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Originally Posted by ohioboxster
Yes Jim but what do you do if the car doesnt detonate? You know even when my car continues to run for trouble free mile after mile I have a feeling there are people that will say "Fluke" or "Lucky". I just dont understand why several people say the car will detonate due to the compression ration. I drive my car everyday without issue. Let me clarify, Im not looking for an endorsement or even convince you of anything but just want you be aware 5 lbs of boost does not make the car detonate.
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Hi,
I never said your car detonates. What I said was that at anything over 8.5:1 CR, detonation becomes a concern. At 11.7:1 CR, it becomes a major concern.
Your car
may be detonating at certain times and you're unaware of it, it may be doing so under certain conditions (Ambient Temp, Load, Fuel Quality, etc.). The DME
may be compensating for it by using a detuned MAP, I don't know, I haven't studied your specific installation and all it's pertinent parts and those of the motor itself. But,
if you are at times detonating, it's probably not enough to grenade the engine, but what are the longterm effects?
The rule of thumb is that anything over 10:1 CR requires Intercooling to be safe from detonation. Porsche doesn't force the M96 engine in any of their cars, what do they know that we don't? The Turbo uses a different engine. The TT engine (M96/70) is similar to the M96, but also very different - 8.4:1 CR.
Adiabatic Heating (a Gas rises in Temp if it is compressed) raises the Charge Temp 11°/1 PSI (10.9°/1 PSI to be precise). So, at 5 PSI, your intake charge is 55° hotter than the Ambient Air entering the Intake. On a 60° day, this means a lot less than on a 98° day, because the Charge temp would be 115° on the 60° day, but 153° on the 98° one. This heated charge will reach a detonation threshold (approx. 268°) a lot sooner in the compression stroke (where it is further compressed and heated) than Ambient Air charge would, which is what the engine was designed for (Pistons, Bore, CR). This means the engine must produce a spark sooner to compensate. How much sooner is it capable of? I don't know. How soon is too soon? Again, I don't know. But, it's the not knowing which concerns me.
Also, one bad batch of fuel and you could have serious problems. That's just too much risk management for me I guess...
Happy Motoring!... Jim'99