Thought I'd ask here instead of Tech/Misc. since you guys are the exhaust experts...

So I've read in a few places that the ideal, theoretical exhaust system is one that continually tapers out larger, from the header/dp back to the muffler. So it'd start at perhaps 2.75" at the dp, then tapers out to 4" at the muffler. Is this true?? It seems to me that the ideal exhaust would be the exact opposite; start out large and continually taper smaller. My thinking is that as the exhaust travels down the pipe, it cools and contracts and would therefore need less pipe volume to maintain velocity. I mean... if the exhaust is continually getting colder/denser, and takes up less volume, wouldn't a continuously larger taper just introduce recirculation zones? Or at least hinder scavenging?

Please, someone help me understand

 

Likely to do with the fact that as it cools, it slows down.
I believe it is referring to that for max power under the curve. For max peak power, a drastic pressure drop across the exhaust outlet as soon as possible works fine. Such as; open headers,etc.

 

Yup. Exhaust slows down as it cools, which means that more volume in the exhaust container allows more exhaust to occupy the space, rather than becoming a restriction. As long as it's further down the exhaust "exhaust scavenging" no longer applies.

Or you can just cut that **** off and go open DP/header.

 

Depends on the application obviously, FI or NA and what the car is built for. My exhaust goes from 2.75 - 4". From what I understand, on turbo cars you want your exhaust pressure to always be lower than your boost pressure to reduce the risk of reversion and grenading your engine.

 

This does seem to be a pretty popular design in regards to headers. Where the primary is stepped to a slightly larger diameter. They usually produce good power too.

*random picture from google*

 

Thats more because the runners only carry gases from one cylinder and the rest carries all the gas.

 

You could do the math by measuring pressure and temp at the exhaust towards the engine and then again at the end of the exhaust. Then put it all in a fluid calculator and find the optimum diameter pipe depending on the state of the contents in the pipe. My guess is it won't make much if any difference in a real world situation on a car.

Eliminating bends in the exhaust and obstructions inside the exhaust itself will make much more of a difference.

 

The problem with saying that -- the slower, more dense gas is flowing the same amount of molecules as the faster, less dense gas. Say you have a fixed tube size - half the temp (in kelvin) means half the speed with a constant lb/hr flow rate. Speed isn't impacting flow as much as drag. More dense gas will drag more, which is why planes fly miles above for max fuel efficiency.

Regardless, header design is far more crucial than exhaust design. You've got things happening extremely fast up there - gas velocity changes, temp & density changes, even pulse waves alter scavenging a great deal. Exhausts just have to be large enough for your powerband.

That said, there are other areas to spend time & money on for greater power gains.

 

Hey yeah, that makes sense. I never thought of that.



I agree, which is why I've designed a microcontroller-based exhaust cutout that I'll soon be releasing . I was just curious about the theory.