The V8 Owners Forum -

Having cooling issues with the 3.5 in my hotrod. New waterpump, thermostat and capillary guage, electric fan behind the rad. There's no heater jyst a bypass loop. Car gets too hot too quick even with the fan on it won't last more than a few miles.
Header tank feeds in on a T just below the waterpump and the tank starts off near empty. When the car gets hot there's a steady flow from between the carbs into the tank and a fair few air bubbles.
Firstly is my set up correct ? Someone suggested that the manifold tower to expansion bottle pipe should go in below starting water level as it could draw air back in. Certainly when it cools down it looks like it's maybe drawing air back.
I know the radiator is too small and thin but space is extremely limited and the front grill/bonnet has already been butchered to get that in. My thoughts are a similar size but much deeper core rad with the fan moved in front to make way.

How hot is the radiator when you stop?

I would suggest you have perhaps made the perfect bypass via your non heater circuit. Tryclamping that hose with mole grips or similar which will then force all the coolant through the radiator

How big is the radiator, in the GT40 it was 30 x 60 cm and 3 rows thick which worked ok

And finally what fan? Some are hopeless and others create a hurricane!

Ian

Hi Ian, it gets to the point where the expansion bottle is full (starts off near empty) and the guage is towards 100. I've only driven the car a few miles a couple of times as it runs like a coo, 2nd time I was too late to stop and it overheated. A lot of its running has been in the garage as I try and sort out the carbs timing and idling. There it keeps getting hotter after the thermostat opens despite the fan being on and a household fan blowing air too.
The current radiator is 460x430x55, not sure of the fan.
Don't really follow how clamping the 6" heater loop would do anything
My biggest concern is the air in the pipe from the tower to expansion, I'm going to try putting a one way valve on it .
Engine compression is good and the oil is clear but I've ordered a sniff tester incase it's gases going to the tank rather than recirculated air.

Ok
In the radiator circuit the biggest restriction is the thermostat.

If you look at the actual open size it is actually not that big

Now the non heater circuit is around 15 mm diameter so not a whole lot smaller than the thermostat.

Water flow will take the route of least resistance so an amazing amount will be going around the non heater circuit and missing the radiator completely.

Back a good few years back MGB’s would overheat but only in the winter when people turned on the cabin heater and as the water took the heater route less went through the radiator and car overheated.

You could make a restriction and fit it in the non heater circuit….I did this with 15mm plumbing bits……piece of copper pipe and a copper stop end soldered in place…..and a 2mm hole drilled in it to allow bubbles to pass but not have an open circuit


Radiator size mine 30x60 =1800 yours 43x46 = 1978 is actually a bigger area


Another thing is your fan wired to move air from the front to the rear…….amazing how often they blow forward and stall the air.

And anything above about 20mph the natural airflow will outdo what most fans move

Thanks Ian, that explanation makes sense, I should be able to cobble up some sort of restrictor. I'll have a look to at the fans performance, it certainly doesn't sound particularly strong.

I've now come to realise that the bleed from between the carbs is a big part of the issue as it just allows warning coolant to go straight to the expansion bottle filling it up. That means no coolant around the top of the manifold, thermostat or top hose so nothing pressurises apart from the bottle. I bled through that pipe then closed it off.
Now the overflow bottle fills slowly from the pipe T'd in just below waterpump and the car takes longer to get too hot. It still fills the expansion tank to a worrying point though and I switch off. This is all at idle so no natural air flow other than a house fan heater.
I have a bigger radiator on order and once I fit that and work out how much room I have I'll upgrade from the 80w fan I have.