Triple pass rads and stationary temperatures

[Dirty Den]

I've got a triple pass rad fitted to my RoadSport A - when it has airflow running over it, the temperatures seem fine . In traffic though, the coolant temperature goes up very quickly. This morning it was in the high 90s before I got moving again, although I have seen it hit 101 before (fan cuts in just under 100).

 

What sort of temperatures should a triple pass rad run at ideally? Coolant level is fine (checked last night). I've read various threads about replacing the thermostat with one that will start the cooling fan at a lower temperature - any advice on what temperature for the replacement thermostat is a good temp?

 

Cheers,

 

Den

 

😬 - Self portrait - still unable to remove the smile!

[Peter Carmichael]

The thermoswitch is a very unreliable piece of kit, which is why a lot of people run a bypass to a manual override switch.

 

It has probably been a while since I ranted about how the triple pass radiator is the worst possible radiator you could choose...

[Shaun_E]

Den,

The thermostat and the fan switch are 2 different things. You can get a lower temp thermostat but I can't see that making much difference in traffic. Some people advocate running a lower thermostat temperature but personally I don't see the point - it won't enable you to lose any more heat - it just starts the heat loss process at a lower temperature (and not the designed running temperature of the engine).

AFAIK the fan switch is only available with one temperature setting.

The temperatures you quote above are quite normal and as long as the temperature comes down easily when the fan cuts in, there shouldn't be an issue.

If you are worried, the simple solution is to put in a fan override switch that you flick on as soon as you get into traffic.

 

Yellow SL #32

[SLR No.77]

Den,

 

Be sure the temp on the gauge is reading the actual temp of the coolant - the sensor is positioned over the manifold and is affected massively by heat from the manifold when stationary. Caterham do a kit (basically a sleeve with sensor holes, and 2 blanking plugs for the old holes) to reposition the gauge and ecu sensors further back along the by-pass or heater hose, giving readings less prone to radiated heat.

 

Stu.

[TomB]

So what wrong with triple by pass rad then?

[Peter Carmichael]

A triple pass rad generates more resistance to flow than a single pass rad. This can give a higher drop in temperature across the rad, which means the heat dissipated is less. Overall system capacity (ability to dump heat) is reduced compared to a single pass rad.

 

Strange that, seeing as most people think a triple pass rad will increase their cooling.

[Shaun_E]

Tom - theory is that the water pipes in the radiator are of a smaller diameter and therefore the flow rate through them is less than with a standard rad. This would seem to be borne out by Den's experience above with the temperature rising quickly in traffic. The radiator is designed for racing so car is always moving therefore not normally a problem.

Please don't take this as gospel though as I've never actually cut up said radiator to measure the dimensions.

 

Yellow SL #32

[Shaun_E]

OK I give up - Peter can obviously type quicker than I can 😬. It was after discussions with Peter at a get to know your seven day that I plumped for the Radtec radiator rather than the triple pass job from Caterham.

 

Yellow SL #32

[SLR No.77]

Well my engine temperature stabilised somewhat following fitment of a triple pass rad

 

Stu.

 

[Dirty Den]

Thanks everyone for all of the replies so far. When airflow is going over the radiator, the temperatures are fine and when the fan kicks in, the temperature does go down. If the car gets airflow back over the rad, then the temperatures go down after that. As Shaun said, I guess in racing there's always airflow going over the rad so the temperatures always stay at a reasonable level.

 

I'd just like some means of getting the fan to kick in earlier if I'm not moving - but as Peter suggested, perhaps having more control over the fan activity is the best way to go.

 

Cheers,

 

Den

 

😬 - Self portrait - still unable to remove the smile!

[Shaun_E]

I was browsing the SVC site this morning and they do an adjustable fan switch. It's a top hose mounting so I don't know how easy that would be to fit. Have a look here.

 

Yellow SL #32

 

Edited by - Shaun_E on 12 Jul 2004 13:10:00

[Brad Tipp]

Why not just run the fan constantly?

 

I've had two thermostatic switches fail on me, resulting in overheating. So I just bypass the thermoswitch and run the fan constantly.

That way I have removed one point of failure and the temp sits at 80 under nearly all conditions. Can't really see a downside either as the fan always ran whenever I was using the car properly, or stuck in traffic anyway.

 

T 1 PPB - Superlight

 

...Well yes officer I'm not arguing, it's just that [insert excuse here]...

 

Edited by - Brad Tipp on 12 Jul 2004 13:26:30

[Peter Carmichael]

I'm not denying that a new triple bypass rad may have, at some point in the past and may in some times to come, cure some people's cooling problems. Whether it solves a problem will depend on what the problem was in the first place.

 

Same as usual I am suggesting that there is a theoretical "right" answer for cooling. I am also pointing out that there are plenty of counter-intuitive "gotchas" related to cooling and radiators.

 

I have written a longish piece that covers a few of the principles. Too much waffle for most people's tastes, I have no doubt, so I have just kept it on file. If anybody wants a thrilling read, I can post it up or email it.

 

There is one general caveat with all cooling problems: We place great reliance on the coolant temperature reading taken at a single point in the entire system, using unreliable sensors, unreliable wiring, unreliable earths and unreliable gauges. Even people who have Stack systems usually rely on a single temperature reading from a similarly dodgy sender and similarly dodgy wiring. The fundamental parameters in a cooling system are flow rate and temperature - flow rate is difficult to measure apart from in purely experimental setups and we aren't even measuring temperature very well.

 

As for the counter intuitive gotcha relating to the triple pass radiator...

 

The air passing over the radiator cannot tell whether the radiator is a triple pass design or not. All it sees is the temperature of the metal surfaces. If the metal surfaces are hotter, the air carries away more heat. You therefore dump most heat out of the system when the entire radiator is at the same temperature. A triple pass radiator slows down the flow of coolant, increasing the temperature drop across the radiator. The triple pass radiator therefore doesn't dissipate as much heat as a similarly-cored single pass radiator for any given inlet temperature.

[SLR No.77]

I'm not sure the fan is rated for continuous use 🤔

[Peter Carmichael]

And running the fan constantly will place an electrical load on the alternator requiring more mechanical input to the alternator which will require more work by the engine which will result in more reject heat.

 

I've often thought that the fan should be driven by road speed and not just by temperature. I'm not sure the fan has any sensible effect above 20mph road speed.