I am looking for opinions on high CR within the limits of 93 and having to run lower timing versus lower CR and more timing, which one would perform best

 

it depends on alot of factors .bore and stroke .cam .to make high static compression ,and make it run on pump gas by creating a lower dynamic compression with the right cam .

 

I have a 418 with 11.5 CR, 215 TFS heads, 234/242/.600/.610 117+2 cam. I use 94 octane and the most timing it will tolerate is 22* above 4400 rpm @WOT. I have 2 heat range colder than stock spark plugs and a 160* stat. My intake valve closes at 52* ABC.

If you had a cam with an earlier intake closing, then the DCR would increase and would require less spark advance. And if you had higher engine temps, that would also require less spark timing.

Russ Kemp

 

What head gasket are you running?

 

.040 Cometic.

Russ Kemp

 

like the other poster, I run a 11.5-11.7, I forget, 416 w/TFS. I run 91 octane calif gas and 22.5 deg. With a STOCK 02Z cam, I ran 19 deg spark, 91 octane.

My 402 I ran 12:1 with 91 octane. Its all in the timing and tuning.

 

I believe that the OP is asking for "theoretical" differences in power between a high-compression motor having to run "shortened" timing to avoid detonation vs. a lower compression motor than could run greater ignition advance, up to the limits of 93 Octane fuel.

Any thoughts on this ?

 

That's what I'm looking for thanks

 

The way to do it is to run correct fuel and correct dynamic compression with correct quench range and plug heat range.

 

Yes, but what happens at the opposite ends of that spectrum . . .

Lower compression with greater spark advance vs. higher compression with less spark advance . . .

Say, a 9.5 SCR motor with a lot of advance vs. a 12.5 SCR motor backed off ... Assuming that 11:1 was "ideal", hypothetically.

Which would make more power, with all else being equal, including the camshaft?

 

Between then I would imagine the 12.5 motor would make significantly more power. There's only so much timing you can efficiently put in a motor, so just throwing insane amounts of timing at a low comp motor does not mean it will keep making power.

If more and more timing meant more and more power, than I guess that would be different. But that's not always the case, a certain cam and heads will only take so much timing to be efficient. At that point, the even though the low compression engine can handle more timing, it's done making more power.

 

Actually it is never the case. More compression (within fuel tolerances) has its limits as well and Higher compression will make more power.

 

Doesn't seem to be the problem here but I know a lot of oldschool guys believed timing advance was power. Realistically in a perfect world the plugs could fire at TDC and only begin burning the fuel and building pressure after the piston changed direction and headed back down the bore. Building pressure before TDC is wasted energy BUT it is a compromise that has to be made because the flame takes time to travel and build pressure.

So I would agree with compression within reason.

 

So is building pressure after TDC when the piston is already on it's way down and the flame front has to chase it instead of using it's power to push it down.

Point is, unless you build instant pressure, there will always be wasted power. That's why timing is not always a determining factor of power, but compression is (as long as it doesn't predetonate).

 

Sounds like you want to argue with me but are afraid to admit we are in agreement?

 

No not arguing, just stating that it's a fact of life with this type of stuff. All of which means that the ICE is limited in how timing affects its power production, thus the idea of a low compression motor with lots of timing does not sound as good as a high compression motor with less timing.