Altitude and tuning issues for detonation

[douglas vincent]

So if you tune you engine at sea level for max power and no detonation and drive up to 2000 feet, will you be lean or rich.

ie who will detonate when the move elevations? The 2000' engine moving to sea level or the sea level engine moving to 2000'?

I ask this because it seems I need to add more fuel to prevent detonation when at higher elevations. Easy to do by just adjusting my MAF signal but I would like to know why I am detonating at the higher elevations.

Thanks

[Legacy777]

It all depends on engine management. If the barometric sensor is working along with the MAF sensor, it shouldn't change a damn thing.

However if you have engine management that doesn't have a barometric sensor, or you've messed with the stock setup......all bets are off.

As for the whole detonation thing. You need to ask yourself.....what causes detonation. too much fuel.

Is there less or more air molecules at a higher altitude.....less.

Therefore you need to lean your mixture out to prevent detonation.

[douglas vincent]

I thought too little fuel caused det?

Along with a host of other things of course which can cause det.

[Legacy777]

Detonation is for all intents and purposes is an uncontrolled burn. The flame front / pressure wave is moving at super sonic speeds. Which is why you hear the banging sound. The pressure wave actually extinguishes itself and then re-ignites as it moves down the pressure curve.

If you had a pressure trace for a cylinder under detonation, it would have a saw-tooth trace after peak firing pressure. The peak firing pressure will be sooner, and much higher then normal.

The catch to this though is that lean mixtures can lead to pre-ignition or detonation. Cylinder temps rise, which can lead to hot spots & pre-ignition. Also, too lean a mixture can lead to mis-fires. More fuel is then added to a cylinder that didn't fire, so you now have a richer then normal a/f ratio. The result can be detonation or pre-ignition.

Read over this article.
http://www.maintenanceresources.com/ref ... alance.htm

It mainly talks about 2-cycle industrial natural gas engines, but the principles are for the most part the same for 4-cylces.....you just have an extra strokes in there.

I've actually taken some classes by the guy who wrote that article. It's good info for sure.