Craig:

Please let me respond in text.
 
<<I'm relatively new to this site and while I do not personally know you, I am impressed with your core engine knowledge.  I am an ex-General Motors Engine Development engineer and I would guess you have significant engine dynamometer development time.  >>

Yes, I have some test stand time on what is probably the most advanced engine test stand in the world! It helps one's understanding. <g>

<<Question... with the IO or TSIO 550 engines or for that matter any aircraft engine... do you have to worry about pre-ignition when running very low RPM's and WOT?  I have seen engines go into pre-ignition for less than 3 seconds and the result was a hole thru the piston.>>

Pre-ignition is only an issue on non-conforming engines or ones with damage. It is most commonly a result of a failed spark plug or helicoil tang sticking into the combustion chamber. Pre-ignition is exceedingly rare in a conforming engine running on conforming fuel unless a spark plug ceramic has failed.

Now let's look at detonation. Detonation is all but IMPOSSIBLE to induce on a conforming, normally aspirated aircraft engine running on conforming fuel. As a matter of fact, we have run engines for extensive periods under light to medium detonation with absolutely ZERO ill effects-----as long as it is not allowed to progress to heavy detonation and continue until it ruins a spark plug. Once that occurs, the failed plug can initiate pre-ignition and THAT destroys the engine in as little as a few seconds. We have run engines in Heavy detonation without ill effects as long as no damage is done to the spark plugs. We have run engines in pre-ignition for very short bursts (usually unintentionally!) to study the phenomenon. It's UGLY.

Aircraft Turbo charged engines will not detonate if they are conforming engines and are run on conforming fuel unless the pilot pulls a stupid pilot trick with the mixture! <g>

The two factors in detonation are heat and pressure. Control either and you are fine. Control both and your engine will reward you with excellent durability and service.
 
<<Also "70dF LOP".....is this 70 deg F lean of LBT (lean best torque)?>>

No. Best torque occurs at approximately 75-80 degrees F rich of peak EGT. We have been completely unsuccessful in demonstrating ANY detonation at any mixture setting lean of peak EGT (using your reference to best torque, that would be more than about 80-90dF lean of Best Torque).
 
<<In automotive engines, low RPM, WOT and lean of best torque would almost guarantee you detonation and sometimes pre-ignition.  Are these engine's characteristics similar?>>

Well, it guarantees a narrower detonation margin because auto engines can run at very low rpms and very HIGH internal cylinder pressures. Aircraft engines cannot run at such low rpms. The high pitch stops on the props will not allow it. In the old turbo-compound monster radials this was an issue since they routinely operated under very narrow detonation margins when everything was right. The advantage that auto engines have is the ability to control CHTs with water cooling. Then, they only must control the pressure. Very low rpms and high power result in extremely high ICPs. ICPS above about 1200 psi will actually blow the spark plugs out of the cylinder. (please don't ask me how we know! We've actually blown them completely over the roof of the engine test stand and onto the ramp!)

There is absolutely NO difference in the combustion event. How would the fuel know what kind of vessel contains it? <g> The biggest problem the auto industry has is the short latency period of the unleaded fuels. As a result of the shorter latency period, the detonation margin of those fuels is narrow by comparison to the old leaded fuels and that is the reason you see the propensity for the auto engines to detonate, or *ping*.

We actually show these phenomenon on a running engine during the Advanced Pilot Seminar. We like to say that defining detonation is a bit like defining pornography... difficult to define with accuracy, but you sure do know it when you see it! <g> We've had a few IV-P drivers attend and since they run these high performance engines they usually are pretty impressed with what they see.

It's interesting to note that the Lancairs, and the IV-Ps in particular, tend to run hot. The baffling isn't as good as it could be. Running a CHT 50 degrees cooler buys you about 5 octane points of detonation margin. THAT is not an insignificant thing.

Walter Atkinson
Advanced Pilot Seminars