Detonation II

[QSHIPQ@aol.com]

Sarge writes:
>...
>In my case, and I believe in others, the detonation occurred between 3-4k RPM
>range, not at 6-7k. My problem would occur as I switched from 'cruising' to
>full throttle, usually seeing detonation around 1.4-1.5 bar on the gauge. Two
>things there: It was actually ~1.8 bar, since the trip computer gauge is
prone
>to lag behind 'actual' readings, and the 2.5 bar PT (part of all 60 hp
>upgrades for TAP, Hoppen, and IA - stock in the S4) creates a relative
reading
>- the reading on the gauge must be multiplied by 1.25; 1.5 gauge = 1.875,
>probably 1.95 actual. 
>>>

A lot of detonation can occur if you have a faulty O2 sensor, and what I
suspect might have happened to SS and many others (including myself at times).
Faulty O2's can lead to failure easily, because the O2 is no longer responding
fast enough or correctly for the mixture needed.  At WOT the O2 sensor is
ignored, the Lambda goes to a fixed value, so rarely is the detonation heard
or felt at WOT.  The transition, especially with really hot mods can prove
deadly (to the motor that is).  When an O2 goes totally whacky, most computers
assign a fixed value to DC (kinda the same on startup, where until a correct
reading the DC is usually fixed at 50%).  This can be fine for most driving,
but the critical on boost (non WOT) can be affected (as in you are really
lean).

Bottom line, if you are running boost mods you MUST ck the following items for
proper function:  WOT switch (3 ohms resistance is significant), Idle switch
(3 ohms sig - remember a faulty idle switch also affects the decel valve
operation), and the O2 sensor.  A good rule of thumb is 30k max out of an O2
(Bosch oem), less for ones beaten severely (I have eaten them in 15k and less,
btw).  Usually, a modded car with a bad O2 has a serious down in mileage when
it's gone, I usually see 6-7mpg drop when O2's are toasted.

Remember, these boost mods are waaaay above (2.0bar and >) what is considered
reasonable effective compression ratios for street cars.  Add in the Pressure
Ratio concept, and those at altitude should watch harder than those closer to
sea level.  But bottom line, High Performance mods are just those, and you
should be aware of the risks involved in running them.  Late MC (89.5>)
equipped cars running 2.0 mods are the most susceptable to fueling failures,
then the S cars (small turbos>high boost), then the early WX cars, then the
early MC cars.  O2 failure, hose failures, throttle switches and fueling
(pump, system and control pressures) are all key to making high peformance
mods safe.  When I hear "detonation" being mentioned in an audi turbo car, you
already are on borrowed time.  There is a problem already.

Your two best ancillary guages that could help you here are a good calibrated
boost guage (don't "probably" your boost readings, t in the actual, it takes
20 minutes to have the real thing) and a 50USD cyberdyne A/F guage.  Don't
ignore those engine sounds either, engine knock means something isn't right
with your car.


HTH


Scott Justusson
QSHIPQ@aol.com