A failed oil pump and/or a failed oil filter have seriously damaged my very strong running '97 engine -- it is fixable but at a rather high cost. I found and purchased a darn good 82K-mi motor from a 2003 AutoTrans Miata that I picked up at a very reasonably price to use as a replacement for my '97 engine. I bought the 2003 engine minus the VVT cylinder head which the salvage dealer wanted to sell separately to another customer and of course that head would not work with my stock ECU anyway. The head from my damaged '97 Miat engine is in very good condition and I would like to use it on the 2003 Miata block (after thorough flushing to clean out any fine metal particles).
I know the '97 head will fit and work on the 2003 block but one complication is that my '97 head was shaved 30/1000s which my race mechanic believes raised the compression ratio on the '97 engine to about 9.8:1 from the factory stock 9:1, and I have some concern that using that moderately higher compression head on the 2003 block that has 10:1 pistons may possibly cause valve/piston clearance issues and/or too high a compression ratio for use 91-octane unleaded gasoline which I wish to continue using because of cost a availability issues with higher octane gasoline where I live. I have checked the cam lift specs for the '97 vs the '03 cams and here is what I found: '97 intake/exhaust lift is 0.318/0.338 inches vs 2003 head of 0.370/0.350 inches suggests -- if I am interpreting this correctly -- some fairly significant extra valve/piston clearance (52/1000s for intake and 32/1000s for exhaust) with the '97 head and cams vs the 2003, and that could compensate for the 30/1000s head shave on the '97 head. Does that suggest adequate valve/piston clearance for my head swap?
Re my concern about excessive compression ratio increase of using the 9.8:1 compression ratio '97 head with the 2003 Miata block which has 10:1 compression pistons, I am not an engineer and therefore do not know how to calculate or estimate the compression ratio resulting from mating my shaved '97 head with the higher compression pistons in the 2003. It would seem that because the heads have been rather similar over the years in terms of the combustion chamber layout and the displacement of the two engines in question is the same, the volume of the combustion chambers would be more or less the same in the two engines as they came from the factory, but the combustion chamber volume would be lower in my '97 engine vs the 2003 because of the head shave on the '97. But other than the actual volume decrease that now exists due to the head shave on the '97 head, the compression in the two cylinder head would be determined by the piston, and without a head shave the '97 and 2003 heads would have pretty much the same compression ratio, about 10:1. But decreasing the volume of the combustion chamber on the '97 head would add some amount to the compression ratio simply because the same amount of displaced air/fuel in the cylinders would be compressed into a smaller volume, therefore using the shaved head on block with 10:1 pistons would add some additional amount to the compression ratio of my hybrid 2003 Mita block with a shaved '97 head. What would be your best "guess" as to what final compression ratio I can expect with the hybrid engine I am building?
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