Thread: Compression Ratio and Octane

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Will a chevy 350 with 10:1 C.R. function with 91 octane pump gas?

Depends on the squish dimension and the intake valve closing point. With a tight squish (0.035" to 0.040") and a cranking compression of 165 psi or less....probably....depending also on your ignition timing....

hey tech read you blurb on crank comp,reason why is that i did a comp test on my 500 caddy engine before pulling it to swap over comp was high 155 to 165 and hit hard 1st pump was 120 to 140 .
i fiddled with the timing and got the thing to run good but anything really over 8 to 9 deg advance the thing wanted to ping on reguler. never had a high comp engine before but i am thinking this thing has some squish.

how much can be told if high comp or not by compression test ??
i would assume cam overlap/duration would also affect this.

al, as you have correctly assumed, the cam has a lot to do with it. More specifically, the intake closing point of the cam lobe because that is what determines how much of the charge will be trapped for compression. You might build an 11 to 1 motor and use a very long cam with a lot of duration (and therefore a late closing point) and build maybe 160 psi (I'm just pcking numbers here to make a point). You might also build a 9 to 1 motor with a very short duration cam (and therefore an early closing point) and also build maybe 160 psi.

The overlap doesn't come into play here except that with a very long duration cam, you will be increasing overlap because you have to open the intake earlier and close the exhaust later to get the higher duration numbers, but overlap doesn't determine cylinder pressure. It determines lope, because the earlier you open the intake, the more chance there is to push burned exhaust gases back up the inlet tract and disrupt the air flow signal at the venturis of the carb at lower rpm's as the piston evacuates the cylinder.

The intake closing point can be changed in the motor by advancing or retarding the cam in relation to the crank. Advancing it will open and close the intake earlier to capture more mixture, building more low-end torque. Retarding the cam will open and close the intake later and result in more high-end torque.

Although I haven't experimented much with retarding the cam, I suppose you could retard it a few degrees, closing the intake later and bleed off some cylinder pressure to lessen the tendency of the motor to detonate. dr_bowtie has played with this more than I have, maybe he'll see this and offer his experiences with retard. If not, e-mail him and ask. I've been pretty lucky I guess, with setting the compression ratio, squish and cam selection to preclude any detonation problems, so my advance/retard experiments have been to gain a little more on the bottom end, not to eliminate detonation.

You can get a pretty good idea of the intake closing and how it relates to c.r. by studying the Crane Cams site and comparing different grinds for a SBC. Look at the 0.050" closing points and the recommended c.r. range of the cam. After you look at enough of them, you'll begin to get an idea of where the intake needs to close to accomodate a certain c.r. Of course there are other areas of cam selection to consider, operating range being perhaps the second most important to me, then whether or not it will work with a stock converter if I'm building a very mild street motor.

The bottom line is that the cam should be the last item you choose in your buildup, after you have installed gears, converter, intake, exhaust and the other engine parts it will take to do the job.

Everybody starts at the wrong end of the car.

Good info Tech...I was going to respond, but after reading your reply, you said basically everything I was going to. Im surprised how many people put, say, 11/1 pistons in their engine (not this case) and think they have 11/1 c/r. There is so much more to it, and like you said, the cam is probably the biggest determining factor in how that engine will run on what octane. The dynamic c/r (running) is what will make it or break it. There is a calculator I can post here if I can figure out how, that will show the dynamic c/r if you know the other factors, but you also need the rod length, bore, stroke, piston dome or dish, etc.....if you dont have the engine apart, or are building it, it wont do any good to guess. The engine Im building now is at just over 8/1 dynamic c/r, and about 10/1 static. Thats about as high as you want to go to use pump gas. Al, you cant tell too very much by cranking compression, but I would say you are about as high as you want to be for pump gas and a street motor. You really have to know what's inside to be sure. John

thanks for the responses guys good stuff to learn and talk about