[Mike S]

I am going to put some 93 pump gas in the race car to drive it around over the winter. I have 12-1 comp and 185 lbs of cranking comp. I usually run c12 in it. Engine likes 38 deg of timing.

What rpm would be max engine load (where i should be careful with detonation)?

I am thinking where it would lug at low rpms. could be wrong.

[chuckies76ta]

Mike: I'd be curious what your dynamic compression ratio is? If your into calculating it, here is one I like. https://www.gofastmath.com/Compressi...tio-Calculator
185 lbs is not too high so 93 octane should be fine. From what I understand going over 200 lbs is race gas territory. I think lugging an engine at low rpm can't be good.

[Mike S]

just figured dynamic ratio using wallace calculator. came out to 9.41.

[steve25]

The most common time for detonation would be in rpm range where the motor gains rpm the slowest regardless of what gear it’s in.
This most likely though in whatever gear the motor spends the most time in.

[chuckies76ta]

Originally Posted by Mike S

just figured dynamic ratio using wallace calculator. came out to 9.41.

From what I understand, that's high. I would be-careful using low octane fuel. I'm just a rookie at this so. I believe mine was 7.8-1
Others with more experience may chime in.
DCR can increase as rpm goes up. This was a little note I wrote down during my build.

[steve25]

The other consideration is engine temp stability.
As mph goes up so does wind resistance and thats weather on the street or at the track adds a lot of increasing load.

54 hp is needed to sustain 75 mph on level ground with a 3600 lb vehicle.
Crank your mph up to 100 mph and now you need 72 Hp, or some 30% more hp .

[Formulas]

the highest torque point is the highest cylinder pressure point which i would bet is the highest probability point for mechanical detonation...

if you have a dyno graph you could see that point set your rev limiter well under that point and sneak up the RPM range until you know one way or the other

the different fuel may have a different BTU rating ??? which might require re"jetting to keep same A/F ratio ??

[slowbird]

Peak tq is highest cylinder pressure. Loose converter and light throttle along with cool engine Temps will all help

[Ashley P]

Years ago I put #48 heads on a 455. I don't recall what cam, probably a 041. I was so eager to drive it that I put about 2 miles on it, including one WOT trot, burning 93 octane. The next day I noticed a blown head gasket. I put a mix of 110/93 octane in it afterward. Some weeks later, a piston cracked. When I took it out, the rod bearing fell out of the cap. So I replaced all my rod bearings.

All because of low octane and a heavy foot for less than 2 minutes. Beware.

[steve25]

Even if your 48 casting heads where the 72 CC version and not the 66 CC version as used on the late 350 HO motors you still had a way to high for 93 octane 11.2 compression in that motor.