Thread: Thermofan over Viscous?

HI kiethyP38
I also have a P38 - standard form and I measure the temperature at the back of the manifold and at the radiator outlet.
Your data is very interesting - the data with the electric setup doesnt make sense because I helped a friend fit late model commodore fans to a RRC classic with a cracked block. They kept it cool under all driving conditions with that affliction until he changed engines. The coolant hoses were rock hard most of the time and he drove it like that for quite a while. His fan power feed was via the original condenser fan wiring which powered both fans on together. What I did notice was that he was not getting max speed out of the fans due to the undersize wiring of the original loom. These fans pulled something like 50 amps on start and about 35 amps at max speed when tested straight to a battery. On my RRC I wired them differently using 60amp maxifuses and much thicker and shorter wiring direct to the battery via a dual 60 amp relay - they ran a lot faster than his - so check your wiring first.
How is the thermo switch plumbed into the system - did you do a pressure check to make sure the system was holding pressure after that modification because you may be losing some system pressure which would affect the pressure at the pump inlet at higher RPM?? PM me if you want some more info on that because system total pressure must be correct to ensure correct pressure exists at the pump inlet to avoid any cavitation as RPM increase. Pump inlet diameter, system pressure, flow rate, and pump design are all closely interrelated as you know.
I can also state with confidence that your data with the viscous fan is completely accurate as I have an accurate temperature sensor mounted at the manifold where the coolant rises adjacent to number 8, and a sensor on the radiator outlet to see how much the radiator drops the coolant temp at various scenarios.
I just recently towed a boat to Cairns Aug/Sept and the coolant in and out differentials are largest in cruise and least at idle with a standard viscous fan.
To investigate the idle case some more, I installed a manual switch to turn on both condenser thermofans without the AC on - it only moved the temperature differential between in and out by 2 degrees( in both AC on and AC off cases)

The whole principle of a VC is that the fan can have a higher /pitch than a fixed fan as they are usually programmed to reach about AFAIR 3000 RPM before they start to slip (when locked).

This is why when people lock up the VC by putting a screw in it they tend to blow blades off.

The VC only locks( albeit partially) at about 94 C so below that it is somewhat freewheeling, based on the temperature airflow through the radiator affecting the spring on the front which expands to close a relief hole .

In RRCs 3.9 the condenser fans come on at 102-104C. based on the switch on the thermostat.

The advantage is that the higher pitched fan delivers more air at low revs, but not obviously as much as an electric fan setup at idle.

You only would see the difference between a VC and electric fans at hot high load conditions such as a 34C-40C driving on dry sand .Under normal conditions the temperature drop across the radiator will be sufficient with either VC or electric fans.
I sometimes think that often people have an old car with say 200KK on it and find the car will not stay cool, and blame the VC , when it can be a combination of old worn VC and partially blocked radiator.

My RRC 3.9 NEVER went above about 98C even climbing long grades towing a trailer. One time only on a 35+ day climbing Mt Pinnibar in low range did it reach 102 when the radiator fans cut in, and it cooled to under 100C.
A clean radiator and a operational VC are all that is needed.

Regards PhilipA

The VC only locks( albeit partially) at about 94 C so below that it is somewhat freewheeling, based on the temperature airflow through the radiator affecting the spring on the front which expands to close a relief hole .

When the engine is hot the fan is never freewheeling, at lower temps it's turning with lower rpm(down to 40%) than the engine but with power and at high temps with at least 85% or the engine rpm, i measured the temperature on the bimetalic coil with laser thermometer imediately after i stopped the engine and it was 85*C while the ECT was 92*C and it locks at above 74... here's a detailed description of how it should work https://www.fanclutch.com/PicsDocs/B...ives_Only).pdf

When the engine is hot the fan is never freewheeling,

Maybe I should have elaborated and said that under lockup temperature the air forced through the radiator will partially drive the fan so that the parasitic drag on the engine is not much.

Regards PhilipA

Originally Posted by peter51

HI kiethyP38
I also have a P38 - standard form and I measure the temperature at the back of the manifold and at the radiator outlet.
Your data is very interesting - the data with the electric setup doesnt make sense because I helped a friend fit late model commodore fans to a RRC classic with a cracked block. They kept it cool under all driving conditions with that affliction until he changed engines. The coolant hoses were rock hard most of the time and he drove it like that for quite a while. His fan power feed was via the original condenser fan wiring which powered both fans on together. What I did notice was that he was not getting max speed out of the fans due to the undersize wiring of the original loom. These fans pulled something like 50 amps on start and about 35 amps at max speed when tested straight to a battery. On my RRC I wired them differently using 60amp maxifuses and much thicker and shorter wiring direct to the battery via a dual 60 amp relay - they ran a lot faster than his - so check your wiring first.
How is the thermo switch plumbed into the system - did you do a pressure check to make sure the system was holding pressure after that modification because you may be losing some system pressure which would affect the pressure at the pump inlet at higher RPM?? PM me if you want some more info on that because system total pressure must be correct to ensure correct pressure exists at the pump inlet to avoid any cavitation as RPM increase. Pump inlet diameter, system pressure, flow rate, and pump design are all closely interrelated as you know.
I can also state with confidence that your data with the viscous fan is completely accurate as I have an accurate temperature sensor mounted at the manifold where the coolant rises adjacent to number 8, and a sensor on the radiator outlet to see how much the radiator drops the coolant temp at various scenarios.
I just recently towed a boat to Cairns Aug/Sept and the coolant in and out differentials are largest in cruise and least at idle with a standard viscous fan.
To investigate the idle case some more, I installed a manual switch to turn on both condenser thermofans without the AC on - it only moved the temperature differential between in and out by 2 degrees( in both AC on and AC off cases)

Thanks for your reply Peter. I appreciate the comments regarding wiring being a potential factor. It’s definitely something to consider and something that I could try in an effort to give more boot to the fan. I have the exact same fan on my Ute (which is a 5.0 VS III Commodore) and it’s measuring about 35 amps on start, settling to about 18amps at full speed. On the setup in the Range Rover I’ve used thicker wiring and a much neater relay setup, with the power wiring to the fan very short and matching the fan controller and fan itself for size. I don’t recall the exact wire size off the top of my head, but I’m open to re-wiring with something bigger, which I will try.

The temp switch is fed into the top hose via that bypass t-piece just before the radiator. I can’t really see it as being either a restriction or altering system pressure in any way. I fitted the switch up before putting the thermo fan in (drove around for a few weeks like this before fitting the thermo). That’s how I established the 7 degree difference between head and top hose temps. The system was bled properly in all instances and is running the usual 50/50 coolant mix.

It’s very interesting to hear the current draw from the commodore fans on the RRC you mention. Do you recall what model commodore those fans were off?

Ill have a look at the wiring and up the size and fuse/relay combo from 40 amp to 60 amp and see what difference it makes.

Cheers
Keithy

some food for thought,

most cars coming out now says have a thermofan.
why did the engineers from different companies all go in this direction?

Originally Posted by Eevo

some food for thought,

most cars coming out now says have a thermofan.
why did the engineers from different companies all go in this direction?

In an east/west engine layout (front wheel drive) with a rad at the front you have (need) a thermofan.
The majority of cars on the road are front wheel drive. (Only Subaru and some Audi's are north/south layout)

Of the RWD platforms, it could be cost, noise efficiency, packaging or a combination?
You can mount the rad/condenser closer to the engine with a thermofan and have the fan mounted ahead as a forced draft fan?
One less pulley/belt to package? etc.

I think the fans were VE V8 fans. I need to access my old posts from about 4 years ago to confirm but I cannot access them. I have sent a post to the moderator.
I would say that they need to be in dual configuration for the P38 - single for you will not do it unless you go to an X5 type.
I have an article from autospeed(now a broken link) that details how he tried all sorts of single fans to control overheating in a Cobra replica - he had outstanding results with a fan from a 1999 BMW X5 - PM me if you want a pdf copy emailed to you.
I use a dual sensor engine guard https://engineguard.com.au/ to monitor radiator inlet/outlet temp - temps entering the radiator are are 95 in cruise and 98 to 100 idling with AC on hot humid - exit from the radiator is 75 degrees at cruise(78 in traffic - more when downwind). The sensor is jammed between the shroud and the alloy housing - so accurate enough.
Since you have already done all the fabricating work I think it would be better to change the thermosat setup like they do in the US on the V8 Discos - this will achieve the results you want with the current thermo fan in place I believe. The threads on this conversion are easily found.
Fitting another thermofan is more cost and more work. The problem with the X5 fan is that you would need to carry a spare in the bush I believe - same with the commodore option however new ones are easily purchased online if your original seized somewhere up track.
Before you do that I would remove one of the condenser thermo fans and retest - in my opinion all they do is block airflow. Check if they are induced into high rotational speed by your new thermofan at idle - if so this may cause flow/heat exchange issues through the radiator at idle - my opinion only - however as I said if you use a manual switch to turn on both condenser fans on the P38 when idling on a hot humid day it only reduces the engine temp(measured by OBD) by 2 degrees - if that.