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· Premium Member
2,536 Posts
AC R25TS is the stock plug. The R43s are for the high compression engines like the 302, some 327s, 350-350HP, etc. They had 10.5:1, 11.0:1, and 11.5:1 compression ratios. If yours is stock, it probably has 8.5:1 or 9:1 CR.

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