Head Swap

wiredgeorge wrote:

The J motor valves are a tad bigger. Not sure if the loss in compression and resulting horsepower would make sense. Perhaps you could put a couple gaskets on to get compression back upadding gaskets lowers compression as its just like doing the opposite of milling the head or cylinders but this would be a headache. If you want to go with bigger valves, buy the valves, get a valve job done on your old head as a decent performance shop could install the larger valves in the old head and you won't have to worry about adapting the cam chain idler arm and figuring out how to bump compression back up. Stock compression is 8.25:1 and that is about where you would end up.

the j head has bigger ports to go with the bigger valves, so just putting j valves in the old head without porting wouldnt be as good as running the j head with the right compression.

Actually Wireman...that was my "rant" on another forum after I read what I considered to be a lot of misinformation....I'll stand by my post...I also did apologize to the guy for hammering on hims so hard..

There is a point where the J head makes good sense and a point where it can't be justified to swap...

larrycavan wrote:

Actually Wireman...that was my "rant" on another forum after I read what I considered to be a lot of misinformation....I'll stand by my post...I also did apologize to the guy for hammering on hims so hard..

There is a point where the J head makes good sense and a point where it can't be justified to swap...

thats why i love this guy(:blink: )we think so much alike!

Whoever wrote the big long post have a number of errors. The most blatant is that a J head can take a 42mm intake valve without changing the seats. NOT SO. 38.6mm. max

Jay ( One who has put a lot of big valves in a lot of J heads.)<br><br>Post edited by: APE Jay, at: 2007/03/10 22:21

Interesting reading, thanks. With the head porting, agree that velocity not cfm flow is more important.
Sent the 84 GPZ head from my race bike to a well known cylinder head shop for a valve job and a "second opinion" It's got 42mm.in and 34mm. ex & was told I'd have to put bigger valves in it to maintain velocity.

Jack wrote:

Interesting reading, thanks. With the head porting, agree that velocity not cfm flow is more important.
Sent the 84 GPZ head from my race bike to a well known cylinder head shop for a valve job and a "second opinion" It's got 42mm.in and 34mm. ex & was told I'd have to put bigger valves in it to maintain velocity.

That would be true only if CFM increased on a scale proportional to cross sectional area.

Velocity if a component of volume and cross section. If volume [CFM] is increased while maintaining the same cross sectional area, then velocity goes up. If CFM increases because of an increase in CSA then velocity can remain the same or even drop. Just depends on the balance of volume and area as to which happens.

In any event, it's not going to mean diddly to most people. All they want to know is the bike went quicker/faster...