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Old 07-14-2011, 11:40 AM
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dynamic compression and regular gas

working through some compression numbers, using the Pat Kelly Compression calculator and I have a question or 2

Engine build is a slant six, 2bbl carb, free flowing exhaust, OS valves, mild porting,staying hydraulic cam, will have 250/260 duration, .441/.443 lift, 45 degrees overlap on 105 lSA
Vehicle is a 83 D-150, short bed, a833

want a low RPM torque motor and want to stay with regular gas.

cc's heads and checked piston recession on stock motor and using the PK calculator got
static 7.97
dynamic 7.31
had recently checked cylinder compression, those numbers were 128 to 150

using the cam mentioned above and shaving the head / block .100 to raise compression and re running the numbers in the PK calculator I got
static 9.3
dynamic 8.3

using the cam mentioned above and shaving the block / head .080 to raise compression and running the numbers again I got
static 9.06
dynamic 8.03

I had found a posting on this forum where it was stated 8 to 9 dynamic compression is idea, 8 favoring the street and regular gas, 9 favoring high performance street and premium gas.
I am wanting to stay with regular,, any thoughts on if the .100 shave is too much or the .080 will leave something to be desired?

had also found a posting here that said
Static Cranking Pressure Performance Implications
Less than 115 PSI Poor low speed performance, poor throttle response, hard starting. Compression Ratio and Cam not matched or worn rings, valves.
120 PSI to 145 PSI Expected range for most stock or modified street motors. This is a good range for a street motor.
145 PSI to 165 PSI Modified street motors. Static pressures in this range should produce good results. This is a good range for a street motor.
165 PSI to 180 PSI Marginal for street motors. Possible hard starting, detonation and overheating.
Over 180 PSI These are all out race engines. A street motor in this range will be trouble. Stick to the track.



so is using dynamic compression * standard air pressure of 14.7 psi accurate to predict cranking cylinder pressure?,, with my calculated dynamic in the stock build of 7.3 * 14.7 should have got around 107,, my results were much better. In the 130 to 150 range.

thanks

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Old 07-14-2011, 01:13 PM
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Is there a question here?
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Old 07-14-2011, 02:29 PM
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obviously not for a dog that can't comprehend this
so is using dynamic compression * standard air pressure of 14.7 psi accurate to predict cranking cylinder pressure?,, with my calculated dynamic in the stock build of 7.3 * 14.7 should have got around 107,, my results were much better. In the 130 to 150 range
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Old 07-14-2011, 02:55 PM
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That is outrageously confusing. 9.5:1 static compression or lower will run on 87 octane. I dont even look at dynamic compression.
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Old 07-15-2011, 03:14 AM
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There are a few other things to think about, including squish, and combustion chamber shape.

Some engines can take a much higher compression ratio than others before detonation becomes a problem, and it is detonation that is what sets the limit.

I guess what I am trying to say is, fuel octane and theoretical compression ratio are only two factors, and there are other things to consider.
There is really no way of knowing for sure how much mechanical compression you can safely build into your engine, or how much margin you really have to increase compression further.

Try it and see is, I know, is not very helpful, but it is really the only way to find out.

I would creep up on it a bit at a time, and not shave too much off to begin with, unless you have a spare cylinder head or two you are prepared to sacrifice.

Even the type of vehicle and gearing will determine how far you can go.
A big engine in a light car with short gears can get away with running a higher compression.

A small engine, with taller gearing, used for towing will work a lot harder and will complain a lot sooner if it is built right on the verge of detonation.

It is really a judgment call rather than a question anyone can really give a definite answer to.
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Old 07-15-2011, 04:59 AM
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both answers were helpful,,thanks
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Old 07-16-2011, 06:34 AM
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for cranking compression,found these general guide lines

Anything under 100 psi = rebuild the engine.
100 to 140 psi use 87 octane
140 to 170 psi use Premium (92 octane)
170 to 200 psi use Race gas (100+ octane)
200 to 240 psi use Super Race Gas (114)
240+ alcohol


also found this calculator that converts dynamic compression to cranking compression, and is correctable for altitude

http://www.rbracing-rsr.com/comprAdvHD.htm

regards
DT
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Old 07-22-2011, 05:10 AM
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Quote:
Originally Posted by DadTruck
for cranking compression,found these general guide lines

Anything under 100 psi = rebuild the engine.
100 to 140 psi use 87 octane
140 to 170 psi use Premium (92 octane)
170 to 200 psi use Race gas (100+ octane)
200 to 240 psi use Super Race Gas (114)
240+ alcohol


also found this calculator that converts dynamic compression to cranking compression, and is correctable for altitude

http://www.rbracing-rsr.com/comprAdvHD.htm

regards
DT
hey,

Thanks for the psi guide!

That will come in quite handy while doing compression checks.
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