# 1966 389 GTO Tri Power with 670 Heads



## jgotzens (Jul 15, 2014)

I've owned this car about 8 months and bought it already restored. I noticed right away that I had 670 heads on it. The engine is a 389 built in June of 66. The car runs solid. No top end noise other than a little lifter noise. Good power and no pinging on 93 octane. I've seen a lot of talk on the forum that say these heads won't work on a 389, or they will, with some machining. Since the car is running good, I guess I have to assume whatever had to be doen to the heads was done properly. My question is should I leave it as is? Is there any reason to change them back to 93 heads or other original 389heads. Could these 670's cause other problems over time??


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## geeteeohguy (Feb 2, 2008)

The big issue with '67 and up heads is that the valves are located in a different position/angle than the '66 and earlier heads. The pistons in a 389 are relieved for the valve angle in the earlier heads. With enough camshaft lift, you will hit the valves with the pistons. Since your car runs fine, you have enough clearance, whether it's due to custom pistons, , or a mild camshaft profile (close to stock). 670 heads are far superior to 093's, and are about 72cc in chamber volume. The 093's typically are less than 70cc, which means you are more apt to ping on pump gas. So, the 093's would give less power and need higher octane to boot. I'd leave well enough alone.


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## PontiacJim (Dec 29, 2012)

The 389 bore size is 4.06". The 400 bore size is 4.12". It would probably make sense that your 389 was bored .060" over and stock sized 400 pistons used. This way you would have the correct valve reliefs in your pistons to match the 670 heads or any '67 and up head. But, unless you know for sure, I would just leave well enough alone.

The 670 heads are rated as an excellent head for the 400.


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## geeteeohguy (Feb 2, 2008)

Back in the day, it was common to punch a 389 out to 4.12 in order to use the cheap, easy to find std. bore 400 pistons. I built one that way in 1987. Only problem is, .060 is max oversize for a mid-'60's 389. No more rebuilds after that without sleeves. The 670 is indeed an excellent head, if you have the the right combo of parts to keep it happy on today's pump gas. Mine are in the garage on a shelf, as I haven't been able to keep them happy for the past 20+ years on California's lousy fuels.


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## jgotzens (Jul 15, 2014)

Thanks all for your replies. I'm going to leave well enough alone as suggested. It's obvious whatever combination of changes required to accomodate the 670"s was done since the car runs fine. I think it's safe to assume that if that were not the case it would be pretty obvious in the way the car runs / performs. Thanks again.


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