Choosing Exhaust sizes
- Thread starter styx1787
- Start date
You can't go too big but unless you are wanting some huge power 3" is plenty and makes a huge difference going from stock.
rumptis;950201 said:
That's actually not true. (And rupmtis, I'm not busting your balls, I used to think the same thing...)
Stovepipe diameter exhaust can actually hurt performance once you go to far. One of the key factors is exhaust gas velocity. You want to keep your exhaust velocity up so it gets out of the system quickly. As exhaust cools, it gets more dense. If you have a huge exhaust, the gas slows down as it fills the large pipe and has time to cool down, become more dense and slow down even more. You don't want to waste horsepower pushing a cool dense mass of exhaust gas out of the tailpipe. This is why riceboys with 1.2 liter stock Hondas tend to lose power with giant fartcan exhaust. It's not backpressure...
Unfortunately, it's damned near impossible to come up with a scientific method that most "do it yourself" guys can use to determine when they've crossed the line from enough to too much. What you want is to get past the point of back pressure but not cross over into slow, heavy exhaust gas.
My general rule of thumb on a Supra is a 3 to 3.5" exhaust up to about 500 RWHP. After that go for a 4" exhaust. Anything over 1000 RWHP, go for 5" oval. So ruptis' general statement about 3" is correct, but the bit about "can't go too big" is dead wrong.
Ceramic coating can help with this btw, anything you can do to keep the exhaust hot while it's in the pipe helps.
Well, I know that with any NA car the idea is to keep the exhaust gas velocity up, because this uses the inertia of the exhaust in the system already to help pull the exhaust from the manifold. It is my understanding that this is the reason for the Civics with the huge exhausts losing power.
The total mass leaving the exhaust tip is the same, regardless of the temperature/density, but of course mechanical energy being equal to M*V*V, higher speed = more energy, but it also takes more energy from the pistons to much the exhaust at higher speed.
Turbochargers change the equation somewhat, because the turbine alters the pulsed nature of the exhaust stream, and a larger exhaust than would otherwise be advisable becomes a good thing. A 3" exhaust on an NA wouldn't be such a good thing, for example.
The total mass leaving the exhaust tip is the same, regardless of the temperature/density, but of course mechanical energy being equal to M*V*V, higher speed = more energy, but it also takes more energy from the pistons to much the exhaust at higher speed.
Turbochargers change the equation somewhat, because the turbine alters the pulsed nature of the exhaust stream, and a larger exhaust than would otherwise be advisable becomes a good thing. A 3" exhaust on an NA wouldn't be such a good thing, for example.
Dan_Gyoba;950247 said:
From my understanding, with a massively oversized exhaust this isn't true. Once you "fill" the larger pipe (hot fast gas in one end, slow cold gas out the other is a definition of a bottleneck and in the middle gas collects) with slower/cooler gas, you're pushing more. But it's probably not that big a difference anyway.
Dan_Gyoba;950247 said:
gofastgeorge seems to disagree with us on that topic:
http://www.supramania.com/forums/showpost.php?p=950242&postcount=11
I'm not an "expert" in exhaust design, so I'm not going to argue with anyone on the topic. I've stated what I know to be true. Perhaps my understanding of the truth on this topic is incomplete.
3" works wonders if you are upgrading from stock. The majority of 7m downpipes are 3", and matching that up with a nice 3" cat back (and 3" high flow cat if you need it) will give you superb results.
Mike: The turbo IS the restriction in the exhaust, the rest could be 6" piping and see no ill effects.
The effects of velocity and scavenging to lowend torque doesn't apply on a turbo car, the turbo is a huge restriction. An N/A car with 6" long tubes right off the exhaust ports won't have as much torque as a car with a proper header and exhaust to produce the velocity driven scavenging.
Backpressure doesn't exist, it's a myth and a lie.
As such, anything larger than 3" is expensive and ground clearance issues will eventually develop...
The effects of velocity and scavenging to lowend torque doesn't apply on a turbo car, the turbo is a huge restriction. An N/A car with 6" long tubes right off the exhaust ports won't have as much torque as a car with a proper header and exhaust to produce the velocity driven scavenging.
Backpressure doesn't exist, it's a myth and a lie.
As such, anything larger than 3" is expensive and ground clearance issues will eventually develop...
anything larger than a 3'' isnt that expensive... especially if you do it yourself. Summit Racing sells everything you need, and pretty cheaply too...
i could made a 3.5 or a 4'' exahust that BIC and ETS make for less than HALF the cost...
Plus i dont see running anything bigger than 3'' on the stock ct-26 of any benefit.... back when i had my eclipse, the stock t-25 turbo (god damn pea shooter) i had a 3'' o2 eliminating downpipe, 3'' cat, 3'' cat-back... i lost power up top because the turbo couldnt keep up... when i went back to a stock o2 housing, stock downpipe and cat, and then a 3'' cat, it felt a lot better...
and on an n/a, you lose bottom end, but you gain top end, that is what i gathered from all of the dynos i've seen...
i could made a 3.5 or a 4'' exahust that BIC and ETS make for less than HALF the cost...
Plus i dont see running anything bigger than 3'' on the stock ct-26 of any benefit.... back when i had my eclipse, the stock t-25 turbo (god damn pea shooter) i had a 3'' o2 eliminating downpipe, 3'' cat, 3'' cat-back... i lost power up top because the turbo couldnt keep up... when i went back to a stock o2 housing, stock downpipe and cat, and then a 3'' cat, it felt a lot better...
and on an n/a, you lose bottom end, but you gain top end, that is what i gathered from all of the dynos i've seen...
Mike
you are right on the flow. There are two contributing factors.
1. As you stated, heat cooling down does indeed effect the speed of the exhaust. (Poodle, even on a turbo charged vehicle
)
2. Fluid dyanmics plays a big role here to. As a gas goes from a certain smaller area to a bigger area. The g, since it obeys fluid dynamics, will have a slower velocity in the bigger area (ie 3 inch going to 5 inch).
Also to add, everyone MUST consider noise. A 4 inch exhaust, no matter how much you try will not be in any way, shape or form, remotely quiet.
you are right on the flow. There are two contributing factors.
1. As you stated, heat cooling down does indeed effect the speed of the exhaust. (Poodle, even on a turbo charged vehicle
)2. Fluid dyanmics plays a big role here to. As a gas goes from a certain smaller area to a bigger area. The g, since it obeys fluid dynamics, will have a slower velocity in the bigger area (ie 3 inch going to 5 inch).
Also to add, everyone MUST consider noise. A 4 inch exhaust, no matter how much you try will not be in any way, shape or form, remotely quiet.
Poodles;952332 said:
Are you sure about that?
The times I've run open downpipe just to see what it's like, the low end felt like shit and a bag full of nothing until higher RPM.
Poodles;952332 said:
i partially agree with this guy over here (just not sure about the myth/lie statement).
Backpressure is needed, but for a turboed vehicle, it's in the hot side of the turbo itself. that's why a car with a larger turbo hot side will have less bottom end power as opposed the same setup with a smaller side, speaking about the power before boost pressure overcomes.
this is how variable geometry turbos are great. depending on the exhaust backpressure, vanes open and close making the hot side of the turbo smaller and bigger at the appropriate parameters.
With a 7m and a CT26, i could see a smaller exhaust being partially beneficial for lower end torque, as the velocity would be there because of the low rpms and lack of mass to the exhaust stream out the turbo, but this is just used as an example, and we're looking for higher end power where you are plentyful of exhaust mass, so a 3inch would be just right for ya with a mild setup under 500whp, as stated before.
OneJoeZee;952547 said:
You need a pipe on there for about 1 meter to keep the velocity high and the temps high. Just think your gas is comming out the turbo straight into cold air. if open headders worked on a turbo F1 cars of the 80's would have run them. You need to keep the gas temp high for better flow to keep the pressure differental from one side of the turbine to the other as high as possible.
There may be people who disagree. Obviously there are people with Civics who think that a 3" or larger exhaust on an NA 1.2L displacement engine isn't a waste.
It's my opinion that a 3" exhaust on an NA doesn't do good things for it, though I conceed that a good header design can make a lot of difference there as well.
Still turbochargers change the rules. A good NA exhaust uses exhaust gas velocity in order to produce low pressure at the exhaust ports of the head. This isn't really possible in a turbo car, as there's this turbine in the way.
Because the turbo works off of a pressure differential between the exhaust manifold/header and the downpipe, the faster the exhaust can clear the downpipe side of the turbine, the more power there is available to turn it. This is NOT best served by an open elbow, but by an orderly flow. The length that this flow shoud be directed varies by the velocity of the gasses.
It's been a long time since my fluid dynamics courses, but I remember the math being enough to give me a headache. I'm pretty sure that there's much more trial and error rather than actual calculation that goes on with the companies that produce these things, and with the combinations of headers, turbos, elbows, downpipes, cats, and cat-backs, there's no way that any solution can cover all situations. Change any of these elements, and it changes all of the other ones as well.
It's my opinion that a 3" exhaust on an NA doesn't do good things for it, though I conceed that a good header design can make a lot of difference there as well.
Still turbochargers change the rules. A good NA exhaust uses exhaust gas velocity in order to produce low pressure at the exhaust ports of the head. This isn't really possible in a turbo car, as there's this turbine in the way.
Because the turbo works off of a pressure differential between the exhaust manifold/header and the downpipe, the faster the exhaust can clear the downpipe side of the turbine, the more power there is available to turn it. This is NOT best served by an open elbow, but by an orderly flow. The length that this flow shoud be directed varies by the velocity of the gasses.
It's been a long time since my fluid dynamics courses, but I remember the math being enough to give me a headache. I'm pretty sure that there's much more trial and error rather than actual calculation that goes on with the companies that produce these things, and with the combinations of headers, turbos, elbows, downpipes, cats, and cat-backs, there's no way that any solution can cover all situations. Change any of these elements, and it changes all of the other ones as well.
OneJoeZee;952547 said:
WORD.
Ive heard its worth a few tenths in the 1/4 but thats because you are at high RPMs 99% of the race.
When I put my exhaust back on a couple weeks ago after running it open for a few days, the car felt stronger all over the RPM range and it definately spooled up faster. Not to mention it sounds MUCH cooler when you can hear the turbo spool instead of just NOISE.
Backpressure is an old term that the old guys used.
They thought that the backpressure in the exhaust actually helped push the piston down (how it did this with a valve in the way I have no idea).
Much more recently they've figured out it's the scavenging effects of the other exhaust pulses actaully PULLING the exhause out of the cylinder and as the cam profile has overlap, will actually pull fresh air into the cylinders. THis makes far more power.
The issue with velocity is that you reach a point where you juct can't flow enough air through the system, and it cuts off the topend power. Only an old SBC that doesn't wind too high, that's fine. On a modern car that spins twice as high (S2000 is a great example of this), the exhaust HAS to flow to take advantage of the power on top.
Supplement it with gearing to keep it in the powerband and that's how you get em to go fast.
There is an article from Garrett somewhere talking about exhaust design for N/A compared to a turbo car, it's a pretty good read...
They thought that the backpressure in the exhaust actually helped push the piston down (how it did this with a valve in the way I have no idea).
Much more recently they've figured out it's the scavenging effects of the other exhaust pulses actaully PULLING the exhause out of the cylinder and as the cam profile has overlap, will actually pull fresh air into the cylinders. THis makes far more power.
The issue with velocity is that you reach a point where you juct can't flow enough air through the system, and it cuts off the topend power. Only an old SBC that doesn't wind too high, that's fine. On a modern car that spins twice as high (S2000 is a great example of this), the exhaust HAS to flow to take advantage of the power on top.
Supplement it with gearing to keep it in the powerband and that's how you get em to go fast.
There is an article from Garrett somewhere talking about exhaust design for N/A compared to a turbo car, it's a pretty good read...
Dan_Gyoba;952582 said:
I run 3" on my NA,
and I have yet to find a faster non-turbo that will out run it.
In fact, I have out run Every stock Turbo Mk3
that I have ever come up against.
The only place velocity is beneficial is in the header primaries.
Smaller pipe will only 'Look' better if the car isn't tuned properly.
When you run a more free-flowing exhaust,
you have less exhaust diluting your incoming charge.
This means you must have more fuel going in,
or the engine runs Lean,
and makes Less HP.
It isn't the slower moving exhaust flow in the pipe,
or the reduced Back Pressure,
that causes people to have less power when they go to a large pipe.
It is the fact that they didn't add more fuel
to deal with the reduced exhaust contamination in the next charge.