[Bob Arthur]

I have heard that an engine that is overbored too much can cause it to overheat. My 389 is bored out to 400 cu in. I have also heard that a newly rebuilt engine will run hot, how much, how long? My engine has 250 miles on it and I am running 230 plus at speed (idles at approx 190). Engine specs below:
1966 pontiac GTO
389 bored to 400
Zero decked
Dished pistons (approx 9 to 1)
093 Heads (67 cc)
Comp Cams XE268H (.477/.480 lift, 268/280 duration)
Water pump (8 hole cast impeller) divider plates at .040
Rodney Red Radiator (spring in lower rad hose)
TH350 with external transmission cooler
Edelbock 1406 (main jets 1 stage rich)
Initial timing 10 degrees btdc, 32 degrees all in, 50 degrees with vacuum advance

[TheSilverBuick]

Is the fan shrouded? And if you can pull 10* off the vacuum advance with out touching the other advance settings to see if there is a difference. Or simply unplug your vacuum advance and drive it that way and see if it runs cooler while driving as a test. I can see 50* being over advanced at cruise (keeps more heat in the engine versus out the exhaust). Is your vacuum advance hooked to manifold or ported vacuum? With 18* of additional advance, if hooked to manifold it could be over advancing at idle too.

[Bob Arthur]

I have a shroud. I am using ported vac. I have run it without vav adv hooked up, same result.

[PontGuy]

How quickly does the temperature jump up when going from idle to cruise? 190 at idle jumping to 230 plus during operation is a big jump, and my street cars have always had more problems holding temp at idle than cruising, so seems odd. (Exceptions- dirt track race cars or radical builds that have a heavy heat load.) Don't know if boring that much can be a factor, doesn't seem like it should be. But by 250 miles the engine should be broken in good enough not to have a big impact on cooling. I've never really seen much of a difference in temps after the initial test drive though general performance can improve a bit as the engine loosens up.

Everything in your post seems fine to me. Timing isn't set up the way I would do it- I like more initial/total and less vacuum advance, but I don't think this would account for your problem. And if the car runs smooth with no hesitation or surging than I would think the jetting should be close enough. If it was a pump or a fan issue then I would expect to see higher temps at idle. But what kind of fan and clutch do you have?

Sounds like maybe a restriction somewhere either in the water flow or the air through the radiator that doesn't affect cooling much until the engine is running at RPMs. Maybe your thermostat isn't opening all the way? Does your car have AC or anything else that would restrict air flow through the radiator?

[Bob Arthur]

AC is not hooked up. I removed the condensor from in front of the radiator. I removed the thermostat until the problem is solved. I have seen posts that the thermostat is required for flow restriction but isn't this an old wife's tale? I am replacing the vacuum advance so I can reduce the amount of advance. I have come to understand that the fan has no effect on cooling above 45 mph.

[chrisp]

Try without a thermostat .