CR Variables

[oldeskewltoy]

stock chamber is 36cc. Uncut head thickness is 116mm (4.566"). .004" cut = about 1/2cc lost

[Dave W]

I think Dan missed the metric to inch conversion.
.010" or .25mm CUT = about 1/2 a cc reduction in head volume.
Dave W

[Bean]

Nah he's right 0.004" is 0.1mm so
π×4,05cm×4,05cm×0,01cm
Which equals about 0.5cc

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[Dave W]

Yup. I stand corrected. Maybe I can not convert to metric :-(
Dave W

[oldeskewltoy]

Back a few years ago(8/14) when I designed a high comp piston for a client, I sent a bigport, and smallport piston into JE Piston to help in designing my pistons. I asked them to measure the pistons, and I got the following back from them....

-3.8cc 9.4:1

+0.25cc 10.3:1

The problem with this... is if you use the same variables for calculating compression ratio... you don't end up with 9.4:1, or 10.3:1

[Project D]

The 9.4:1 pistons may have a little bit more deck clearance than the 10.3:1 ?
IF you take:
bore: 81mm, stroke: 77mm, HG: 6.60cc (82x1.25), CC: 36cc, deck clearance ?

9.4:1 -> with a piston volume of -3.8cc, you get 0.15mm deck clearance. (9.411)
10.3:1 -> with a piston volume of +0.25cc, you get 0.05mm deck clearance. (10.312)

Or you can do the reverse formula:
bore: 81mm, stroke: 77mm, HG: 6.60cc, deck clearance: 0, CC ?

9.4:1 -> 396.78/8.4-6.60= 40.63, with piston volume of -3.8cc -> CC= 36.83cc
10.3:1 -> 396.78/9.3-6.60= 36.06, with piston volume of +0.25cc -> CC= 36.31cc

[Bean]

iirc when you did my head - you ended up with 36.75cc not sure how much you removed but it's safe to assume a stock head is less than 36.5 at least for smallport. Though I think small and big are the same in this regard

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EDIT: my head was brand new

[oldeskewltoy]

Stock 16V chamber is 36cc, besides my measurements, it is listed in the TRD Bible....



Typically I add somewhere between 3/4cc and 1cc to a chamber when I deshroud it. On your head (Bean) since it was new, I likely didn't have it machined (forgotten). Once the head get machined, the volume drops.

I did some checking of compression heights after answering above... it seems both the largeport, and smallport pistons use a compression height of 30.5mm. In addition, the compression height on conventional rod Atlantics is often 1.201" (30.505mm).

While rods do change, the center to center measurement is 122mm(4.803") for all the rods - rod length appears to be consistent.