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Typical SP250 engine operating temperature / thermostat clarity...

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Typical SP250 engine operating temperature / thermostat clarity...

Post by Agnello11 » Mon Jun 10, 2019 6:38 am

Having finally managed to get our lovely old SP250 running properly (see separate thread for that saga!), we want to make sure it continues to do so for years to come....

With that in mind attention has turned to the operating temperature of the engine.

We do not have the original fixed fan arrangement on our car, having a Kenlowe with adjustable thermostat instead and it would be useful to hear other's experience of where to set the Kenlowe thermostat for best resuslts. Currently I feel the Kenlowe may be running too long and I might have it set too 'cool', but I don't know where I should be expecting to see the needle on the Smiths temperature gauge.....

We have the standard Smiths 90/185/230° F gauge and sender and the rest of the cooling system is also standard, with a re-cored radiator etc. I am used to Smiths gauges of this type being supplied with power via a voltage stabiliser regulated at 10v, yet I note no stabiliser fitted to our SP250 anywhere.... is that correct? Should there be a stabiliser to supply regulated gauge voltage or are the gauges supplied at battery (and thus the variable charging) voltage?

Where should I expect to see the needle on my gauge? It spends a fair amount of time hovering around the 185 level, pushing higher on the motorway, but I am wondering whether that higher reading is as a result of higher charge voltage at the higher engine rpm and thus a higher gauge reading ... or whether it really is reflecting higher engine operating temperatures...

The next question concerns the thermostat. What temperature thermostat is regarded to be the best compromise nowadays? and should the thermostat be the type with a 'weep' hole to aid self-bleeding / allow some circulation even when closed? I can't say what temp. thermostat is fitted currently because I don't know verbatim, but given the price of them I am tempted to simply acquire one, with members guidance, that would be considered correct and just change it so I can relax about it in future.

We have had an infrared temp gun on the engine / thermostat housing / radiator etc. which showed the gauge is over-reading a little (hence querying the voltage stabiliser) but that also served to confirm that we weren't 100% sure what sort of actual temperatures we ought to be seeing at the various measuring points....

All references and pointers appreciated!

Ian Slade
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Re: Typical SP250 engine operating temperature / thermostat clarity...

Post by Ian Slade » Mon Jun 10, 2019 9:59 am

Work on the theory all gauge needles are vertical at 70mph/3500rpm with the battery in the charged position.
Owner since the 70's, Genghis is slightly to my left.

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Re: Typical SP250 engine operating temperature / thermostat clarity...

Post by JohnM » Mon Jun 10, 2019 11:32 am

Hi Larry,
firstly, to put your mind at rest, the type of guage fitted to the SP does not need a voltage stabiliser - it's of a type known as "Wheatstone Bridge" which self-regulates for voltage changes, so don't worry about that.

secondly, your temperatures seem pretty well in line with the "norm". Although I'd expect the temperature to drop back from the 185 if you run "cruising" on the motorway rather than on power, that could depend on the thermostat you have in the car.
As an example, mine has a cool (often called "summer") 74deg thermostat fitted, so water will start to circulate round the rad as the gauge reaches about 165deg, but if yours has a hot ("winter") stat fitted, water will only start to circulate at about 190deg on the gauge - pretty well exactly where yours is running. The choice of stat is yours - a 74deg like mine means the system will generally run cool, as the SP system is pretty efficient when you're driving, but that means the heater's not very hot either! An 88deg gives you nice warm feet, but for me gives the system less time to cope with a sudden heat load, like dropping from a steady 70mph down to 10 or less in an M25 traffic jam! An 82deg would probably be a good compromise, and I intend to try one at some stage.

As to your Kenlowe setting, my preference is to set the fan stat so the fan comes on just as the temp needle leaves the end of the white arc on the gauge going towards 230. This seems to be a good compromise between the fan not running too long and the temp not getting too high. The fan should then run until the needle comes back to about 1/8" inside the white arc and then go off. Set like this, mine will sit on the drive idling all day without overheating ( and more importantly, it'll do it in a traffic jam!). In normal running, it should be rare for the fan to come on unless you're consistently running below about 30mph when there isn't enough natural airflow through the rad.

Hope that helps, and it sounds like your SP is in great fettle to use over the summer - enjoy!

cheers, John
John M in Middlesex, NW London
1962 SP250 "B" Spec

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Re: Typical SP250 engine operating temperature / thermostat clarity...

Post by John-B » Mon Jun 10, 2019 11:32 am

I think you are worrying too much. as long as the needles are all nearly vertical, as Ian says, then just relax.

When my car was returned after Phil Glennerster had restored the engine, the temperature gauge was usually a bit lower than 185 degrees, but the gauges are hardly ever precise. As long as the coolant is very hot I don't think a variation of ten degrees is enough to worry about. My temperature gauge does read higher on a fast run in very hot weather (like 1st June), but then my Kenlowe fan comes on for about five minutes. The ammeter tells me when it is on. It always comes on for about five minutes when the engine is turned off.

If the gauge does get right to the top 230 or more, stop and check for leaks!

Phil fitted a 76 degree thermostat (fail = closed).

Make sure you have blue coolant (Bluecol). Don't keep topping it up to the top. It will expand and as long as it settles at the bottom of the expansion tank filler tube when cold, leave it.

I was told by Phil that if you set the Kenlowe fan to come on at a higher temperature (to let the car run hotter), the temperature at which it will go off is lowered, ie the band between on and off is increased at both ends. I think this would mean that once the fan had come on during fast run on a hot day, the coolant temperature might never get low enough to make it go off! Set the Kenlowe so that it never comes on except on a really hot day or when idling, and it goes off after about five minutes.

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Re: Typical SP250 engine operating temperature / thermostat clarity...

Post by Agnello11 » Tue Jun 11, 2019 9:34 am

Excellent advice and comments! Thank you gentlemen 8-)
Given what's been said I would guess I have an 88 degree thermostat fitted and that would probably fit with where I am seeing the gauge readings.... useful tip to look for vertical needles when cruising, thank you.
Having nudged the Kenlowe it sounds like I may have it just about right now as it comes on just about at the end of the 185 'arc'. It's never been terribly efficient and runs for quite a while, but does seem just about up to the task. One of my jobs on the 'to do' list is to change the Kenlowe 'puller' fan for a more modern 'pusher' and mount it within the air ducted side of the radiator.... I've always been a bit dubious about fans that try to 'suck' cool air through the radiator without a proper cowl to manage ariflow as it's so much easier for them to suck air from everywhere else around, so I think it's worth a go to see if I can reduce fan running times by pushing air through it instead.

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