Turbo exhaust theory discussion
 

Hey guys,

Someone who is reasonably well respected locally claims that his brz turbo kits utilise 2.5" exhausts because the cars made MORE pwer than 3". He freely admits that this is a controversial claim.


Someone please tell me under which circumstances he could be right, huge huge a/r + low boost?

Cheers

I asked if it was huge AR and low boost he said:

"Kinda.
We have done a few variations, to be honest, about 75% of the efficiency comes from runner length, diameter and equilibrium. We started off using huge A/R but now we have them down to small/medium sizes."

What do you make of this?

Sounds like neck talk.

Is it a full 2.5" from turbo to tip?
What size turbine housing?

Questions sent.

I think he's full of it . . . or should at least be able to back up his argument with theory or maybe a dyno run against a dump tube.

OTOH, if the 3" exhaust was a bent-up POS . . . .

I've always gone by this: https://www.miataturbo.net/engine-pe...-theory-71503/

Turbo exhausts are like butts. Bigger is better.

He's in la la land.
Engine to turbo runner size, shape, and placement, as well as collector design - now those can def affect the shape of the curve, but after the turbine, it doesn't matter at all. Some will argue that you need a "trumpet" after the exducer to promote velocity, but even that is trivial and I've yet to see actual results proving it actually improves anything other than "in theory".

Flow = power
You cannot flow less in a pump and make more power. It defies ALLTHELAWS

Unless he did something crazy like run less spark advance and fatter AFR's at lower laod in the map, in which case LOLOLOL

Conservation of mass theory tells you that the faster the fluid flow, the lower the pressure. So given a constant flow rate the 2.5" exhaust would have faster moving gases and less pressure. Faster flow means more Momentum in the gases which can be good (scavenging), however at the same time faster flower means more drag around the perimeter of the pipe... Also the change in diameter effects the reynolds number which could cause more turbulent gases, more turbulence = more drag. Also the transition from the turbine to downpipe is likely accompanied by an increase in cross-sectional area. A rapid increase in area could cause an unfavorable pressure gradient opposing fluid flow direction and cause boundary layer separation. This may cause weird pockets of turbulent gases and pressure variations at the turbine exit.

All that stuff is confusing, but the take away is there are trade offs between back pressure from restricted area vs fluid velocity, and maybe there is some spot in the power band that benefits from the smaller diameter. I'd just ask him to dyno both and prove it with numbers that are significant and can't be explained away with random error in measurements.

No. Bigger is better post turbo

Do I need to build a 2.5" exhaust to test against a 3" exhaust?

Yes

Well fuck then. Let me go talk to the guy I told I would not build one with a 2.5" exhaust and see if he is still interested and would let me dyno with it before I sent it out.

None. Impossible to tell why his data is skewed, but it is. He's a fool for believing it, and you would be a fool to believe him.

No you don't. The less restriction post-turbo, the better. Well accepted, proven with dyno graphs on MANY applications, and can be shown on paper with a little math as well.

I would say no, and that i would do it. But I only have 2.5" logs from when i had a 5 speed. And that is not an apples to apples comparison. More like a granny smith to red delicious comparison.

Stopped reading right there. The whole point of the larger exhaust is to increase flow rate.

If he saw that, then there's something else that varied between the tests. It's possible to build a crappy 3" exhaust that flows less well than a good 2.5" exhaust, a nice metal core cat on the 2.5" vs a cheap/crappy ceramic one on the 3" could explain it.

--Ian

You stopped too late. First stop should have been conservation of mass. He's probably referring to Bernoulli and that's conservation of energy.

The only way its possible would be is if he could scavenge THROUGH the rear housing/turbine right?

Dann

you can't scavenge through a turbine unless you put a vacuum on it lol