Compression Ratio

sammy · Post by **sammy** » 17 Feb 2009, 22:54

Hey im trying to work out the comp ratio of MY G180W.

I have measured the combustion chamber of my head to be 55cc.

Standard bore so 84mm right, and 82mm stroke.
Standard pistons.

Can someone help me fill in the rest for this calculator?

http://www.csgnetwork.com/compcalc.html

Hope you can help, Cheers

bad 44u · Post by **bad 44u** » 18 Feb 2009, 12:50

if its got standard pistons they have a dome on them so you need to subtract the dome from the 55cc. is the standard comp ratio no help or have you changed things? i just sed the calc you put a link towith your values and got 8.6:1 with out adding the dome the standard cr is 9:1 so this means the dome is about 3cc

sammy · Post by **sammy** » 18 Feb 2009, 15:48

Standard comp is no help, as i want the compression of this engine as it has been bought apart, so basically unknown compression.

So what should i enter for piston deck clearance?

EDIT: The CR shouldn't be any lower than standard.

Cheers

archangel62 · Post by **archangel62** » 19 Feb 2009, 00:41

Basically you'll need to know the cc of the piston crown protruding from the block. This can also be calculated if you know the stock head cup cc, but if you knew that you could calculate the compression far easier anyway.

I'm afraid I don't have numbers for you but if you know either of these things (protruding piston crown cc, or stock head cup cc) I can calculate it.

sammy · Post by **sammy** » 19 Feb 2009, 08:09

I have Measured the combustion chamber of the head, so the bit with the valves. Is this what you mean by head cup? It measured 55cc anyway.
Cheers

bad 44u · Post by **bad 44u** » 19 Feb 2009, 21:06

you need to assembly th bottom end and measure how far the pistons hang out the block and work out its cc then minus that from 55cc and thats your combustion chamber size.
i think you need to get a plasticine mould of the top of the piston then measure the cc the same way you measured the combustion chamber.
you'll probably need to get a head gasket made so the decide what thickness you'll use and enter that into the calculator.
why do you need the exact comp ratio half a point either way wont make that much of a difference as long as the valves don't hit the pistons.

sammy · Post by **sammy** » 19 Feb 2009, 23:14

Its because im considering running gas and therefore possibly increasing CR. So that i can work out what needs to be done.
Also i would like to know the compression ratio just to know it