Variable Restrictiveness Exhaust

[G. Scott Ponton]

Ord Miller wrote:

>I use high flow cats.  For the 1 or 2 HP difference, I really don't think
>it's worth the risk of legal problems or injuring my environmental
>consience. ;)

>On the other hand, I am wondering if I could improve low rpm torque if I
>could add a variable restrictor??

I would tend to agree with ORD. I have run vehicles with and with out cats.
They seem to have very little if any effect on power output. I think this
comes from the early units they used in the start. Back in the 70's the cats
were resterictive enough you didn't need a muffler. The cats they have today
flow as well as a straight piece of pipe.
    I have built and installed a 3" crossover and single high flow cat
system for a 96 Z28. The engine was a stroked LT1 with ported heads,
aftermarket cam, headers etc. and a centrifical blower boosting to 15 PSI.
On a chassis dyno it made 605 horses at rear wheels. Passed emissions test
here in Dallas with .01 CO and .00 HC at idle and .00 CO and .01 HC at 2500.
    Biggest gain was installing a cat back system. 3" with Flowmaster.
Second biggest was the 3" crossover. The cat had no effect at all so was
left in after it was installed. This project is the reason for my interest
in learning how to tune OBDII systems. In order to make it drivable ended up
with an Accel DFI unit. Although I have no problem with it on the whole, it
is a royal pain to tune with all the mods which were done to this car. Also
it eliminated the EST system. On this thing it is definetly needed.

Scott