As the title states: what is the approximate static compression ratio of a B18C1 head, block and rods with JDM Integra Type R pistons (13020-P73-J00)? Assuming stock stroke, 81.5mm bore and no extensive milling done to the head or block.

I've been searching through decade old threads and I always come across conflicting information. Some people say its 11.3:1, others say its around 11.6:1 to 11.7:1... even various calculators I've tried give different results.
I also have seen people claim that P30s from the JDM B16A yield a slightly higher compression, and vice versa. I'm not sure who to trust honestly. Maybe there was a sticky or some obvious thread I missed but maybe I'm blind because I haven't found it. I'm aiming for having 11.7:1 in my GSR, and I was initially considering going forged with Wisecos with Brian Crower lightweight rods, but I'm heavily reconsidering due to oil squirter clearance, more machine work involved, and most importantly: money. Can someone confirm the true static compression ratio of JDM ITR pistons in a GSR?

 

According to the Zealworks calculator, your static compression would be 11.56:1 with a stock height (unmachined) GSR head and block and a .030" head gasket. If you use a stock OE .026" thick head gasket, you would increase compression to 11.70:1. If the block or head have been machined, then the compression will get higher. The JDM P30 piston DOES have a bigger dome on the top of the casting, however, the installation height is slightly shorter than that of the P73-00 piston and will actually make slightly LESS compression than a P73 piston will. Now, IF your cylinder head or block has been milled, you might consider the P30 piston instead of the P73 piston because it will sit deeper in the block and provide greater valve clearance... which means you have a safer rotating assembly and still make good compression.

 

Thanks for the well-informed reply. That definitely clears things up. In hindsight, I think what I saw was people confusing/mislabelling JDM ITR pistons as having 11.3:1 in a GSR, when in reality they were really referring to the USDM ITR pistons. But I saw so much of that happening that it confused the hell out of me... And this may come off as a newbie question, but would basic resurfacing of the head and/or block for the sake of cleaning/making them level constitute as milling? I'd think that I could avoid piston to valve issues and the need to clay the motor even with my Buddy Club Spec IV cams if I used P73-J00s... or are the lobe profiles so large on those that getting my motor clayed would be necessary regardless of piston/dome?

 

Your focus on a few tenths of compression is misguided. Removal of any material from either the block or the head casting IS considered "milling", and this is a critical error when using large camshafts or oversized valves. If you must mill a block or head, then the amount of material removed should be replaced by a head gasket with an equal increase in thickness. The key is the science here... reducing deck height or head thickness will INCREASE the chance of valve to piston contact because the valve stem center plunges closer to the edge of the piston, and thus, valve margin contact with the piston top increases because the valve relief in the piston dome is no longer in the proper place.