My father built full out drag racing 351c's for decades. I have built several over the years and use a super simple cooling mod that actually makes 351c's actually run a little on the cool side.
Assuming radiator shroud is in place and a good radiator, fan, and water pump - OEM quality is fine.
Remove the thermostat:
Make sure the brass bypass is still installed - it is there for a reason - do not remove or block it off.
Source some thin wall .75 od pipe. We buy cheap aluminum pipe at Lowes.
Cut a short section of this pipe so that when it is inserted and bottomed out into the block below the brass restrictor it sticks up of a 1/4 to 1/2 of an inch above the hole in the restrictor.
Insert this short section of pipe into the restrictor - I have never epoxied or sealed the tube into the restrictor as it is "loose interference" as is. We have also ever so slightly squeezed the tube to make it barely out of round to make it a bit tighter but usually we just drop it in. Once everything is assembled it can not come out anyway.
Install the normal 351c thermostat that you get from any auto parts store - it will be the wrong one as no one makes the correct t-stat for a Cleveland. Even the "correct" and expensive Cleveland ones do not actually work correctly. They do not open far enough to actually enter the hole in the restrictor and block the coolant flow.
Put everything back together and see what happens.
How it works:
Without the additional tube the t-stat can not open enough to close off the bypass when completely open. The auto part store ones also have a smaller diameter tip and even if they opened enough to engage the hole in the restrictor there is still a considerable gap around the edges.
Enough coolant still bypasses the radiator that the engine runs hot.
This additional tube shortens the distance the t-stat has to open before it blocks the coolant flow through the brass restrictor. The tube also reduces the diameter of the passage so the commonly available t-stat will actually block the hole.
Once up to temp the t-stat opens and is now firmly bottomed out against the top of the added pipe. Most if not all of the coolant flow is now directed through the radiator.
This end result is that everything works as FORD designed it to.
Most cooling upgrades actually end up pushing the coolant through the radiator faster than it takes to radiate off the heat. The coolant needs to be in the radiator for a period of time to actually cool off.
We have a 71 351C, 4v closed chamber heads, 10.5 to 1 compression, fairly aggressive cam, completely stock cooling system, with the extra little pipe added.
It barely gets above the cold line even at full stop idle, in drive, with the A/C on. Just drove it on a 97 degree day and it never made it even close to the halfway mark.
Hope this helps!
- Paul
