by 85AW20v » Wed Feb 08, 2017 7:57 pm
Copied this from MR2 OC - posted by Malcolm. Pasted here so he doesn't have to type it all out again!!
Jeez what a lot of nonsense! Sounds like the guy from the Summit technical department is about as clued up as your local pimply-faced Repco salesman. I can't believe any technical article would propagate the myth that coolant moving too quickly through a radiator gives you worse cooling.
To answer the original questions, Ash; I agree that you should definitely check out the state of your current radiator - the AW11 one is virtually identical to the SW20 and they're both more than adequate for your power plus quite a bit extra.
As for the oil cooler - first step there would be to look at oil temps - generally you can handle oil temps up to 130-140 degrees C, but if you're hitting that on the road then it probably means you'll be massively overheating it if you take it to a track. If your oil temp is close to coolant temp or even below it, there's no point separating the system off.
And a few general comments about the physics behind radiator/cooling system sizing: generally, there are diminishing returns for increasing radiator size and/or coolant flow rate: a radiator that is twice as wide will not reject twice as much heat, a radiator that is twice as thick will not reject twice as much heat, a coolant flow rate that is twice as high will not reject twice as much heat. This will always be true, but how steeply diminishing the returns will be for each change is hard to know without actually testing.
To understand the reason for these diminishing returns you need to realise that radiators are all about temperature differences. When air first hits the radiator it might be 80 degrees cooler, once it's traveled through 10mm of core thickness and absorbed some heat the temperature differential might be 50 degrees, once it's gone through a further 10mm of thickness the differential might be 30 degrees. For every bit of extra thickness you add, the more hotter the air becomes and the less efficiently it cools.
From my experience with cooling system testing on various cars and similar vehicles, the biggest limitation tends to be airflow through the radiator. Because air is much less dense than water/coolant and has a lower specific heat capacity, you need a shitload of it in comparison to the coolant flow, and usually it ends up being the biggest limitation. Interestingly, this can lead to the situation where making the radiator thicker, and therefore creating more resistance to airflow, can actually cause the cooling of a vehicle to get worse! Not likely the case on your average road car with a large frontal area and thin core, but if your starting point is a 38mm core and you replace it with a 76mm core, it can happen (I witnessed this on a vehicle I was involved in the development of a couple of years ago).
Cheers
See ya
Simon
85 MR2 20v