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I've got a similar setup in my PV's motor. Well, B21FT dished pistons at least.
I think the typical B20E head that has had a few thousandth's taken off in a skim or two will have roughly 45cc combustion chambers*. To the best of my knowledge the dish on top of a B21FT piston is 12cc's. Add to that the bore (92mm) and stroke (80 mm) and head gasket thickness (varies, depending on type, and can't just measure a new gasket as they crush down somewhat, unless copper). Then measure the TDC piston to deck clearance accurately, and take those numbers to a calculator such as this: http://www.csgnetwork.com/compcalc.html
Which will tell you that (assuming a .034 'thin' head gasket and 0 deck clearance**) you may be around 10.3:1.
* - After 35 years, not much reason to assume a head is still in stock configuration anymore. Best to CC it yourself and see for sure. I cc'd the chambers on my very spiffy R-sport head and found *yikes* only 32.5cc's. Apparently it had been used on a race car and shaved, shaved, shaved. And then shaved a bit more. With the head off, take a little grease and lightly smear around the valve openings (to prevent seepage). Then tilt the head slightly (chamber facing up) and grease lightly around the edge of the chamber, and place a small piece of glass over the opening, leaving just a small gap at the top. Then use a syringe with enough CC's to fill the chamber in one shot (to avoid inroducing error by refilling the syringe) and fill it up, noting how many cc's it took. Repeat once or twice to make sure you get the same figure. Then check all 4 chambers to check for variances, just to be OCD about it. On my R-sport head I ended up using a carbide burr and taking out about 6 cc's of metal, unshrouding the intake, matching the sides to the 92mm bore, and trying to regain as much volume as practical.
** - Deck clearance is how much the pistons either protrude out of the top of the block or remain recessed within it at TDC. Typically a stock motor will have the pistons slightly recessed. They are unlikely to have 0 deck clearance or protrude unless they aren't stock pistons or the block has been shaved slightly. An important consideration in this area is the 'squish' factor. If you look at a cylinder head you will notice that it is a central depression in the head, surrounded by to protruding flat areas. When mounted on the engine, the top of the piston gets very close to these flat areas at TDC. If it gets *really* close (typically, in a static, cold measurement, no less than .030 inches clearance) then this sudden pinch at the sides of the chamber at TDC really *snaps* the air into the center, causing lots of turbulence and swirl. Turbulence and swirl is generally bad for airflow going into and out of a cylinder, but in the chamber, it is a very good thing, it helps promote a more even flame propogation, and prevents detonation. In many cases, a motor with higher CR and more squish will resist pinging better than a motor with less squish and lower CR. Sometimes people try to reduce pinging by using a thicker head gasket to reduce the cr, and end up just killing the squish by spacing the head out further from the piston and it pings worse.
For what it's worth, after working on it, my R-sport head still only had 38cc chambers, so the 12cc dished pistons were a necessity. Even then, that calcualtor shows it as having somewhere around 11.2 CR. But it doesn't ping. I'm using an Isky VV-81 cam currently. I really wanted to try a KgTrimning KG19, but they were out of stock when I was ordering parts. Ah well, based on my recent experiences with cams it might not be too long before I need a new cam anyhow...
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I'm JohnMc, and I approved this message.
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