moparcanuck
Deceased, But not forgotton
My stock 440 runs between 170 and 180 PSI. Can compression ration be calculated from this?
How to calculate compression ratio?
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Roadbird
Well-Known Member
No but this formula can help.
1. Determine the displacement of your engine. Displacement formula is:
BORE X BORE X STROKE X .0031416 = DISPLACEMENT
Example: 92 x 92 x 82 x .0031416 = 2180cc
2. Determine the swept volume of one cylinder:
Example: 2180cc divided by 4 = 545cc
3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.
Example: on cylinder #1 you measure and find you have .020" deck height.
BORE X BORE X DECK HEIGHT X .01996 = CC"s
Example: 92 x 92 x .020" x .01996 = 3.378cc's
Measure each cylinder.
4. Measure the volume in each cylinder head. To do this, use a piece of plexiglass cut to fit in the cylinder head to cylinder mating area. Drill a 1/4" hole in the center of the plexiglass. Lightly grease the edge of the plexiglass and install in the head. (Spark plug and valves have to be installed) with a syringe graduated in cc's fill the cylinder head chamber up with a light weight oil. Record the measurements. Repeat for other three chambers. Average cc volume of a new head chamber is between 47 and 51 cc's.
5. You now have all the measurements to determine your compression ratio.
one cylinder swept volume + deck cc + head cc
deck cc + head cc
Example: 545 + 3.378 + 48 = 596.378
3.378 + 48 = 51.378 = 11.6:1
In this example, 11.6:1 is the actual unadjusted compression ratio for one cylinder. Add cylinder shim thickness which will increase the deck cc volume and refigure compression ratio. This must be done for all cylinders. Average deck height shim for use on a 2180cc engine will be between .150" and .185". We use a .177" shim for AVGAS and a .216" or .256" shim for unleaded premium auto fuel. Lower is better for increased engine life and fewer valve related problems.
Be advised that this is for a 4 cylinder, so for a V8 you have to measure all 8 cylinders to get a true accurate compression ratio.
1. Determine the displacement of your engine. Displacement formula is:
BORE X BORE X STROKE X .0031416 = DISPLACEMENT
Example: 92 x 92 x 82 x .0031416 = 2180cc
2. Determine the swept volume of one cylinder:
Example: 2180cc divided by 4 = 545cc
3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.
Example: on cylinder #1 you measure and find you have .020" deck height.
BORE X BORE X DECK HEIGHT X .01996 = CC"s
Example: 92 x 92 x .020" x .01996 = 3.378cc's
Measure each cylinder.
4. Measure the volume in each cylinder head. To do this, use a piece of plexiglass cut to fit in the cylinder head to cylinder mating area. Drill a 1/4" hole in the center of the plexiglass. Lightly grease the edge of the plexiglass and install in the head. (Spark plug and valves have to be installed) with a syringe graduated in cc's fill the cylinder head chamber up with a light weight oil. Record the measurements. Repeat for other three chambers. Average cc volume of a new head chamber is between 47 and 51 cc's.
5. You now have all the measurements to determine your compression ratio.
one cylinder swept volume + deck cc + head cc
deck cc + head cc
Example: 545 + 3.378 + 48 = 596.378
3.378 + 48 = 51.378 = 11.6:1
In this example, 11.6:1 is the actual unadjusted compression ratio for one cylinder. Add cylinder shim thickness which will increase the deck cc volume and refigure compression ratio. This must be done for all cylinders. Average deck height shim for use on a 2180cc engine will be between .150" and .185". We use a .177" shim for AVGAS and a .216" or .256" shim for unleaded premium auto fuel. Lower is better for increased engine life and fewer valve related problems.
Be advised that this is for a 4 cylinder, so for a V8 you have to measure all 8 cylinders to get a true accurate compression ratio.
Seax440Dodge
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In Short: no, because cylinder pressure can be higher or lower depending on how much duration your cam has
Meep-Meep
дворянин
Not reliably. Reason is the cranking PSI is dependent on the actual compression ratio and the cam timing - specifically when the intake valve closes. If your car has decent idle vacuum (12 or better), and it doesn't idle like a drag car, then it might be safe to say you don't have a lot of overlap in the cam. From that you can suspect your cranking PSI is due to a fairly early intake closing point and a moderate compression ratio like 9.0:1. There are a few other clues like initial timing requirement and if it runs on pump gas without issues.
Grabinov911
Well-Known Member
Hey guys teach me something here: If I assume MoparCanuck s at 175 PSI (average) in the cylinder during cranking, I can divide this number by 14.7 (Standard Atmospheric Pressure at Sea Level) and I get a factor of 11.9. This means the cylinder reaches 11.9 times atmospheric pressure during cranking. Doesn't that make the compression ratio 11.9 to 1?
Seax440Dodge
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No
- - - Updated - - -
reason being, this would only work in a perfect cylinder where valves close exactly as the piston reaches the bottom and stays closed the whole way, and if no air bleeds out from the valves, or between the piston and cylinder wall. Not to mention the difference between if it's hot or cold. Plus, according to the math problem, if you had a compression ratio of 1:1, you would be pushing 14.7 psi. So there isn't any REAL way of telling an engines compression ratio without getting the specs for everything
- - - Updated - - -
reason being, this would only work in a perfect cylinder where valves close exactly as the piston reaches the bottom and stays closed the whole way, and if no air bleeds out from the valves, or between the piston and cylinder wall. Not to mention the difference between if it's hot or cold. Plus, according to the math problem, if you had a compression ratio of 1:1, you would be pushing 14.7 psi. So there isn't any REAL way of telling an engines compression ratio without getting the specs for everything
moparcanuck
Deceased, But not forgotton
I like 11.9 to 1. :hello2:
Grabinov911
Well-Known Member
So Roadbird's formula holds if the valves are closed at "Bottom Dead Center" and remain closed all the way to TDC. In other words the entire swept volume is compressed. So this is essentially a way of calculating the "maximum compression ratio" of the engine.
If the valves are open during part of the upward stroke of the piston, then a part of the total swept volume is not compressed (the air in the cylinder just runs out of the open valve rather than compressing). As a result, the numerator in Roadbird's formula, which includes swept volume, goes down. If the numerator goes down, the ratio goes down. The longer intake the valve stays open during the upward compression stroke of the piston, the lower the compression ratio is going to be.
The actual compression ratio of the engine is the ratio between the volume of the cylinder AT THE TIME THE INTAKE VALVE CLOSES OFF THE CYLINDER DURING THE UPWARD TRAVEL OF THE PISTON ON THE COMPRESSION STROKE and the volume remaining with the piston at at Top Dead Center. Huh... Cool... Not sure I ever thought of it that way before.
If the valves are open during part of the upward stroke of the piston, then a part of the total swept volume is not compressed (the air in the cylinder just runs out of the open valve rather than compressing). As a result, the numerator in Roadbird's formula, which includes swept volume, goes down. If the numerator goes down, the ratio goes down. The longer intake the valve stays open during the upward compression stroke of the piston, the lower the compression ratio is going to be.
The actual compression ratio of the engine is the ratio between the volume of the cylinder AT THE TIME THE INTAKE VALVE CLOSES OFF THE CYLINDER DURING THE UPWARD TRAVEL OF THE PISTON ON THE COMPRESSION STROKE and the volume remaining with the piston at at Top Dead Center. Huh... Cool... Not sure I ever thought of it that way before.
Roadbird
Well-Known Member
That is why they call these 4 stroke engines: Compression stroke, Ignition or power stroke, Exhaust stroke, Intake stroke (air fuel mixture enters the combustion chamber) and repeats itself again. Basic mechanics guys. Don't forget to multiply all 8 bores for a V8 engine before the stroke and 0031416 will give you the displacement for your engine.
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