Determining Static Compression Ratio - Help

[Cinnabar325is]

I'm putting together my 2.8 stroker (M52 crank, e rods, short skirt i pistons) and I would like to calculate the static compression ratio of the motor.

I know there are six values I need to take into account and some will be difficult for me to determine:

1.) Stroke: 84mm

2.) Bore: 84mm

3.) Compressed Headgasket Thickness: Anybody know this value for an OE Elring part? Uncompressed it measures 0.075". I've ready anything from 0.060" to 0.070" compressed for these motors...

4.) Deck Height (in this case defined as:the distance between the top of the cylinder bore and the top of the piston, when the piston is at TDC): My understanding is that my pistons will sit 0.5mm LOWER than a standard M20B25 at TDC. But I need to know this value for the stock motor.

5.) Piston Top Volume: No good idea on how to measure this myself, as these are complicated piston tops... hoping somebody can fill in.

6.) Combustion Chamber Volume: I roughly cc'd it at 42cc but I was not using a real cc kit. What is the standard M20 volume? I've read 43 and 47cc. Also, my head was likely skimmed by a shop while the last owner had it, but I have no official record of that.

Thanks for any help!

[whodwho]

Here is a nice compression calculator to play with and some of the numbers filled in for you :)

http://www.zealautowerks.com/index.html?setup=6,m84.00,m206.0,m84.00,40.00,-22.00,m26.6,m140.00,m1.75,m0.000,7000,0,0

[Cinnabar325is]

Here is a nice compression calculator to play with and some of the numbers filled in for you :)

http://www.zealautowerks.com/index.h...0.000,7000,0,0

Nice, thanks for the link. I'm playing around with it... but it's giving me 7.01:1 CR. There's no way that's correct, others with this motor have quoted anywhere from 9.1 to 9.7 CR.

The only info of which I'm really uncertain is the piston dome volume. Does anyone have that info?

Any of the other bits of info I asked about up top would be great too!

[Cinnabar325is]

Okay, I did a whole bunch more fact checking and changed some numbers a bit. But I still can't be certain they're all correct! Everybody seems to tell a different story.

Here's what I'm using:

Bore: 84mm
Stroke: 84mm
Cylinder Block Deck Height: 206.7mm
Combustion Chamber Volume: 40cc
Piston Dome Volume: -9.5cc (came from a guy at VAC, but don't know if that's right for certain)
Piston Compression Height: 34.09mm
Con Rod Length: 130mm
Compressed Headgasket Thickness: 0.068 inches
Amount Head Has Been Milled: 0.05mm (just a guess as it was resurfaced)

Does anybody agree or disagree with these figures?

This gives a CR of 8.5:1. Again, that can't be right according to the general consensus... what gives?

[whodwho]

Block is 206mm
Piston is 34.2mm

[Cinnabar325is]

Won't argue with you on the piston, but every figure I've ever seen for the deck height is 206.7mm?

[whodwho]

You hear the deck heights of 206.7 for an i block and 206.2 for an e block

If you add up the stack height:

1/2 stroke + rod length + Piston Compression heigth

40.5 + 130 + 35.7 = 206.2 for an eta stack
37.5 + 135 + 34.2 = 206.7 for an i stack

That would make them sit flush and we know that is not the case :)

[Cinnabar325is]

You hear the deck heights of 206.7 for an i block and 206.2 for an e block

If you add up the stack height:

1/2 stroke + rod length + Piston Compression heigth

40.5 + 130 + 35.7 = 206.2 for an eta stack
37.5 + 135 + 34.2 = 206.7 for an i stack

That would make them sit flush and we know that is not the case :)

Okay, so in that case, an 'i' piston sits 0.7mm out of the block at TDC? The value I'd run across was 0.015" or 0.38mm. Again, conflicting figures... haha.

In my case the stack height is:
42 + 130 + 34.2 = 206.2 or the same as an eta which is interesting.

With your new numbers, I'm getting 9.07:1 which is still below other's claims. I'm starting to wonder at the accuracy of the other calculations.

[whodwho]

Here are some good threads about this, he has a couple others you can search up

Good one with pics - Keep the 206mm deck in mind and it will start to convince you :)

http://www.r3vlimited.com/board/showthread.php?t=157679

http://www.r3vlimited.com/board/showthread.php?t=167174

http://www.r3vlimited.com/board/showthread.php?t=165696&highlight=Pistons

http://www.r3vlimited.com/board/showthread.php?t=150058&highlight=mahle