Exhaust setup question

[THAWA]

I saw a picture and explanation of some headers somewhere, cant remember where though and they were interesting to me. They started out smaller near the ports and the size enlarged a few inches away, like say it went from 2" at the ports to 2.5" a few inches downstream. The reasoning behind this was that as the gas cools it expands and so to keep the velocity the same the pipe diameter was made larger. That makes sense to me, but the real question is, would it work in a real world application?

[BAC5.2]

Do you think the exhaust gas would cool significantly in those few inches?

I don't think so, and on a turbo setup, the exhaust only needs to go a foot or two before it's allowed to expand.

Heat wrap the headers and crosspipe and uppipe and I doubt you'd have to worry about the exhaust getting to cool.

I wouldn't think the exhaust would cool enough to justify that. Maybe on an N/A, but I don't know about within a few inches of the exhaust port.

[Kelly]

The reasoning behind this was that as the gas cools it expands and so to keep the velocity the same the pipe diameter was made larger.

That deosn't make since to me. Why would you want to keep the same velocity? Wouldn't you want to increase velocity?

This brings up another thought for you to ponder. How could you make a variable diameter up pipe? To increase pre turbo exhaust velocity at low RPM, then open up at higher RPM for more flow, in the persuit of less turbo lag.

[BAC5.2]

Turbolag is overrated.

Under driving where turbolag would make a difference, there is very little, if any. I'm at full boost in the next gear by the time the pedal hits the floor. If you had some stupid monster turbo, you should have a built gearbox, and you can tune your ratios to compensate for lag.

[THAWA]

Hmmm, maybe a butterfly valve in the middle of the uppipe like in the throttle body? or what about adding another pipe controlled by a valve? Like it's just more tubing that goes from one part of the exhaust to the other. Open it and the area effectively increases lowering velocity, close it and the area decreases increasing velocity. Does that make sense?

I dunno about the headers, I just saw it somewheres and wondered what it was all about.

[Kelly]

Turbolag is overrated.

Under driving where turbolag would make a difference, there is very little, if any. I'm at full boost in the next gear by the time the pedal hits the floor. If you had some stupid monster turbo, you should have a built gearbox, and you can tune your ratios to compensate for lag.

Maybe the problem is my driving style, and what I expect. Im used to little cars with quick throttle response. My turbo also has a lotta miles on it, its next on the chopping block.

1st gear is too short, when 2nd comes, theres lag (granted it only takes a second) but theres no low end. 3rd is fuckin awsome, and 4th is better. I rarely use 5th, cause my car sounds so fuckin hot runnin at high RPMs, and all the powers up there. Plus I live in a city.

But there is lag in all turbo cars. Ive drivin 996 turbos, and they lagged. The 3.6 single Porsche, had so much lag, that when the turbo kicked in, everybody wrecked em cause they had the throttle wide open, and they only made em for a year.

My Idea for the variable up pipe, was a flap. Kinda like a tounge when you whistle. one moving part.

[G-reg]

The answer to that problem has been addressed with variable geometry turbo's, kinda like your down pipe idea but right at the turbo inlet. I have only heard of these on big stuff like semi's and other commercial things....Like jet engines

[professor]

gas doesn't expand when it is cooled...its the other way around, so that theory needs a little work

[THAWA]

Oh my, I can't believe I completely missed that.

[tris91ricer]

gas is technically an aqueous thing.. right?

[BAC5.2]

Damn, how did I miss that? Lol. Damn.

Jason - Turbo Lag is only an issue from a roll on, in gear. As soon as your cooking, the lag is insignificant compared to the gains.

Roll in 1st, hit it, and lag becomes an annoyance, once everything is boiling and you grab 2nd, the lag doesn't really make a difference. It isn't like you are sitting there waiting for seconds while the turbo spools up. You are generally at full boost before the pedal hits the floor again.

And when you are sitting in gear in high RPM's, it doesn't matter either. Downshifting to 2nd, and as soon as the clutch is out, and the pedal is on the floor, your making boost and away you go. Coming into a turn at the top of 3rd, sitting at 5k, you are spooled up, again, before the pedal touches the metal.

[93forestpearl]

PV=nRT

(pressure)(volume)=(# moles)(gas const.)(temp)

Gas is definately a fluid. I'm excited to take fluid dynamics next semester. Then I'll know exactly what is going with alot of what is going in these air pumps we call engines.

Check out cobbs site. They have some really interesting articles on exhaust theory.

http://www.cobbtuning.com/tech/exhaustdesign/index.html

[legacy92ej22t]

I've never heard of a variable diameter up-pipe but CES makes a variable diameter DP! The DP01. The main dump is like 2.25" at the turbo and slow tapers up to a full 3" by the time it's at the mid-pipe cat. It's for smaller turbos like the VF11 or 10 and the TD04h-13G. It helps with low end torque supposedly.

[THAWA]

Well that's a fixed diameter. What Kelly was talking about was making the effective size of the exhaust change depending on rpm.