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Re: XR racing, and downpipes...

Dave Compton wrote:

> > On a side note, I just wanted to publicly state that I believe Scott
> > is correct on the 3" downpipe not helping much without changing the
> > cat-back too.
> Except, that experience shows that it works.  And what is "much"?  7-10 HP?

I haven't heard any before and after numbers showing a 7-10hp gain for
changing ONLY the downpipe.  I've just heard people talk about how much
better it "feels".

>  Tell that to people who spend $200 on a ignition that gets em 2 maybe 3
> horsepower.

Yeah, I hope they are happy with their investment, too.

> I have one, and it works, I know.
> If you don't have one, you cannot know.

I have a full 3" system pipe, cat, muffler, and all.  I got a pretty
big gain when I took out the stock cat and continued to run the rest
of the exhaust stock.  I got a SMALL additional gain going to the full
3" with high flow cat.  Either adding the high flow cat added a lot of
resistance back into the system, or else the downpipe didn't really gain
me THAT much at the power levels I am running.  Had I been running 
significantly more horspower, I'm sure I'd have seen a bigger gain.

> There must be more at work here, than theories can explain.

I'm sure someone has a theory for every possible explanation.  Fluid
dynamics is not black magic, though.  As long as your flow is not too
close to Mach 1, it's pretty much a linear pressure drop per unit of
flow rate (psi/cfm) kind of thing.  That's why Corky Bell said in his
book that his guideline was too keep flow lower than about Mach 0.4.

Using this guideline, a straight 2.25" pipe can flow over 400hp worth
of air, 2.5" is over 550hp, and 3" is nearly 800hp before going
non-linear.  This approximates Mach 1 as 1000ft/s and 1hp as 1.5cfm.
So, we definitely want to stay below those limits.

Obviously, there are still losses in the linear region, but they can
be easily calculated because they are linear.  Below 400hp, 3" pipe
will only regain you half the pressure drop that 2.25" pipe is creating,
and will only regain you about 1/3 of the drop that 2.5" pipe is
creating below 550hp.

So if you ran a straight 2.25" pipe with no muffler, cat, etc., and
got a hp number, and then changed to a same length 3" pipe set up the
same way and got a 5 hp gain, that would imply that you should see
an additional 5hp gain with no pipe at all.  Makes sense, should be
verifiable.  However since we're talking about full exhaust here, the
downpipe is probably less that 25% of the total resistance.  If we
call it exactly 25%, and gain 10hp by cutting that 25% in half, then
that implies that there's 10 hp to be gained for every 12.5% of resistance
removed.  So, if we ditch everything after the downpipe (the other 75%)
we should see a 60hp gain with no other changes!  I doubt it.

Take this FWIW, the only number that's really arguable is what percent
of the total drop is caused by the downpipe.  I'd bet money it's LESS
than 25% on a stock system, which would support what I'm saying even
more.  Joe, and then Cory, and then Scott have all said some abrasive
things, but that doesn't mean they're wrong.  Spend your money wisely...