jh static compression ratio

[CStryker]

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

[antichristonwheels]

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

[WaterWheels]

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.

[ny_fam]

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

[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?

[Glegor]

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..