the ratio for modern gas engines is between 1:10 and 1:15, for Diesel without turbo, compressor etc is between 19:1 and 23:1.

The cutoff ratio is the ratio of the volume after combustion to the volume before combustion. From the above equation, it is observed that, the thermal efficiency of the diesel engine can be increased by increasing the compression ratio, Ro, by decreasing the cut-off ratio, rc, or by using a gas with large value of γ.

9-53C Cutoff ratio is the ratio of the cylinder volumes after and before the combustion process. As the cutoff ratio decreases, the efficiency of the diesel cycle increases. 9-15 The four processes of an air-standard cycle are described.

around 90psi

An engine with low compression in one cylinder will present itself by having a misfire and running rough while no compression in all cylinders will cause the engine not to start. There are many different low compression symptoms which will cause the engine to run abnormally.

You should start to worry if one of your cylinders is getting close to 100 psi. Most new engines in good condition compress at around 175 psi. Suppose that #4 cylinder were at 120 psi, the difference ratio would be 31.4% which would indicate excessive wear in the cylinder.

Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings. TIP For a cylinder below 100 psi, pour 1 teaspoon of engine oil into the plug hole and retest. If the reading jumps, the piston rings are worn. If not, think valve problems.

The stock compression for a 454 with 781 heads and flattops is around 7.75:1 cr. Then the bigger cam with the later intake valve closing point will reduce cylinder pressure even more. I would except to see 110 to 125 psi numbers. This engine will like lots of initial ignition timing.

The compression test can be done either hot or cold. A hot compression test is done with the engine warm to ensure all the parts are up to temp and the clearances are as expected. If you suspect damage you might just want to perform a cold test instead of letting the motor sit and run to warm up.

150-170 psi

For example, if you have a manometer reading of about 15 and your compression ratio is supposed to be 10:1, then your PSI should be 150, or 15×10/1.

Measure the water it took to fill the cylinder with the piston at bottom dead center, and then divide that by the amount of water needed to fill the cylinder with the piston at top dead center. The ratio of the two different volumes is the compression ratio.

By definition, the compression ratio is the total swept volume of the cylinder with the piston at bottom dead center (BDC), divided by the total compressed volume with the piston at top dead center (TDC).

The generally accepted gauge for adding compression is that one full point of compression can add between 3 to 4 percent power. So, if an engine is making 50 horsepower and we add a full point of compression (from 11 to 12:1 for example), this could potentially push the power to 51.5 horsepower.

The petrol engine produces 83bhp, which is higher than the 75bhp output of the diesel engine. However, the diesel engine produces 190Nm of torque, 77Nm more than the 113Nm output of the petrol engine.