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

On Tue, 25 Nov 1997 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.

Is that right????  

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


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

Agree partly.  If the cat and cat back werent such a large restriction in
proportion to the head pipe, I would agree.  But the head pipe is not much
of a restriction compared to the rest of the system, when stock.  I agree
that a 3" head pipe would be a benefit once you installed a  3" muff and
cat and 3" pipe to match.  But I dont see a 3" head pipe helping when the
restricion of the stock muffler and cat is 10x the restriction of the
headpipe, and considering the cat back has jus as much 2.25" pipe as the

 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 may sound harsh, but do you actually read the "other sides" posts?
Must not cuz you didnt reply at all, and it directly shot that theory down
to the point that noone I know agrees with it.  But for the 10th time, to
recap, the differance in volume is not a big number.  I did it last week,
forgot it, if you are curious its 3.14159 squared times the radius times
the lentgh, and divide by 1728.  that will give you the volume in cubic
feet of each pipe.  The volume differance was WAY under half a cubic foot.
Now figure that a 140 cubic inch motor at 3500 rpm at 0 psi of boost flows
enuff air to over come the advantage of that expansion chamber in aobut
.03 seconds.
Real expansion chambers, like the ones that are on two strokes, expansion
chambers that work, if you notice are 2 to 3 times the displacement of the

 This is why
> will gain alot of lowend power with the 3'' downpipe. The larger volume will
> also cool the exhaust gas better before it enters the cat. 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. 

You wouldnt be blowing nubers up would you?  ric said 6 or 7 hp....hmm,
musta been someone else rite?  And like I said, I dont doubt it helps on a
modded exhaust..

> 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.
Well, I am going to pull the downpipe completely off the car next time.
Sould be better than a 3".  And dont worry, over x-mas break I am gonna
make my 3", I hope.  Its a matter of getting the time to devote to
something that wont make my car faster...jus quieter.


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-71 Pinto, stock 2.3 turbo, 13.67@98, radials
	8.1@85 1/th mile on slicks
	Big intercooler ON!
	T4 Compressor soon to be on!

-85 Merkur, let the modding begin!