Raising evo compression : V-Twin Forum: Harley Davidson Forums

Harleyd · 09-14-2009, 04:21 PM

I have a 99 evo reman motor from the motor company with 6K miles on it (no warranty) and want to bump the compression up to 9.5 - 9.8:1. I have an ev27 cam for it which will be installed and also going to do a street port and polish job. My question is, should I gain the added compression by milling the heads or go with 9.5 pop up pistons as well as a .030 head gasket. Ideally I would like to get the compression 9.7-9.8. I want to keep it under 10:1 for fuel requirement reasons.

I have only been able to find 9.5 pistons in the forged type. Since this will just be a street motor I didn't feel forged is needed but that is the only thing I could find. If I am correct the forged pistons need to be fitted a little loose to account for growth when at operating temp. I also read and heard that they are noisy when cold, will the slapping around when cold put undue wear on the cylinders and reduce the life of the motor? I am hoping to get 100K out of this motor. I'm looking for a strong reliable street motor no racing.

One more question. Does anyone have any idea what kind of power numbers this combination will produce?

Thanks for all your help.

99 Softail Standard FXST
Mikuni HSR42
SE roller tipped rockers
Factory ignition

wfolarry · 09-14-2009, 06:54 PM

If you want a cast piston KB has one. As far as forged pistons being noisy I have JE pistons in mine & they don't make any noise at all. It's all in the details.

fastfatcow · 09-14-2009, 07:48 PM

dont forget, screaming eagle still makes a cast 10.1 comp. piston for the evo. when i bought mine 6 months ago .005 was not available anymore but .010 and .020 were. i went .010 over and cost was less than 150.00 for the kit. pistons, rings, wrist pins, and circlips. that was with stock heads.

Cheapdad · 09-14-2009, 08:15 PM

My builder used the SE pistons..at 10 over.
It's a great build...looks like mine! I was a little cautious with the ping issue...but 2500 miles later (with a rejet and 1 step cooler iridium plugs) I am VERY happy!
Before the compression and head work, I dyno'd at around 70 hp and 75 tq with the mik 42, andrews 27, ignition, and old SE pipes. I have no idea what it is now--but it is a very nice step up from those numbers!
oh yes--get her tuned WELL...it makes a difference.
Cheapdad

4XLT · 09-14-2009, 08:17 PM

my opinion is to have the heads shaved for the desired compression, while their off send them out to one of the site sponsers to get set up properly with a mild port and polish. leave the pistons alone...

power wise you shoudl be well into the 80's for hp and upper 80's low 90's for tq..

i had that cam in my bike before my build and loved it.. with nop other mods it took my evop from 58 to 71 hp, remember that was no mods whatsoever, no carbtuning no head work .... nothing

dsvracer · 09-15-2009, 09:24 AM

Be careful on running to much compression with the ev27. that cam creates a lot of compression on it's own so if you add more it will run better but it will start harder and it will want to pull the cylinder studs out of the cases. While the heads are off have a set of compression releases installed to ease the starting. they will also make the battery and starter last longer. 9.5 is all you will want with that cam. My evo heads and the 27 cam easily make 95+ torque and 85-95 hp depending on the exhaust. any questions just ask. dsv

Harleyd · 09-16-2009, 01:18 AM

Hey guys thanks for the replies.

DSV, interesting info about the compression with the 27 cam. Being a bolt in cam I wouldn't think it would have that effect.

Anybody know what would be an opitmal compression ratio for the 27? Based on DSV reply 9.5:1. If this is so I like it, all that alki in the gas sucks even if it is only 10%. I have noticed a difference in all my vehicles, mainly in mileage and pinging on my bike, had to play with timing a little to stop it.

One more question. Being that there is only 6K miles on this motor is it really important to have the cylinders honed for the new pistons, no scratches or scuffs on the bores, just like new. If so, does it need to go to a machine shop that does hd all the time? I know of a real good shop that does high dollar race motors (cars), vw and porsche etc... After all, all we are doing is putting fresh crosshatch back on the bores and fitting the pistons. I can't imagine it woud be that different from automotive machine work.

Thanks again for all your replies.

Hillside · 09-16-2009, 06:17 AM

That cam, or a Wood 6 will hover at 80 hp and 90 ft/lbs with a good valve job on those heads, set @ about 9.8.
As Larry said, no noise from forged pistons here either. That is mostly an old wives tale, or improper set-up by the not-knowing.
Scott

Harleyd · 09-18-2009, 04:48 PM

Once again, thanks for the replies.

I think that I have decided what I'm going to do. 9.5:1 forged pistons, valve job from one of our site sponsors, and a .030 head gasket. I'm going to let an indy do the jugs. I am hoping this will put me in the 9.7-9.8 range, I really want to stay just under 10. I am curious though, my cylinders are currently .010 over, the indy recommended to go another .010 to insure a perfect fit with the new pistons. How will this effect the final compression ratio being that the bore will be growing by an over all .020. Would it still be safe to use the .030 head gasket, or will it bring me up to 10.0:1 or more? I think that the .020 makes the 1340 a 1350.???

Oh, does it matter about piston brand Wisco, JE, KB.... Anything to look out for?