At what point is 3 inch exhaust required?

[monza]

This is interesting 9 Ball. I'm a firm believer in keeping the exhaust as small as possible. Do you have any specific examples where you tested this? It would be good for reference as the bigger-is-better debate always comes up.

I have a real life example of this, it really shows all the variables that go into the best exhaust. It's not really a simple 'flow' issue as I would have thought. Need to consider transition point in the headers, tuning length of headers/collectors and how it all reacts to the exhaust. (I've researched this so much my head spins)



Recently with FUeL (525 RWHP) we changed the headers to fit with a new DSE front clip upgrade and left the exhaust (2.5) exactly the same. With no other changes we lost about 15hp and torque peak moved from down to about 4000 from 5200. The new headers technically better 'should' have increased performance, so I would have thought? The old exhaust was 2.5. Un-bolting the old exhaust on the dyno we gained back the loss? So the conclusion was the 2.5 exhaust with the new header was chocking it out. Every thing has been now changed to a 3.00 exhaust and it 'better' have solved the issue! Still waiting for the new dyno time and results-(fingers crossed)

pretty new exhaust pic...

[1968Maro]

I enjoy the technical aspects of this so let me take a stab at what I believe is going on.

Venturi Effect

http://www.youtube.com/watch?v=Z4AbzQyLPcE

When air exhuasts from the combustion chamber at high speed it will create a venturi effect only when a pressure differential is created by first restricting the flow and increasing the pressure at the heads then decreasing the pressure and increasing the flow to create a vacum and help to empty the exhasut chamber more effiecently.

Two large an exhaust and you loose this effect.

Two small an exhaust and you restrict the gas flow and do not allow the engine to vent.

How does that sound?

[CarlC]

I'll loved FD vs. vibrations. I hated that class. I too did a similar quickie calc just to verify the same when doing mine last year.

If memory serves a Z06 does not have 3" exhaust, but closer to 2-1/2". The aftermarket exhaust systems that "go-big" do not up the horsepower, but instead go for sound and wow-factor.

[wmhjr]

CarlC - You've got to remember that motors are big air pumps, and that different pumps make power in different ways. I believe it would be a real mistake to assume that because a Z06 has a 2.5" exhaust (if it does) then other motors only require that. First of all, it's making less power and far less torque than many of us are making. Second, it's doing it at higher RPMs. I'm making more torque at 3000rpm than the Z06 makes ever. But beyond that, factors like valve geometry, cam profile, compression and lots of other stuff become variables.

I will agree that many cars end up with oversized exhaust. Yet many others would do well with an increase. And I do not agree that aftermarket exhausts that "go big" do it for the "wow factor" or sound.

1968Maro - you've got a start. But don't forget that unless you're running "true duals" with no X-Pipe or H-Pipe, exhaust pulses from one side assist in the evacuation of gasses from the other side.

[wmhjr]

BTW, at least according to http://www.corvettemuseum.com/specs/2008/index.shtml the Z06 has 3" exhaust.

[tonykim]

I think a good part of it is simply testing and getting the best combination of parts and the right tune. on my fox stangs I found that with a 450 hp NA 331 stroker that going from a 2.25" flowmaster 2-chamber catback to a 2.5" magnaflow showed no change in peak power and almost no change in peak torque. i was really surprised. and on my other car I'm making 360 rwhp with a supercharged 302 and have 2.25 exhaust behind a 2.5" x-pipe. It's quiet and crisp sounding and seems to be well balanced.

My feeling is that above 500-525 hp the 3" exhaust starts to show an improvement on power without loosing torque assuming the engine is a smaller displacement (under 400 cid). However, the hedders, fuel system and ignition have to keep up with the increased power potential as well. otherwise, you'd be making noise not power.

[DRJDVM's '69]

What about going from a 3 inch to the muffler but 2.5 after?

I may need to do that since my "up and over the rearend" is gonna be tight....

[TnBlkC230WZ]

What about going from a 3 inch to the muffler but 2.5 after?

I may need to do that since my "up and over the rearend" is gonna be tight....

As the exhaust cools and gets denser, you can use a smaller pipe

[tonykim]

sounds good to me. 3" over the axle can be a real pain.

[383hq]

I went through this a few years ago, when deciding to go twin 3"over twin 2.5"
Vizard has a very good article available on the internet - I've posted the link to it here for reference.

- in essence it talks about flow and what can be considered as acceptable losses within the exhaust system.

- straight pipe flows about 115cfm/ sq in
- zero loss required 2.2cfm per hp (less than 1% of total power produced by the engine is then lost due to back pressure).
- straight pipe only with <1% loss 3"twin was circa 700hp, 2.5"twin was around 500hp.

Check the numbers I've put here it was a while ago - I do remember then hunting for muffler flow numbers on the internet to ensure the saleman speak did all match up

Vizards article can be found here:

http://www.popularhotrodding.com/eng...exh/index.html