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Chasing Overheating

Finding why a V8 Cobra replica was getting hot under the collar

by Graham Pring

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This article was first published in 2009.

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What do you do when you have a home-built Cobra replica that’s just had an engine upgrade – and overheating has become a huge problem? You just chase it down, step by step.

The car was originally fitted with a Holden ‘blue’ 308 V8. This modified engine produced 185 - 198 rear wheel kilowatts at 5500 rpm and the car ran a 12.7 at 105 mph drag slip time. 

The 308 gave mild and reasonably fast performance with relatively good fuel economy; however, when idling in very hot weather, it was borderline in the cooling department. But when moving, there were no indications of overheating. 

The cooling system has been extensively tested (see Water Pump Testing) so its behaviour is pretty well known – especially as to water pump performance.

The decision was made to build a stroker motor to get a lower stall speed for better cruising rpm and fuel economy (again relatively speaking, of course!), plus more torque to keep my interest up.

I used a similar 308 block and took it out to 383 cubic inches. After making clearance in the block for the stroker crank’s longer throw, plus all the necessary items to get the 308 out to a 383, time came to get everything together and run the engine in.

Uhhh, Ohhhh

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A couple of hundred kilometres later, I noticed that the coolant temperature gauge showed that things were getting hotter than with the 308. This occurred mostly at slower speeds. At city or open road speeds there was only a slight problem, where the cooling system appeared to run a little warmer than I would have liked. 

But at idle the problem was major.

Testing confirmed that if the gearbox oil was at 80 degrees C and the engine oil temperature was at about 87 degrees C (ie normal readings), and if the car was stationary, then I was in trouble.

Previously, with the modified 308 engine, the coolant temperature would (just) cope when idling, even on a 42 degrees C day. 

But now it wouldn’t cope even when the day temperature was only 14 degrees C!

I talked with some friends and acquaintances who also had stroker motors in their Cobra replicas and found that all of them had overheating problems at slow speeds or when stationary – problems they didn’t have prior to the stroker crank engines being fitted. Some had Ford motors and some had Holden motors, all V8s. 

They knew their machine’s limitations and drove accordingly – something I didn’t want to do.

First Changes

One of the first things I looked at was the type of coolant. It appears that water is the best coolant in terms of heat collection/dissipation; however I added a little commercial coolant to stem corrosion.

I have a bypass system which controls the temperature of the coolant into the engine, not out of the engine, so the first modification was change in the coolant thermostat from 82 degrees C to 71 degrees C. 

This helped if I had been at any sort of speed for a while, but not if I had been driving around in city traffic. At anything above 60 km/h there weren’t any cooling problems. 

Testing on my car years ago had proved that at those speeds, sump oil temperature was reduced by 12 degrees C if I removed a very small front spoiler which I’d fitted under the front of the car - so I knew what could be achieved with better cooling airflow.

The radiator was a modified outlet, three row, copper-cored, Volvo 264 unit which was sealed as best as possible around the outside and had a fan shroud with 16 inch puller fan on the engine side.

Stationary airflow through the radiator occurs only through the action of the electric fan (there’s no engine-driven fan), so I went searching for an answer relating to fans.

Asking radiator repairers and performance cooling ‘specialists’ gave various replies including:

• try a different radiator

• try a different fan

• try twin fans instead of the large 16 inch unit

• we’ll make you a large aluminium radiator

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The fan that was fitted was of unknown manufacture and specifications, other than it was advertised as a 17 inch twelve years ago and purchased from a large performance equipment supplier. At the time, it was the biggest fan I could find. Judging by tip to tip measurements, these days it would be called a 16 inch fan.

But how powerful was it? An indication of how much work the fan is doing can be gained by looking at how much power the fan motor is consuming. I got out my ammeter and measured 6.9 amps current draw when the fan was connected to a fully charged battery off the car.

Looking on the web at fan manufacturers’ data, it appeared that 7 amps wasn’t much current draw for a high performance 17 inch (or 16 inch...) fan. Some of the listed models used 7 amps to run just a single 10 inch fan at around 800 cfm. That probably meant that my 16 inch fan was pulling only 800 cfm.

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And then when I saw a 16 inch SPAL fan advertising 3000 cfm and requiring 30 amps to run it, I figured my fan was producing only a very small airflow. 

In other words, compared to other more powerful fans available on the market, the amperage draw for my fan was very low for a fan of that diameter. If I had correctly interpreted the fan data I had seen, there appeared to be an almost direct relationship between current draw and airflow.

There were no specifications written on the motor of my fan, so I had no idea how much current flow it could handle, however if I modified the fan to produce more flow, then at least I could see if a larger airflow was going to help with the overheating dilemma.

This meant that I didn’t have to purchase another fan and the modifications could be done with very little expense, especially in my case where I had a heat gun and a multimeter capable of reading up to 20 amps.

I took the fan out, dismantled it and measured the pitch angle of the blades. These proved to be 7 degrees. The hot air gun was then used to increase the pitch of the blades, with each configuration checked for current draw.  

About 17 degrees pitch angle was tried first, resulting in 10 amps current draw off the car – an increase in power of 45 per cent!  When tested on the car, with engine and alternator running, the current draw rose to 11.5 amps.

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I then changed the fan pitch angle to 22 degrees (resulting in 12.2 amps off-car current draw), then to 30 degrees, which gave 14.5 amps off the car and 15 amps with the car running. 

The motor seemed to handle this 110 per cent increased current draw very well and there appeared to be no loss of rotational speed, although I did not test the speeds.

Back on the Car

So the fan power had been more than doubled, and when I put my hand behind it, the airflow certainly felt like it had been doubled.

But testing the car showed that this modification had made extremely little, if any, difference to the performance of the cooling system!

Perhaps fans weren’t the answer... What next then – maybe the performance of the radiator?

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I used thermocouples to test the temperature drop across the radiator, finding that only a 4 degree C drop was occurring. I tried the same test on one of my ‘normal’ (read: Holden) cars and the result was 17 degrees difference from one side to the other – with the tests made on the same day and in the same conditions.

I then spent time spent looking at the cooling capacity difference between aluminium and copper radiators.

I also thought about the size of the tubes in the radiator - as to whether they were too large in diameter and the flow through them was too slow and not turbulent enough, thus not getting the heat into the metal of the radiator.

Once again the questions were asked of ‘specialists’, all of whom said that aluminium is the answer - however no-one could quantify it.

Checking various sites online resulted in even more frustration, with those sites selling aluminium products espousing its greatness while one site, US Radiators, which sold both, actually had some data on both which made interesting reading, especially when looking at different approaches. (See http://www.usradiator.com/default.htm.)

So if changing to an aluminium radiator was an unproven – and potentially very expensive – option, what about altering the design of the existing radiator?

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If you don’t have a transmission warmer/cooler in the radiator, then your radiator can be converted into a triple pass radiator. This makes the radiator effectively three times as long, but with each ‘pass’ occurring through a smaller number of tubes. 

The figure the web gave was that a triple flow design would have a 15 percent improved temp drop over a conventional single-pass radiator. Asking locally I came up with the answer of 3 percent by the one and only person in the radiator game to put a figure on whether it was worth doing.

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Ignoring the local’s advice, I removed the radiator and had it converted to a triple flow design. This cost about AUD$100 and I was told by the specialist that there were no blockages in any of the tubes. I then got out the thermocouples and retested the radiator performance. 

The temperature drop across the radiator was now about 17 to 19 degrees C, a massive difference to the single pass unit at 4 degrees. This probably meant that the speed of the coolant flow through the radiator had been increased and the flow through the tubes had gone from laminar to turbulent, therefore releasing a lot more heat into the metal from the coolant.

Prior to the triple-pass conversion, the coolant would take 12.5 minutes at idle to go from 89.5 to 102.2 degrees C. After the conversion, I stopped the test after 20 min 45 secs as the temperature had reached only 95 degrees C.

I was happy, although all the tests were conducted at only about 14 degrees C ambient temperatures. 

Pressures

Using a pressure gauge I then tested the thermostat-closed pressure of the system at the radiator cap, which in most systems these days is the lowest pressure point in the cooling system.

I found that idle pressure was 8.7 psi and, as I had a quite restricted bypass hose, 29 psi at 6000 rpm . Too high!!

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In a conventional system, the bypass hose, which is quite small, is used in engines to allow the coolant to circulate inside the block and keep the block temperature relatively stable until the coolant heats up and reaches thermostat opening temperature, whereupon the coolant can then circulate through the rest of the cooling system through the large bore hoses. Some thermostats close off the bypass as they open the main radiator flow path, but not mine.

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Remember that my system has a thermostat in the engine supply hose just before entering the pump (and so engine block), so the temperature of the coolant should always have been the same going into the block, unlike a ‘conventional’ thermostat system where the thermostat allows hot coolant to flow through the radiator and enter the block at whatever temperature it is after passing through the radiator.

This pressure testing was only done because the take-off point for the radiator cap was now to be the first pass of the radiator, not the last as was the case prior to the triple pass operation. This meant that the radiator cap had to operate at a higher pump pressure because it took pressure to get the coolant to flow from the first pass (where the cap was) through the last two passes of the radiator prior to arriving back at the water pump.

With a 110 kPa (15.9 psi) radiator cap and a pressure in the system of 200 kPa (29 psi), the radiator cap would relieve and the pump would expel coolant into the overflow bottle, which when full would then overflow to ground.

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Removing the restriction in the coolant bypass hose and retesting when cold resulted in 25 kPa (3.6 psi) and 100 kPa (14.5 psi) figures - which I was much happier with as the radiator cap was set to relieve at 110 kPa.

Retesting of overheating at idle then gave a time of 12 minutes to get to 95 degrees C, and 30 minutes to 100 degrees. (The reason for the decrease in time to get to 95 degrees was because I was bypassing more hot coolant through the bypass hose rather than through the radiator.)

Conclusion

Modifying the fan proved to be a cheap and easy approach to increasing radiator airflow. (Of course, you’d expect the life of the motor to be reduced as it is now working twice as hard.) But in the case of the Cobra, improved airflow made no difference to the cooling system performance – the car still overheated at idle. 

Modifying the radiator to change it from a single to a triple-pass design gave an immediate, major improvement, while removing the bypass restrictor stopped the propensity of the car to blow water out of its overflow bottle at high revs when cold. 

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