Hi Bobby;
I have seen several turbo "detonation danger" charts over the years.  Heck if I can find any of them now.
Some searching should turn one up.
Detonation also depends on engine load - ask Lynn about NA detonation at idle speeds.

Well, my driver has proved several times (in case I didn't believe it possible) that you can detonate an engine at idle speed. But the event still depends on charge temperature. The charge overheating in a location remote from the plug(s) and beginning to burn after the first plug has fired, is a detonation. So, if at idle speed you suddenly go to full throttle, And your ignition system does not remove nearly all advance BTC then a nearly ideal mixture, will detonate, because the chamber volume is expanding too slowly and resulting cylinder pressure becomes too high and you have the ideal condition for detonation.

 

In the case of the race car, ignition was fixed at 27 degrees BTDC. So it was not difficult to detonate the engine, if the clutch was fully released, and the engine slowed to walking speed. First gear had a 1.96:1 ratio, (about third gear in a street car). So the engines idles at 2,200 RPM, and a bit of throttle, and a bit of clutch would get you there, but the metal clutches are grabby, and it is a talent to maintain that idle RPM and never bog the engine with the throttle open much above the idle setting. 

 

So, the constant load of a prop is similar to the load of the race car at walking speed. Should the throttle open suddenly, with the engine speed very low, detonation can occur. Cylinder filling at just off idle speed will be close to 100%. So what is the ideal ignition advance for 100% cylinder filling at say 600 RPM?

 

Perhaps 8 to 10 degrees or more, after TDC. Ignition advance at the RPM where the engine is used is based on developing maximum cylinder pressure at the ideal crank angle (leverage) for best HP. That angle is a function of engine design and unaffected by RPM.  

 

Normally aspirated engines, both piston and rotary can be detonated on occasion with no damage. It does clean off the carbon, and make a puff of black smoke out the exhaust. Sustained detonation over heats all of the pieces until preignition takes over, and you no longer need an ignition system. In just a few more revolutions you will not have an engine. The irons crack through the dowel holes at each end, and release the oil into space, or more likely onto the exhaust manifold.

 

In a boosted engine you can carry that high cylinder filling right into the working RPM, and ignition timing becomes critical as does charge temperature control. So a NA engine might be doing great with 25 degrees BTDC.

 

A boosted engine might be near death at 20 degrees BTDC. We raced NA engines for years at 20 degrees and had no problems at all. A rotary needs less advance than a piston engine. The crank is turning three times faster than the rotor. So from outside looking at the degree wheel on the crank, it looks like the dwell time near TDC is three times longer than a piston engine. So, my Fiat with 14:1 compression ran 35 degrees of advance on 100 Octane race fuel.

And my 12A runs 27 degrees on race fuel and 25 degrees on 93 Octane pump (car) gas.

 

 

Lynn E. Hanover