Jay wrote,
>Well, I just asked the same question. There is not an exact consensus Our
>resident guru Jarrid suggests not going over 1.75" or else risk loss of
>scavenging from too little velocity. Note also that unless you enlarge
>your exhaust manifold itself, it will serve as the smallest bottleneck,
>thus making any change to the rest just an exercise in making the car
>louder (which is what a the mechanic on my 911 told me). I do know a guy
>who builds racing Lotuses. He suggested 2", and is completely satisfied
>with the 2.25" on his street/race Lotus Elan. I have yet to get it done
>since it hasn't stopped raining long enough for me to drive the car without
>a top.
>
>Jay
I'll stick to my guns on the 1.75 inch rule.
I have a 2 inch system in my car now, and its really too loud, and
static exhaust pressure measurements show that there is very little
back pressure, which is great for high RPM opperation, but poor for
low RPM tuned scavenging.
The velocity of the exhaust system is important to make the headers
work as they are designed, and to increase the breathing of the engine.
As in most things MUCH bigger is seldom MUCH better, and since the exh
velocity decreases by 21% with only a 10% increase in tube diameter, you
can see that tuning can change greatly with only a small change in tube
size.
I regret putting in the bigger tube for two reasons,
1) The car is very loud.
2) The car could have had more low end torque, which it sadly lacks.
In the cars current state of tune, the engine is just too fragile to
pull the RPMs it needs to make use of the larger exhaust system
and other go fast goodies, so these niceties benefits are lost.
The lotus analogy just doesnt hold in comaprison to an alpine engine.
The Elan would have used a twin cam cortina engine with more stock revving
potential
than any alpine ever gets to see. In that case you would comprimize some
low
end torque (with larger tube diam) in order to reap the HP benefits of the
high revving engine.
I agree that the manifold can be a major limiter on the alpine, and if
headers
are used, the Y branch should be modified so that its OD matches the size of
the
rest of the system.
So to sum up...
Unless you are building up a 8000 RPM or a 2 liter alpine engine, stick
with a smallish tubing system.
Ive seen the stainless kits available from S.S., and I have been impressed
with
the sound, I think that performance should also be reasonable.
Jarrid Gross
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