Quick Q for the motor guru's

[joshuaeric]

When I purchased my ski, it came with an extra head and multiple sets of domes.

I remember reading that there is a quick and easy formula to calculate the CC that certain domes would produce.

I currently have the following:

780 CC
7 mil stroker crank
?? domes (I am not sure of the size that is currently installed, but was given the size of the engine when I purchased it, so I believe the dome size can be calculated from that).
I am not sure if it was bored over, but I believe it was. I think it has to be to reach 780CC (from a 701), correct?

So my question is, how can I calculate the CC that will be produced with certain domes?

If you need anymore information, let me know.

Thanks!

 

[SuperJETT]

Dome size has nothing to do with displacement in cc's, only with compression.

Also, I've never heard of a 7mm crank. 4,5,6,8,10 yes, not 7.

Displacement in cc's is (radius of cylinder)^2 * pi * stroke * 2, so for 81mm bore and 68mm stroke, that's 40.5^2 * 3.1415927 * 68 * 2 = 700.8cc, or a stock 701.

 

[RMBC Freeride]

Actually Jett, there are some 7mm's out there. I came across one a while back (on ebay or something).

 

[njfl]

Here's a thread with some discussion on displacement, explaining a little about dome size too:

http://www.x-h2o.com/threads/12691&highlight=displacement

 

[joshuaeric]

Ok, that helps a ton Darin. Btw, your avatar is driving me bonkers.

So if I am currently running 100 octane, what would you guess is the dome size? I imagine the only proper way to tell is to take the head off and look.

The ski is Kilowatt Ken Wood's old ski, and I believe it was built in 97.

That is the formula I was looking for, thanks.

 

[SuperJETT]

Actually Jett, there are some 7mm's out there. I came across one a while back (on ebay or something).

Learn something new every day. Weird.

 

[SUPERTUNE]

The only true way of knowing the dome cc's you need, is to fill the combustion chamber with engine assembled and at TDC (top dead center) will give you your FV (fill volume) and then calculate the engine displacement using the exhaust port height as the stroke length.
Displacement cc's + fill volume in cc's, then divide by the fill volume cc's will then = C/R (compression ratio) like this 9.2:1
Then use this compression ratio to match to the port mapping, ignition timing, w/curves and pipe set-up against your peak target RPM's.