12 replies to this topic

[Coopershift]

I have a 1275 engine overbore 0.020" to 1293 with P21251-20 pistons, 12G940 head STD, 286 Kent cam, LCB, and something else
I would like to have a 10,5:1 to 11:1 CR
I have been reading "Tunning the A-Series Engine" 3º Edition and I need some information....(page 222-223)

Bore: 70.625mm or 70.61mm STD ?? I've seen a lot of different numbers
Stroke: 81.28mm or 81.33 ??

Head Gasket: GEG300---3.8cc
Piston Dish: 8.4cc
Cilinder Head: 21.4cc
(*)Ring Land Volume: I don't have any idea
(**)Volume above piston and below block deck: the same, no idea!!!

I need this information (*) (**) to set properly the CR

[Guess-Works.com]

There's a thread somewhere which details how to discover the above, and also a spread sheet for working it all out, but as a 21251 is a 8.8 LC piston, unless the block has been decked or the chamber volumes reduced (by skimming) then expect your compression ratio to be iro 9:1

I'll have a look..

[blacktulip]

standard turbo pistons + 12g940 head std = 9.4-1 compression. i would say it would be around that now.

[Coopershift]

There's a thread somewhere which details how to discover the above, and also a spread sheet for working it all out, but as a 21251 is a 8.8 LC piston, unless the block has been decked or the chamber volumes reduced (by skimming) then expect your compression ratio to be iro 9:1

I'll have a look..

do you have the link??
yes, I think is too low

[Ethel]

Is it assembled? The most reliable way would be to measure it.

[Coopershift]

Is it assembled? The most reliable way would be to measure it.

yes, but I have the head off

[Guess-Works.com]

Sorry could not find the thread, well within 5 minutes of looking, but it's there somewhere...

[Ethel]

You'd want the head off,

Use a graduated pipette to fill a combustion chamber with paraffin or similar. Put a piston to tdc and do the same to a cylinder - use a known quantity of plasticine or grease that doen't dissolve in what your chosen fluid is to seal the gap above the rings. A piece of perspex with a hole in is handy and easier than getting things totally level. Just add the quoted figure for the head gasket

[Phaeton]

Sorry could not find the thread, well within 5 minutes of looking, but it's there somewhere...

Would it be the one you put in the FAQ's?

www.afracing.com/cr.xls

Alan...

[Guess-Works.com]

Doh!

[Jammy]

Just out of interest, where do people get pipettes nowadays??

[alexcrosse]

ditto

[Guess-Works.com]

I use a syringe which I got with a injet refil kit...