Compression ratio and balance factor 1

[mick93]

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.

 

[thundair]

IMHO ...Harmonics... raise or lower the percentage factor of balance.


Cheers

I don't know anything but the people that do.

 

[Tmoose]

Cylinder pressure, etc are "internal" forces.

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.

 

[mick93]

it is a stroker with S&S 4.5 inch stroke flywheels, like i said nothing else was changed apart from the comp ratio and cam, the motor was together a few years before the change and was fantastic, it was balanced when it was stroked and vibration was never an issue. Now the vibration gets progressively worse with increased RPM, i have checked all mount points, everything is in good condition and tight. the compression ratio is now around 11.5:1. thats all i can think of is the effect the extra compression may have had and if it can be solved by changing the balance factor % next rebuild. i have heard of some v-twins having counter weights upto 70% reciprocating weight in high performance engines

 

[patprimmer]

It seems to me you mixed up some parts on reassembly after the compression raise procedure.

Regards
Pat
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[mick93]

Just for the record, all parts went back into their previous locations, all parts were marked and assembled as they came apart, cylinders were removed and shortened, pistons re-ringed, cam change, thats it.

 

[patprimmer]

Well I have modified a lot of engines and I never saw a compression change throw an engine out of balance.

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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[GregLocock]

Simple diagnostic

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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[mick93]

if i was to describe it, it is a significant change in the
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.

 

[patprimmer]

If the pistons are in the wrong way, the engine makes a tad more power due to less skirt friction, but the engine gets noisy due to piston slap. I was not suggesting this was your problem, I am simply saying things do get assembled incorrectly at times, often in regard to a detail where someone is unaware of the consequences.

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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[NormPeterson]

Would it help to index the plugs and/or cut any overly long side electrodes back a bit?


Norm

 

[marcusaurailius]

This may be a long shot, but it happened to me once,

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

 

[patprimmer]

Certainly skimming the cylinders is a normal method for increasing compression on an air cooled VW engine. There is normally a large safety factor on production engines. On a VW you need at least 0.032" between the piston at TDC and the head. On a Harley you will need a lot more due to the bigger bore and longer rods and stroke. I would think at least 0.040" and probably 0.045" clearance to avoid it touching and crushing the piston ring land and nipping a ring.

Regards
Pat
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[thundair]

I would withdraw the harmonics idea if you cannot drive through the vibration as rpm's increase.

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.

 

[PEW]

"HD motors are notorious for unstable combustion and miss fires".

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.

 

[patprimmer]

The miss fire in Harleys with two cylinders and a single carby result from the V angle and consequential charge robbing because of the uneven firing sequence.

British twins had a uniform 360 deg of separation between firing points.

Regards
Pat
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[PEW]

And ignition by Lucas..... ;-)