# Thread: jh static compression ratio

1. OK, either the stated 8.5:1 static CR for the JH motor is way off or I'm doing something horribly wrong.

After making all of the relevant measurements, I come up with a 6.7:1 CR using this calculator: http://www.kb-silvolite.com/calc.php?action=comp

Here are the measurements I'm using:

combustion chamber volume: 33.5 ml (cc)
piston bowl volume: 36.5 ml (cc)
gasket compressed thickness: 1.75 mm = 0.06809 in
gasket bore: 3.245 in
cylinder bore: 3.184 in
deck clearence: -0.018 (- indicates above the deck)
stroke: 3.385
intake valve closing: 53°
rod length: 5.670 in

Do any of these sound horribly wrong? I am using water instead of oil to measure the volume, but the head volume comes out to roughly what it's supposed to...

edit to correct compression ratio calculation

2. combustion chamber volume: 33.5 ml (cc) 28cc
piston bowl volume: 36.5 ml (cc) i dunno seems like way too much. you are saying the piston dish is bigger that the chamber in the head. I think your chamber cc is too much as well but any experts chime in. I will calculate as flat top piston.

keep all your units metric and consistent to match 28cc:

gasket compressed thickness: 1.75 mm = 0.06809 in .75mm .075cm
gasket bore: 3.245 in 82.5mm 8.25cm
cylinder bore: 3.184 in 81mm 8.1cm
deck clearence: -0.018 (- indicates above the deck) .45mm .045cm
stroke: 3.385 86.4mm 8.64cm

max volume = swept volume + deck + gasket + chamber

(4.05 x 4.05 x pi x 8.64) + (4.05 x 4.05 x pi x -.045) + (4.05 x 4.05 x pi x .075) + 28

445.23 -2.32 + 3.86 + 28 = 474.54

min volume = deck + gasket + chamber = -2.32 + 3.86 + 28 = 29.31

ratio = max/min = 474.54/29.31 = 16.2:1

Obviously this is too high however a few cc make a big difference. So get the correct number for the piston dish and also I think the piston should be below deck but never built a JH and you will get a more accurate number.

I got the .75mm gasket thickness from Darrell Vittone years ago so I always use it in my calcs. I think your chamber cc is too much as well but any experts chime in. Thats why I used the 28cc

3. I believe you were using bad numbers and maybe mixing up inch and metric. I plugged in numbers I have gathered over the years from taking measurements, more or less averages as things can often vary a little, and came up with a static (all I did) figure of about 9.8:1. That would be about right as I have always read and heard that VW based their static compression on a cylinder head which is at it's minimum allowed thickness. Here are the numbers I plugged in:
Cylinder head volume - 29.5cc
Piston dish volume - 10.5cc*
Head gasket thickness - .069"
Head gasket bore - 3.24"
Deck - .007"
Stroke - 3.40"
* This is the dish ~14cc minus the heal (squish pad) ~ 3.5cc which makes doing the math a little shorter.

Cylinder head volume an vary depending on a few things as can the deck but the differences are most times rather small, but enough to change the results by up to a compression point maybe.

4. I have the data and the calculations here from my rebuild.
JH with JH pistons
then JH with MK2 GTI pistons

http://www.scientificrabbit.com/node/14

5. Thanks guys. I must have been off a fair bit on my piston measurement, because it's off compared with what I've found elsewhere by quite a bit. Either way I decided I wasn't going to be able to mill the head enough to get 10:1 or higher compression. So, I went to a pick 'n pull and snagged some pistons out of a '90 cabriolet, which has a 10cc dish according to my measurements (and verified by ny_fam's site). This should get my compression up around 10:1 according to the factory and confirmed by my new measurements.

That gives me a dynamic compression ratio right around 8.5:1. I'm running a solid TT 288 cam and intend to run pump premium (91 octane). Am I wrong in thinking this should work out pretty well?

6. Originally Posted by CStryker
Thanks guys. I must have been off a fair bit on my piston measurement, because it's off compared with what I've found elsewhere by quite a bit. Either way I decided I wasn't going to be able to mill the head enough to get 10:1 or higher compression. So, I went to a pick 'n pull and snagged some pistons out of a '90 cabriolet, which has a 10cc dish according to my measurements (and verified by ny_fam's site). This should get my compression up around 10:1 according to the factory and confirmed by my new measurements.

That gives me a dynamic compression ratio right around 8.5:1. I'm running a solid TT 288 cam and intend to run pump premium (91 octane). Am I wrong in thinking this should work out pretty well?
would work better with more compression..

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