3.6 dome size

[kirkf]

Can someone tell me how many CCs a standard 964 piston dome is?
Also what is a typical cylinder head volume of a 964 head?

Thanx
Kirk

[ChrisBennet]

I measured 85.2cc head volume on a 3.6 I rebuilt.
I measured 36.7, 42.4, 38.9, 33.0cc for dome volume (for the same piston). Obviously, with such a wide range of numbers I wouldn't put to much stock in their accuracy.
-Chris

[kirkf]

Thanx Chris,

Here are my calculations for my compression ratio:

COMPRESSION RATIO = (V1 + V2 + V3 - V4) / (V2+V3 - V4)
WHERE:
V1 = SWEPT VOLUME
V2 = DECK HEIGHT VOLUME
V3 = CYLINDER HEAD VOLUME
V4 = DOME VOLUME

3.6 L
BORE = 100mm
STROKE = 76.4 mm

CYLINDER VOLUME = PI/4 * BORE² * STROKE
CYLINDER VOLUME = 0.7853982 * BORE² * STROKE

V1 = SWEPT VOLUME
V1 = BORE² * STROKE * 0.7854
= 10cm² * 7.64cm * 0.7854
= 600.05 cc

V2 = DECK HEIGHT VOLUME
V2 = BORE² * STROKE * 0.7854
= 10cm² * .095cm * 0.7854
= 7.46 cc
(.95 mm DECK HEIGHT MEASURED DURING REBUILD LAST YEAR USING CRUSHED SOLDER METHOD)

V3 = CYLINDER HEAD VOLUME
V3 = 89.57 CC (MEASURED WITH BURETTE)

V4 = DOME VOLUME
WITH PISTON 1cm BELOW DECK, THEORETICAL VOLUME (WITHOUT DOME) WOULD BE:
= BORE² * STROKE * 0.7854
= 10cm² * 1cm * 0.7854
= 78.54 cc
MEASURED VOLUME USING BURETTE = 44.4 CC
DOME VOLUME = THEORETICAL VOLUME - MEASURED VOLUME
= 78.54CC - 44.4CC
= 34.14 CC

COMPRESSION RATIO = (V1 + V2 + V3 - V4) / (V2+V3 - V4)
= (600.05 + 7.46 + 89.57 - 34.14) / (7.46 + 89.57 - 34.14)
= 662.94 / 62.89
= 10.54:1

The deck height was measured when I rebuilt the engine. Although the pistons were machined to lower compression, I dont see how the deck height measurement would change, as only the dome of the piston was machined.

The problem is this:
My target compression ratio was 9:1. I had SoK machine the domes of the pistons down. Since then I have had the piston tops coated with thermal barrier, and the side skirts coated with dry lube.

Now I am doing a compression check and finding numbers that I would have expected if the pistons were stock.

Obviously 10.5:1 is too high for my supercharger project.

Kirk

[kirkf]

The 964 heads apparently are typically ~90cc from factory, so my results are in line with that.
I decided to redo the measurements for the piston dome. The results are:


Machined Piston: 33.068 cc dome volume
= 10.38:1 CR

Stock Piston: 36.242 cc dome volume
= 10.87:1 CR

Obviously SoK blew the machining, and only took off .5 points of compression, instead of the 1.5 points they were supposed to. (they said 9:1 was their target CR )

Result: I have 6 beautifully machined pistons with thermal barrier coated tops and dry film lubricant to use as paperweights.



Kirk

[kirkf]

Anybody have any ideas on how to fix this?

Kirk

[Steve@Rennsport]

Kirk,

Removing the coating is not difficult; one VERY carefully tapes off the top ring lands and carefully bead blasts the coating off. I use minimal air pressure to accomplish this.

The real issue is more complex. I've never machined a lot off these pistons and I am unsure one can safely remove enough material to hit your target CR, considering forced induction usage. I don't have one here to measure the dome thickness, but you might need different pistons to finish your project.

[kirkf]

Thanx everyone for the replies.
I'll have to decide whether I want to pour more $$ into pistons with 40k on them or just buy new aftermarket pistons with the correct CR.

Sigh.

Kirk

[Steve@Rennsport]

Kirk,

If you want to use a custom piston to finish this properly, I'd be happy to help.