Calculating 2jzgeT compression ratio

[p5150]

THESE MEASUREMENTS HAVE BEEN REVISED. INACCURACIES HAVE BEEN DELETED. SEE THE LAST POST.

Im starting to think that the stock CR is closer to 9:1, not 10:1.

Everybody says that the 2JZGE is a 10:1 stock CR engine.

For all of your sakes I have taken measurements of all aspects of the engine and included already known specs. The block has been re-surfaced and so has the head.

Piston dish volume (including area around rings) - 12cc
MY Piston deck height - approx .006 inches (this can vary depending on how much you have removed for resurfacing)
Combustion chamber volume - 43cc
Bore - 3.386 inches
Stroke - 3.386 inches

Head gasket bore diameter - 3.425 (for an 87mm Cometic)
Gasket thickness - xxxxxxxxxx

Now plug it into something like this:

http://www.csgnetwork.com/compcalc.html




But im having trouble getting the "stock" CR reading of 10:1 to come out correctly. Im coming up with a 9:1 ratio for a STOCK head gasket by measuring my engine..... I know the measurements are right so what is the problem here? Ideas?

Is the CR of the 2JZGE really 9:1?

 

[p5150]

No inputs?

 

[RacerXJ220]

Interesting calculations. Others have suspected this, but no one has posted numbers. Boostlogic sells their standard 3mm headgasket for the 2JZ-GE to change the comp. ratio to around 8.5:1, so claimed. Shoot them an e-mail, maybe they made a mistake????

Have you tried using another CR calculator to verify accuracy?

 

[p5150]

Interesting calculations. Others have suspected this, but no one has posted numbers. Boostlogic sells their standard 3mm headgasket for the 2JZ-GE to change the comp. ratio to around 8.5:1, so claimed. Shoot them an e-mail, maybe they made a mistake????

Have you tried using another CR calculator to verify accuracy?

Yeah - I thought that maybe the software was jacked on the calculator I was using, but other calculators gave me identical results.

 

[RacerXJ220]

have you tried calculations without using the area around the rings?

 

[p5150]

You should try to play with it - I tried removing a few cc's from the piston dish volume, the combustion chamber, varying the thickness of the head gasket, etc and I cant get it to come out to 10:1 for the life of me.

For the volume between the piston wall and cylinder im only calculating approximately .13cc or .008 ci - that wont make a significant difference

 

[RacerXJ220]

Did you get the answer you wanted off of SF?

 

[p5150]

Did you get the answer you wanted off of SF?

I am replying now. It will be a few because I have to do some math and measurements.

 

[tissimo]

what was the engine out of.. I've heard the supras are 10.0 and the scs are 9.5.. dunno if there really is a difference.. Maybe a different stock gasket?

 

[p5150]

http://www.supraforums.com/forum/showthread.php?p=4843076#post4843076

Here is a copy of the link - you can see on the pictures attached to this that the piston dish volume is bigger than what the guy on SF says it is. I also have a couple of other different measurements..... Thats the beauty of discussing this stuff. We can find out where the errors lie and meet in the middle. He has helped me figure out some stuff.

Pics #1 and #2 show that the block is level in both directions.

Pic #3 shows 80cc in the graduated cylinder. Dont forget to look at the bottom of the meniscus - remember science class?

Pic #4 shows me pouring the oil into the valve cutouts for a preliminary measurement. This measurement determined how much can be poured in before spilling out of the valve relief cutouts on the side of the piston. Pic #5 shows completion of this task. See the next post for results

 

[p5150]

Explination of pictures - be sure to look on the previous post........

Pic #6 - Clearly shows that AT LEAST 6cc is required to fill the piston valve cutouts. Keep in mind that at this point, all fluid has remained on the piston. By adding more oil it will spill out the side of the piston valve cutouts and EVENTUALLY leak past the top combustion ring. That is why I chose a somewhat thick oil - but not too thick - it still had to be able to settle in the graduated cylinder. We also have to consider the entire cutout parts as the "dish" of the piston.

In case anybody is wondering, I let all of the oil settle in the graduated cylinder and run off of the glass walls for a few minutes before I re-took the measurements.

Pic #7 - Shows the piston dish filled to its brim. This has to be done rather rapidly because - as stated before - the oil will EVENTUALLY seep past the .003" piston side clearance and past the ring. Total, it took approximately 10 seconds to pour the fluid level.

Pic #8 - Shows the results - AN APPROXIMATE PISTON DISH VOLUME OF 9-10cc


I want to thank scott aka motorheaddown on SF for helping me realize that there is an error with the piston dish volume I had before...

 

[RacerXJ220]

Well at least you know what YOUR compression ratio is going to be!!! lol

Nice work.

 

[GrimJack]

Where / how did you get the combustion chamber size? That's the only thing I can see that you haven't explained your measurement methods beyond any doubt...

 

[p5150]

Where / how did you get the combustion chamber size? That's the only thing I can see that you haven't explained your measurement methods beyond any doubt...

I measured the CC the same way - but with valves and plug installed. I will measure them againwith a plexiglass sheet and take pictures when I get the head back from the machine shop next week or so.

 

[p5150]

For all of your sakes I have taken measurements of all aspects of the engine and included already known specs. The block has been re-surfaced and so has the head.

These measurements are very accurate - I used a graduated cylinder, syringe and measured/remeasured several times. Volume measurements were very consistent. The piston dish volume was measured with the piston installed in the block. It is nearly impossible to accurately measure the piston dish without it being installed.

Piston dish volume (including area around rings) - 9cc
Piston deck height - approx .006 inches (this can vary depending on how much you have removed for resurfacing)
Combustion chamber volume - 46cc
Bore - 3.386 inches (86mm)
Stroke - 3.386 inches (86mm)

Head gasket bore diameter - 3.425 (for an 87mm Cometic)
Gasket thickness - xxxxxxxxxx

Now plug it into something like this and be sure to put the (-) signs where they belong for the Java app:

http://www.csgnetwork.com/compcalc.html

WITH AN 87MM BORE HEAD GASKET, THE HEAD GASKET THICKNESSESS (IN INCHES) BELOW WILL YIELD THE FOLLOWING COMPRESSION RATIOS:

.228mm/.009" = 10:1 (approximate stock HG thickness)

2mm/.079" HG = 8.57:1

3mm/0.118" HG = 7.94:1

I took great care to measure these accurately and I stand by these volume measurements. They were very consistent.

 

[bountykilla0118]

so what is the thicness you plan on running?

 

[p5150]

.074" 87mm bore cometic

 

[fstlane88]

Hate to bump this really old thread for this but, a buddy of mine is considering purchasing a NA-T SC and the car in question has a GTE stock HG on it. What I'm trying to figure out is what the CR might be. The block and head have been milled so it's not going to be exact but something close will help. So we have an idea.

 

[p5150]

You can check your compression with a compression gauge and it will give you a good idea of your ratio. Just compare it to the specs for the GTE vs the GE.

 

[jmcboost]

So, for the guys (like me) running the 3mm MHG, what is going to be the noticeable differences and the long term effects to the engine and engine power?