What causes the engine temp in a Stag to fluctuate

[sujitroy]

Over the weekend I went with my son to Mojave desert. (He's big into rockets and was testing a propulsion system) We drove for hours in my V6 Honda minivan. Average temp was 90-100 degrees. . The weather was hot and I can only assume the water through the system was quite hot as well, but the temp. gauge didn't move one bit from normal.

If I had to list things to do on the Stag to get the temp. gauge to be more stable and I'd like to keep the car stock, what should look at? My auto Stag's temp gauge fluctuates, more than my manual with OD, but he engine speed is working much harder. Maybe if I drive my manual at 4k rpm constantly, I'd see the same temp. fluctuations.
My Mk1 manual is running a stock mk1 rad. pump and fan with a header tank. I have rubber strips around the rad. No bonnet pad
My Mk1 auto is running a stock fan, a Super Gill Mk2 rad. with cowl and a header tank. I have rubber strips around the rad. Bonnet pad.
I think both cars are running correctly.
Are other owners of BW35 Stag notice similar fluctuations in temp. gauge readings as you drive?

Sujit

[flying farmer]

When I had my first Stag it was an auto and I also noticed fluctuation in the temperature gauge depending on the type of driving.

When it blew up it was rebuilt as a manual but very much modified so I couldn't make a fair comparison.

What might be significant though was the difference in fuel consumption, it was about 30% better. That meant 30% less heat generated so that has got to make a difference.

I think another factor in modern vehicles is the way the temperature gauge operates. I have read that on many moderns the temperature gauge is driven from the engine ECU rather than being a direct measurement of the engine temperature. this gives the manufacturer the ability to stabilize the temp readings so it always reads the same unless a pre determined point is reached, then it will tell you there is something going wrong.
If you get stuck in traffic with a lot of moderns you will see no change in engine temperature despite the electric cooling fan cutting in and out. There must be a temperature change , its just not shown.

i have found in most of my Triumphs the difference between an 82 degree thermostat and an 88 degree thermostat is about one quarter of the gauge. On my Triumphs where the cooling fan is triggered by the engine ecu there is a 3 degree C difference between on and off. As the temp gauge is the original Triumph item the gauge does display this difference, it is about 1/8 of the gauge which agrees with the difference between the different thermost readings

Neil

[wilf]

^^ What Neil says - moderns don't show real temperatures, just "synthesised" ones. Stops people from complaining about temperature fluctuations under warranty.

In reality, the Stag gauge is reflecting engine load vs cooling capacity varying with ambient/airflow through the radiator, is all.

When it stops varying, then you will know it is broken.

[Tdafforn]

It's funny that people are talking about this. I was thinking about it today. I guess in essence the system has a heater (the engine) which produces heat at differing amounts when running. The radiator is able to dissipate heat dependent on water flow, airflow and the difference in temp between the air and the water. That means there are 4 or so variables.

engine heat output (which is essentially water temp and flow rate)
radiator cooling (which is based on air flow and temp differential between air and water)

At rest ticking over heat output from the engine is low, fllow is low and cooling relies on fan airflow and air temp. If your car can cool at idle at the max air temp in your region then the fan is good.

At cruise speed cooling should rely only on Rad airflow (and air temp) and the heat output of the car. In theory the latter is pretty set by the engine so if the car isn't cooling at cruise it is either water or airflow through the Rad

when the engine is labouring (e.g up steep hills), the engine heata output goes up as does the water flow as you tend to drop a gear. Air flow drops as the car is going slower. If the temp increases it means engine heat output is exceeding Rad heat dissipation. This is not an issue for most uphill as there will be a down hill after. However, if you are doing alpine passes then the solution would be to enhance airflow by either a spoiler or fan to push more air through the Rad. With the later solution youths the fan needs to push more air than the ambient airflow from the car just moving. Another solution would be to increase the size of the Rad. Going wider or higher would work but not possible really in a stag. Going thicker also helps with more tubes, even better, but not as good as larger. This is because the air gets hotter as you go through the Rad so the back tubes see less of a temp gradient between the water and air (like driving on a hotter day) . So increasing thickness is diminishing returns.

Interestingly, there is also an influence of air speed and I some cases slowing speed is better as it allows more heat transfer. I have a costing designed bonnet on my midget with a small frontal opening which opens out inside the bonnet and the cooling is excellent. Even running a 1.8 L engine.

I think this isthe way it works. Happy to be corrected.
tim

[Phil S]

Something that an aussi owner put on here a while ago was that they had removed about 4 inches of the bonnet seal along the back edge to allow for better air flow through the engine bay, an easy way to test this would be to pop the bonnet in test situation to see if this adds to cooling. Any comments from the bonnet louvre owners on this?

[sujitroy]

When I had my first Stag it was an auto and I also noticed fluctuation in the temperature gauge depending on the type of driving.

When it blew up it was rebuilt as a manual but very much modified so I couldn't make a fair comparison.

What might be significant though was the difference in fuel consumption, it was about 30% better. That meant 30% less heat generated so that has got to make a difference.

I think another factor in modern vehicles is the way the temperature gauge operates. I have read that on many moderns the temperature gauge is driven from the engine ECU rather than being a direct measurement of the engine temperature. this gives the manufacturer the ability to stabilize the temp readings so it always reads the same unless a pre determined point is reached, then it will tell you there is something going wrong.
If you get stuck in traffic with a lot of moderns you will see no change in engine temperature despite the electric cooling fan cutting in and out. There must be a temperature change , its just not shown.

i have found in most of my Triumphs the difference between an 82 degree thermostat and an 88 degree thermostat is about one quarter of the gauge. On my Triumphs where the cooling fan is triggered by the engine ecu there is a 3 degree C difference between on and off. As the temp gauge is the original Triumph item the gauge does display this difference, it is about 1/8 of the gauge which agrees with the difference between the different thermost readings

Neil

Thanks for the explanation.
Sujit

[sujitroy]

It's funny that people are talking about this. I was thinking about it today. I guess in essence the system has a heater (the engine) which produces heat at differing amounts when running. The radiator is able to dissipate heat dependent on water flow, airflow and the difference in temp between the air and the water. That means there are 4 or so variables.

engine heat output (which is essentially water temp and flow rate)
radiator cooling (which is based on air flow and temp differential between air and water)

At rest ticking over heat output from the engine is low, fllow is low and cooling relies on fan airflow and air temp. If your car can cool at idle at the max air temp in your region then the fan is good.

At cruise speed cooling should rely only on Rad airflow (and air temp) and the heat output of the car. In theory the latter is pretty set by the engine so if the car isn't cooling at cruise it is either water or airflow through the Rad

when the engine is labouring (e.g up steep hills), the engine heata output goes up as does the water flow as you tend to drop a gear. Air flow drops as the car is going slower. If the temp increases it means engine heat output is exceeding Rad heat dissipation. This is not an issue for most uphill as there will be a down hill after. However, if you are doing alpine passes then the solution would be to enhance airflow by either a spoiler or fan to push more air through the Rad. With the later solution youths the fan needs to push more air than the ambient airflow from the car just moving. Another solution would be to increase the size of the Rad. Going wider or higher would work but not possible really in a stag. Going thicker also helps with more tubes, even better, but not as good as larger. This is because the air gets hotter as you go through the Rad so the back tubes see less of a temp gradient between the water and air (like driving on a hotter day) . So increasing thickness is diminishing returns.

Interestingly, there is also an influence of air speed and I some cases slowing speed is better as it allows more heat transfer. I have a costing designed bonnet on my midget with a small frontal opening which opens out inside the bonnet and the cooling is excellent. Even running a 1.8 L engine.

I think this isthe way it works. Happy to be corrected.
tim

There's a freeway near my house, there is a relatively long gradual dip in it. At the dip, the ocean fog passes through it and the temp. difference before and after can be upward of 10 -20 degrees F. When I drive through the dip, the temp gauge goes down.

Sujit

[Tdafforn]

Something that an aussi owner put on here a while ago was that they had removed about 4 inches of the bonnet seal along the back edge to allow for better air flow through the engine bay, an easy wa.y to test this would be to pop the bonnet in test situation to see if this adds to cooling. Any comments from the bonnet louvre owners on this?

Venting to the back of the bonnet is an interesting one. This is a high air pressure area so not an ideal zone for sucking out heat. However it probably has a positive effect despite this.
tim

[Tdafforn]

Long run down hill and I can get the temp to drop compared to the flat..engine putting out less heat but air temp and airflow amd waterflow unaffected.
tim

[Wheelz]

I think the cooling margin is a big factor, i.e. if the cooling system has spare capacity. What I mean is that it can still keep cool without overheating even on the hottest days. So when driving well within those conditions, the thermostat is almost totally in control of the temperature, and the only fluctuations you should see is the difference between opening and closing temperatures, perhaps 10 degrees, and this might not show up as much movement on the gauge.

I think that Stags that show a much wider variation are in effect struggling to keep cool and are at the mercy of the varying driving and ambient conditions.

Only my theory, but interesting to learn that moderns don't really show the actual coolant temperature.

[trunt]

Agree on the cooling margin, that’s the key. My EWP keeps tight control of the temperature but the 3 degrees between setpoint and fan coming on is definitely visible, not much, it’s about a needle width for me so maybe a thermostats range would cause quite a movement? My thermostat is fully open in the summer (82 stat, 90 setpoint) so I don’t notice that affecting the needle. However I do see that when it’s winter cold and that thermostat opens my needle drops significantly.

[trunt]

One thing that surprised/alarmed me not having a mechanical engine fan was how the electric fan always came when driving at lower speeds after a highway run. Until it was pointed out that the high heat output of the engine at speed was well within the cooling capacity with the corresponding high airflow, but as soon as you slow down all that heat needs the electric fan to catch up..

[Wheelz]

One thing that surprised/alarmed me not having a mechanical engine fan was how the electric fan always came when driving at lower speeds after a highway run. Until it was pointed out that the high heat output of the engine at speed was well within the cooling capacity with the corresponding high airflow, but as soon as you slow down all that heat needs the electric fan to catch up..

Could that be due to water pump inefficiency? If the gap between vanes to cover is too great, efficiency at low revs might be poor.

[trunt]

Could that be due to water pump inefficiency? If the gap between vanes to cover is too great, efficiency at low revs might be poor.

It’s an EWP… so just the latent heat from fast running that still needs to be cleared out.. Normally cruising at 30 mph or so the fan doesn’t run even on 35 degree days. It does turn on if I stop at a light though!

[Tdafforn]

I think the cooling margin is a big factor, i.e. if the cooling system has spare capacity. What I mean is that it can still keep cool without overheating even on the hottest days. So when driving well within those conditions, the thermostat is almost totally in control of the temperature, and the only fluctuations you should see is the difference between opening and closing temperatures, perhaps 10 degrees, and this might not show up as much movement on the gauge.

I think that Stags that show a much wider variation are in effect struggling to keep cool and are at the mercy of the varying driving and ambient conditions.

Only my theory, but interesting to learn that moderns don't really show the actual coolant temperature.

Cooling margin is very important. My TR6 has a wide margin and hence the stat is in control at all times bar waiting in traffic. Even driving up alpine passes. Stag is more marginal with temps rising on long steep inclines but not enough to activate the electric fan. Obviously an engine driven fan would reduce this effect.
tim