[colin]

What is normal compression in EVO motors? I'm doing post Winter tune up work on my 97 FLHTCUI and am getting readings of 95 PSI in both cylinders. The motor is stock, except for exhaust, high flow air cleaner and a Power Commander. There are 65K miles on the odometer so the old girl may be getting a little tired.

[AFNurse]

Originally Posted by colin

There are 65K miles on the odometer so the old girl may be getting a little tired.

Can't help you with the rest of it, but I know my father in law would be stressed to hear "tired at 65K"!!! He has a 1987 Softail Custom with 90+K....not sure, maybe he has rolled the 100K.....not for sure. He takes good care if it and doesn't hot rod, but does take quite a few long road trips over the years! Hope yours measures out ok!

[mwelych]

Not sure of the exact specs, but 95psi in both cylinders sounds awesome... I have 40K on mine and it is a strong runner. If you take good care of her and don't hot rod, she'll last a long time. I ride to work every day, but I also take her on long rides as often as I can. That's what she likes the most...

[chucktx]

95 in both cylinders is good reading. when you have one that is quite a bit higher than another is when you need to worry. being the same and the milage on it, it appears to have been well taken care of.....continuing the same will result in many miles of smiles!!!!

[cowboy]

I have to agree, 95 in both cyl's is real good , just keep good oil in her & ride away , you can also double check with Larry's custom cycles there in Alb & he'll tell you the same , he's just off central east of wyoming , he's also been around for over 20yrs building HD & the likes

[colin]

Thanks for your information evryone. I guess I'll have to look for another excuse to open up the motor and do some tweaking!

[AFNurse]

Originally Posted by colin

Thanks for your information evryone. I guess I'll have to look for another excuse to open up the motor and do some tweaking!

CUZ...... or JUST CUZ........ that would be as good a reason as any if you want to open things up to tweaking!!! Good luck!

[madcatter]

as long as pressure is close to the same in both cylinders theres no reason to worry

[colin]

Here is a "how to" from the Andrews Products website. I'm on the low end but still within the limits outlined here so all is good!

HOW TO MEASURE STATIC COMPRESSION PRESSURE:
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 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. These high static compression pressures are a
result of compression ratios set too high, intake cam duration too
short or the intake cam timing set with too much advance.

Low compression pressures translate to poor low RPM torque. Causes for low compression pressures are late intake cam timing,
too much intake duration or a compression ratio set too low.

Modifying an engine by changing pistons, camshafts or compression ratios will 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 sup-ply stores or well equipped motorcycle shops sell them. To measure static pressure, the procedure is as follows:

With a warm engine, turn off the fuel from the tank, make sure the choke is off and the transmission is in neutral. Remove both
spark plugs and insert the gauge adapter in one spark plug location. Now, holding the throttle wide open, turn on the starter motor
and read the pressure indicated on the gauge. Then repeat the procedure on the other cylinder. The figures on the next page give
some idea as to the significance of different pressure readings. Also some gauges have a rubber tip instead of a screw in adaptor.
Either way, reading the static pressure of your engine should be fairly easy. Generally, higher static pressures mean more
torque at lower RPM ranges but the trade off is that above a certain point (around185 PSI) the risk of detonation enters the picture. What happens at higher RPM is less predictable and can’t be easily determined from a static pressure reading. What is important is that the cam timing, duration and compression ratio all be properly matched so that each compliments the others.

ENGINE TUNING INFORMATION

STATIC COMPRESSION AND ENGINE TUNING


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 reduce low RPM torque and power. Too much duration in a camshaft is not necessarily a good thing.

Static Compression Pressure Ranges and Engine Performance:

Less than 115 psi: poor low speed response, hard starting. Pistons and cams well not matched or worn rings, valves.
125 to 145 psi: OK for stock or modified street motors. On the lower end of the best range for a stock motor.
145 to 165 psi: OK for modified street motors. Static pressures in this range will be best for stock street motors.
165 to 180 psi: Marginal for large displacement street motors, possible hard starting, detonation and overheating.
Over 180 psi: These are high performance numbers. Street motors over 185 PSI may need compression release.
The above numbers are not absolute but the point is that static compression is important. Proper matching of cams and compression ratio will allow engines to be modified for more performance and still run smoothly in street engines.