head questions

got the engine stripped and head apart and stripped, cleaned. needs a valve job but looks ok.
thinking about skimming the head to up the cr a bit, how far can i go ofr a road bike running super unleaded fuel?
going to give the ports a working over and polish/match the chambers. any thing else i should do while i'm in there?
cheers rob

i wouldnt go a whole lot more than just cleaning up the surface of head to get it flat and squared up,dont go too crazy porting bigger isnt always better.if you want to up the compression a little id measure how far down in bore the pistons are and have the bottem of cylinder block milled just a little,after removing sleeves of course!:laugh:

why not just mill the top of the block with the sleeves in?
doesnt hurt to clean up the case surface as well if u have any leakage issues.
Hhat motor are u talking about? if its a 900 you could bore it to 1015.

the list is endless.
your head is the biggest restriction in your motor.
can't go if it don't flow.
spend most of your money there.
i've seen 1045 motors beat the hell out of 1200's because they had good work done to the head.
don't skimp on a critical part.

judgie wrote:

got the engine stripped and head apart and stripped, cleaned. needs a valve job but looks ok.
thinking about skimming the head to up the cr a bit, how far can i go ofr a road bike running super unleaded fuel?

That's 92 octane these days. This is purely a guess based on my old hot rodding days from the 60's. I recall comp ratios around 10.5 or 11 to 1 were about as high as we went and it took about 100 octane to run that. If you are restricting to 92 octane, I would take a guess at maybe 9.5 to 1 ballpark?

To know how much you can shave the head you have to know the combustion chamber volume at bottom stroke and TDC. The comp ratio is the ratio of the BDC volume over the TDC volume. You also have to calculate how much the compressed (TDC) volume changes for amount cut on the bottom surface of the head.

Make sure you have enough valve to piston clearance.

as far as decking the head goes i wouldn't go more than .020". porting is quite an art but if you don't get to agressive you can do a good job but what ever you do don't polish the intake ports please, just clean up the casting flash and don't take much off the floor if you can help it one of the most important areas are the bowls do a good job blending those in and the head will work good for you.and by all means if you don't think you can do a good consistent job on those ports please involve a professional as it is very easy to mess up when porting and it is very hard to keep all the ports flowing the same the cylinder head is the most important part of the engine don't be afraid to get pro help. i don't know how to do stomach surgery and i wouldn't be afraid to ask if i needed help.

cheers for the answer
i build aircooled vw race engines and do all my own head work.
my own 2ltr aircooled ohv motor makes 145bhp @ the wheels on the dyno
was more after any thing to watch out for when porting the heads, can see that the port shape is restricted by the valve spring pockets. the finish to the valve seats is horrid. was suprised to see 3 angle valve seats though
we can get 98 octane fuel at the pumps in the uk so it will be fed a diet of that so i'm guess i should be able to go to 10.5/1 on the cr with out to much trouble.
i'll see about getting the head on a flow bench and see what works
cheers rob

I run 10.5:1 on 92 octane.no problems.