Andrew

I know that is a stupid question. :lol:

I think it is when the motor is running to lean, but how do you know? Does the car make any kind of noise?

I drove my old car for the first time in many months, and when I was slowing down with the car in gear, I would hear very faint backfiring noise. Not the backfire that sounds like a shotgun, gut a faint "puff, puff" kind of sound. Is this detonation? :cheers:

verskel

Pre -detonation causes a 'pinging' noise either from weak gas in a high compression engine, running very lean, or the timing advance is way off.

Garrett1972

During normal combustion you are supposed to get a nice controlled burn starting at the spark plug, and moving across the combustion chamber. Detonation happens as the flame moves across the chamber the pressure starts to rise, and the remaining unburned fuel explodes. This is like a hammer comming down on the top of your piston. It sounds like some one pinging the side of your engine with a small hammer. Normally it happens when the pressures are highest, or during hard acceleration. so I dont think what you are describing could be detonation.

Andrew

Thanks for the info. Learn something everyday. It sounds like the piston actually starts down before all of the fuel is burned. Is this correct? My problem definitely does not sound like a ping.

Is this sound causing any harm to the motor, or should I not worry about it?

Grinchia

Diesel engines operate on nothing BUT detonation...When the fuel/air mixture is rapidly compressed, it's temperature rapidly rises and it explodes. This is intended and carefully timed in a diesel, and no spark is necessary, hence no plugs.

In a gasoline engine that's running very high compression (or low octane fuel), the risk is that the fuel air mixture will detonate as described above before the sparkplug fires. The result is the cylinder firing while the piston is at the wrong point in it's upward travel, forcing it back downwards before the rest of the rotating gear is ready. Not good!

low octane gas detonates at lower temperatures and pressures than high octane gas, and therefore is not suitable for high compression engines. Higher octane gas is actually LESS sensitive to explosion secondary to heat and pressure, and therefore will combust more reliably at the moment it should in sync with the spark, not before.

This leads me to wonder...why couldn't you use a little diesel fuel mixed with your gasoline for a cheap octane booster?

-Roy

Andrew

:lol: Good idea!! And much cheaper than high oct fuel.

Thanks for explaining that. I can see how this is not good on the crank and connecting rods.

LiveandLetDrive

Diesel fuel burns much slower, which is why it doesn't cause harm for it to "detonate" I think. The harm of detonation has nothing to do with firing at the wrong part of the stroke, the secondary explosion causes extremely high cylinder pressures, which tends to blow holes in things.

-Chris