Caterham Radiator Maximum Cooling Capacity

Caterham Race Radiator Fan Shroud

I’d purchased a cheap, second hand custom made, aluminium Caterham radiator designed for a full race 2Ltr Duratec engine. Although it had seen some stone damage it was essentially in good order.

I had a cheap eBay radiator but I wanted to use this on another Hot Rod project and this one seemed a better fit for my needs.

To make sure it had good air flow, I spent a few evenings straightening all the cooling fins.

It was hand made for a track only, highly tuned 2Ltr, that ran without a fan and a flow restrictor, rather than a thermostat. It fits snugly inside an original Lotus S2 nose cone. 

One advantage of the Caterham radiator would be the top hose would be slightly down hill towards the inlet manifold. When fitting a radiator, the outlet from the engine to the radiator top hose inlet should be slightly lower or at worst level. The radiator outlet should be the highest part of the cooling system.

I wanted to know if it was big enough to cool a 4.6Ltr V8.  When I bought the Caterham radiator, I couldn’t measure it before hand, so it’s maximum cooling capacity was a mystery.

But first I needed to sort a few other things out.

Caterham Race Radiator Fan Shroud


There have been a number of thermostats fitted to Rover V8’s over the years, not just the temperature but the location and type. Early cars had an 80°C / 176°F thermostat mounted inside the elbow on the inlet manifold. Later P38 4.6 engines had a plastic 96°C / 204°F thermostat housing in the bottom hose. Apparently, the reason for moving the thermostat was for faster interior heating on cold mornings. In a P38, within 20 seconds of starting hot water would flow to the heater. Some people on the forums have also suggested that the 96°C thermostat was for emission reasons, this would be interesting to test as, in theory, the engine would be burning fuel less efficiently. Although less efficient, it may be producing less harmful emissions.

There’s plenty about thermostat choice in this book:

According to this book and endless forum sites, Rover V8 engines do not fair too well over 80°C / 176°F. Reliability is seriously compromised and power is significantly reduced.

The Rover V8 produces the most power between 65°C & 75°C (149°F & 167°F). This is considerably lower than the P38 4.6 Range Rover’s 96°C.

The P38 Range Rover 4.6 was notorious for overheating, hose failures, slipped cylinder liners and cracked blocks. Despite several redesigns, the problem was improved but never really cured. If you have a 4litre block, they are lower grade than the 4.6Litre ones and overheat and destroy liners for a past-time. If you have an early 4.6 Range Rover chances are it will have overheated at least once. After 4 over-heats, you’re not just looking at slipped liners the block will probably crack and will be fit for the scrap-heap. If you have a 4.6, many recommend changing the cooling fluid for high quality coolant very regularly.

I’m running a P38 4.6 Rover V8, but to avoid overheating issues, I chose to fit a ETC4761, 74°C / 165°F thermostat to the inlet manifold. With this thermostat the engine should run at around 76°C / 169°F.

With a 74°C thermostat, I chose a 79°C / 88°C Fan Switch (INTERA033866). If this works, I might try a slightly lower switch temperature, 76°C / 86°C (50250), as fitted to some TVR’s, to see if it still cycles.


Fan Switch

I fitted a Fan Switch into the threaded mount on the radiator top tank, but you can get inline hose outlets.

Part Number: INTERA033866
Manufacturer: Intermotor
On temperature: 88°
Off temperature: 79°
Thread: M22 x 1.5
Contact: N.O. (Normally Open)
Spanner: 29mm
Spade Terminals: 6.3mm

50250 Fan Switch

The idea behind a fan switch is to activate the fan(s) when the water in the radiator exceeds the desired operating temperature. Once the water has returned to ‘normal’, the fan should turn off (open circuit). This is known as ‘cycling’.

To get the fan to cycle, you need an off temperature, a few degrees  (5°C) above the ideal temperature. If you were to chose a Fan Switch with an off temperature below or the same as the ideal temperature, once the fan came on, it would not turn off until you turned off the engine.


Radiator Cooling Capacity

I’d previously calculated, that if I used a fan shroud, my cheap eBay aluminium radiator was still too small. Hence me trying the Caterham item.

Initially, I’d made a couple measuring errors, with my cheap eBay rad calculations. I’d taken the overall dimensions and not the core dimensions. – all sorted now though

See :

Equation for Calculating Car Radiator Size

Start with 2 cubic inches of core for every cubic inch of engine. Increase or decrease that value by the following factors:


  • 0.1 for a vertical flow radiator core
  • 0.1 for an in-line engine
  • 0.1 for a small trailer towing
  • 0.1 for a 2 row radiator
  • 0.1 for double evaporators
  • 0.2 for outside temperatures of 105°F (40.5°C)
  • 0.2 for a medium trailer towing
  • 0.2 for a small engine fitted to a heavy car
  • 0.2 for a radiator fan with diameter less than 90% of smallest dimension
  • 0.3 for air conditioning
  • 0.3 for no fan shroud
  • 0.3 for an antique car with small engine compartment
  • 0.4 for large trailer towing
  • 0.6 for a diesel engine


  • 0.1 for remote transmission cooler (not within radiator)
  • 0.1 for standard in-line transmission
  • 0.1 for a single row radiator
  • 0.1 for a V6 / V8 engine
  • 0.2 for a spacious pickup truck engine compartment
  • 0.2 for outside temperatures less than 90°F (32.2°C)
  • 0.2 for a full fan shroud
  • 02 for a horizontal flow radiator core
  • 0.3 for a large engine in a small car

My Caterham radiator core dimensions:

44cm (17.3″) x 24cm(9.4″) x 6.5cm(2.5″) = 419ci (6864cc)

My Cheap Ebay radiator core dimensions:

35cm (13.7″) x 34cm(13.4″) x 5.5cm(2.2″) = 399ci (6545cc)

  • Electric Fan (12″) radiator (9.4″)
  • No trailer
  • add 0.1 for vertical flow core
  • add 0.3 for an antique car with small engine compartment
  • add 0.1 for an in-line engine
  • add 0.2 for aluminium radiator
  • subtract 0.2 for outside temperatures less than 90°F (32.2°C)
  • subtract 0.1 For V8 engine
  • subtract 0.3 For large engine in light car
  • subtract 0.1 for standard in-line transmission
  • subtract 0.1 for remote transmission cooler (not within radiator)

No fan shroud:

2.0 + 0.1 – 0.2 + 0.3 + 0.1 – 0.1 – 0.3 – 0.1 – 0.1 + 0.2 + 0.3 =  2.2

  • add 0.3 No Fan shroud

Caterham: 419ci ÷ 2.2 = 190ci (3120cc) Engine
eBay Rad: 399ci ÷ 2.2 = 181ci (2966cc) Engine

With full fan shroud:

2.0 + 0.1 – 0.2 + 0.3 + 0.1 – 0.1 – 0.3 – 0.1 – 0.1 + 0.2 – 0.2 =  1.7

  • subtract 0.2 for full fan shroud

Caterham: 419ci ÷ 1.7 = 246ci (4031cc) Engine
eBay: 399ci ÷ 1.7 = 234ci (3950cc) Engine



With a 4,552cc (277.8ci) Rover V8, I’ve got a few issues. If I can increase airflow through the engine bay, maybe I could remove the ‘0.3 for an antique car with small engine compartment’. When there is restricted airflow, the whole engine bay heats up, preventing heat dissipation. If I can force masses of air through the tight engine compartment, hopefully I’ll be able to completely refresh the air in a few seconds.

If I can’t increase heat dissipation above 80°C, I’m in trouble and will be slipping liners, like every other P38 owner. If I can keep close to 80°C and below 90°C, I’ll be happy.

419ci ÷ 1.4 = 299ci (4902cc) Engine

The least I can cope with is 419/277.8 = 1.5

419ci ÷ 1.5 = 277.8ci (4552cc) Engine

Alternatively, I need a 472ci (7739cc) radiator (44 x 24 x 7.3mm) – That’s only 8mm thicker than my current one – doh! How close I was!

Although it’s probably too small for my needs, I’ve already made a fan shroud.

I know, should have done the sums first!

Caterham Race Radiator Fan Shroud

That 12″ fan is mounted behind the radiator, sucking the air through. I have some silicon rubber edging to seal the shroud to the radiator, ensuring all the air sucked by the fan is pulled through the radiator. Inside the nose, I will make another set of shrouds, to ensure air can’t pass around the sides.

To prevent unwanted corrosion, I have used large rubber mounts to insulate the radiator from the chassis.

I also have a 30 row oil cooler with a 9″ Fan and shroud. It will be mounted to a scoop on the side of the engine bay, drawing cool air, through the oil cooler onto the exhaust manifolds. I’ll put a similar fan, without cooler onto the opposite side.

Oil Cooler 30 Row With Fan

There is just about room to mount another 12″ fan in front of the radiator, running in the opposite direction, blowing air through the radiator. If I have problems then I’ll try that too.

I will cover the exhausts in titanium heat wrap as this, according to some, performs miracles.

There is room on the scuttle to fit a couple of 9″ fans (on a separate temperature switch) to blow air out of the louvred bonnet. In traffic, hopefully there will be a huge amount of air being forced through the engine bay.

In short although I currently have one 12″ Fan and two 9″ ones, there is the opportunity to fit three more.

This is the second attempt at a fan shroud, the first was a patchwork of steel. The reason was, before I started, I had no idea how it would look when finished.

Caterham Race Radiator Fan Shroud

I think you can see why it ended in the bin.

Hopefully, all those fans won’t drown out the V8 rumble, which is the only reason why I don’t have a 4 cylinder.

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