Originally Posted by Attack

What octane does 9:1 compression need?

Originally Posted by Gary Anderson

With all the descriptions, I'm feeling kind of "simple".
1) Lean codition reslts in detonation
2) Detonation causes increased cylinder pressures
3) Incresed cylinder pressure causes increased temps
4) Incresed temps melt stuff
BTW, anything that increses cylinder pressures (increased CR, advanced ignition timing) can increase temps.
Simple enough?
Gary

Originally Posted by mcollinstn

Being lean, in and of itself, does not burn engines up.

Chrysler intentionally used incredibly lean fuel mix back in the 80's on their larger motors and advertised them as "lean burn" engines. It was essentially a cheap way to eke out a few extra mpg on their oversmogged and choked v8's. But they didn't burn up.

Currently, there is a company who modifies AIRCRAFT engines to use a turbocharger to blow a highly lean mix thru engines at cruise speed. Exhaust gas temps drop dramatically and engine efficiency improves a significant degree.

Similarly, a lawn tractor with a dirty fuel filter can mow your 2 acres stumbling and choking on itself from running lean and it can do this for months on end.

Ever run out of gas in your Accord (or whatever you drive or have driven)? Of course you have. Did it burn the motor up?

So what's the dealio ??

The quest to make power. THAT's the dealio.

In our quest for more power, we run our engines closer and closer to their detonation thresholds. Higher compression, advanced ignition timing, more boost. All of these things increase cylinder pressure, which is what we use to create motion in our engines.

The cylinder pressure/fuel octane relationship is critical when you get close to the "edge".

To make horsepower, it has been proven time and time again, that it takes a bit more than the theoretical "chemically-optimum" ratio of fuel to air. This is the case in all gasoline engines that run at rpm higher than a lawn mower (interestingly enough, a low rpm motor makes best power AT the chemically optimum ratio). It has been surmised that the additional fuel does two things: 1) at higher speeds, the burn is incomplete so additional fuel provides a balance to allow the portion that IS burned to be closer to optimal AND 2) the additional fuel acts as a heat buffer, providing a bit more substance to the charge so that combustion chamber hotspots can transfer some of their heat to the additional fuel in the charge.

As far as actual heat of combustion, for a given amount of air, as you progress from rich to lean you will slowly increase the temperature of the burn. As you pass the "optimal" point, temperature will rise a bit more sharply. Beyond that point, however, the temperature starts falling again. Consider a cutting torch with oxygen and acetylene (fuel mix doesn't have to relate to gasoline only). There is a point in the fuel/oxidizer mix that produces the "white-hot" flame. With gasoline in a motor it is the same thing. You can have a flame with too much fuel or not enough fuel, but there is a spot where it is just right.

Anyhow, when a motor is being run closer to its detonation limit, a SLIGHTLY lean condition will cause combustion temperatures to rise sharply which will cause cylinder pressure to rise. If this causes the pressure/octane relationship to cross the borderline, you are now into light detonation, which is incredibly hard on your components from both a temperature loading standpoint (detonation sends cylinder pressures and temperatures WAY up, which is a chain-reaction) and a mechanical impact loading standpoint. Light detonation is sometimes difficult to detect, but it will turn into Heavy Detonation in short order. Heavy Detonation does not go unnoticed for very long.

Boom (or more commonly Tap Tap TAP KNOCK KNOCK KNOCK BaGaddaBaGaddaBaGaddaBaGadda ThUNK!).