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Re: SVO: XR racing, and downpipes...

Mp23cc@aol.com wrote:
> I find it real funny that anyone who has ever  installed a 3'' downpipe on
> their car find it runs much better. 


> Fluid charts or better yet a theory tells
> us it does not work, well  so much for rocket science.  


> A 3'' downpipe will
> act as an expansion chamber with the exhaust.

Yes, an insignificant amount.  I'm not saying that you're not seeing 
ANY gains with a downpipe, but I'm not understanding why you're sure 
that the expansion chamber theory is the reason.

> It will reduce the exhaust flow
> turbulence out of the turbo. 

This is definitely a possibility, because this is the kind of thing
where fluid dynamics DOES turn into black magic.  Even the experts
start guessing at this point...

> As you step on the throttle, the turbo starts to
> spool up. How fast will depend on how much exhaust gas passes by the exhaust
> turbine wheel. If you can move more exhaust gas by the turbine wheel the
> turbo will spool up faster. 

All true.

> If you have a bigger downpipe then stock you will
> have less flow  resistance and less turbulence, both will help flow more
> exhaust. 

But if you have a big cork at the end of the downpipe (cat-back),
the lower flow resistance is insignificant.  Turbulence?  Who knows...
At any rate, there are small, but verifiable gains, so SOMETHING has
improved a little.

> Now the turbo spools up to speed faster even with the stock cat.
> With the 3' downpipe you have more volume to fill between the turbo and the
> cat. The backpressure will build slower then with the stock downpipe. By the
> time the backpressure reaches the same backpressure as the stock set up the
> turbo has spooled up much higher then it would have been before.  This is why
> will gain alot of lowend power with the 3'' downpipe.

I don't think it's because of the extra few hundredths of a second for the
back pressure to rise, I think it's because the pressure is slightly lower 
at all flow rates due to the turbulence factor(?)/slightly lower resistance.
This is at a critical point on the turbo's map where a slight increase
in pressure drop across the turbine can make a big difference.

>  The larger volume will
> also cool the exhaust gas better before it enters the cat. 

Only if the pressure is lower than it would have been otherwise.
That cat at the end of the pipe is going to keep the pressure very
close to what it would have been with the smaller pipe.

> When G-tech tested
> on a Merkur with a dual Borla system, our 3'' downpipe added upto11 hp over
> the 2.5'' Borla downpipe. Just unbolt the Borla 2.5'' downpipe and installed
> our 3'' downpipe.

Up to?

> More lowend, mid range and more topend, the 3'' downpipe works great on any
> 2.3 turbo street car. Even you Scott with your opend exhaust would find more
> power. You may not have much backpressure with you open downpipe exhaust
> system. But you do have more turbulence and this will be less with a 3''
> downpipe. If I were you and I AM NOT, I would take your $25.00 and build your
> self a 3'' downpipe and try it out. Hold on real good, this time the tires
> will be alot higher off the ground.

Actually he should see more gain than anyone running a full exhaust,
because it's the rest of the exhaust system that adds enough resistance
to make the downpipe size not critical.

That's what I originally recall this to be about.  It's not that good
exhaust isn't important to a turbo motor, it's VERY important.  The
question is whether there's any point to a 3" downpipe when the rest
of the exhaust is left stock.  I can buy that it's POSSIBLE that for
turbulence reasons or whatever, someone could see 5hp.  Then the
question is: "Is that gain worth the money?".  Only the individual
can decide, but I'm thinking they'd get a lot more bang for the buck
concentrating on the cat and possibly muffler first.  In fact, I
think that at sub-400hp performance a GOOD full 2.5" system with a
very good cat and muffler would perform so close to a similar 3"
system that it would be worth sticking with just for packaging and
noise reasons.  That's what the numbers suggest, anyway...