Motech
_-_-_-_-_-_
- Joined
- Mar 1, 2008
- Location
- Santa Cruz, CA
- Moto(s)
- aprilia RSV Mille: BMW R1200RT: F650-GS: GPZ-Ninja 1000R: Bandit 600: Seca 650:
- Name
- Nancy
A question was raised recently about how much compression pressure to expect to see on a motorcycle engine. It's a good question, not so easily answered due to varying factors.
Gone are the days of generic specs. Used to be, cars and bikes alike, you could assume 140-150 psi to be good, 125-130 as borderline, 90-110 will cause misfiring at idle, and 80 psi & less will cause misfiring at all engine speeds. With technical fuel & ignition management advancements, variable intake runner, valve and cam timing systems, etc... we've seen compression ratios and horsepower output climb higher and higher, all while simultaneously reducing tailpipe emissions to levels so minute we can no longer consider suicide in an enclosed garage. With all these advancements, cranking compression pressure specs can vary anywhere from 130 psi to over 200 psi, and old-school compression expectations are out the window.
As a result, it's really hard to find accurate compression specs for vehicles--especially motorcycles--as manufacturer's are hesitant to publish them due to so many variables and, frankly, the relative uselessness of compression testing in modern powertrain diagnostics. But with this simple formula, all you need to know is your engine's compression ratio, and you can very quickly calculate how much cranking compression pressure your engine should produce, no matter if it's an old carbureted trail thumper or a modern EFI repli-racer.
Cranking Compression Formula:
AP x CR x VE = CP
Example (my bike, 2001 Aprilia Futura, 998cc v-twin)
14.7 x 11.75 x 1.15 = 198.62 psi
Another Example (2005 Suzuki DRZ400SM, 398cc single)
14.7 x 11.3 x 1.15 = 191.02 psi
Another Example (2007 Kawasaki Ninja 500R, 498cc I-twin)
14.7 x 10.8 x 1.15 = 182.57 psi
One More (2006 Honda GL1800 Goldwing, 1832cc boxer-6)
14.7 x 9.8 x 1.15 = 165.66 psi
I doubt anyone outside the factory engineers can provide acurate compression specs for most modern engines, and I wouldn't even want to try and track them down. But I found each of these bikes' compression ratio specs with 3-4 mouse clicks, and since the other two factors are fixed (at least at sea level), it makes it pretty easy to figure out what your optimum base cranking pressure should be.
Remember though, all compression tests should be performed with wide-open throttle, fully charged battery, ignition and fuel injectors disabled, and all spark plugs removed on multi-cylinder engines for best results. About a 10-15% loss could be considered normal on a healthy, high-mileage engine, but 10% or more higher results are immediately suspect (altered valve timing or carbon buildup). And on multi-cylinder engines, 10-12% consistency is more important than actual numbers.
S.A
(Thanks to Leon Algers of Professional Technicians Seminars, division of Standard Motor Products Inc.)
Gone are the days of generic specs. Used to be, cars and bikes alike, you could assume 140-150 psi to be good, 125-130 as borderline, 90-110 will cause misfiring at idle, and 80 psi & less will cause misfiring at all engine speeds. With technical fuel & ignition management advancements, variable intake runner, valve and cam timing systems, etc... we've seen compression ratios and horsepower output climb higher and higher, all while simultaneously reducing tailpipe emissions to levels so minute we can no longer consider suicide in an enclosed garage. With all these advancements, cranking compression pressure specs can vary anywhere from 130 psi to over 200 psi, and old-school compression expectations are out the window.
As a result, it's really hard to find accurate compression specs for vehicles--especially motorcycles--as manufacturer's are hesitant to publish them due to so many variables and, frankly, the relative uselessness of compression testing in modern powertrain diagnostics. But with this simple formula, all you need to know is your engine's compression ratio, and you can very quickly calculate how much cranking compression pressure your engine should produce, no matter if it's an old carbureted trail thumper or a modern EFI repli-racer.
- AP=Atmospheric Pressure (about 14.7 @ sea level)
- CR=Compression Ratio
- VE=Volumetric Efficiency (constant, always a factor 1.15)
- CP=Compression Pressure
Cranking Compression Formula:
AP x CR x VE = CP
Example (my bike, 2001 Aprilia Futura, 998cc v-twin)
14.7 x 11.75 x 1.15 = 198.62 psi
Another Example (2005 Suzuki DRZ400SM, 398cc single)
14.7 x 11.3 x 1.15 = 191.02 psi
Another Example (2007 Kawasaki Ninja 500R, 498cc I-twin)
14.7 x 10.8 x 1.15 = 182.57 psi
One More (2006 Honda GL1800 Goldwing, 1832cc boxer-6)
14.7 x 9.8 x 1.15 = 165.66 psi
I doubt anyone outside the factory engineers can provide acurate compression specs for most modern engines, and I wouldn't even want to try and track them down. But I found each of these bikes' compression ratio specs with 3-4 mouse clicks, and since the other two factors are fixed (at least at sea level), it makes it pretty easy to figure out what your optimum base cranking pressure should be.
Remember though, all compression tests should be performed with wide-open throttle, fully charged battery, ignition and fuel injectors disabled, and all spark plugs removed on multi-cylinder engines for best results. About a 10-15% loss could be considered normal on a healthy, high-mileage engine, but 10% or more higher results are immediately suspect (altered valve timing or carbon buildup). And on multi-cylinder engines, 10-12% consistency is more important than actual numbers.
S.A
(Thanks to Leon Algers of Professional Technicians Seminars, division of Standard Motor Products Inc.)
) Sure, it might work for catastrophic compression loss, and then only as a rough indicator, but never very useful for diagnosis of marginal loss. For that, smoke injection is the shit. 
