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Posted by: DavidSweden Jan 19 2021, 03:47 PM

Hello,
What is the maximum allowable compression ratio difference between cylinders? 2056 motor AA big bore

Thanks in advance

Posted by: rfinegan Jan 19 2021, 03:57 PM

I believe under 50 to 60 psi will have problems and not fire/start.
As far as difference, I would think more than 25% between cylinders would indicate a problem. And 10 % between cylinders would be normal (OK)...OOPS this is a mechanical problem /Question for a build about Compression Ratio/s,CC/Deck Height. Which variable is giving you the problems. The volume of the Heads or the Deck Height?

Im going to say 10-15% But perhaps it will have to do with the measuring instruments you are using and how accurately they are used. Im sure a veteran engine builder can give you better real world values. But this is just a guess.

Posted by: Jake Raby Jan 19 2021, 09:40 PM

What values are you seeing? Are you performing ng the test with the throttle wide open, or not?

Posted by: DavidSweden Jan 20 2021, 10:39 AM

I am installing some new heads because of damage to one of the spark plug holes and at the same time installing a 2056 AA piston & cylinder set. The engine is a stock 2.0 (stock cam) with D jet

The new heads have been fly cut at some time- cylinder 1 & 2 = 59cc and 3 & 4 = 57cc.
Deck height = 1.2mm on all cylinders, and the AA piston valve cut outs = 7cc.
This gives a compression ratio of 7.84:1 on cylinders 3 & 4 and 8.03:1 on cylinders 1 & 2

This is with no shims and no head gasket, so unless I get the cylinders or heads machined I am stuck with these low compression values.

The compression ratios on each side of the engine are different I guess this is ok but I just want someone who knows to give me an opinion.


Thanks in advance

Posted by: rfinegan Jan 20 2021, 11:14 AM

what does the work out as far as % error on your CC of the heads (3.5%) or so?

Posted by: rfinegan Jan 20 2021, 11:15 AM

that is pretty low compression ratios

Posted by: Jake Raby Jan 20 2021, 03:00 PM

I thought you were trying to get a value for a compression test.

Based on the data you have in your last post, you'll need to disassemble the short block, and flatten/ lower the case decks accordingly.

Posted by: DavidSweden Jan 20 2021, 03:20 PM

At the risk of showing my ignorance isn't there an easier way? if I got some flat top pistons without the valve cut outs it would up the CP 8.5:1
Also if my engine is stock how come the CP is so low, is 1.2mm deck height excessive?

Thanks in advance

Posted by: rfinegan Jan 20 2021, 03:37 PM

I have read a lot of the AA cylinders have to be turned on the lathe to deduce the deck height to 1.00mm (.0400 inch). Good new is no shims will be needed. Sound like you can get the chambers in the heads taken care of too by cc'ing the chamber's (open the small one) or fly cutting the big one to match the smaller head.
Research this sight/ web for additional information on 2056 builds

Keep it up David, There is a lot of GREAT info on the page. You have one of the best resources and over all great guy checking on your thread already.
Unfortunately the parts you purchased are not just drop in parts, and will need a little fitting

Posted by: Dave_Darling Jan 21 2021, 04:53 PM

Not for a 914. Stock compression ratio for the US-spec 2.0 motor is 7.6:1 . I wouldn't go over 8.5:1 in an otherwise-stock motor.

You want the CR difference from cylinder to cylinder to be as small as you can get it. More even means less overall wear and generally better power and more smoothness.

--DD

Posted by: stownsen914 Jan 22 2021, 11:35 AM

If I read your post right, wouldn't the fix be to cut the head for cyl 1 & 2 so it matches the other one, and get the volume both at 57cc?