After trying a few different fans, I have concluded that anything less than a famous brand fan is a waste of time.
14" is the largest fan size which will fit inside the frame of radiator, although if you made a cowl and mounted the fan on the frame of the radiator, a slightly larger fan could fit.
Fans with curved blades are quieter and slightly less efficient than straight blade fans.
Fans are often directional, it wouldn't be advisable to go for an bidirectional fan as there may well be a loss of efficiency.
On the SR8 one pull fan can be mounted on the rear of the left-hand radiator, and a push fan can be mounted on the front of the right-hand fan (due to the position of the oil cooler on this side).
I tried a number of fans around the £50 mark. The first was a 160W curved blade, which seemed to blow a decent amount of air, but the air was directed sideways out of the fan rather than straight behind it - it had nowhere to go. Also when I was testing this fan and had it in my hand, there was a worrying amount of vibration. It seems that the blades were not uniformly made, and the vibration would eventually rub a hole in the radiator. Chinese rubbish.
The second fan was a 220W straight blade fan. Strangely it blew noticeably less air than the curved blade one, but in the right direction. Normally the blades go right to the edge of the housing, but in this particular fan I could put my finger between the two. More Chinese rubbish
Eventually I went for a high-performance 200W Spal straight blade pull fan (VA08-AP70LL-23MA), the difference was immense, it blew an impressive amount of air, ran much sweeter and vibration was minimal. I was blown away...
Due to the 17A draw for this one fan, I'm adding a separate power and relay line as a precaution. This would definitely need done for a twin fan setup.
Thanks Stephen. Great write up.
The ECU seems to bring the fan in at around 95 degrees and out at 90, this lets the oil temperature creep a little higher than I would like. The ECU is locked, so a fan controller unit was bought - Davies Craig Digital Fan Switch Controller 0444. ( https://www.ebay.co.uk/itm/Davies-Craig-Digital-Thermatic-EWP-Fan-Switch-Controller-0444/193361266295?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649)
It seems like a pretty good unit, easy to operate and displays either Centigrade or Fahrenheit. The temperature can be set to basically whatever you want, and when the water temperature reaches that value the first relay is activated, and ten seconds later the second relay is activated. They stay activated until the temperature is 5° below the preset. It is completely automated and the unit retains the setting after the power is taken away from it.
This time I went for a 14" push fan (Spal VA08-AP10/C-23S) to be mounted on the front of the right-hand radiator. There was a minor fitting issue, in that the duct which runs through the side pod, is tapered up towards the top of the radiator. This means it isn't room for the full height of the fan in the mouth of the duct. We trimmed a little off the fan body and it fitted well. The alternative would be to go for a 12" fan.
This fan has a 5.5A current draw rather than the 17A on the left side (956 CFM vs 1623 CFM). The fan controller has its own independent temperature sensor which is pushed into the fins of the radiator, it reads about 5 degrees below the engine temperature, but this can be compensated for with a different preset temperature on the controller.
I've mounted the controller on the right-hand bracket that holds the side pod in place, it is accessed and viewable through the ECU panel, and attaches securely to the bracket. It is currently set for a maximum temperature of 85°C which keeps the oil nicely in check.
For prolonged sitting in traffic, this fan on its own isn't enough to stop the temperature eventually rising, but it does do a good job of keeping it in check 99% of the time with a low current draw. The two fans could be wired to this controller, but I thought it safer to have two independent means of cooling the car, just in case.