964 engine rebuild 11.3:1 compression

[fstockcarrera]

I'm beginning a rebuild on a 92 964 race engine. I would like to build to 11.3:1 compression. I appears that we can not get there just fly cutting the heads. We are starting with virgin used heads that will completely be rebuild will competition springs and new Mahle p's&c's.
Question
1. Can we cut the cylinder and if necessary machining the piston domes to get 11.3:1 without compromising the piston dome or ring land strength or integrity.
2. what deck height is reasonable for this application
3. Will the chain tensioners be able to accommodate the
change in height of approximately 1mm
Thanks Bob

[ChrisBennet]

I'm tearing down a '91 964 with a bad piston today. I might be able to slice the piston in half for you if you want to see how thick the piston is in places. I'm curious myself. Let me know.

Why not stretch the rods and address the deck height issues if any?

Did you calculate how much you need to raise the piston to go from stock (10.4:1?) to 11.3?

-Chris

[fstockcarrera]

Hi
It's much easier to cut the jugs.

I would be very interested in the dome thickness.

A have a rough idea that every .0010 gets you approximately .2 in compression.

Bob Scotto [

[Wayne 962]

If you're going for custom compression ratios, and you don't want to worry about a whole mess of other problems (cutting heads, chain lengths, etc.), then I recommend a set of custom pistons from JE...

-Wayne

[fstockcarrera]

Thanks Wayne but in PCA club racing I don't have that luxury.
Will the tensioner handle up to .0040 decrease in height and a ultimate deck height of probably 1.00 mm
Thanks I'm awaiting your book
Bob

[ChrisBennet]

Sorry Bob, the piston looks too mangled to be of use.




BTW: What PCA class allows a 9 point compression change (10.4 to 11.3:1) but not different pistons?
-Chris

[fstockcarrera]

Yikes! What happen? The Factory spec for the 964 c2 is 11.3:1 even though they are not in reality. Have we met @ LRP. I run the Tag Heuer 964 cup. Bob

[ChrisBennet]

Originally posted by fstockcarrera
Yikes! What happen? The Factory spec for the 964 c2 is 11.3:1 even though they are not in reality. Have we met @ LRP. I run the Tag Heuer 964 cup. Bob

The factory figures are way off. Measure your head and piston volume, calculate the compression ratio and you'll see.

I'm not a racer but I would think that any compression ratio increase large enough to make a difference would also make the motor non-stock.

I might have met you at a DE event in the past. White '86 Carrera. Last year I started wearing blue devil horns on my helmet.

-Chris

[fstockcarrera]

I agree the factory #'s are way off, but
I'm allowed to build to the factory spec. I don't have my #'s in front of me today. Bob

[ChrisBennet]

Originally posted by fstockcarrera
I agree the factory #'s are way off, but
I'm allowed to build to the factory spec. I don't have my #'s in front of me today. Bob

The factory spec for 964 cylinder heights is 82.750mm (group 5) or 82.770mm (group 6) so if you have group 6 cylinders now you could shave them to group 6 spec and pick up .02mm there. The spec allows refacing the heads by .1mm twice so you pick up .2mm there. That only adds up to a maximum of .22mm. That's going to be far short of your 1mm target - if you want to remain in spec that is.
-Chris

[fstockcarrera]

I 'm assumming a deck height of .0039 or 1.00mm.?? Have not recieved the new P's&C's yet, minus -.20 would give you a deck clearance of .0031 which seems low. The head machinist does not to lower the clearance below .0036 base on a 1mm deck height. I'm using standard rods and new Mahle P's&C's. I really need to measure the deck height. Do you have an idea what it should be??
Thanks Bob

[ChrisBennet]

Originally posted by fstockcarrera
I 'm assumming a deck height of .0039 or 1.00mm.?? Have not recieved the new P's&C's yet, minus -.20 would give you a deck clearance of .0031 which seems low. The head machinist does not to lower the clearance below .0036 base on a 1mm deck height. I'm using standard rods and new Mahle P's&C's. I really need to measure the deck height. Do you have an idea what it should be??
Thanks Bob

I don't know what it should be. I don't know what the minimum deck height should be. I've been told that you don't want it greater (yes, greater) than 1mm/0.04" because it increases the chance of detonation in the resulting pocket along the outer edge of the piston.
-Chris

[Dan J]

Bob,
You can easily get there by cutting the cyl's. You do need to assemble the lower end first to figure your deck. You also need to have your heads done before you measure your chambers. All the motors are a little different so you have to wait to do the math. Leave yourself a little wiggle room because the "whistler" always reads a little higher then your calculations
Dan J

[john_colasante]

Good stuff Dan.

[KobaltBlau]

Originally posted by fstockcarrera
What happen?

Someone set up that motor the bomb!

[ChrisBennet]

Originally posted by fstockcarrera
What happen?

Sorry, I missed that question. I'm still tearing the motor down so I'm not sure. I suspect the previous owner juxtaposed the piston and valve concepts.

Originally posted by KobaltBlau
Someone set up that motor the bomb!

All your base are belong to us!

-Chris

[john_colasante]

Originally posted by ChrisBennet
All your base are belong to us!

I'm glad I'm not the only one who still finds that somewhat funny.

[KobaltBlau]

Originally posted by ChrisBennet

All your base are belong to us!

-Chris

exactly! I still think it's funny too... we get signal!

[john_colasante]

C'mon guys, what are the secrets to getting 11.3:1 in a 3.6?

[ChrisBennet]

Originally posted by john_colasante
C'mon guys, what are the secrets to getting 11.3:1 in a 3.6?

Different pistons would do it.
I think what is happening is an assumption was made that the factory compression "spec" was just a little high and that a few tweaks could bring it up to "spec".
In reality, the factory compression "spec" is a lot higher than actual (~10.4:1 actual vs 11.3:1 factory).
I think one would need to ignore all the other dimensional specifications to modify cylinders/pistons/etc enough reach the factory compression "spec".
IMO, I don't think any racing body is going to accept a motor with 9 points higher compression (than a truly stock motor) as "stock".
-Chris