One of the things mentioned is time. It takes time for the flame travel. One aspect is the differences at rpm and time, say between 3000 rpm and 6000 rpm.

At 3000 rpm the combustion rate has twice the time available as it does at 6000 rpm in the same space. Detonation takes time to occur. So there is less of a chance of detonation at 6000rpm than at 3000rpm. So detonation has more of a chance to occur at lower rpm levels. Check out the 2nd to last paragraph in the article quoted by Automotivebreath.

 

But if the ignition advance is set correctly, there's more advance at higher RPM to compensate for the flame travel time. The time period is relatively constant. Differences in volumetric efficiency can raise cylinder pressures at moderate RPM. That could be a more significant contributor to detonation.

 

interesting....
i like that idea of volumetric efficiency and cyl pressure at different rpm, higher pressure means higher temp as well.... it would be interesting to see what rpm detonation occurs at in different setups.

maybe even develop like a test if you could predict the approximate rpm of where it will most likely detonate at at

 

Yes ignition advance requirements increase with RPM but not directly proportional. If that were the case and advance requirements at 3000 RPM were 30 degrees then at 6000 RPM the advance requirements would be 60 degrees.

That’s not the case at all. With race only applications it not uncommon to lock the ignition advance and run the same setting from idle all the way up to max RPM.

The reason this is possible is with increased piston velocity of higher RPM, mixture movement increases. Compression happens faster and squish velocity increases, the resulting increase in cylinder turbulence shortens burn duration. Therefore detonation is more likely at low RPM, the end gas has more time to absorb heat.

There are other factors, as Injuneer mentioned VE has a huge impact on detonation. A simple example is to close the throttle when detonation begins, when the throttle closes VE goes down as well as cylinder pressure and the detonation goes away.

Another huge factor is engine load, with a load increase cylinder pressure rises and detonation is more likely.

I test this often, raising the dynamic compression ratio until the engine is on the edge. Detonation will show up first at launch, next is at the high gear shift when RPM drops and engine load is very high.

 

cool, i'll keep that in mind. thnx automotivebreath

 

i run up to 13 to 1 compression on pump gas. there is a lot to consider though. the camshaft that i run are in the .280 to .290 at .050 duration. that kills the cylinder pressure tremendously at lower rpms. the cranking compression should be (my motors) under 195-210 to survive. the ring gaps are larger for nitrous, the pistons(and most everything else) have coatings on them and the radiuses are smoothed. the heads are designed with the pistons in mind( use the heads to make the compression). i will not street race with pump gas. i try to use a local 95 octane spot.
there are limitations on what i can do but it is possible. i fill the car up with 110 at the track. fuel octane is the percentage of what you have (110 plus 95 = 102.5) good enough for what i run

the timing is set low due the efficient heads(28 degrees max)

if you want some of the best reading out you need to check out rehermorrison. they have great info about motors.

i will be in the pump gas drags probably next year(dang divorce)
i can't give specifics on my combos but it is possible

 

Hi Byron,
Are you able to run WOT with this combination and pump gas?

How did you establish the 28 degrees timing, detonation limitation or maximum performance?

 

I don't believe the above at all and neither should anybody else. I've run cams of that duration in 350ish engines for heads up racing with high compression and they won't even shut off if you throw pump gas in there, and that's irrespective of timing or load.

 

did any of you check out the pump gas drags. there was some engines well over a 1000 hp. there a lot of tricks to getting serious power out of pump gas. i run only 11.7 max on motor specifically built for pump gas only. the motor with 13 to 1(actually 13.11 to 1) was not built for pump gas only but i was saying that i could drive it with pump gas. i did say that i will not street race with that motor. and it was not a 350ish cube motor. he was asking about the maximum cr with 93. i would never recommend a 13 to1 motor for 93 but will say that if done right you can drive them. i replaced a few pistons on trial and error but i lived over 45 minutes away from the track and despise trailer queens. at just over 2mpg i could not afford or keep enough race fuel handy to make the trip to the track. if you are carefull and do not race you can get away with it.

again i would recommend around a 10 to 11to1 compression as safe for 93. but not with old school tech and only with the newer heart shaped heads and more efficient heads.

there is alot about timing that most people don't know and 28 degrees timing is about right on some motors. check out some of the pump gas cars and you would be amazed at the high compression and low timing on some of these cars.

 

the 13.11 to 1 motor ran mid 11's in a 78 chevy truck. i did a lot of street racing and nothing says uh oh like getting beat by a truck. rust and all.