Overbore and boosting

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Mar 17, 2026 | 02:05 AM

Sullyret

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Overbore and boosting

Hi guys I have a '90 miata that is turboed and a '99 engine I am putting forged internals and some other things into to then put into the '90 and eventually upgrade the turbo. I'm wondering what the maximum bore is for boost as I see most people go 84 or 84.5mm but supertech offers 85mm pistons at 9:1 compression which sounds great if the block can take it. Is anyone out there running F/I with 85mm pistons? Will I be able to pull this off or should I stick with the smaller pistons? I will be mainly on pump gas 93 when I can find it and 91 when I can't unfortunately. Plans for a flex fuel sensor and moving somewhere with e in the future.

Mar 18, 2026 | 05:15 AM

sixshooter

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Don't bore more than needed in case you ever need to bore again. Displacement gain is tiny. 0.2 psi of added boost would easily be more than the difference. Wall thickness prevents hot spots, too.

Mar 19, 2026 | 09:01 PM

Blkbrd69

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From: Orlando, FL

Stay as small a standard bore as possible for future cleanup and reliability.
Sonic test block.

Mar 20, 2026 | 01:33 PM

Jesse99James

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From: Minneapolis, MN

This was exactly my thinking on my MSM engine rebuild, plus I found a set of NIB, 83.5mm forged pistons on eBay from another Miata owner at a price I couldn't pass up
.