Radiator Caps - 7 vs 13

[Greta]

Some time in the production of Spitfire's Triumph changed from 7 lbs to 13 lbs radiator caps.

My MK II has a newer style full width radiator and relatively new hoses - should I retain a 7 lbs cap - as running at higher pressure may blow the heater matrix and perhaps shorten the life of the water pump - or can I safely switch and thereby through higher pressure reduce my boiling point ??

[drofgum]

Greta,

I have owned my Mk2 for 34 years and only used a 7 lb cap the entire time. In contrast to the later Spitfires it runs on the cool side. So I have never seen the need for a 13 lb cap.

Cheers,

Paul

[RobPearce]

The 13lb cap was not intended to reduce running temperature. Quite the opposite!

Higher pressure in the system INCREASES the boiling point of the coolant, which lets you get away with a hotter engine, which in turn improves combustion for better emissions and economy.

[glang]

Why not try running with the 7lb cap Greta and if it lifts too early ie you feel the engine isnt really overheating change to the 13lb item?

[Bitumen Boy]

Many years ago we had several Acclaims as well as my Herald. One time we needed a rad cap for an Acclaim which should have been 13lb or possibly even 15lb, but in a pinch I nicked the 7lb cap off the Herald as a temporary measure. It worked fine until the correct part was obtained a few days later and the 7lb cap returned to the Herald.

I don't think it makes a lot of difference to be honest!

[Clive]

The extra 6lb makes about 10degrees (C) difference to the coolant boiling temp. So a 7lb cab will boil pure water at about 110, and the 13lb at about 120.

50% antifreeze will increase both those by about 6 degrees. 7lb/116degrees, 13lb/122degrees

So if temps stay lower than that it won't make a jot of difference. The higher pressure one is there to stop boiling under extreme conditions.

[Rosbif]

Quoted from cliftyhanger-

 

So if temps stay lower than that it won't make a jot of difference. The higher pressure one is there to stop boiling under extreme conditions.

I agree.

The thermostat opens well below 90° so if the cooling system is working as it should there shouldn't be any effect between a 7 or 13 lb cap in practice, i.e. they wouldn't open. It is only in exceptional conditions that a 7 lbs cap would open and extreme conditions that a 13 lb cap would open. My 13/60 has a 13 lb cap which is the standard item for this model so I guess Triumph wanted to add an extra buffer to overheating before the cap vented.

[glang]

Dont forget that the cap has to lift as the engine warms up to dump the expanding water to the bottle. So which ever cap you use the entire system including head gasket, water pump and hoses will have to withstand its rated pressure.... 

[RobPearce]

Quoted from Rosbif-

 It is only in exceptional conditions that a 7 lbs cap would open and extreme conditions that a 13 lb cap would open.

Simply not true. As glang pointed out, the cooling system on Heralds and Spitfires is designed without any expansion room (no gaseous header) so the coolant will always exceed the cap pressure once it warms up. Liquid water expands with heat and is near-as-damnit incompressible. It overflows into the desertising bottle, to be drawn back in as the engine cools down.

Modern cars (and even the later Triumphs like the Dolomite) use a different design of cooling system, where the cap is on a separate expansion bottle, which provides a compressible air buffer. On those, you may not get the cap opening.

[Rosbif]

Quoted from RobPearce-

Quoted from Rosbif-

 It is only in exceptional conditions that a 7 lbs cap would open and extreme conditions that a 13 lb cap would open.

Simply not true. As glang pointed out, the cooling system on Heralds and Spitfires is designed without any expansion room (no gaseous header) so the coolant will always exceed the cap pressure once it warms up. Liquid water expands with heat and is near-as-damnit incompressible. It overflows into the desertising bottle, to be drawn back in as the engine cools down.

Modern cars (and even the later Triumphs like the Dolomite) use a different design of cooling system, where the cap is on a separate expansion bottle, which provides a compressible air buffer. On those, you may not get the cap opening.

Sorry but you are wrong. The 13/60 does NOT have a bottle therfore if the cooling system does overheat and liquid is vented it is lost. If your comments were correct then everytime I took my 13/60 out I would need to top up the coolant when I returned, which clearly isn't the case.

You are correct in that the liquid isn't compressable but the air above it is. A correctly filled radiator has a level of air above the liquid which can be compressed hence no venting. Occasionally I have overfilled my radiator and the excess is vented, but that is all.

Some models of Triumphs do have a reservoir bottle and any vented liquid can be sucked back in as you say. I don't think the bottle is there on the basis that everytime the car is used liquid vents to it, it is there as a reserve and stops the total loss of liquid should the system overheat.

[RobPearce]

If your 13/60 doesn't have a bottle then it's missing part of the system Triumph designed it to have. I am not wrong.

[glang]

well having a quick look I can see a bottle listed for Spitfires but not Heralds which is very strange as I thought the cooling systems were pretty identical. The water still has to expand on heating (I calculate about 1/4L on my Vitesse) so if a bottle isnt required on a Herald theres only two possibilities:

The radiator isnt completely filled so theres an air pocket or the hoses somehow flex enough to absorb the expansion....

[PeteStupps]

I was intrigued enough to look at the 13/60 parts diagram on Canley's website - it shows an overflow pipe but no bottle (unlike the Spitfire) https://www.canleyclassics.com/triumph-herald-13/60-radiator.

My old MG had the same setup, and whenever I filled up the radiator there would be a trickle from the overflow next time it warmed up. Presumably this then created an expansion air gap in the top of the radiator, which is sufficient at normal running temperature. 

[glang]

yes perhaps it depends on the height of the radiator as I imagine the Herald being further back than the Vitesse is a little higher so it can run with an air pocket (and no bottle) and not stop the heater working. In the Vitesse when the heater stops working I know air has got into the system....

[Rosbif]

Quoted from PeteStupps-

I was intrigued enough to look at the 13/60 parts diagram on Canley's website - it shows an overflow pipe but no bottle (unlike the Spitfire) https://www.canleyclassics.com/triumph-herald-13/60-radiator.

My old MG had the same setup, and whenever I filled up the radiator there would be a trickle from the overflow next time it warmed up. Presumably this then created an expansion air gap in the top of the radiator, which is sufficient at normal running temperature. 

Spot on, treat yourself to 

[Rosbif]

Quoted from PeteStupps-

I was intrigued enough to look at the 13/60 parts diagram on Canley's website - it shows an overflow pipe but no bottle (unlike the Spitfire) https://www.canleyclassics.com/triumph-herald-13/60-radiator.

My old MG had the same setup, and whenever I filled up the radiator there would be a trickle from the overflow next time it warmed up. Presumably this then created an expansion air gap in the top of the radiator, which is sufficient at normal running temperature. 

Spot on, treat yourself to 

[Rosbif]

Quoted from PeteStupps-

I was intrigued enough to look at the 13/60 parts diagram on Canley's website - it shows an overflow pipe but no bottle (unlike the Spitfire) https://www.canleyclassics.com/triumph-herald-13/60-radiator.

My old MG had the same setup, and whenever I filled up the radiator there would be a trickle from the overflow next time it warmed up. Presumably this then created an expansion air gap in the top of the radiator, which is sufficient at normal running temperature. 

Spot on, treat yourself to 

[Rosbif]

Quoted from PeteStupps-

I was intrigued enough to look at the 13/60 parts diagram on Canley's website - it shows an overflow pipe but no bottle (unlike the Spitfire) https://www.canleyclassics.com/triumph-herald-13/60-radiator.

My old MG had the same setup, and whenever I filled up the radiator there would be a trickle from the overflow next time it warmed up. Presumably this then created an expansion air gap in the top of the radiator, which is sufficient at normal running temperature. 

Spot on, treat yourself to a glass of something - as long as it isn't anti freeze 

[glang]

Can confirm that everytime I use the Vitesse the bottle level goes up when hot and then back down as it cools. The overflow pipe is also always full of liquid and logically when I remove the rad cap its full to the brim. In fact when you look at a rad cap its not just got the thermostatic element but also another little reverse sprung valve that is pulled open whenever theres a drop in the system pressure. In the end coolant is going back and forth to the bottle with engine temperature changes such as those produced by hills, traffic and stopping after high speed runs🤪

[Rosbif]

Quoted from glang-

Can confirm that everytime I use the Vitesse the bottle level goes up when hot and then back down as it cools. The overflow pipe is also always full of liquid and logically when I remove the rad cap its full to the brim. In fact when you look at a rad cap its not just got the thermostatic element but also another little reverse sprung valve that is pulled open whenever theres a drop in the system pressure. In the end coolant is going back and forth to the bottle with engine temperature changes such as those produced by hills, traffic and stopping after high speed runs🤪

I wonder why Triumph did away with this for the Herald? Maybe they just thought as the Herald had an air gap the bottle system wasn't needed.

The filling instructions for the 13/60 radiator, after a change of coolant etc, actually do say to fill it to the bottom of the filler orifice. This explains why on the first run afterwards there is a small quantity of coolant vented.

In the periodic checks for the radiator water level it says to :-

top it up to one inch below the filler neck. This will allow for the expansion of the coolant as the engine warms up and is particularly important if an anti-freeze mixture is being used.

All of this doesn't help the origin question for a Spitfire - 7 lb or 13lb.🤔

[glang]

Got to say I prefer the Herald system (basically most cars have that arrangement now) for its simplicity....

[nang]

Slight thread drift here but :-

In the '70's I lived and worked in the snow country in New Zealand. Back then anti freeze was uncommon apart those of us who were subject to freezing temperatures and was more  expensive than at present.

I think it started off with Landrovers, you could buy a kit consisting of a water overflow bottle, clamps and a washer that fitted between the radiator and the cap. This was necessary to keep the vacuum in the radiator so the water (coolant) would suck back into the radiator from the overflow bottle when things cooled down.

Being Landrover these kits were quite expensive so we ended up making our from cordial bottles and larger ones for the likes of RL Bedfords and other trucks. The important bit was the rubber washer. Without it no vacuum in the radiator and coolant was not sucked back up.

This was a most successful project. All the locals vehicles were so fitted, radiators stayed full.eliminating air in the system and no loss of antifreeze.

Now seems to be a standard set up in modern cars.