One time someone told me that the more compression that I build into an
engine the less durable the bike becomes. Now is that true or not ? How much
compression can i build before i have to give up my stock bike durabelity,
if the opening statment holds water? The reason I ask is because I am thinking
about a head job "NO NOT THAT KIND OF A HEAD JOB" I am talking about having my
SE 103 cvo heads reworked. Now get your mind out of the gutter and tell me what you think............................................. ....kev

 

need to put this over in the engine section of the Forum--the motorheads hand out over there

 

Ask Dalton and Dawg... MaineUltraclassic too. Those gents know their stuff. I'm about to boost my 107 from 9:8 to 10:1; according to the builder no loss at all in reliability. I think it also depends on who does the head work. Mine was done my Dan Baisley and his work is impeccable.

 

Higher compression alone won't hurt dependability. Several motorcycles come stock with higher compression than a stock Harley. The myth about unreliable performance motors is perpetuated by people that don't know how to ride them.

Depending on how high you bump the compression what you'll find is 1) better performance 2) difficulty starting (solved with compression releases & an up graded starter) 3) you may find that the ride becomes a little tweaky (not a bad thing unless you want a smooth, gentle, tourer)

My FXR is 10.5 : 1. It's not really happy idling around quiet neighborhoods at 2000 rpm, but it's great above 2500 rpm! The bike is 22 years old, & the build is 10 years old.

 

...general rule of thumb these days is..(and remember, I said GENERAL), any Harley over 10.5 compression is asking for issues...what kind of issues?...starters, detonation(leading to fun things like melted wrist-pin bushings),..and other general PIA problems...a lot has to do with the overall build supporting that level of compression, (clutches, tranny etc)...personally I have not seen any builds these days at, or over 11:1 comp....bottom line...stay at 10:1 and life will be good....

 

There is a trade off on everything,Harley builds them the way they do for a reason.

 

You can run higher compression over 10:1 as long as you use the complementing gear to make it work,aka cam with more duration.The higher compression on it's own does not hurt the motor.Just do your homework and do it right.Both of my big inch bikes are over 11:1 and are a blast to ride.Good Luck

 

Yep, cost and the EPA.

 

Head work such as porting and polishing is not going to change your compression ratio. The only head work that would change it is by shaving the heads which will only add a couple points. Compression bumps occur when you add pistons, and for most that means going to a flat top or domed piston which you can comfortably target between 10.5:1 to 10.75:1 without consequence. With the higher of the 2 compression ratios you will require compression release's in the head to reduce cylinder compression when cranking the motor to start it or you will kill your starter.

The next step in the equation is to add larger cams with higher lift and longer duration to keep the valves open longer. This will help when running higher compression and boosts the horsepower and torque. Cams vary in degree and lift so this requires complete analysis of your targeted HP/TQ and riding styles before one sets out to build power. The other things you need to look at is the elevations you ride at, your riding habits such as solo or 2-up and where you rev the bike to. The final thing you should consider is what octane of gas you run from 87-92 octane.

These questions can be discussed with any reputable builder, and I don't suggest you run out and get a set of heads ported before considering the above first. DOC; User Alias (HDMD) is down in Florida and he posted some 10 pertinent questions that should be asked of a builder or tuner when one is considering engine work. If they don't answer your concerns up front, then you best be looking for another builder. Perhaps someone here has the questions DOC posted a while back and can repost them for your reference.

Hope this info helps you out.

Forgot...I will be running 11.6:1 in my new build and I have seen guys runnning near to 12.0 but at these high elevations of 5000' one can get away with that (deduct .3 compression for every 1000' above sea level due to less air density). Sea level won't like higher compression since it will ping when it's hot and that wreaks havoc on bearings and bottom ends so you're best to stay in the mid 10's unless you're looking to build killer power. Race bikes will run higher compression such as 14:1 but we all know how often those motors come unglued and what upkeep is required to fresen them every year.

 

Agreed a "simple" compression boost with pistons and heads within the 10-range not a problem. I think the "problem" lies with performance over-kill: making it a drag bike for the street which compromises driveability and longevity. High performance bikes only have to make a 1/4 or 1/8th mile run. I guess if you have the money and time to upkeep for a monster, the skies the limit. I'm of the school that an AC, good set of pipes, mid-range cam, and 10.1:1 pistons and heads all tuned to each other isn't a bad set-up for the street and getting to work everyday. Or a Muscle would be nice...