compression in a cylinder

[scroshba06]

the book says it's 17-1/2 to 1 compression. is there a formula to find out compression in the cylinders..... for example 450 psi

 

[Diesel Freak]

are you looking for cylinder pressure after compression??

If so,

P2 = P1(V1/V2)^y

Where

P2 = cylinder pressure at TDC
P1 = cylinder Pressure at BDC (safe to assume 14.5 PSIA)

V1/V2 = Static Compression Ratio
y = Cp/Cv = 1.39 (for air at 100F)

 

[ENafziger]

are you looking for cylinder pressure after compression??

^^^^^ Correct.

So, if you're looking for max in cylinder pressure at TDC from turning the engine over by hand, it would approximately be:

P = 14.5 psi * (17.5) ^ 1.39

= 775 psi

This is somewhat subjective to valve overlap, piston ring sealing, temperature, the exact atmospheric pressure at your altitude, etc

--Eric

 

[VMacKenzie]

When I did a compression check on my '96 the readings were 520-550psi. Original head gasket, 355,000 on the engine at the time.

 

[Diesel Freak]

When I did a compression check on my '96 the readings were 520-550psi. Original head gasket, 355,000 on the engine at the time.

I bet we could work backwards to get the actual VE of the engine!!!

 

[Diesel Freak]

hmmm on a 12 valve...VE could be as low as 0.74

 

[BgBlDodge]

Mine were 500psi across the board. That was around I wanna say 140-150k miles and numerous runs above 1600*

 

[VMacKenzie]

hmmm on a 12 valve...VE could be as low as 0.74

Don't know if that's good or bad but at least we have a number LOL