Less than 115 psi: poor low speed response, hard starting. Pistons and cams not well matched or worn rings, valves.
125 to 145 psi: OK for stock or modified street motors. On the low end for a stock street motor.
145 to 165 psi: OK for modified street motors. Static pressures in this range will be very good for street motors.
165 to 185 psi: Marginal for large displacement street motors, possible hard starting, detonation and overheating.
Over 185 psi: High performance numbers. Motors over 185 PSI may need compression releases and/or octane booster.
The above recommendations are not absolute but the point is that static compression is important. Proper matching of cams and compression ratios will allow engines to be modified for more performance and still run smoothly in street applications.
The figures below give some idea as to the significance of different pressure readings. Generally, higher static pressures mean more torque at lower RPM ranges. The trade off is that above a certain point (around185 PSI) detonation enters the picture. What happens at higher RPM is less predictable and can't be easily determined from a static pressure reading. For the best overall engine performance, compression ratio, cam timing, duration and fuel system tuning must be correctly matched. Static or cranking compression pressure is what each cylinder
experiences when the starter motor is turning the engine or when
the engine is running at idle RPM. Please don't confuse static
compression with "compression ratio" which has to do with how
much volume remains in the cylinder combustion chamber when
the piston is at the top of its' stroke. Static compression and compression
ratio are related but the definition of each is different.
Compression pressures that are too high can result in difficult
starting and detonation or "pinging" which in turn can cause
engine damage. Modifying an engine by changing pistons, camshafts or compression
ratios will all have a direct effect on static or cranking
compression pressure. If the static compression pressure is too
high or too low, the engine will not run as well as it should and in
some cases, the resulting problems can be serious. Static or
cranking compression can easily be measured with a compression
testing gauge. Cost is usually less than $25.00 and most auto
supply stores or well equipped motorcycle shops sell them. When
installing cams with high lifts and long durations, a few general
observations are worth keeping in mind. Remember that additional
cam duration can produce more usable power but too much
duration may actually hurt overall performance. The problem of
poor engine response begins when too much duration results in
lower cylinder compression pressure (at low RPM) which in turn
can greatly reduce low RPM engine torque and power. Too much
duration in a camshaft is not necessarily a good thing.
What causes high static compression pressures:
1. Compression ratio set too high.
2. Intake cam duration too short.
3. Intake cam closing point advanced too much.
Compression pressures which are too high can result in detonation
or "pinging" and possible rapid starter motor wear.
What causes low static compression pressures:
1. Compression ratio set too low
2. Intake cam duration too long
3. Intake cam closing point set too late
4.Worn piston rings and/or burned valves
Compression pressures which are too low will result in poor low
RPM torque and throttle response.
How to Measure Compression Pressure:
With a warm engine(not hot, just warm), static compression pressure
can be measured using the following procedure:
1. Turn off fuel valve
2. Make sure choke is off
3. Transmission in neutral
4. Remove both spark plugs
5. Insert pressure gage adapter into one head
6. Hold throttle wide open
7. Turn engine with starter motor (or kick start bar)
8. Measure cylinder pressure
9. Repeat procedure for second cylinder
If the throttle is not held wide open and the choke
is not off, the resulting pressure measurement will show a false
low reading. Also some gauges have a rubber tip instead of a
screw in adapter. Using either type of gauge, measuring staticpressure in your engine is not difficult.