will stock radiator cap cause overheat/boilover issues?

I've got a '66 with a 289. Put in a larger cam & hi-flow water pump + an aluminum dual-plane 4bbl intake (yes, the manifold gaskets are installed correctly) under a Holly 600 carb. Otherwise block is stock bore with heads off a '74 302,
Are the head gaskets installed with the front at the front?

also have an electric fan that cycles on at 180 degrees
Not as much airflow as you'd get with a thermal clutch fan, most likely.

pulling air through a 3 row aluminum radiator
Good.

and auxillary transmission cooler for and aod.
Probably more than you need. Is the cooler on the inlet or outlet?

Not that it helps operating temp., but dropped to a 160 degree thermostat.
Get rid of it and put in a proper 190°.

Operating temperature stays around 180/190 untill engine is under load then begins to climb to 210 then 220 and boils over.
Boils at 220°? Your cap is holding almost no pressure, then. It shouldn't boil until something like 250°F. Or is it just streaming out the overflow hose?

Takes about 10-15 min. for temp to come back to 180/190 when engine is not under load. Hi-flow water pump is new and is operating well-maybe too well? Could the coolant flow rate be too fast?
I don't see how.

I've heard that if it's too fast it doesn't draw enough heat out of the block as it passes through the water jacket.
I've always thought that claim a bit silly.

Also, I'm using a stock radiator cap on the aluminum race radiator-is that sufficient?
It's a pressure release cap. The speed of the car or the grooviness of the radiator doesn't matter.

How high are you filling the radiator? When cold, the level should be 1" below the filler neck. Any more, and it'll puke out at full temp even if the system is perfect. The stuff expands when hot.

My two thoughts are either your running lean under load or your timing is off.

gmeinholz said:

Sorry for the lengthy response delay, very busy these days. The heads were installed by the previous owner and never overheated until I made the other mods. The electric fan moves a LOT of air. auxillary trans cooler is plumbed post radiator trans cooler and is mounted on front of radiator. Coolant level is never higher than an inch under filler neck but is lower than upper tank (just under a quart) when cooled down. Boilover starts with overflow but often times comes out of cap as well. What should operating temp for this motor be?

Click to expand...

Hi,

I would relocate the tranny cooler out of the air stream of the rad. Next, I would add a coolant recovery system, then, a 8-10lb rad cap, if non-AC.
I've never been a fan of a rad spilling it's contents, then, replacing it with air as it cools. Air does not absorb heat, coolant does.
Happy Motoring!

you are not saying what electric fan you have, at least diameter and how many amps it takes. make sure correct rotation is set. I would go with 16 psi cap too. How do you measure engine temp? OE gauge, afternarket mechanical/electrical)?

.....I'll add, if you do go to the higher rated cap, check all of your hoses and their connections.
Happy Motoring!