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ron eckman 01-13-2004 03:02 PM

Compression Ratio
Hello everyone and thanks for making this site so cool. The knowledge you all share is very helpful especially to a backyard mechanic like myself.
I'm dialing in my 355 trying to get the most out of it. My last thread I asked about if I could run a small blower with 10:5 compression thats what I was told I had. I just bought a compression gauge and tested all the cylinders. My results vary from 160 - 170 is this an accurate way of determining my compression ratio? If my compression turns out to be more like 9:5 how much boost my I be able to run?
Also I have another holley question. I just bought a vacuum gauge and a air fuel mixture gauge with an 02 sensor. Should I tune my mixture screws to the highest vacuum or should I tune it with the air fuel mixture gauge. Any advice is appreciated and thanks for your time

camaroman7d 01-13-2004 03:25 PM

If you were to have an actual 9.5:1 engine you could probably run 4psi of boost maybe 6 with forged pistons. Go here and look at the recommendations most companies will tell you that an "effective" ration should not exceed 12.4:1 on pump gas. This is a little conservative and will depend on many other things. If you are seriously thinking about putting a blower on it and you have forged pistons decent rods/bolts and crank. If you have 64cc heads, I would swap them out for a set of 76cc heads and lower the compression to a blower friendly level. A blower is a big investment, you might as well make the most of it if you are going to bolt it on. This means a compression level of 8:1 - 8.5:1 (maybe slightly higher).

Huskinhano 01-13-2004 05:31 PM

Besides what camaroman7d said, you're measuring cranking pressures. They are different then compression ratio and is not a valid way to determine CR. Cranking pressure have an awful lot to do with cam timing. You could have a 7.5 CR motor make higher cranking compression then a 11:1 motor! All cam timing. It's also important to put 2 & 2 together and realize that with a mild cam timing that a 7.5 motor uses to create high cranking pressure would be disasterous in a motor with 11:1. You'd create stratospheric pressures that would diesel the motor. Conversly is true also, meaning a cam intended for a 11:1 motor would cause such low cranking pressures that the car would be even more of a dog on low end. This is why cam manufacturers tell you what kinda mechanical ratios to run on their cams.


camaroman7d 01-13-2004 06:25 PM

Huskinhano, All VERY good points that most people don't understand or consider.

hemiford 01-13-2004 06:43 PM

Idle feed restrictions are somewhat of a compromise on Holleys, as I've found out. Manufacture recommendations of 1/2 - 1 1/2 turns on the idle screws are pretty good to follow. I use both idle sound and vacuum to set mine. Ideally you want to have an air fuel ratio of 15 or 16:1 at idle and low load (cruising). At full throttle, full load, ignition advance all in, you want about 12:1 for max performance without pinging, or surging. That O2 sensor is actually better for finding your WOT jetting than anything else. I have had some luck canibalizing several carbs of different sizes to get a combo that works good throughout the range. I have about 20 parts carbs laying around my shop. This requires a lot of time patience and a good idea of where, in the power curve, your mixture needs improvement. Use your O2 sensor to get the WOT jetting right and adjust bleed screws by vacuum gauge and ear.
Good Luck

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