[Brokenbones]

With a cp 11:1 piston what the maximum i can shave out of the head to don't go past 11.5:1 and also without slamming the piston in the valves. Thank you!

 

[boost addict]

Decking the head, that's a good question!

 

[Brokenbones]

This is something we used to do on 400ex when we were poors teens

 

[fletch814]

One thing you might want to look into is the timing chain it is a serious week spot on the 700xx motor. I raced with Matlock for a couple of years and his broke three times that I can think of. Alba racing sells a kit, but I am pretty sure its just the gears, chain, guides, and tensiener out of a 650r bike motor. You have to replace all the parts, but size difference is very noticeable, and worth doing.

 

[boost addict]

Thanks for the advice:yay:

 

[Brokenbones]

Yeah i seen this on their web site $$$$$$$ damn. I guess it's a good upgrade but i don't ride 1000 miles wide open but im much concerned about the tensioner...

 

[08700xx]

One thing you might want to look into is the timing chain it is a serious week spot on the 700xx motor. I raced with Matlock for a couple of years and his broke three times that I can think of. Alba racing sells a kit, but I am pretty sure its just the gears, chain, guides, and tensiener out of a 650r bike motor. You have to replace all the parts, but size difference is very noticeable, and worth doing.

Yes thank you for the advice/info. I guess I should do that upgrade now before it gives out on me.

Welcome to the club, glad to have you and your knowledge :rock:

 

[09RappySE]

why do you want to do this? you deck the cylinder, not the head, you should'nt do this unless you head is warped.....or unless you want to reduce the combustion chamber size.

 

[Brokenbones]

The idea is to get more compression, about 11.5:1 with a cp 11:1 piston

 

[09RappySE]

The idea is to get more compression, about 11.5:1 with a cp 11:1 piston

Correct...and you do that by decking the cylinder not the head.

 

[Brokenbones]

What is the difference?

 

[smo0oth1]

What is the difference?

A cylinder is allot cheaper than a head. You would always want as much meat on the head as possible, You could have head gasket troubles if you cut it down to much near water jackets etc.


To the OP, Why not just purchase a 12:1? You could simply deck some of piston dome if you are dead set on 11:5

 

[smo0oth1]

With a cp 11:1 piston what the maximum i can shave out of the head to don't go past 11.5:1 and also without slamming the piston in the valves. Thank you!

Is your goal to get 11:5 or do you have a bad face on the head and in need of shaving it?
If your building an old XR head you should have the machine shop CC the chamber first to see if it has been cut down before. From there you can figure your compression and calculate any further numbers.

 

[EHSRACING]

really you should have a piston made to the correct size by decking a cylinder you change cam timing more than you realize. on a single cam its not a big deal to do a little bit if you have an adjustable cam sprocket but if you dont you would be suprised what .020-.030 does for timing. Same thing with using more base gaskets. By decking the head you will elimiante some combustion space but depending on the pistonyou may nee to make up the extra space there for valve relief. This is an area that takes some rd to really figure out what works.

In my opinon decking the cylinder is a bad idea unless its done to meet certain building requirements you should always have a certain range of piston to deck clearence if you dont you need to get a piston with a corrected deck height

 

[Brokenbones]

Ok i just realised the timing issue, back in the day i had a vw 16v and the head to 11:1 but i used an adjustable cam gear to retard it a bit. Anyway i was looking to get the most cr without going with a 12:1 piston. I don't wanna use racing gas when i'm trail riding cause some days we can fill it up 2-3 time when it's winter.... We have over 10000 miles of all season federate trails

 

[DL700]

the difference in power from 11:1 to 11.5 is not worth touching either the cyl or head....if your not gonna take .030+ off to correct for deck height of the piston or machine due towarpage then its not worth the time or money....

and WHy push the edge of pump gas compatability to its limit.... for a max gain of less then 1 hp ???

 

[Brokenbones]

You are right but it's always cool too have more experienced opinions from nice people who wanna share knowledge

 

[Brokenbones]

My friend has a 12:1 in his 450r and seems to take well with 91octane? It is because it's smaller displacement?

 

[09RappySE]

My friend has a 12:1 in his 450r and seems to take well with 91octane? It is because it's smaller displacement?

450's are a totally different animal....they can get away with more compression on pump gas.

 

[Brokenbones]

For what reason?