.030 Over & .015 mill??

I can't tell you the compression ratio that would result, but I can tell you that I did a 0.030 overbore and a 0.010 head mill on my engine, and 93 octane has never been a problem in the 7 yrs/16k miles I've driven it. That includes the round trip journey to Nashville and back for the 40th anniversary bash.

Modern replacement pistons are desighed so that the compression ratio is not changed from stock. An overbore will not change the compression ratio. The milled head will have little effect on the final compression ratio. You have little to worry about. Put it together, drive it and enjoy the feeling!

Your compression should increase slightly with an overbore. Compression is determined by many factors such as bore, stroke, deck height, size of valve reliefs, ring height, combustion chamber, gasket thickness. Those are the basic ones to get you close. There are many calculators online to check this.

http://www.race-cars.net/calculators/co…calculator.html

I will agree with Murf; boring doesn't change compression in the case of .030 stock hipo motor.

When they manufacture a replacement piston for an engine such as you have, the compression ratio is known and the new piston is configured so that the overall compression ratio is not changed. While it is true that all things equal an overbore will increase the compression ratio since it is volume at tdc over volume at bdc, the people who make pistons know and understand this and compensate for the additional volume. All will be well, and if you could run it before it will be just the same compression ratio. Do not forget that math will only get you in a bit of a mess with this, because in truth you will not begin to compress anything until the valves close, and that does NOT happen at BDC. The camshaft in your engine determines when compression begins, and the more agressive the cam grind the later compression begins and thus the lower the actual compression ratio will be. If you do not hammer the engine you can run a hipo cam and 10 1/2 to one pistons on regular gas with no ill effects. When you run at full power things will suffer with low octane gas. Build your engine and enjoy it.

This may be true with some OEM pistons, you really cannot generalize though. Some replacement pistons will have dual or single valve reliefs. Ring height and valve relief size are not the same from one piston company to another. On a street engine, small differences will not affect performance. When it becomes a really big deal is in sanctioned racing where there are limits in compression ratios. Just as an example, I had forged 4.040" flat top pistons from BRC, SRP and Diamond. Not only were the piston designs very different, they all spec'd slightly differently in all areas. Performance piston manufacturers really are not out to copy an OEM piston. At the very least, aftermarket performance pistons have much larger valve reliefs to account for the larger valve heads on the market.

You could spec a thicker head gasket. I went the opposite way with a thin .028" Detroit Gasket head gasket to get my compression to where I wanted it. You may find some as think as .053". Typically SBF head gaskets are around .040".

Here is another link.

http://kb-silvolite.com/calc.php?action=comp