compression ratio increase? - Harley Davidson Forums: Harley Davidson Motorcycle Forum

bobthebiker · 09-19-2008, 09:27 AM

After looking at the harley web page and seeing the stock compression ratio for the sportsters being only about 8.9:1 I have to ask, has anyone milled the heads and increased that ratio to something like 11.0:1 or more?

I simply ask because from my point of view, that 8.9:1 is a low figure, and some pretty nice gains could be had simply from bumping that number up a few notches.

so, has anyone ever done something like this?

rodeking · 09-19-2008, 12:42 PM

I went to 11 to 1 on my 03 RKC. Had cylinder pressure of 235. Bad detonation and hot starting, even on 93 octane. I changed to 10.5 to 1. with .060 cometic head gaskets. This got it in the 10.2 to 1 range which ran fine and without any problems. Cams can make a big difference as well as other engine mods. Higher compression alone will have only marginal gains. Stick wth components and mods that will work together. Running stock 2007 ultra with tw6 cams. Runs great with stock compression. Just did 4300 miles to left coast and backi to N. Al. Even on 87 Octane, which is all we could get in some places, there were no detonation problems. Make sure you use the right spark plugs or the anti-detonation unit will not work. I also had a big problem with octane boosters fouling plugs whin I was running high compression.

**Kainam** · 09-19-2008, 01:03 PM

Running 10.5:1 on my bike, no detonation on 91 octane.

Cams work best with a certain compression range, simply bumping up the compression without changing cams would be asking for detonation.

bobthebiker · 09-19-2008, 01:36 PM

ahh, wasnt aware of that issue. I'm used to doing 2 stroke motors and making adjustments for my buddy's 70's machines, so that hadnt occurred to me.

**Kainam** · 09-19-2008, 03:37 PM

Much simpler way to wake up the motor would be to get a "bolt-in" cam. Much easier to swap cams than to pull the heads and machine them or to swap in some hi compression pistons.

bobthebiker · 09-19-2008, 08:21 PM

You bring up excellent points. In my heart, I am, and always will be the guy who just does things to see exactly what any given modification will do, and how the motor will react to that mod. I get a huge high out of experimenting. I swear its a freakin drug, but without the criminal risks(usually)