Can anyone explain why this is?

 

I guess there's 60 people as clueless as I am?

 

free bump

 

it seems that this would be untrue? or there is another issue? If this were the case wouldnt the people installing these exhaust cut outs be having boost issue's????

 

From what I've read around it's not untrue. Theory has it that backpressure is bad for turbo cars however people that ran without exhaust had problems running the car like in the above example.
One of the reasons I've heard was that little backpressure is good because it actually creates flow and the exhaust piping extracts the fumes with higher speed than without piping therefore helping boost.
I wonder if someone can get technical about it.
It's obvious that lots of backpressure (resistance to flow) is bad for the turbo car since you want your exhaust fumes to go out as fast as possible.

 

I suspect it is because the turbo is designed with a specific pressure ratio (air in pressure / air pressure out) across the "scroll". That ratio is for the stock system. When the system is modified and the backpressure is removed, you change the aerodynamic operating characteristics of the "scroll - actually works like little airfoils or wings - and lose turbo efficiency.

 

redcat: I think that was it.
But I remember reading somewhere but forgot where that even make it with tuning they couldn't work as good as with the full exhaust.
And for the exhaust, they found a specific width of the pipes to be optimal and going wider didn't do anything (at least for their power level).

 

SoR i've also heard from some well respected tuner (forgot who) that if you run 2.5in from downpipe to exhaust you will actually increase performance than adding a 3in turboback, but most people still buy 3in just in case they might get a turbo upgrade later on

 

I think Al runs a 2.5 with his turbo upgrade.

Thanks for the Bump SoR

 

When you change the expansion ratio across the tubine scroll, you'll effect both pressure ratio and flow. This will have a direct effect on the air incidence angle . . . the angle the air hits the leading edge of the scroll airfoils. Get the angle too far off, and you will impact the airfoils ability to generate lift (that means torgue to run the buisness end - the compressor - of the turbo).

An analogy is an airplane wing. It is designed for "cruise" conditions. It works at other conditions (take-off, approach, etc.), but not nearly as well. Airplanes have variable airfoil geometry (flaps) to compensate for this "off condition" running. Don't think your turbo does.

Regarding bigger points, at some point the system is "chocked" (most flow you can fit through the smallest restriction in the system), hence the need to go to more thorough and costly upgrades.

 

That's post quoted is silly. I know lots of ppl that run 20 lbs. or higher with a full 3" exhaust on a dsm. The biggest problem with that fat of a pipe on a dsm is controling boost creep. I've never heard of someone having trouble keeping the boost up, rather keeping it from going over the set limit.

 

Don't think it's silly at all. I never said it wouldn't work, but might explain why the original post said there was some issue with no backpressure / no boost.

 

I've noticed that some axleback upgrades mention a similar phenomena... boost drops slightly when the straight thru muffler is installed. I never understood why. The Pruven website mentions this and Al told me the same thing at a recent Maryland meet. Seems like boost ought to increase when there is a greater pressure differential across the hot side of the turbo. Less back pressure should equal greater delta.

 

Satori, I have no doubt that removing some back pressure will in fact cause the turbo speed to increase and more boost will be generated. This is because the Mistsu Engineers have designed the turbo with “margin” into the design. This is what prudent engineers do for any production configuration. However, aerodynamics are finicky and unforgiving. An airfoil will generate the most lift right before it generates no lift. This is a basic fact of aerodynamics. The message is, some backpressure reduction will buy you more power. A little more than “some” and I believe the turbo will not work effectively. I am not a "turbo" expert, but do have some experience with high speed / high performance turbomachinery, and I have seen this phenomenon.

Regarding the pipe diameter discussion . . . At some point it does not matter what size pipe you install (forgetting what I’ve said above for a minute). The flow is fully expanded, and you could have a 20” diameter pipe on and it would make no difference. Flow is max’d at the limiting diameter in the system (flow is chocked) and now more flow is possible now matter how hard you try. That diameter is clearly bigger than the 3" pipe as you have suggested.

 

Second to last line should read, "Flow is max’d at the limiting diameter in the system (flow is chocked) and no (not "now") more flow is possible now matter how hard you try."

With a wide open discharge, the choke point with max flow is somewhere upstream.