will get more low-end in the bargain. Not only will your car launch better and make more power at "driving" rpms and probably turn a quicker ET, it will be quiet enough to drive to the track. Most people choose not to believe this, because they don't want to, they want their car to be LOUD because it gets them attention or makes them feel studly, and they will argue the point to justify being loud despite plenty of real world evidence to the contrary. Harley riders are another example of noise for the sake of noise.

If you want loud for the sake of loud, that's fine, but just be honest with yourself. You're not going faster, you're mostly just making noise. I've been as guilty of this as anyone, but when you get smacked in the face with enough evidence, you realize the folly of image vs reality.

TA

 

Same here....guilty as charged! I have a Magnaflow hi-flow cat, and a Magnaflow bullet muffler (straight through design), BUT IT ISN'T LOUD ENOUGH, for ME! I also like the old Flowmaster 40-series original SOUND, not just the final volume.

Honestly, mine is QUIET! My radio EASILY overwhelms it, and I have no amp, no huge speakers. Simple stock-sized, aftermarket replacements.

I have a video a friend was shooting. Didn't even know he was, so I was "just driving". You can BARELY hear my car. So quiet in fact, I can't even be certain it WAS my car, and not another car, just driving by.

 

i did dyno it since my cousin owns a shop we were messing around with it, the exact power gain was around 18hp form completly stock system to headers 3" y-pipe and out

 

I just had to put my 2cents in here. Backpressure is bad, mmk? First and foremost, engines are air pumps. Restrictions in the air system result in less power. Period. Running the engine with no intake and no headers would be the simplest air system to construct. The reason why we have intakes and exhaust is to take advantage of the fact that an engine works in PULSES and FREQUENCIES.

If both the intake and exhaust are tuned to operate at a desired frequency (RPM's), you can gain power by increasing the engines efficiencies at those frequencies. Cutting the exhaust tube shorter lowers the tuned frequency and therefor lowers the power band (or the exhausts contribution to power).

Much like pipe organs, larger runners produce lower peak tuned torque. This applies to intake and exhaust.

Back on subject

To answer the original observation that gutting the cat makes it feel slower, the platinum grid in the cat reflects the exhaust pulses due to it changing the effective density. This makes the exhaust "seem" shorter than it is. This shortness may have tuned the exhaust to help bottom end. Removing the cat would make the exhaust seem longer, which could have pushed the tuned frequency too low to be usable. This would make it feel slower.

Exhaust systems explained