Question for Gurus concerning exhaust size.

[GM Guy]

I was wondering, since 3" is the size of the biggest downpipes, what is the advantage, if there is at all, of necking up to 4" for the rest of the exhaust?

Does it make it sound cooler, or does it actually lower egts?

can one get away with a full 3" exhaust, or is it best to do the 3" DP that necks up to 4"?

This has always intruiged me, and i thought i would try and find out! :thumbsup:

 

[3500GMC]

Something to do with gasses doing different things when cooling down... Kinda like stepped headers for gassers.

No technical info available at the moment.

 

[WarWagon]

The 3" is all that will fit there. So you have high friction from high speed gas there. Then it goes to 4" the gas slows down for the same CFM. Recall you have like 750 CFM cold going into the engine that expands like 47 times when hot. (Numbers off the top of my head so don't quote it.)

So you get the biggest effect of restriction on this hot expanded air. This is why you want to spend $$ on the exhaust first and intake 2nd.

So less MPH on the air is less fiction and less back pressure. It is worth about 1 MPG or more in some applications. The turbo down pipe is 3'. The rest of the exhaust is like 12'. So 2/3 of the exhaust is less restrictive with the 4" pipe.

You could experiment with EGT's but 4" is best and 5" doesn't make any change over 4" as I have read.

 

[buddy]

The goal with a turbo is to have as little pressure on the exhaust side, because you get boost by a differnce in pressure from engine side to exhaust side. The pipe causes backpressure which means it takes more engine pressure to make the desired boost, longer spool time and less efficient.

The earlier the exhaust exits and the more it can expand within the pipe before it exits, the less pressure on the exhaust side there will be

 

[JMJNet]

The advantage of exhaust mod is to get rid of the pinched stock exhaust. The DP is pinched the most, so it is supposed to be 2.75" diameter but there is bottleneck everywhere, it is only as good as the smallest diameter. Then when it goes to the back, it is also pinched whenver it has to go under the frames.

So, in conclusion, anything that is not pinched is better. Most of the vendor sells a mandrel bend system 3" DP, transitioning to 4" exhaust. 3"DP is about what you can fit there without rubbing to the engine block, the fender well and the firewall. It only has less than 1" clearance between them.

 

[GM Guy]

well, thanks for the explanation. Now i know there is a point to the necking up to 4".

 

[midniteplowboyy]

I think the main thing is getting a 3" downpipe, after that a straight piped stock exhaust or 3" exhaust is all you need. The further from the engine the more the exhaust has cooled needing less space and lowering velocity, going 4" lowers velocity even more and may even hinder performance from turbulence. Just my thoughts, no dino to back them up, I think a full 3" is all thats needed, but a 4" has a quieter better mellower sound.

 

[buddy]

having greater velocity on the exit exhaust exactly means it is of higher pressure, and therefore greater backpressure. this velocity is not required in our turbo diesel that has no exhaust scavenging. so although you may not see any gain from 5" pipe on a 6.5td, because our engine doesnt produce that much power, there will be a gain to 4". now if you were designing a 6.5td that ran 5000rpm with twice as much fuel input then there would be benefit to 5" pipe.