hohner0006
Is there a rule of thumb when it comes to cylinder pressures in relation to known compression ratio's? For example, what should the cylinder pressure in pounds be for a 12-1 compression 407 ci amc? Do these pressures remain constant (according to comp ratio) in say a 401 vs a 468 BB chebbie? In other words would you see 185# pressure in a 12-1 401 and a 12-1 468 engine as well? The reason I'm asking is I did a compression test on my 407 and the pressure was not as high as I thought it should be @ 12-1(although it was constant across all cylinders). What are your thoughts on this?
fuzz401
I had 180 to 187 on my 12:1 motor when I pulled it when it was new I had around 220 to 240
tufcj
No table or anything like that. Cylinder pressure is very dependent on cam design (lift, duration, timing) as well as compression ratio.
Bob
tufcj
AMX69PHATTY
Compression Ratio is the Ratio between Cylinder Volume at BDC and TDC.
Static Compression Ratio is calculated from actual cylinder volumes at BDC and TDC.
What's measured with a Gauge is really what's called Dynamic Effective Compression Ratio.
For a given Compression Ratio, Cylinder Pressure as measure with a Compression Gauge
will be the same on all Engines regardless of Engine Size or Engine Displacement
with some variation caused by the Camshaft Intake Valve Closing Angle Timing.
To convert from Gauge reading to Dynamic Effective Compression Ratio:
"Standard" atmospheric pressure is 14.7 psi ( which equals "1 Bar" in Metric Pressure )
( it varies daily with elevation and barometric pressure )
1 - Get a pressure reading on the Gauge from an engine cylinder
2 - Subtract 14.7 psi from the gauge reading
3 - Divide the remainder by 14.7
4 - Result is the Dynamic Effective Compression Ratio.
for 12:1 CR working backwards, add 1 to 12 = 13 times 14.7 = 191 psi gauge reading
or if gauge reads 185 psig then subtract 14.7 (15) and 170 / 14.7 = 11.56:1 Dynamic CR.
PSIG Pressure Gauge & PSIA Pressure Absolute are two different things
and that difference comes into play when doing this I think.
At least that's the way I understand it fwiw.
:idea:
