Everyone needs to keep in mind that the term "backpressure" is a misnomer. I get
the impression that when folks think of "backpressure", they envision a potato
in the exhaust pipe.....or something along those lines.
Probably without using the exact engineering terms that I don't feel like
looking up at this time of the night, the actual effect are the exhaust pulses
that produce the return wave up the pipe, which helps to further extract exhaust
gasses. The extraction happens at a lower rpm in a small diameter pipe, and at
higher rpm in a bigger pipe.
You can play with the PCM programming al you want, but there are some basics of
physics here that tuning can't get around. Sure, the factory tuning can cause
some weird stuff to happen when the pipe diameter and size changes, and big
pipes can make the engine perform more poorly with the stock tuning, but big
pipes (either physically, or by removing the cats) will definitely move the
torque curve farther up the rpm band.
Opening up the exhaust system n a small /a engine like the SHO will probably
always show a peak HP increase on a dyno, but it will be at the expense of
killing the torque below that point. And you can't get this back with EEC
tuning.
Ron Porter
-----Original Message-----
From: shotimes-admin@autox.team.net [mailto:shotimes-admin@autox.team.net] On
Behalf Of van Oss
Sent: Tuesday, August 22, 2006 10:16 PM
To: Adam Parrott; shotimes@autox.team.net
Subject: Re: [Shotimes] new y-pipe purchase
Thank you Mr. Parrott.
It's been known for 1.5 decades that SHO engines (normally aspirated, at
least) need some backpressure to make their full torque potential. If
needed we can go back to the literature. Stop saying (a) like you know it
because what you say is false. Most SHOs (outside perhaps of those blown or
turboed or maybe cammed) need at least some backpressure or else they lose
torque.
What you say in (b) might be true, I don't know. I do know that the fine
for driving a catless car on the street (which is pertinent to the origin of
this thread), among other consequences, makes it a bad idea to drive an SHO
catless.
But you do whatever you want.
VO
----- Original Message -----
While we're at it, I figured I would also add that:
a) SHOs (like most gasoline engines) do NOT require backpressure to make
power.
b) any "loss" that is experienced or measured from the use of a catless
Y-pipe
on a V6 SHO can almost always be regained with a little bit of EEC tuning.
Adam
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