Like the title says, how do i calculate how thick of a decompression plate I need to get my desired compression ratio? Also, are there any companies in the USA that will custom make one if I send them a head gasket as a template? I need to go from the stock 11.5:1 to around 9.5:1 on my cbr954rr.

 

I am pretty sure cometic will make anything. I have seen people buy multiple mls hesdgaskets and take the shims out of them and stack them to lower compression on apps where custom stuff was too expensive.

 

Could be a viable option, looking at their website customs section it looks like they will make aluminum spacers as well so that should do the trick. Any Idea how I would calculate how thick I would need the spacer to get my desired cr?

 

http://www.carcraft.com/techarticles/calculate_compression_ratio/

Math that's a pretty good write up. You just have to figure out volume and do some division.

 

Well I calculated it up and what I come up with is that I need a 1mm spacer (is that about a normal thickness spacer?) to take me from 11.5:1 to right around 9.5:1. Do you think it would be best to use a 1mm spacer or see if they can make a 1mm thicker head gasket? The stock head gasket is right around 1mm thick so if I did use a spacer I would not be bale to run a head gasket on top of it, which I hear a lot of guys dont and just use a rvt type sealant.

EDIT: well sense I only need an extra mm it looks like cometic offers an mls gasket in .07" thickness which should put me at 9.6:1 which will work for me so looks like that will be the best way to go?

 

Depending on heads.. You could unshroud valves a little too..

 

A bit off topic, and It may not be an issue, but another thing to check is cam timing after adding shims....

 

Shit, that's right on topic. This is true, although normally a very small amount. I would want to calculate this too. Do you already have adjustable cam gears?

 

I do not have adjustable cam gears, does 1mm of extra thickness throw it off that much? If so I can buy a set of adjustable cam gears, they are cheap for this bike, like $50 for both, def not going to make or break the build lol.

 

It will advance the cams a bit. You might try it as is and see if you like it or not. I milled .050" off of the head of my race car and it seemed to like the retarded timing, but it's NA and I needed more top end. Your bike might like a little more low end.

 

Yah I could use the low end pickup anyway sense im probably not going to see boost till 4-5k rpm sense my bike red lines around 13,000.

 

on a 2.0 4cy .100 is hardly noticable on the butt dyno from my past projects. I was trying for the highest compression possible so had the head milled as far as the machine shop would go. But I also gained power from the higher compression so who knows.

 

I never actually checked it, but my thinking is the smaller the diameter of the cam gear, the more it would change. Although probably not enough to make a difference.

 

true but it should move less being a twin cam I would think sense the chain wraps over both cams

 

I'm trying to wrap my brain around that and got a headache lol.

 

lol, the larger the sprocket the less change 1mm will make. Sense the 1 chain wraps around 2 sprockets it in essence is like its one big sprocket so the change should be less.

 

Won't this make the cam chain also tighter ? or is there a tensioner on it.

 

There is an automatic tensioner on it. Should be fine with a 1mm rise.