S4 Ohne Kats

["Frank Amoroso" <FAmoroso@svbank.com>]

Hello all,

I finally got around to doing the after testing on the cat bypasses (raw 
data as well as my impressions follow). Once again this is on a '93 S4 (Ur) 
w/ 65,000 miles, Hoppen Stage III (RS2 manifold, RS2 turbo, Lehman eproms) 
pumping 30 psig max boost. The exhaust is stock save for the recently 
installed Stebro cat bypasses, ahem, I mean "Euro resonators."

Testing was done using four extremely accurate instruments: gtechpro, VDO 
pressure gauge, radio shack sound level meter, and my butt. As well as a 
pair of more questionable instruments: my eyeballs. It is rather difficult 
to keep one eye on the road, one eye on the boost gauge and yet a third on 
the back pressure gauge, unless you bring a friend.

This table probably won't email well. Tough.

Before (stock)
Backpressure @ 16 psig of boost:   3.25 psig
Backpressure @ 30 psig of boost:   6.10 psig (avg., see needle comment)

Max boost 1st            Sub 20 psig
Max boost 2nd            Sub 22 psig
Max boost 3rd            Sub 25 psig

Bhp, gtechpro @ the wheels:   292 bhp
Instantaneous forward G's:         0.70g

Loudness (idle):              64 dBs
Weight of Kats:               25 lbs, 14.5 ozs

After (bypassed)
Backpressure @ 16 psig of boost:   N/A
Backpressure @ 30 psig of boost:   3.25 to 4.00 (needle bounces violently)

Max boost 1st            20+ psig
Max boost 2nd            22+ psig
Max boost 3rd            30 psig

Bhp, gtechpro @ the wheels:   295 bhp
Instantaneous forward G's:         0.68 g

Loudness (idle):               67 dBs
Weight of bypasses:      14 lbs 6.5 ozs

The back pressure really didn't start to build until the higher boost levels 
and at 30 psig of boost the back pressure gauge read between 3.25 psi and 
4.00 psi (needle bounces rather violently, more so in the after test). This 
represents between 2.1 psi and 2.75 psi less back pressure than the stock 
system. However, I feel that the most important aspect is the lack of any 
measurable back pressure under 20 to 25 psig of boost. From a seat of the 
pants standpoint it is blatantly obvious that lag has been reduced. Higher 
boost levels are attainable in the lower gears as the turbo has more time to 
build boost prior to the 7,000 RPM redline.

The bhp and instataneous G figures are attributable to several factors that 
affect the read out of the gtechpro. Ambient conditions as well as on boost 
vs off boost transition. In fact, the gtechpro literature warns of reduced 
accuracy on turbocharged vehicles given the way it measures power vs the way 
a  turbo delivers power. While peak power levels are probably up a bit with 
the bypasses installed the power delivery has changed. What I am getting at 
is that there are too many variables at play to discern any real info from 
the before and after Bhp and G figures. Graphs on the other hand would tell 
a more accurate story. G's over RPMs would show the bypassed version with a 
steeper graph earlier on.

As far as sound and noise go the exhaust is not that much louder than stock. 
from a quantitative standpoint probably 3 dBs, from a qualitative standpoint 
not much different from stock. To be honest, I was hoping for a more manly 
rumble. I can, however, extrapolate from this that the stock center 
resonator and rear muffler baffle the hell out of the exhaust (yet still 
flow well for a stock system). Apparantly team door handle didn't work on 
the S4 exhaust?

Normal disclaimers apply (any similarities between this and reality is 
purely coincidental), I am just seeking the facts and I thought I would 
share them with you. Bottom line is that bypasses add to the fun factor of 
my particular setup.

YM, however, MV,
Frank--