Re: Causes of detonation

[Sargent Schutt <sargent@novagate.com>]

I am in the habit of using the largest gear I can, with smooth shifts and
prudent use of tranny synchros, but I wouldn't fear the redline on an
occasional basis, or even for a track day or two.  

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. 

So, the _final_ detonation occured at lower rpm (I was at about 95 mph in 5th
(~3800 RPM) and 1.6-1.7 bar (actual 2.0, maybe 2.1 bar, or ~1 bar of _boost_).
 And whatever caused the detonation was not a function of high RPM. The
detonation would typically start after 3k RPM and 1.4 bar on the gauge (=1.75;
1.8 bar actual?). 

Based on the expert opinions here, it seems likely the excess oil in the
combustion chamber may have been a significant factor in my case, and the oil
in the IC may have further prevented proper cooling of the intake air as it
passed through the IC, which may have been a little warmer than normal due to
excess friction in the turbo related to the bad oil seals. Result: Hot spots,
perhaps increased compression, and hot intake air - an effective 'detonation'
recipe. Whether the rotor may have also played a role, well, perhaps, but it
certainly wasn't the sole asassin in this case. It may have added fuel to the
fire, or helped the fire to 'erupt' a tad too soon.  

Rods stretching/tensil stress? I suppose repeated and consistent high-rpm use
might make them more brittle (184k miles on mine). But it seems to me one or
more may have been bent by detonation/compression, and repeated detonation
finally snapped one. Also, as an aside, the motors that have failed have (to
my understanding) lost either the #1 or #2 rods, not 3, 4, or 5. What's the
reason for that? 

I will know more about how the rod was bent/broken etc next week when the new
parts arrive and the old motor is removed and torn down. Hopefully specific
reasons for failure will be a little more apparent at that time.

Regards,

Sarge


Orin Eman wrote:
> 
> Another thing to consider with the 3B is the high redline - 7000 RPM
> with the limiter being about 7200 and setting the overrev code
> at 7440 RPM (Note the Bentley notes possible engine damage if this
> one is set).  (BTW, in comparison, the 10V ECU cuts the fuel pump
> at 6700 RPM.)
> 
> High RPMs are very stressful to rods - puts them under considerable
> _tension_ around TDC.  Then they are under considerable compression
> on the power stroke and around BDC.  Usually, the tensile forces
> are greater than the compressive ones _and_ rods (steel in general)
> are stronger under compression than tension, so there is a greater
> risk in increasing RPM than increasing boost _up to the point that
> detonation or pre-ignition occurs_.  Once you get detonation, then
> all bets are off if the ECU can't stop it and stop it quickly.
> If you get pre-ignition, then the burning mixture is going to be
> trying to force the piston down when it's still going up...
> 
> There is the question of how you could get uncontrollable detonation.
> There are limits of how much the ECU can retard timing.  Could
> carbon buildup in the cylinder do it?  Could high RPM excursions
> stretch and weaken the rods slightly, increasing compression ratio
> at the same time?
> 
> Things to ponder.  Anyway, personally, I'm going to be setting
> a mental redline more like 6500 RPM...
> 
> Orin.