Cooling has always been marginal on my 70 big block. Engine is bored .60 over, with a blueprint solid lifter cam 10.25 : 1 compression - tri-power - side pipes.

10 years ago, I added a 7 blade fan w/heavy duty fan clutch, added an electric fan to the radiator and had a custom 30 quart 5 core radiator installed. took great care in sealing up all "leakes around the shroud to force all air through the radiator. Also widened the cutouts under the nose to bring in more air. Also removed the guts out of the thermostat bu left the core in place to keep coolant in the radiator longer for additional cool off . Also have a 26 lb 231 degree pressure cap on radiator and a later model recovery tank. Running 43RT plugs and 12-14 degrees BTDC and an ingitor breakerless ingintion.

I do have the headlight black covers installed and a front license plate in place.

After all this work, the car has cooled reasonably well for the last 10 years- usually in the 190 - 200 range on summer days at speeds up to 70 MPH.

4 weeks ago I replaced the 3:08 rear end with a 3:70 rear end. I also had a major tuneup done to include adjusting valves and recallibrating carbs.

Since this was done - the engine temp just climbs. It goes to 210 fairly quickly and then with both stop and go and cruising it will slowly climb to 235+ and does not go back down. After stopping the pressure over 231 causes the cap to release and boiling coolant fills the recovery tank.

The water pump seems to be pumping OK - no leaks ....

Any ideas? could the added RPM from the 3:70 rear end add enough detonation heat to destory the cooling balance? any suggestions to correct??

Thanks

 

Changing to a 3.70 rear will increase the friction loss due to the RPM change. A little. Shouldn't be a big increase.

More likely, the tune up changed the ignition timing or the carbs were leaned enough to raise the temp.

I have a 540 in my 75 with a 3.55 rear and am running a Dewitts radiator and dual Spal fans. Haven't had a temp problem of any kind so far. (knock on wood)

Langg

 

It's counter intuitive, but your fan(s) are blocking airflow at highway speeds.

My setup (I also have a 454, about 520hp or so):

A be-cool 2-row aluminum radiator, about 15 years old. Spal dual electric fans. No mechanical fan. 180F thermostat. I converted to the later overflow tank system rather than expansion tank using a '78 plastic tank.

The fans are wired to run half speed whenever the engine is running and switch to full speed at 215F.

No front plate, but the headlight shields are installed.

It cools amazingly well. I drove it to work last week in 92F weather, AC on with some traffic and it never budged off 180F, the fans never switched to high.

I think you would find better cooling if you ditched the mechanical fan and went with the spals. They have rubber flaps that open whenever the air pressure in front of the radiator is higher than behind, like driving at highway speeds.

I also think you should consider an aluminum radiator. A 5-core brass radiator will not let very much air through it.

That's if you don't find anything wrong with your tune, of course. Retarded ignition timing or a lean carb will also make it run hot.

 

I think it may be your tuning change too...maybe running a little leaner generating more heat.

My '70 has a 555 w/ 10.3:1 & 3.83 rear and I have no cooling issues using a DeWitts twin Spal and a Stewart highflow waterpump.

Check your T-stat too by pulling it and doing the old stove top test with a thermometer to see if it's really opening where it's supposed to. Double check the top radiator shroud seal too, don't want air moving over the top of the radiator.

 

Stewarts hi flow pump and a pressure balanced tstat.

Check your tuning too.

 

with noonie. also personally I'd be running a 190-195 T-stat

 

I don't have a BBC, but giving the situation I'd guess that the mechanics retarded your timing. I mean if it worked before, I wouldn't figure that a gear change would heat the engine up that much.

 

Are you running vacuum advance off of full manifold vacuum? You should, if you aren't. Definitely some cooling to be picked up there.
Re-check your initial timing with the vacuum disconnected and plugged, it should at least be the 12-14 degrees you had before.

 

My 70 LS5, same gearing, 3:70, behaves about the same. Temp stays around 210 when cruising back roads. I noted it tends to climb slowly when driving on the highway for extended periods. My assumption has been the radiator is marginal. The PO had it recored just before I bought the car, early 80's. I don't know what core they used.

My T-stat is 180. I tend to doubt the ignition timing is retarded, I get detonation when I push it hard.

 

This.

Check your timing and vacuum advance.