Hi compression or low

[ldfly112]

Still struggling with way too much compression on my 1961 flh im considering going with low compression pistons 7.5 compression ratio I need advise on this issue. How much power loss should I expect and what other issues could come into play. And what pistons should I purchase. Any advise will be greatly appreciated

[dukekleman]

IDFLY,

Static compression ratio is based off the pistons compression height, Bore, Stroke, Head gasket and combustion chamber volumes.

Dynamic compression is then based off the above static compression ratio, Engine RPM and Camshaft timing.

Any power loss you ask about will vary based on the camshaft you are currently using in your motorcycle. Lowering the compression ratio will reduce throttle response and move your torque curve up in the rpm range.

Hope this helps,

Duke Kleman

[T. Cotten]

Still struggling with way too much compression...

You mean pre-detonation, Idfly?

Like knocking when you lug it under load?

Or just too hard to kick?

....Cotten

[ldfly112]

The problem started after the heads were rebuilt when i got the motor back together i noticed a lot more compression when kicking it over not bad when cold but when it is hot i can not kick it all the way thru im 250 pounds. my thought that too much material was removed when milling the heads. any thoughts THANKS

[otis71]

I run low compression on any of my bikes I rebuild. Power loss is barely noticeable to me, they kick over easier and generally start easier too.

[DEEBS47Chief1991]

And they run much cooler.

[dukekleman]

IDFLY,

When I build Panhead series engines, I build them accordingly to the crankcase numbers. FLH models get higher compression pistons and FL models get lower compression pistons.
I've done a lot of the FLH series and agree that they do kick over harder, especially on a proper tolerance and rebuilt engine. Nothing unacceptable however.

So the question is why does yours seem unbearable?
It could be many things...

IDFLY Quote: " The problem started after the heads were rebuilt when i got the motor back together i noticed a lot more compression"

My question: Where only the heads rebuilt at this time?
Where the pistons replaced at this time?
Was the entire engine rebuilt along with the heads?

Duke Kleman

[T. Cotten]

... my thought that too much material was removed when milling the heads...

Ordinarily, Idfly,..

Headgasket surfaces are usually only "milled" (more easily 'ground') to relieve where the headbolt inserts have deformed, or 'pulled', beyond the original deck, and to clean up inevitable warpage over the ports; Usually this isn't significant enough to affect the compression.

The mystery is why you only feel the problem when the motor is 'hot'.

...Cotten