12.5:1 vs 13.5:1 piston kits

Im rebuilding my bike and noticed that Wiseco sells a 12.5:1 (stock) and a 13.5:1 piston kit. And the 13 is $1 cheaper!! How big are the gains when going from 12.5 to 13.5? What about gas octane? Any negatives to switching. The bike is totally stock.

Ed

When i had my 426, i did a rebuild with the 13.5:1...from idle to probably about 5000 RPM it did feel a little bit stronger and seemed to help with roll on power...its not a huge gain, but i guess it helped a little bit. It ran fine with it though...didnt need to change anything and still ran fine on premium pump gas. I had that, the hotcams intake and exhaust cams, and a thunder alley on mine and it freakin hauled the mail and then some!

i guess i felt more of a gain than jeffro, but my bike run fine on pump gas too. there are no downsides, but it works really well with the hotcam.

Awesome kit! Go for it! I run mine on race gas. It seems to like it a lot better then pump gas. But it will run on pump gas no problem. I noticed a stronger hit in the bottem end.

As long as the gas can stand it without knocking, compression is one of the best things you can give a four stroke.

Ironically, the biggest gains will be at low speed, where it will also be most likely to first exhibit any tendency to knock, but modern ignitions cover that situation pretty well. More compression makes the intake pulse stronger, which helps civilize agressive cam timing.

Four-strokes don't suffer a loss it top end pull from higher compression like two-strokes sometimes do either.

It just depends on the fuel, but apparently, that's been working OK for these other guys.

Where's a good place to get one of those piston kits? I have mine apart and was contemplating doing that same thing.... Never hurts to add more power! :cry::cry:

ever hear of any problems from putting this piston in? A local shop said when they put a higher comp piston kit in some Honda's - they blew the top studs out? (whatever that means).

So its pretty safe and you all would reccomend it?

Ed

I haven't heard of any problems associated with bumping up the compression except occasional problems getting quality fuel.

If raising the compression would pull out cylinder head studs, don't you think that the huge contingent of supermoto CRFs would be tossing their heads off like a bunch of young strippers? Anything that increases horsepower also increases cylinder pressure (that's why it increases horsepower). This increase in the pressure of the fuel burn far exceeds any increase in cranking pressure brought about by the compression itself.

Sounds like someone with high compression pulled a stud, so naturally, it had to be because of the compression.

All together my little CRF has about 14:1 compression. So i dont think yours will have a problem. The reason i have so; high is cause my stro;ker and the high comp piston. Stock in this bike was 9:1 and i went to 12:1. Plus the stroker which equals 14:1 :cry: or something near that. Havnt had anyproblems yet. And hope to never have problems. :cry:

All together my little CRF has about 14:1 compression. So i dont think yours will have a problem. The reason i have so; high is cause my stro;ker and the high comp piston. Stock in this bike was 9:1 and i went to 12:1. Plus the stroker which equals 14:1 :cry: or something near that. Havnt had anyproblems yet. And hope to never have problems. :cry:

Have you poured the motor(cc'ed) or are you just guessing on the compression? Just curious.

All together my little CRF has about 14:1 compression. So i dont think yours will have a problem. The reason i have so; high is cause my stro;ker and the high comp piston. Stock in this bike was 9:1 and i went to 12:1. Plus the stroker which equals 14:1 :cry: or something near that. Havnt had anyproblems yet. And hope to never have problems. :cry:

Have you poured the motor(cc'ed) or are you just guessing on the compression? Just curious.

I havnt done a compression check but i had people calculate it some how on what it might be. But i know my new piston is a 12:1 And the stock was 9:1, so thats a big increase for a little bike like this. And with the 9.5mm stroke increase so it increased the compression even more so its some where around 14:1.

like jeffro said earlier i noticed a lil'more hit on the bottom end BUT mine seemed like it would stall easier than before,i got used to it after a few hours and now its a non issue.probably one of the best bangs for the buck since your replacing a piston anyhow

The only way to calculate your compression ratio is the clearance from the top of the piston to the bottom of the head at bottom dead center, minus the clearance top of the piston to the bottom of the head at top dead center.

Of course it takes an effort to find out how much clearance there actually is. Am I right, Tryce? :cry:

Or you can use this if you dont have all your calculations already.

http://www.csgnetwork.com/compcalc.html

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now