Thermostat Mod

[Forrest Gump]

Here's what I've done to my thermostat to resolve a problem....



After rebuilding my engine and making it a 383, I had been getting a puzzling temperature issue. As the engine warmed up the temperature just kept climbing higher and higher, up to an alarming 220 -230 degF. Just as I was about to switch the engine off, it would start to drop back to a more comfortable level. When driving, if I stepped on it even for a short time, the temperature would spike again. I've got a big electric fan cooling a stock radiator. ally heads and a high flow pump.

My initial thought was either a sticking thermostat or an airlock. I removed the 180deg thermostat and confirmed that I had already got a 3mm hole drilled in it to prevent an airlock. I did a test in a pan of boiling water that showed it to be opening fully.
As an interim measure I tried running the car with no thermostat and sure enough, no spiking temperatures but instead generally running too cold. Temps a bit all over the place, suggesting that a thermostat was needed to regulate it.

I tried a new 160 thermostat, again with a 3mm hole drilled in it to prevent an air lock. Result was exactly the same - temp shooting up alarmingly high on warm up and also when stepping on it.

I decided to to try an idea - I drilled a series of 5mm holes around the thermostat flange. Theory being the holes will allow an amount of water flow even before the 'stat opens and then when it does open, throughput is increased further. Result - no more temperature spikes. The engine still warms up normally and then stabilises at 170. If I gun the motor the temp will rise but the fan will bring it back down.

 

[Oneball]

What water pump have you got?
On a SBC the bypass is in the block. There should be a small hole in the one leg of the pump.

 

[Forrest Gump]

I have questioned the pump as that is the only other change to the cooling system. It’s a “proform“ pump from Summit, not sure now that it‘s fitted if it has the small hole. So you think maybe I’ve compensated for a possible lack of bypass by drilling the stat?

 

[Roscobbc]

Had similar issues to Andy re. temperature 'spikes' - looking for a 160 degree thermostat that I sure was knocking around somewhere I found an ancient Nissan one in my son's stash of crap from when he lived at home years ago and messed around with turbo Nissan Sylvia's - it was an 'adjustable' Nissan 185 degree thermostat. Adjustable in as much as the central 'pin' is threaded allowing the central disc to be rotated in and out on the thread and adjusting the water flow at low temperatures - and effectively offering the same flow 'by-pass' as Andy's modification.

 

[Oneball]

Yep the holes will be fine. The bypass hole is only about 5mm in diameter. It just allows hot water to circulate to both sides of the pump and stop air locks.

Drilling holes is what I’ve done to mine as the Moroso water pump has no bypass. People have been doing it since someone invented the thermostat without issue.

 

[Forrest Gump]

That’s interesting...I wish I’d asked you a while ago! I’ve found the solution without really understanding the cause of the problem.
I don’t run a heater or bypass hose so if the pump has no bypass hole then that makes sense now.
I’ve not seen anyone else do this mod though. Most comment that if you drill anything more than a 1/4” hole in a thermostat then you might as well not bother having a stat at all.

 

[Oneball]

That’s interesting...I wish I’d asked you a while ago! I’ve found the solution without really understanding the cause of the problem.
I don’t run a heater or bypass hose so if the pump has no bypass hole then that makes sense now.
I’ve not seen anyone else do this mod though. Most comment that if you drill anything more than a 1/4” hole in a thermostat then you might as well not bother having a stat at all.

Only the BBC have a bypass pipe as stock. It’s in the block on a SBC. If you bin the thermostat entirely you have two issues. 1 it’ll take a long time to warm up, bad for economy etc. 2 you end up with a very large hole where there shouldn’t be one. Not quite so much an issue on a SBC but on some engines it causes flow problems and overheating a-series is an example.

 

[Forrest Gump]

Yep, cooling is a vast topic. I understand all that about too much flow results in the water not staying in the radiator long enough to lose its heat etc.

I wonder why it is that the top and bottom radiator hoses are much larger diameter than the thermostat‘s aperture.

I really thought that my stock radiator would struggle to cool my modified engine but in fact it copes okay. I can go on a mixed road drive and not need to switch on the radiator‘s fan as long as I’m not stationary too for long. I took the car to work on the hottest day of this year 35 deg - just to see what would happen and we made it okay with town traffic.

Fan when needed, is a powerful Kenlowe, clipped straight onto the radiator, no shroud. It seems to work. I did try a full radiator shroud but found it pushed the engine temperature up at general road speed - so must have been reducing airflow through the radiator. It probably made the fan cool more effectively but also made the car run hotter generally so counter-productive. I took it off.

 

[Roscobbc]

Fan & shroud not an issue on stock engines as fan (viscous) is running all the time. Its only when shroud is removed that problems start. There seem to be no GM figure available for the CFM flow of a stock set-up. Not convinced by various aftermarket twin fans assemblies that bolt to the rear of the radiator and effectively 'block' free passage of cooling air where the fan assembly (outside of the swept fan area) is a 'dead' area in terms of airflow. These set-ups seem to trigger fan operation far more often than perhaps would be the case if mounted in some form of shroud/plenum with the fan sitting away from the radiator surface.