Re: "Chipping" your car?

[glen%hera@mail.smc.com (Glen Powell)]

Well, I think the small loss of low-end torque is due to exhaust gas dilution
of the intake charge. The exhaust and intake valves normally have some
overlap. THis overlap means that the exhaust and intake valves are both 
open at the same time during the end of the power stoke and the beginning
of the exhaust stroke..... NO, that's not right..... it the end of the
exhaust stroke and the beginning of the intake stroke. But, in any case,
this overlap results on some exhaust gas dilution of the intake charge
and results in some lost power, torque and MPG. Reducing the backpressure
reduces the intensity of pressure pulses in the exhaust system that are
timed and designed to reduce the effect of exhaust gas dilution, so
decreasing backpressure to get more power. torque and MPG at higher revs
(where you really want and need it) can reduce torque, etc. at lower revs.
The effect is small in the 4000Q and these cars have plenty of low-end
tourque anyhows and are geared low as well. I can definitely notice a
difference on the race track. I increase backpressure a small amount
for a typical autocross that uses the mid-to-high RPM range. I open the
exhaust up completely if the track uses only very high RPM for the whole
track. I run with a little backpressure on the street, it's quieter and
off-the-line and low-to-mid RPM response is better. If all by street
driving were at 80-100 MPH I would run it wide open. We can dream.  :)

-glen