Tech Information > Technical Discussion
Compression Ratios, what do they mean?
WOODY@DDLLC:
Fordboy & Dean: Two of the best and most concise posts on this or any forum! :cheers:
There's certainly more to suck, squeeze, bang, blow than meets the eye! I subscribe to the 7 event cycle myself. :-o
Don Terrill of SpeedTalk.com has just put together a very good book "The Horsepower Chain" on how his ENGINE Pro software works! The math and practical advice! Check it out guys! :cheers:
Captthundarr:
Big thanks to Mr.Boy and Mr.L.A. as I am in process of yanking the engine in Amy's car to bump up the gratuitous production of heat. These posts seem to filter the useless chatter that frequently accompanies the search for info to help guide in the decision making process.
Frank
wobblywalrus:
Some numbers for street/race would be useful for two valve engines with hemi style heads and domed pistons vs four valve pent roof heads and flat pistons. This is a nice thread.
Dean Los Angeles:
For a frame of reference:
Normally aspirated engines : Maximum BMEP in the range 8.5 to 10.5 bar (125 to 150 psi), at the engine speed where maximum torque is obtained. At rated power, bmep values are typically 10 to 15% lower. A 4" piston has 12.57 square inches. At 150 psi that's 1885 lbs of force.
--- Quote ---BTW: 15/15.6 bar (217 to 226 psi) is generally accepted as the upper limit of what can be achieved with normally aspirated engines, regardless of rpm range.
--- End quote ---
Honey, you ain't neva gonna see them numbers.
Blown engines : Maximum BMEP in the 12.5 to 17 bar range (180 to 250 psi).
Top Fuel dragster engines: 80–100 bar (1160 to 1450 psi).
Compression ratio is the computation of the ratio between two volumes. Pressure ratio is the ratio between the starting pressure and the ending pressure. We assume that an adiabatic compression is carried out (i.e. that no heat energy is supplied to the gas being compressed, and that any temperature rise is solely due to the compression). We also assume that air is a perfect gas.
Compression ratio 2:1 3:1 5:1 10:1 15:1 20:1 25:1 35:1Pressure ratio 2.64:1 4.66:1 9.52:1 25.12:1 44.31:1 66.29:1 90.60:1 145.11:1
manta22:
Dean;
My guess is that the compressed gas actually transfers some of the heat of compression to the piston, cylinder, & head, so the process isn't perfectly adiabatic--- not isothermal but not really adiabatic either, somewhere in between.
Regards, Neil Tucson, AZ
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