Wiseco Pistons and 9:1 CR

sorcereur

sorcereur

MKIII Revolution Inc.
Mar 31, 2005
86
0
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NC
The wiseco pistons available for the 7M increase the compression ratio to 9:1 from what i read. Special orders need to be made to have stock compression.
I've searched and seems a few people are running wiseco pistons. For the ones that are running it, what have you done about the compression ratio, or you didn't know/care about it? Are you just running less boost, and/or race fuel?
I know i can run lower boost and not have to worry about it. But for big boost, lower compression is better.
How much does 9:1 compression change things?
What size headgasket can be used to compensate and lower the compression ratio? What's the formula to figure that out for all you engineers out there?
 
I

IJ.

Grumpy Old Man
Mar 30, 2005
38,728
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I come from a land down under
Lifted from a site but this is how I calculate it>>

Don't forget you need to calculate the volume of the Gasket as well and add that in.

1. Determine the displacement of your engine. Displacement formula is:

BORE X BORE X STROKE X .0031416 = DISPLACEMENT

Example: 92 x 92 x 82 x .0031416 = 2180cc

2. Determine the swept volume of one cylinder:

Example: 2180cc divided by 4 = 545cc

3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.

Example: on cylinder #1 you measure and find you have .020" deck height.

BORE X BORE X DECK HEIGHT X .01996 = CC"s

Example: 92 x 92 x .020" x .01996 = 3.378cc's

Measure each cylinder.

4. Measure the volume in each cylinder head. To do this, use a piece of plexiglass cut to fit in the cylinder head to cylinder mating area. Drill a 1/4" hole in the center of the plexiglass. Lightly grease the edge of the plexiglass and install in the head. (Spark plug and valves have to be installed) with a syringe graduated in cc's fill the cylinder head chamber up with a light weight oil. Record the measurements. Repeat for other three chambers. Average cc volume of a new head chamber is between 47 and 51 cc's.

5. You now have all the measurements to determine your compression ratio.

one cylinder swept volume + deck cc + head cc

deck cc + head cc

Example: 545 + 3.378 + 48 = 596.378

3.378 + 48 = 51.378 = 11.6:1
 
sorcereur

sorcereur

MKIII Revolution Inc.
Mar 31, 2005
86
0
0
NC
Cool... If my math is correct, with.....
83.5 Bore, 91 Stroke, 84 MHG Bore, 2 Compressed MHG thickness, 40cc Combustion volume, -16 Piston volume, and .20 Piston Deck
I should be around 8.3:1 CR... Or use a 1.2 MHG and bring it up to 8.8:1 CR.
hmm....
 
sorcereur

sorcereur

MKIII Revolution Inc.
Mar 31, 2005
86
0
0
NC
Makes more sense if negative because the pistons are above the deck at TDC.

Ok... that changes the CRs to 8.7:1 with 2mm MHG, or 9.3:1 with 1.2mm MHG.
Still within good range for me with 2mm HG
 
T

turbodriz

mk3 onwer
Feb 25, 2006
471
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newyork....N.c
I hate to be the odd ball and I do not know if what I am going to tell you can apply to supra's to ( I don't see why not thou) we tend to run High compression on our turbo hondas for spool time. I have seen 10:1, 9:5:1 and still run 25psi. Jus t remeber c16 baby.
 
Dziuggy

Dziuggy

eurotrash
Mar 31, 2005
169
0
16
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san francisco, ca
dziuggy.com
turbodriz said:
I hate to be the odd ball and I do not know if what I am going to tell you can apply to supra's to ( I don't see why not thou) we tend to run High compression on our turbo hondas for spool time. I have seen 10:1, 9:5:1 and still run 25psi. Jus t remeber c16 baby.
Click to expand...

yes you can run high compresion on 7m (like Gena's red car) but the motor has to be built very well. lower cr is much safer and you can make it up by only couple pounds of boost
 
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