Compression ratio and balance factor
Compression ratio and balance factor
(OP)
I have an old Harley, recently i upgraded the cam to a performance cam with extra lift and duration. I increased the compression to get the static compression back up to where it needed to be to get good low rpm performance (170psi).
Apart from the cam change, the only other change was to the compression by shortening the cylinders, same pistons.
The bike vibrates really badly now when before it did not. I have heard that extra compression requires a change to the percentage of the balance factor as the extra compression is effectively additional reciprocating weight that needs to be offset by the counterweight, any comments appreciated.
Apart from the cam change, the only other change was to the compression by shortening the cylinders, same pistons.
The bike vibrates really badly now when before it did not. I have heard that extra compression requires a change to the percentage of the balance factor as the extra compression is effectively additional reciprocating weight that needs to be offset by the counterweight, any comments appreciated.





RE: Compression ratio and balance factor
Cheers
I don't know anything but the people that do.
RE: Compression ratio and balance factor
Balance factor plays with inertial forces, that are "external" forces.
Is this new vibration throughout the rev range, just at idle or low rpm, or ???
Sportster, big twin, stroker (80 inch flywheels or S&S, or ??), flywheel weight ?
Check the motor mount bolt tightness lately?
Sensitive to half throttle vs full throttle?
At some point I'd check the pinion shaft runout to evaluate how true the crank is.
RE: Compression ratio and balance factor
RE: Compression ratio and balance factor
Regards
Pat
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RE: Compression ratio and balance factor
RE: Compression ratio and balance factor
I have seen plug leads put on wrong, counter weighted flywheels put on out of index, cams installed with incorrect timing, pistons installed with the thrust side the wrong way around, the wrong sized bearing shells to name a few.
Are you sure the extra compression has not instigated a misfire, or that one cylinder is timed correctly but the other is out of phase.
Regards
Pat
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RE: Compression ratio and balance factor
Has the vibration increased at full throttle and on overrun and on a gentle accel?
If it has, you might have a mechanical balance problem. If it is just at full throttle, it is more likely to be some combustion problem.
Cheers
Greg Locock
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RE: Compression ratio and balance factor
way the bike feels, it tends to pulsate and vibrate more than it did becoming progressively worse as the rpm increases, perhaps the flywheels are now too light for the compression the motor now has, the bike runs perfect by the way, just vibrates alot more. How do you determine a balance factor for the purpose of balancing an engine?
pat, discribe for me the effect of a piston being in the wrong way? if Harley pistons go in the wrong way the skirts hit at the bottom of the stroke.
RE: Compression ratio and balance factor
It now sounds more like harshness than mechanical balance. That could be unstable combustion or mechanical flexing of parts or even mounts due to the extra power, or even the extra noise creating a feeling or perception of harshness. Extra compression can certainly decrease combustion stability and HD motors are notorious for unstable combustion and miss fires.
I would close the plug gap down substantially to say 0.018" gap and see what happens. Also keep the ignition wires well separated.
Regards
Pat
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RE: Compression ratio and balance factor
Norm
RE: Compression ratio and balance factor
has ..both.. the heads been skimmed the same amount,!!!!
you say that you shortened the cylinders, is this normal way to go with Harlys,!!
as shortening the cylinders, will result in the pistons sticking up past the gasket, may be hitting the heads!!!
as I am not that to familiar with them, but i would have thought that you would skim the heads, as said above,. to increase compression ratio
regards Marcus
RE: Compression ratio and balance factor
Regards
Pat
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RE: Compression ratio and balance factor
I will agree with others as to crank alignment I am building a 111 S&S motor right now but I have a one piece plan bearing crank, I am glad I don't have to try and dial the flywheels in on this motor.
Cheers
I don't know anything but the people that do.
RE: Compression ratio and balance factor
An American saying that really makes me smile! In UK, where we had temperamental old Beesas, Triumphs and in particualr, Velocettes, we said our bikes merely "had character".
Paul W.
RE: Compression ratio and balance factor
British twins had a uniform 360 deg of separation between firing points.
Regards
Pat
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RE: Compression ratio and balance factor