legacycentral bbs

what is the formula for calculating CR?

You can't really calculate it unless you know the entire geometry of the combustion chamber. Usually you measure it. You TDC a cylinder, fill it through the spark plug hole with motor oil or something similar, then suck/pour it out and measure its volume. Do the same at BDC, then subtract the volume of a spark plug from each number (you can measure that by dipping the electrode into a graduated cylinder of motor oil and seeing how much the level rises; it's probably negligible though), then divide the BDC number by the TDC one.

the ejcalcs website appears to have all the data i would need for the volume of the cylinder for the ej25 and the head volume for the ej22. in fact it has a lot of data. so i need to know exactly what parameters i need to calculate the CR.
all data is from:
http://wac.addr.com/auto/obs/turbo/ejcalcs.html

90 ej22
Quench Vol (cc) 11.2
head. vol. (cc) 40 this value was estimated

00 ej25
bore 9.96 cm
stroke 7.9cm
disp 616cc
gasket bore 9.96 cm
gasket thickness .0058cm thin sohc gasket
or
gasket thickness .152cm thick dohc gasket
piston volume 6cc

so whats next?

side notes:
00ej25 head volume is 50cc and quench volume is 4.5

a stock 00ej25 has a CR of 11.2:1
and a stock 90ej22 has a CR of 9.4:1

i think that if i use the thick dohc gasket i can get away w/high octane pump gas and no mods. but i could just be overly optimistic.

You could try it and if it runs like ass, you'll can try better fuel.

Steve

this was posted on special stage tell me what you think:

"Well, there are two types of CR, geometric and effective. Usually what you hear is the geometric CR. Effective CR is based on when on the compression stroke the valves actually close. If the valves closed at BDC, then effective CR and geometric CR would be equal, but since that is not the case....
Geometric CR = (V + Vc)/Vc
V = volume of cylinder = ((pi*diameter^2)/4)*stroke
Vc = chamber volume, which is the volume left when at TDC

So.... working backwards from the EJ22, we find that V=553cc
CR=9.5 = (553+VC)/VC
8.5VC = 553
VC for EJ22 head = 65.1 cc close enough for me

So for the combo of EJ25 block and EJ22 head

V for EJ25 = ((pi*9.96^2)/4)*7.9 = 615.5

CR = (615.5 + 65.1)/65.1
CR combo = 10.5 approx.

Correct me if I'm wrong"

Nick Collins
VW GTI Group 2
www.teamoneil.com
www.bildon.com
www.yarrowsport.com

so this doesnt seem too far off base? it looks like i could get away with using the stock ecu and what not.

here is another site. but there are some values i am uncertain of. any help filling them in would be great.

http://not2fast.wryday.com/turbo/compre ... sion.shtml

like con rod length and Piston Clearance, Intake Close ABDC, Top Ring Height this is all for a na ej22 and ej25