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I need to get some things nailed down about engines. Correct me if I'm wrong on these.
FACTS:
1. The higher the compression, the more energy and more HEAT is produced.
a) As turbo pressure rises, compression rises.
2. Correct advanced timing (BTDC) will create the right amount of energy and heat.
a) To much advanced timing will fire the mixture too early and create less energy but more heat.
3. Knock happens because the air/mixture is compressed to much and some of it ignites before the spark happens.
A) Higher octane gas stops this pre-detonation
4. Retarding the timing (Closer to TDC) will create less energy and less heat.
5. Retarding the timing while raising compression will keep the energy and heat levels relative.
6. Hotter air has more energy to spin the turbine wheels
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I'm not thinking about engine timing here yet, I'm assuming it's not moving...
During cruising, the compression is 9:1. So the energy and heat is relativly low. With foot on the floor, the compression is going up along with the heat of the exhaust with more energy being created. This higher exhaust heat will spin the turbo turbine even faster, creating more compression and more energy/heat.
This will go on until the compression goes up high enough, and the engine will start knocking because some of the air/fuel mixture is detonating before the spark plug fires.
So how does retarding the timing stop knocking? If my fuel/air mixture pre-detonates, the engine timing isn't going to change that, only the better fuel can stop that.
My only guess is that by retarding the timing, you waste some energy by letting the piston get closer to the top and then fire the mixture.
The combustion has less energy and less heat.
But, it's too late, the pre-detonation already happened!
At least it won't continue after the first one.
However, Saabs don't retard the timing, at least with the C900, they cut the boost.
It's up to us tuners to keep the timing correct because the computer doesn't know anything about that.
I'm sure there is alot more to the whole process...
Jeff & Royce's SAAB Extravaganza
, Thu, 12 Apr 2001 01:01:20
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