Cross flow cooling systems

One of the biggest changes made in special building in the late ‘50’s was the move from traditional styled cars such as the Ulster with a vertical thermosyphon radiator to low bodied cars like the SPEEDEX 750 and Silverstone. The low bodywork needed a significant change to the cooling system with a low crossflow radiator, pump and head tank.

It’s perhaps best to start this off by describing the two different types of system.

Thermosyphon

We learnt at school that gases or liquids if heated become less dense and can rise through cooler areas by the buoyancy created. Similarly cooler liquids are denser and fall. This principal is used in thermosyphon cooling. Heat generated in the engine is picked up by the coolant and rises out of the top of the cylinder head progressing up the top hose into the radiator header. Coolant in the radiator is loosing heat and tends to fall to the bottom where it is lead back to the engine at the lowest point. Thus the system works well and responds to changes in load etc by using the laws of Physics as Scotty from Star Trek would have put it. However the system does need certain aspects to be maintained. The radiator top has to be higher than the water outlet on top of the cylinder head with as near a vertical top hose as possible, within reasonable bounds the higher the better, certainly no less than about 4 or 5” and not too far in front of the engine to keep the upward slant of the hose. If the water level drops such that the radiator and top hose are not flooded, the circulation will stop. Large pipes are needed to ensure the limited motion of the coolant isn’t restricted.

The standard design of Austin Seven radiator etc incorporated all these features. Some of the more highly tuned sports engines also had a water pump gear driven from the front of the engine.

Hot water rises through engine

Cold water falls down radiator

Side view

Cross flow systems

With sports car design and the 750 Formula cars evolving into lower sleeker styles in the late 1950’s, specials such as the SPEEDEX followed suit. This necessitated changes to the cooling system to incorporate a low bonnet line. The result was the use of cross flow radiators. These are named to reflect the way the water moves across the radiator rather than going from top to bottom, this style does not induce any thermosyphon movement. In addition, to aid filling, swirl out any bubbles and give a reserve of water a header tank is put into the system at the outlet from the cylinder head. This neatly fits onto the bulkhead behind the engine and is the highest point in the system. With there being very little thermosyphon action from the engine a water pump is necessary to circulate the water. This is best illustrated by a simple sketch.

ENGINE

PUMP

HEADER TANK

View looking down onto engine

There are a number of things to consider when moving to this system from a conventional thermosyphon one. Firstly you definitely need a pump to circulate the water otherwise it will boil. Most specials of this type in the ‘50’s and ‘60’s used the Ford 10 ‘Export’ pump that neatly fits into the bottom hose. However the hard part of fitting it is giving it an adjustable secure mounting and also the drive belt. Specialist companies in the 60’s produced a steel or aluminium V belt pulley to fit in place of the Austin flat belt pulley. The SPEEDEX front shock absorber bracket also incorporated a mounting for the pump.

The best place for the pump is in the bottom hose where the water is coolest. It will work in the top hose but could be prone to the flow being interrupted by bubbles etc. Good news is that these Export pumps are being remanufactured and are available from Small Ford Spares. They also remanufacture the aluminium version of this pump made by Aquaplane but it costs more.

Whilst it is possible to rig up a top hose from a forward pointing top water outlet, SPEEDEX incorporated a reversible top water casting that could face either forward or back. This solution was followed by others such as the Supaloy and LRM heads. See the section on cylinder heads.

The bulk head mounted header tank is obvious, it also makes a good location to mount a temperature gauge sender. Note the rearward facing water outlet from the cylinder head to the header tank. The second pipe on the header tank leads down over the manifolds to the radiator. Just visible in front of the dynamo is the water pump with a V belt drive from a special pulley.

ENGINE

There are alternatives to the Ford pump if you have the time and ingenuity. I’ve heard of Ford Fiesta, Hillman Imp and Citroen Ami pumps being used. Most modern designs of water pumps are mounted into the cylinder blocks, to use them will involve getting a suitable back plate made up.

Another alternative is to use a pump out of a washing machine. Get rid of the mains voltage motor part of it and rig up a suitable drive.

I’ve seen electric water pumps advertised. This could be quite a neat solution as you don’t need to have a belt drive — as long as the dreaded dynamo will supply enough current at 12volts I’ll hastily add!

Just to be different I’m building a pump based on a washing machine pump driven by an old windscreen wiper motor. Whilst not easy to describe as compact or elegant it does have the advantage of having cost nothing! A previous one I built pumped an impressive amount of water when immersed in a bucket but I didn’t see it on a running car. Should be an interesting little side project though!

Something else to think about with a crossflow system is airflow. Overheating even when the mixture and timing etc is OK, especially at higher speeds and engine loads is usually attributable to poor air flow through the radiator. Make sure most of the air that enters the front of the car is ducted through the radiator core with baffles. It doesn’t have to be 100% sealed etc, as long as most of the flow is ducted towards the radiator it’ll be fine. On a similar note the system works well when the car is moving, but it’s usually a good idea to have a fan for when you get stuck in a traffic jam or are stationary for a long period. An electric one from a modern car out of the scrap yard/breakers is a simple solution. Another reason for going for 12 volts though.

If all else fails there’s products available that increase the efficiency of the cooling, water wetters I think they’re called. I’ve heard they are effective.

Here’s what typical crossflow systems look like;