17 replies to this topic

[Dog]

Helo all!

I'm trying to work out my CR...

20ml Combustion chambers
9cc dish pistons
deck height is 0.5thou (piston edge is 0.5" down the bore)
Gasket is 3cc
Standard 1275 stoke crank
bore is 73.3 x 82.6 stroke

Can someone help me

Edited by Ðogmatix, 25 March 2007 - 01:07 PM.

[mini93]

already told you

[Dan]

(piston edge is 0.5" down the bore)

Bloody hope it isn't.

What type of pistons? Need to know the ring land volume for an accurate figure.

[Road Runner]

Mine is around the same as you and is about 10.2 -1

[GraemeC]

I make it somewhere around 11.5:1 using .7cc for ring land volume. Have you measured the 0.005" deck height?

But using 73.5mm bore and 81.2mm stroke, which I think you mean I get slightly less (still over 11:1 though). You'll need to watch the fuel quality, espeicially if you time it in for good power.

[Dog]

Yeh I meant 0.005" deck height at TDC

I'm using powermax pistons, and the stoke was taken from the deck to the crown of the piston at BDC...

If I am running 11:1 - I'll need 98ron fuel yeh? shell optimax here I come lol

[Retro_10s]

shell optimax here I come lol

Not available anymore. you mean Vpower (it's al the same)

[Dog]

wooo Vpower

[Dog]

Can someone rework this out moi?

22cc Combustion chambers
9cc dish pistons
deck height is 0.05" (piston edge is 0.05" down the bore)
Gasket is 3.8cc
Ringland is 0.7
bore is 73.5 x 81.2mm (std 1275) stroke

I think thats all you need?

[Leonard]

i get 11.12 , checked using two different programs. just google "compression ratio calculator"

[Guess-Works.com]

10.56:1

or...

Chamber volume @ bdc / Chamber volume @ tdc

Chamber volume @ tdc = volume of Head (22cc) + volume of dish (9cc) + volume of head gasket (3.8cc) + volume of free area between top of piston and 1st piston ring (0.7cc - well that what I assume you mean by 0.7 if not measure diameter of piston crown and distance from crown to top 1st ring) + volume left in bore when piston @ tdc ( 0.005" @ 73.5mm = 0.538cc )

or... 36.038cc

Chamber volume @ bdc = 36.038 + bore engine CC (stroke * area of bore)

or... 36.038 + 344.52 = 380.56

Therefore

Ratio = 380.56 / 36.038 = 10.56

[TOMMO0302]

Does anyone know the cc of the head for a 12g 940 from a A+ Metro...if that makes any difference?

Also, when considering your CR, is it mainly in regard to the cam? Hotter cam needs a higher CR i get that, but do other components come into it? For example, if you had cheaper pistons would you want to keep the CR down to avoid blowing a hole in them?

[danwebster]

I thought the standard 1275 stroke was 81.3, which gives you 9.7:1.

[The Matt]

I believe it's 81.23, but I may be wrong?

[danwebster]

Mine has 81.33 stamped on it, although its an EN40b forging and not a standard crank.