Compression Ratio

[Anonymous]

Wondering how you work out the compression ratio if there is a table or do i have a wrong compression gauge, have 4.1 carb , the compression pressure for all the pots was approx 1120 kpa at max cranking speed, I don't know how to work out the ratio

Joip

 

[xctasy]

Dats rooly god (in my best Kywi uccent).

You use the electronic abbacus and just divide that nasty metric number, 1120 Kpa, by 6.89, and get 163 psi of pressure. Very okay to me.

The CR is possibly about 9:1 or so, pretty standard for a 'Con. My Falcon kicks out that with a longer duration (~256) cam. There is no easy way to establish it because atmospheric pressure can play around with it a lot. Clevelands with 10.7:1 used to hit the 190-200 psi mark even with the long duration 290 degree cam. Nasty!

 

[Anonymous]

The actual compression ratio must be calculated or measured. It is the total volume of the cylinder. combustion chamber, gasket thickness and piston ring crevice volume at bottom dead center. Divided by the total cylinder volume at top dead center. It can be calculated if you know your cylinder head cc's and piston pop up or dish cc's. Cranking comp is not the same thing and will vary mostly (assuming no other mods to engine componets) due to cam events. A radical cam will give less cranking comp than a stock one. This is part of the reason why you see cam manufactures recomending a certian comp ratio to be within when using a particular grind.

 

[Anonymous]

Ok thanks for the help, i just didnt understand when some of you talk about different ratios how you work it out, i thought it might have been calculated from a compression test

I think they need to put a dunce hat in the emoticons section lol

Joip