XL Cranking Compression

[cbass]

So far nobody has been able to answer this question. The cranking compression spec on the 1200 rubbermount sportsters is 200-225 PSI. The cranking compression seems very high. This is not a typo as I have recorded the cranking compression on my internally stock sportster and both cylinders came out to 215 PSI. First, what exactly contributes to the very high cranking compression of a rubbermount sportster when the static compression is only 9.7:1? Second, how is the rubbermount sportster able to get away with a cranking compression that is so high? Everything I have read has said that you do not want to run a cranking compression that high on a street bike, but harley is running it stock on the rubbermount sportsters.

[Mike]

It's a function of the early cam timing and short duration. With an intake closing of 26 degrees ATDC (using specs published by Andrews for the 91L D cams), the corrected compression is 9.4 which could equal a cranking pressure of 200+ (depending on how fast the motor spins, ring seal and other factors). Main reason XL's can accept this higher pressure vs Big Twins is the weight. Since the motor tends not to be as heavily loaded as a BT it will be less likely to detonate.
Mike

[cbass]

Although the rubbermounts claim to have W cams I believe they were revised. Here is the specs that I have been given for the rubbermount W-cams:


Lift
.476" / .476"

Duration
223 / 240

Timing @ .053" Lift
Open / Close
INT. 05 / 38
EXT. 46 / 14


I think that is pretty accurate the the intake closing of 38 would be consistent with the advertised torque peak of 3500 rpm.


Either way, the rubbermounts are not light bikes compared to earlier sportsters. You have 1200 custom weighing 585lbs wet (slightly more with the EFI bikes). That's the weight of an older dyna and it's getting away with a cranking compression of 215 PSI.

[Mike]

Sorry, I was using 883 "D" specs.
Mike