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  #16 (permalink)  
Old 11-03-2005, 06:50 PM
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I used to hate physics in college, until recently I found my professor is a car guy, and will answer any and all questions pertaining to formulas/equations that are automotive in nature. I asked him about compression ratio, and there are two ways to figure it, both which I'd heard before, but didn't truly understand. The simple compression ratio formula is as follows:

CR= ((pi x b^2 x s)/4 + Vc)/Vc

b=bore
s=stroke
Vc= Combustion Chamber Volume (Include volume of gasket)

The above formula will give your static compression ratio. Compression ratio as you can see from above is basically the ratio of the volume when the piston is at BDC over the volume at TDC.

You can't truly have an accurate compression ratio from your pressure value, because you can't take into consideration valve timings and such. Also, a hot engine will increase compression, as will RPM with rod stretch. Use the above formula and you'll have an accurate ratio.

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  #17 (permalink)  
Old 11-03-2005, 06:51 PM
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So Larry, not to get way off topic, this explains in easy terms why a 305 would "wake up" if you slid a stock cam for a 400 in the place of the factory 305 cam. Correct? If the situation were reversed, a stock spec 305 cam in a stock 400 would barely let it run, right?


Still learning, Chet.
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Old 11-03-2005, 07:30 PM
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You got the right idea but with cams in production engines, special HP stuff aside, there's not much of a difference between them.

A better example would be a popular cam like Comps XE274H.

Stick one in a 283 and you'll need a fair amount of compression, 10-1 at a minimum, to keep the engine from losing too much low speed power. It would rev very well.

Use the same cam in a 400 with the same compression ratio and, if it didn't detonate itself to death, it would be a torque monster but wouldn't make power past 5000 rpm.

This is assuming all other variables are equally matched.

Larry
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Old 11-03-2005, 07:38 PM
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Gotch ya. I was thinking at work today too about how a bigger bore and thinner headgaskets would affect proper cam selection. Too many variables right now for me to explain with not-enough sleep. It was a looonnnggg day at work.... I need a beer.

In a while, Chet.
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Old 11-04-2005, 11:14 AM
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cold knock's info is correct and

one more variable that makes a big difference....altitude/air density... a 170psi at sea level motor will read about 150psi at 5,000ft
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Old 11-04-2005, 12:13 PM
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Larry has nailed it. When compression testing, I also wire the primaries open so the engine can breathe.
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Old 11-04-2005, 02:40 PM
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If you want to calculate cranking compression, here is the formula:


r =rod length in inches
b =bore in inches
s = stroke/2 in inches
Vc = clearance volume (chamber+gasket+piston+deck volumes) in in^3
IC = intake valve closing angle @ .050 (from Cam Card) in degrees
Pa = atmospheric pressure in psi

Have fun with the math! (I reccomend Excel)
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Old 11-04-2005, 02:52 PM
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Quote:
Originally Posted by orangef4
here is the formula:
Einstein lives!!!
Thanks orangef4.
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Old 11-04-2005, 05:22 PM
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Quote:
Originally Posted by orangef4
If you want to calculate cranking compression, here is the formula:
Thats a good one .. thanks

BTW Einstein used to build engines ?
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Old 11-04-2005, 05:29 PM
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Quote:
Originally Posted by camaro 77
Thats a good one .. thanks

BTW Einstein used to build engines ?

That is a really one, but now I have to wonder when Einstein started using computers...

In a while, Chet.
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Old 11-05-2005, 06:34 AM
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orangef4's formula works great on a perfect engine....100% volumetric effiency

"can" pull 30Hg on the intake stroke and has "0" air pressure loss cause the rings seal perfect even though the piston is cocked during the compression stroke and the cylinder walls are perfectly straight, etc. etc. etc.

that don't happen in the real world....
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  #27 (permalink)  
Old 11-05-2005, 08:16 PM
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According to that formula my gauge is wrong. I really have 233psi instead of 160, in my Geo.

Larry
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Old 11-06-2005, 01:31 AM
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Now all you have to do is properly time the camshaft for the maximum dynamic compression of the specific combination..
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  #29 (permalink)  
Old 11-06-2005, 03:30 AM
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and do to take the compression guage to a local instruments house a have it calibrated/validated before you begin......or atleast test/check yours against a buddies for a same reading/same cylinder....

coldknock....maybe the math is correct, if it was a "perfect" motor it would be 233psi? Geo motors ain't perfect! 160/233=68% perfect
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