Kevin in Atlanta

Scenario: Driving (86.5 Auto) at a high rate of speed doing 80 at 3k (2.54 diff) Press accelerator to push a bit faster and I get pinging. Not everytime, though.

What causes pinging and what should I check. I am tempted to try out a spare set of brains. My brains are custom chipped.

Pinging I've had before was eliminted by replacing the Ignition brain. Seems the vacuum control module was the problem.

I have a vacuum gauge and air/fuel meter to monitor.

TIA

Curt

Uh... What kind of gas are you running?? And whats your timing set to???

Normy

-"Pinging" or detonation is caused when the fuel air charge explodes rather than burn smoothly. There are numerous causes for this, the obvious one being poor fuel.

This detonation is critical in turbocharged cars; pistons are so highly stressed in these engines that a second or two of strong detonation at high rpm/boost can [and often does...] cause holes to be burned in the tops of the pistons.

Many turbocharged engines [and M28 engines from '87 on up] have knock sensing systemes that literally listen for the tell-tale sounds of detonation. An easy way to reduce detonation is to retard the ignition, which reduces the amount of time that the hot expanding gasses are in contact with the piston, head and cylinder walls, thus reducing overall temperatures. Since temperature is the primary culprit in detonation, it is quickly and easily eliminated.

Other causes: Hot spots in the cylinder, possibly caused by either small imperfections on the piston top or head, excessive carbon fouling, or excessive compression.

I'd pull your aftermarket chips and replace them with stock- if your car still does it...then try some additives to remove carbon deposits.

Normy
'85 S2 5 Speed

ViribusUnits

Can't an overly lean mixture cause pinking?

IIRC, there are two "octane" numbers for a fuel. One is under rich conditions, the other under lean conditions. The lean one is alway smaller than the rich one. Too lean a fuel mixture = predetonation.

Kevin in Atlanta

[quote]Originally posted by Curt:
<strong>Uh... What kind of gas are you running?? And whats your timing set to???</strong><hr></blockquote>

I always use Amoco Ultimate.

Normy

Try using some Chevron Techron additive first. If it still does it, then you should look at the chips.

-Excessively lean mixtures burn very slowly, and have little excess fuel to absorb heat by vaporizing. This leads to excess heat and detonation.

Normy
'85 S2 5 Speed

Incendier

I don't want to quibble, Normy, but it's not so much the nature of the explosion as the timing of it. Pinging is usually caused by pre-ignition, i.e. the a/f mixture firing before the correct moment of the stroke.

Timing, octane, hot engine. ??


<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

ViribusUnits

I thought it was both.

IIRC, as the engine compresses the air/fuel mixture, hot spots, low octane, or excessily lean mixtures should cause the ignition of the fuel, before the proper time. It also does it in a less than perfect mannor. The resulting early, chaotic burning of the fuel results in potential dammage to the engine.

To get around this, you can lower the compresion, get rid of the hot spots, run at low throtal settings (lower compression), increase the octane rateing, and/or advance the timeing. By doing the first 3 methoids, you remove the problem, by doing the last one, you make the fuel burn in it's proper methoid, preventing the choas.

IIRC, the correct solution depend on exactly what is happening in the engine.

Jack '84 928s

Go get 2 gallons of toulene or xylene at sherwin willams paint store or just go to a couple diffrent ones and they will have it and mix that with about 6 gallons of high octane gas and if that doesnt get rid of pinging because it will in a turbo then something is really wrong... Try it, it can be considered a very highly refined gas there are some threads on it in the 944 turbo forum i use it my 928 about 1-2 weeks for about the past 3 months and it will help.. toulene is 114 octane and xylene is 117