I have been arguing with myself recently and am looking for others opinions relating to the size of an exhaust on a given turbo and it's effect on the power band.

The end all question:

Can the exhaust on a turbo be *too big* and hurt performance on the car, possibly low/mid range?

I have always read and heard that the turbo itself is all the back pressure an engine needs. That said, you want to spool the turbo as fast as you can to build chamber pressure. Comparing apples to apples, lets take a generic 60-1 P trim with a 1.0 AR housing. Obviously there is a point of diminishing returns but a 4" exhaust should allow the turbo to spool sooner than a 3" which should be sonner than a 2". Right?

Everyone will generally agree that 2" is too small, but would the 4" *hurt* performance over a 3" or 3.5" DP?

-Chris

 

I think you want the pressure drop to be as great as possible through the turbine... that's just my feeling though.. no hard data.

A 4 inch outlet might also make a lot more noise than a 3 incher... so that might be a consideration for some that drive around where police or neighbors will have a problem with it.

B

 

i wonder if you get into a diminshing return kinda thing. ie on a stockish t2 going from 2.5 to 3" is like night and day, but i doubt you'd be able to notice a difference on a 4" from a 3"

 

On a turbo exhaust system, you want as little backpressure as possible. Same for NA, but like j9fd3s said, it gets to a point where bigger isn't always better. I'd go with the 3".

 

Well... I already have a 4" setup. And it the works very well ~470RWHP. Make ~20psi by 4000 RPM.

But while I was tunning the tunner made the comment that he thought I'd pick up more low end if I had a smaller exhaust and I wanted to know if anyone else bought that theory. I've read from Garrett engineers that 3" is actually too small for 400+HP cars. Not that people can't do it... just that it wasn't as efficient.

The second part is:

Currently I have a .84 exhaust housing with a GTQ wheel and a heavy street port. Manifold is fully divided, tunned etc. However, when I first built the manifold I didn't have the turbine housing, so it turns out that the inlet to the turbine manifold is significantly smaller than my mani runners.... like 30-40%. So much infact that I can't port the housing sufficiently to eliminate this "wall"

So, all else being equal (and I realize this is kinda guessing) I'm thinking that a larger (1.0 A/R housing) with a smooth transition to the manifold, would probably spool as well as the .84 with a 30% "Wall" inducing turbulence right before/at the turbine housing.

Any guesses on that?

 

Won't the 1.0 housing have the same ID inlet?

 

I don't have a good answer for your question but I do remember that Gordon Monsen tinkered around with putting an electronic valve in his exhaust stream for this reason. If I remember correctly he had it partialy closed at lower RPM/load/something to generate a little backpressure and it would open up once everything was spooled. I don't remember his observations though..

As for the turbo question, I'm with setzep, The flange should be the same size.

 

I don't see why. I was accidentally shipping a 1.15 housing the first time and the port entries were much larger.

If the A/R is larger then for a given radius at entry the area would need to be larger. Have you seen contrary to this with other turbine housings?
Hmm, I'll have to search for this. Thanks

 

^ That was me again under the wife's name

 

The higher the pressure differential between the turbine inlet and turbine discharge, the quicker the spool and more overall turbine effiency. Anytime you lessen backpressure after the turbine discharge, your spool will speed up (less lag) and more flow will result. The bigger the better in respect to turbo exhaust. There is a point were all bigger does is increase noise. The larger and less restrictive the exhaust system, the more torque and faster spool will result. In other words........... NO going smaller in the exhaust after the turbo will not result in any gains in power / torque anywhere in the power band but decrease it.

All standard T4 flanged Garrett based turbines have the same inlet size. Hence only 2 gaskets available (ok, 4 that I know of, there’s a T4 Euro flange that has a slightly more rectangular bolt pattern, but same inlet size holes {both open and split}) the open volute (non divided) and split pulse (divided). All the gaskets have the same inlet area holes. If there were different area inlets per turbine A/R, there would be a specific gasket for each A/R, and there isn't.

The inlet area divided by radius (radius in respect from the inlet center to turbine center)A/R explanation is a bit misleading when dealing with Garrett based turbines. That particular theory of A/R is more applicable to compressor housing A/R. If you compare turbines of the same frame, they all have the same inlet area and the radius (inlet center to turbine center) varies slightly. The real difference between A/R is that it necks down after the inlet flange more or less abruptly depending on what A/R you have.

~Mike..................

 

Thanks, I guess that's exactly what I thought. Too bad no one, that I have seen, has done a comparision as to exactly where the cutoff for performance vs noise is concerned on the DP size.
Interesting on the area entrance to the turbo. I would have sworn that the 1.15 had alarger opening than my .84. I do realize that there are only 4 gaskets (divided and un divided) but my divided gasket is significantly larger than the opening in the turbo... which furthered my belief that the opening size varied with A/R size. Unfortunatley the only pic I have is a side shot that shows the scrolls. Oh well, I appreciate the input.

Now I need to decide if it's worth the extra $$$'s to experiment with a larger 1.0A/R turbine housing on my setup.

 

I normally run a 3.5" turbo back w/ one muffler.

I was planning on adding a presilencer so I bolted on my old 2.5" downpipe/presilencer.

The low end took a BIG hit in power w/ spool being ~500rpm later. W/ the 3.5" I was just getting positive boost ~1,500rpm and 3-4psi by 2,000rpm.

To exit my work it is a slightly uphill tight turn w/ a bit of chunked asphalt. W/ the 3.5" I would give it about 15% throttle and ease off the gas as the clutch caught so I didn't spin the tires and stay out of boost. I killed the car 2 times yesterday w/ the 2.5" leaving work untill I learned I had to give it ~20% throttle and add more as the clutch caught and I still wasn't going to lose traction unless I floored it.

Normally w/ the 3.5" I cruise town just over idle and pulling from stops I shift up ~1,800rpm using light throttle to stay out of boost and I still pull away from "normal" traffic. I have competed in "stop light Grand Prix" w/ pick trucks (average redneck here) just shifting before 2,500rpm (8psi boost)- it is loud enough they really think I am racing...

W/ 2.5" I can not accelerate smoothly from anything below 1,500rpm- though to be fair it may just need different fuel maps. It is quieter w/ much less low frequency noise.

 

There's an old saying.

"You can never have to big of an exhaust on a turbo car"

If you rescrict it on the way out it won't boost as fast.
SO if you go with a smaller exhaust you will actually lose your bottom end.

I like to use transition cones on my dp's.
For example if the exit on the turbo is 3" i would use a 3" to 4" transistion and make the dp 4".

 

I think that bloke may have said to use a smaller exhaust, so the exhaust gasses have good velocity before the turbo kicks in (basically using N/A tuning on a turbo)

But as we all hear, in a well-known fast-food chain:

Would you like that supersize sir?



slap a bit of steel drainpipe on it!

 

Thanks Ian and Enzo. That's exactly what I was thinking and felt. My 4' DP setup really isn't loud at all. I'm running through a dual 80mm catback that seems to muffle the sound well. I Get a bit on a exhaust leak sound up front, but that's surely just from my 16Ga DP.

[QUOTE
I like to use transition cones on my dp's.
For example if the exit on the turbo is 3" i would use a 3" to 4" transistion and make the dp 4". [/QUOTE]

At this point I wish I had done that. Running the 4" Vband that I am using is makes going to a different turbine housing pricy. I'm alreayd paying for machining for the GTQ turbine wheel, but adding the 4" Vband to the mix gives me a $400 turbine housing So I"m struggling with how badly I really want to try a 1.0 over my .84.

Thanks for the insight. Maybe I"ll try the transition this go around.

-Chris

 

Somewhere I read that having too big piping causes better cooling of the exhaust and thus can slow down the passing gasses. Fluid dynamics... never was good at it, but it kind of makes sense.

I think there is a sweet spot but noone can determine it.