Exhaust stuff, here kitty, kitty...

["Scott Mo." <scottmo@teleport.com>]

Kerry,

You said this about that:
> Thanks for the response. Your downpipe experience has me intrigued. I
> should have included in my post that replacing the stock exhaust >manifold with the RS2 manifold made for very significant (subjective) >results (with no other mods, Glen :-)). Hoppen claims 10hp, which is >believable, but the quicker spoolup is what I really like. If a >kittyectomy were to produce a similar increase, I'd be a happy camper. >Unfortunately, this is likely to remain hypothetical since I seem to >recall the cats are not simply bolted on the way they are in the other >cars. 

One test you can do to get some idea of the capability of your exhaust
system after the turbo, that is fairly straight-forward, is to check the
exhaust back pressure when you drive the car up to redline in third
gear. (make sure you have lots of running room) You can use the sniffer 
pipe that is connected to the down pipe just after the turbo to connect
up the pressure gauge.

You need to use an accurate gauge that reads from 0-10 psi, the older
mechanical fuel pump gauges can be used for this. You should route the
hose into the car and have a helper watch the gauge as you drive the
car. You may need to insert a small diameter restrictor in the line to
keep the pressure gauge needle from vibrating wildy due to the exhaust
pulsations.

You can also do some brief baseline tests where you hold the RPM to
3,000 or 4000 or 5000 with the brakes on at WOT. Don't do this test for
very long, only long enough to read the gauge, a few seconds... These
baseline tests can be used later when/if you upgrade the exhaust to
compare. 

If you are running stock levels of boost, the exhaust system is likely
ok as long as the cats are not getting restricted as they age. I would
expect the back pressure to be ~2-3psi at WOT/max RPM. This is what I
found on my car with 1.4 bar boost.

If you are running higher levels of boost, then you may find the back
pressure rising up past the 3psi mark under full throttle/max RPM.

On my 89 200TQ running 1.8 bar, with the stock exhaust, cat converter
installed, I was seeing 6.5 psi at WOT/max RPM in 3rd gear. Temporarily
removing the cat and installing a straight pipe, reduced the back
pressure to 3 psi! The difference in turbo spool up time was very
noticeable, I started hitting the over-boost cutout on a regular basis. 

This leads me to believe that my original cat with 110k miles was very
restrictive but I don't know how well a brand new OEM cat would do when
running the higher boost levels. A better idea, as Graydon mentioned,
would be to increase the exhaust system size (2 3/4 or maye 3 inch) and
use a generic high flow cat instead of spending a lot of money for a
stock replacement cat.

HTH
Scott Mo.