2 different yz/wr pistons from wiseco???

I was just looking in the dennis kirk catalog and see there is 2 different pistons you can get for the wr 400 from wiseco. There was a 12.5 to 1 and a 13.5 to 1 compression ratio. Anybody have anything to say about them - good or bad. What is the stock yamaha piston at? I was told to post this on this side. :cry:

Stockers are 12.5-1, or 12.3 in the '05. Compression is one of the best hop ups you can do to a four-stroke. It will increase power throughout the entire range, BUT, since you said "on this side", which causes me to guess you ride a WR, octane can become a problem. If you do a lot of low speed woods riding, that will be important to you because the bike will be most likely to knock from low octane at low speeds, rather than at higher rpm. See if anyone has experience with this in a WR that they can share the results of.

Quite a few YZ owners have run 13.5-1 pistons and are quite happy with them.

i've got one and i love it. But i race and use ultimate 4 so pinging is not an issue unless i runnin her extremely lean!

I went with the 12.5:1 because I felt the 426 reved freely with it (more so than the 13.5). The 13.5 will give you a little more oomph down low and mid but it seemed to make the bike die a little on top and sign off earlier. Maybe just my opinion but I love the way the 426 revs with the 12.5 better. The 13.5 made it run more like one of those red bikes! LOL

Just my opinion.

That's interesting. What you describe is a phenomenon associated more often with 2-stroke engines (in which it is quite pronounced) than in 4-strokes. I've never had a 4-stroke engine that I bumped compression on rev less well than before. (except for Triumph twins. 12:1 pistons for them look like rifle bullets because the combustion chamber was so deep. They were too pointed, and messed up airflow and flame travel at high rpm)

Anyone else have the same experience Satch did? Just curious.

I think there might be an ignition timing jetting issue involved with the

"Doesn't rev as easy or high thing" I built a few TT 500 s in the 70s that

used the Venolia 12:1 Pistons as well as the 10:1 set-ups other than weight differences there shouldn't be any rev problems..... :cry:

BC

http://img.photobucket.com/albums/v93/billcalicura/ASCOTBOUND.jpg

I loved my 13.5.1.

I went with the 12.5:1 because I felt the 426 reved freely with it (more so than the 13.5). The 13.5 will give you a little more oomph down low and mid but it seemed to make the bike die a little on top and sign off earlier. Maybe just my opinion but I love the way the 426 revs with the 12.5 better. The 13.5 made it run more like one of those red bikes! LOL

Just my opinion.

Yea...can somebody chime in on this?.....Im starting to notice some oil blow by in my silencer. Im assuming that my rings are worn...so I'll go with a new piston kit. Don't know if I should go stock, aftermarket, or high compression aftermarket??? Any suggestions?...I do about 95% mx. What kind of fuel or additives are you guys with the 13.5:1 using? Also, Is 89.00 a good price for the wiesco 13.5:1 kit?

Thanks,

Mike

Yea...can somebody chime in on this?.....Im starting to notice some oil blow by in my silencer. Im assuming that my rings are worn...so I'll go with a new piston kit. Don't know if I should go stock, aftermarket, or high compression aftermarket??? Any suggestions?...I do about 95% mx. What kind of fuel or additives are you guys with the 13.5:1 using? Also, Is 89.00 a good price for the wiesco 13.5:1 kit?

Thanks,

Mike

I noticed more power through out the entire powerband. It noticed no diffrence in the top end, except for more power. It pulled like a bat out of hell ALL the time! I've had that thing pinned in 5th for over a min. Were it feels like it is going to explode and it still had amazing power. Go with the high copression! I used the Klotz 113 race gas. My bike never liked to run on pump after that. That seems like a great price if my memory serves me right.

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