High comp head ?'s

[marcthompson]

I've had a r&d head, which came off of a 750sxi, sitting on my parts shelf for a few years and now I wanna put it on my sxr, but I'm not sure how much it will raise comp... It's marked as being 26cc domes. I'm currently running a stock bore/stroke motor, stock carbs rejetted for the factory wet pipe I have on it, vforce 3 reeds, tau ceti flame arrestors, ATP waterbox, '06 electronics with timing advance jumper, lightened TL stock flywheel... I'm at about 1400', I took a compression test and it showed 145 up front and 144 in rear, this was on a cold motor tho. I would like to run 91 octane so I'm thinking this head may be too aggressive for me. Anyone have any idea how much it would raise compression and how much I could go and still run pump gas? Any info would be great, thanks

 

[phdatsun]

I ran 175# on 91 oct pump on my sxr for 3 seasons with no problems. If it were me I would add more compression. 145 is prob about stock.

 

[TheBuzzard]

the sxr has 185 psi stock at sea level. those 26cc domes are way too small for sea level but you may get away with it at elevation. another problem is the squish relief. the 750 has 80mm pistons and the sxr has 82mm. the 750 domes were not designed to be ran on the 800

 

[brad95]

How can you tell what cc the head is? I have a r&d head sitting here but the only markings are a 7 punched onto the top of it. I'd like to know the compression of it. Cheers brad

 

[djkorn1]

Me too....no idea what size head I have on there. How many cc's are a stock head? My R&D (no idea size) gives me 155.

 

[TheBuzzard]

i'm just guessing here but the 7 could mean 27cc domes. the sxr comes with 185 stock at sea level. 27cc domes are some pretty high comp for the sxr. i've got a joeys/tbm here with 25cc domes and it was the head off joeys limited sxr that was in watercraft illustrated 2003

 

[djkorn1]

How many cc are the stock heads?

 

[kx454]

Bump that baby up into the 175-180 range.

It's been a long time but I wanna say my SXR had 30cc domes stock.

It is not hard to check cc, all you need is a piece of class, graduated cylinder, and a C-clamp

 

[brad95]

Cheers, a mate recons from memory the 7 is 27 so I'll go with that, does it pay to measure the actual cc or could I bolt it on with an old gasket and give a cold compression test to see what psi I'm at?