Cam, CR, compression testing lbs

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Mar 20, 2013 | 07:48 PM

calwldlife

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St. Jude Donor '22

Cam, CR, compression testing lbs

Curious how a 10 to 1 motor will give 210 220 lbs
of pressure on a guage.
14.7 lbs of atmos so, 10 (14.7) to 1 =147 lbs?

Mar 20, 2013 | 08:48 PM

63mako

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St. Jude Donor '08-'09

A 10 to 1 motor could show #140 or #220. It all depends on the cam specs specifically intake valve closing point. A 10 to 1 engine that has a compression test of #220 likely won't run on pump gas.

Mar 20, 2013 | 11:07 PM

cv67

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St. Jude Donor '05

Mar 21, 2013 | 12:55 AM

larrywalk

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From: St Louis MO

Quote:

Originally Posted by calwldlife

Curious how a 10 to 1 motor will give 210 220 lbs
of pressure on a guage.
14.7 lbs of atmos so, 10 (14.7) to 1 =147 lbs?

The compression process is nearly adiabatic, which means that ideally no heat is lost... When the piston rises, the rising compression also causes the temperature to rise which further increases the pressure seen on the gauge. I'm not going to try to run through the thermodynamic equations (been a long time ago), but the peak pressure will be a lot higher than a simple ratio of pressures. And, of course, as Mako commented, the gauge values are greatly affected by when the intake valve closes with a later closing (as in a big cam) decreasing the value seen on the gauge.

Mar 21, 2013 | 01:23 AM

calwldlife

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St. Jude Donor '22

built a 302 ford
10.5 to 1
210 lbs
282 duration, 480 lift, 113 lobe, polished iron heads.
took some effort to jet the carb and curve the dist.

did similar with a couple LT1s
1st a flat tappet one then a roller.

all done years ago before the interwebs and info about cam science.
after reading the dyn CR threads, made me wonder about
cyl pressures and what if any the ideal pressures would be.

Mar 21, 2013 | 10:35 PM

cardo0

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From: Las Vegas - Just stop perpetuating myths please.

Wrong science.

Quote:

Originally Posted by calwldlife

Curious how a 10 to 1 motor will give 210 220 lbs
of pressure on a guage.
14.7 lbs of atmos so, 10 (14.7) to 1 =147 lbs?

1 atmosphere = 14.7 lbs/in2 at standard temp (68*F) and at standard density (0.0765 lb/cu ft). Gauge press is press above/greater than atmospheric press. Compression ratio is change in volume meaning if static volume at TDC = say 4.27 cubic inches (or 70cc) then swept volume plus static volume for 10 to 1 is 42.7in3 (700cc). Pressure follows the ideal gas law: PV=nRT where P = press, V = volume, n = amount of substance in moles, R = gas constant, T = temp.

Compression test should be done with engine warm or hot (good luck removing the plugs hot) to maximize ring seal. Per D. Vizard for a good performance engine should be close to 200 psi (gauge press) and no less than 180psi. Max press for street engine should be <220psi.

Good luck cal,
cardo0

Mar 21, 2013 | 10:41 PM

calwldlife

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St. Jude Donor '22

Quote:

Originally Posted by cardo0

Mar 21, 2013 | 11:27 PM

63mako

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From: Millington Illinois

St. Jude Donor '08-'09

You are not just compressing air. It has gasoline mixed in. Liquids don't compress. As I said #220 likely won't run on pump gas. Might if everything is perfect. Quench, Al heads, Fast burn chamber design, A/F ratio, Cool thermostat, CAI, Right timing curve, Ect.