maximum compression ratio, 91 octane

VTStang66 · Jul 25, 2007

I'm building a 302 for my car, not for daily driving, but still streetable. Everything I have read seems to indicate that the higher the compression ratio, the better the car will run. Of course too much and you detonate.

So what is the most you can run on 91 octane with aluminum heads? I'm going to get my heads decked so I can get pretty much any ratio I want once I do the math. What other factors come into play? I know elevation, temperature, humidity, and a lot of other things like that probably tweak the theoretical ratio a little, but I'm just wondering about a safe non-detonating ratio to run in general. I'm going to run some nitrous too if that helps.

ForceFed70 · Jul 25, 2007

Cam selection will play a role in this. But I'd say max would be 10.5:1 on 91 octance.

D.Hearne · Jul 25, 2007

There's just too many variables here to say. Timing, also comes into play. If you want to play it safe, don't go over about 9.5 to 1. My 10.4 to 1 331 with Canfield heads sounded like it was crushing rocks the first time I ran it, and that was 87 octane. After that episode, nothing but 93 octane Chevron Premium touched the carbs. In a pinch, I probably could back off the timing some to run 89.

fasttback · Jul 26, 2007

Yes lots of variables to consider, but to keep it simple I'm running 10.7:1 in my 352 on 91 octane.

rbohm · Jul 26, 2007

too many variables, but with the right head, cam, rod and piston combination, and the right tuning, you can build a street engine to run 87 octane, and still have 11:1 compression.

dennis112 · Jul 26, 2007

Yep, it is all in the combination.

I used to knock a little @ 36 degrees total advance with the CC 282S cam and 9.6-1 compression and aluminum heads. During the winter I decked the block to get 10.2-1 compression plus bought a custom cam tailored to the same basic combo. Since then, I have run the same timing with no hint of knock/pinging plus I have more overall power with a smoother idle/higher vacuum from what is a more aggressive cam.

brianj5600 · Jul 26, 2007

I don't think that the last bit of compression is worth the risk of running race gas in a street car. If compression limits optimum timing you will be losing HP. Don't be over conservative, but don't get greedy either. The cam you choose will decide the compression ratio.

rbohm · Jul 26, 2007

brianj5600 said:
I don't think that the last bit of compression is worth the risk of running race gas in a street car. If compression limits optimum timing you will be losing HP. Don't be over conservative, but don't get greedy either. The cam you choose will decide the compression ratio.

that depends on what the combination wants. for example horsepower tv built a supercharged hemi that made best power when the total timing was limited to 28 degrees.

static compression ratio does have an effect on what ignition timing, and what octane rating you should use in your engine, but the real key here is the dynamic compression ratio. i have yet to wrap my head around the mathematics though.

2bav8 · Jul 26, 2007

My 10.8:1 motor runs fine on 91 octane. If I run a little less timing I can even use 89 octane, which comes in handy for long distance trips to Knotts Berry Farm every year.

Its all in the combo, aluminum heads, custom cam shaft, lots of gear, proper timing curves etc.

Hack · Jul 26, 2007

If you are having a custom cam made for your engine (or off the shelf) - the guy who grinds the cam for you is the best to ask. He will know all the information to make a judgement. If you don't trust the parts monkeys who may answer the phones for some big cam company, your machinist can help if he has all the info. A cam guy is best, though.

As stated above, dynamic compression is key. Dynamic compression is a term that refers to the fact that intake and exhaust valve events will overlap on a high performance cam. That valve overlap bleeds off some cylinder pressure at low to mid rpms, which increases an engine's resistance to detonation.

If you put a low performance "rv" cam in the engine, it might ping like crazy at the same CR as a higher performance cam runs great.

D.Hearne · Jul 26, 2007

dennis112 said:
Yep, it is all in the combination.

I used to knock a little @ 36 degrees total advance with the CC 282S cam and 9.6-1 compression and aluminum heads. During the winter I decked the block to get 10.2-1 compression plus bought a custom cam tailored to the same basic combo. Since then, I have run the same timing with no hint of knock/pinging plus I have more overall power with a smoother idle/higher vacuum from what is a more aggressive cam.

If you're trying to talk up a custom cam, you made a poor cam to compare it to. The 280S is a 40 year old cam, it was never designed for today's conditions. It's not a fuel efficient cam and barely streetable compared to today's cams. If you had chosen one of their more modern grinds like the XE series, you would have likely gotten the same results as the custom grind by figuring what you wanted from it and doing some research on your own.

brianj5600 · Jul 26, 2007

rbohm, my point is that if the motor make the best power at 34*, but it pings. You end up having to run 31* to get by on pump gas, and loosing power over a slightly lower CR with the proper timing.

65ShelbyClone · Jul 27, 2007

Just for reference, I have about 9.5-9.8:1 compression, aluminum heads, Crane Compucam 2031, 12 degrees initial, and my setup doesn't ping on 87. It rattled just a little bit going up a grade when the water temp got over 210, though.

When it did ping, the outside temp was about 100, BTW The 2031 is also a smog cam without much overlap to bleed the compression off.

D.Hearne · Jul 27, 2007

Just for kicks, the 5.0 in my 89 Ranger is a factory reman, with likely a 9 to 1 or less ratio, ported, polished E7's, Cam is a factory F4TE hyd roller(this one's got a 116* LSA and no overlap) with lift improved with 1.7 rockers. It does ping on 87 octane and 12 degrees initial timing and not a whisper running 89.

rbohm · Jul 28, 2007

brianj5600 said:
rbohm, my point is that if the motor make the best power at 34*, but it pings. You end up having to run 31* to get by on pump gas, and loosing power over a slightly lower CR with the proper timing.

that depends on how much you have to drop the compression ratio. a few degrees of timing is no big deal compared to more than a couple tenths of a point fo compression. also many times if you slow the advance curve a bit, you can keep the total timing where it should be.

brianj5600 · Jul 28, 2007

Everything depends.

maximum compression ratio, 91 octane

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