What's the correct radiator cap pressure for a Mk3 Spitfire?

[1969Mk3Spitfire]

Should the car run with a 7 or a 13 psi cap? 

My "search" skills aren't very good and I can't find a definitive answer.

Is there a down side of using 13 psi, as my car has this cap fitted?

Many thanks for any help

 

[johny]

The downside is that you can run with a more overheated engine, possibly without noticing it, before steam comes out to advertise that you risk doing damage if you dont stop.

I believe your car had a 7psi cap as standard and it was the later models that Triumph designed to run hotter to improve emissions...

[NonMember]

8 minutes ago, johny said:

The downside is that you can run with a more overheated engine,

Err... no. Not really. The engine isn't "overheated" until the steam escapes; before that it's just running in a slightly warm condition. Your temperature gauge will have more chance to warn you about the cooling failure before things get to a "you really must stop NOW" condition.

The 13PSI cap will marginally reduce the amount of coolant that gets cycled in and out of the overflow bottle. It will also put a small amount of extra strain on the hoses, radiator and heater matrix, which will only be a problem if they're rather past their best.

[johny]

14 minutes ago, NonMember said:

Err... no. Not really. The engine isn't "overheated" until the steam escapes; before that it's just running in a slightly warm condition. Your temperature gauge will have more chance to warn you about the cooling failure before things get to a "you really must stop NOW" condition.

The 13PSI cap will marginally reduce the amount of coolant that gets cycled in and out of the overflow bottle. It will also put a small amount of extra strain on the hoses, radiator and heater matrix, which will only be a problem if they're rather past their best.

Err... yes with a 13psi cap you can run the engine at a temperature above that at which steam would have come out with a 7psi one!

[1969Mk3Spitfire]

35 minutes ago, johny said:

I believe your car had a 7psi cap as standard.........................

Do you know that for sure?

23 minutes ago, NonMember said:

The 13PSI cap will.............also put a small amount of extra strain on the hoses, radiator and heater matrix, which will only be a problem if they're rather past their best.

This is probably at the root of my concern.  My radiator and all hoses are new but the heater matrix looks original.  I've just removed the heater box as the motor wasn't working.  Thanks to advice on here, I used a bicycle inner tube to pressure check the matrix while it was out.  It was fine but it's not a job I'd particularly like to do again so I'm not keen on putting stress beyond the original design through the old heat exchanger.

I'm guessing that 7 psi caps are readily available from the usual suspects?

[johny]

Canley Classics certainly indicate the 7lb cap for your car, part no. GRC112

[Mad4classics]

The Spitfire MK3 spare parts catalogue lists 13lb fitted from approx FD46853, 7lb before; it also lists a temperature sender change at the same point.

However, fit whichever cap you're going to feel most comfortable with.

 

[NonMember]

1 hour ago, johny said:

Err... yes with a 13psi cap you can run the engine at a temperature above that at which steam would have come out with a 7psi one!

... without it releasing the steam and therefore without overheating. Thus a 13psi cap make you LESS likely to overheat, not more. The engine doesn't care much whether you're at 95 or 100 and the higher pressure won't actually make it run hotter, but it might save you from having to stop if you get stuck in traffic and the high gauge reading is the first thing you know about, say, a broken fan.

The factory didn't raise the cap pressure because the engine had changed - it really hadn't - but because it allowed them greater tolerance as long as the cooling system materials were up to the job.

[johny]

I think we must be confusing 'overheat'! For me its whenever the engine is operating at a temperature above its optimum as the oil thins and its pressure drops so increasing the risk of metal to metal contact. Triumph designed the engine to run up to a temperature that cant be exceded because the pressure of the cooling system allows the coolant to boil. If you change that pressure with a different cap you can now run the engine above the limit that Triumph set.

According to the steam tables the temperatures reached are roughly 111º and 119ºc for the two caps respectively. Not a great difference and its up to the owner to decide how big the risk is....

Wouldnt like to say what changes, if any, Triumph made when going to the higher pressure cap. 

[NonMember]

I think you're misinterpreting the purpose of the cap. Venting the steam absolutely DOES NOT prevent the temperature going up - quite the opposite! The pressure cap allows you to get above 100C without that happening but can't be set too high because the system isn't designed to withstand too much pressure. The correct engine temperature is controlled by the thermostat - a higher pressure won't make any difference to that, except in as much as it keeps local hot-spots under control (with actual liquid coolant to remove the excess heat, instead of a nasty gas bubble) for longer.

[johny]

No, once the cap opens the coolant temperature cannot rise anymore. Its impossible because if the system pressure doesnt increase then the water must change into steam at the temperature dictated by the steam tables. This turning into steam is actually an effective form of cooling (very early IC vehicles used it) as more energy is removed than by just warming water but of course when the coolant is lost, and the engine continues to run, the temperature is free to rise until lubrication failure takes place. 

The thermostat is only effective if theres spare cooling capacity and when thats used up the thermostat is fully open and the only limit to temperature rise is the opening of the cap.

[NonMember]

And the moment you become reliant on evaporating your coolant you are overheating, because you are losing the capacity to circulate the heat, even if not yet catastrophically. If that happens a bit later, even at a higher temperature, then you have more chance to prevent damage, not less.

The early IC engines that used evaporation also had very inefficient fully open thermosyphon-only cooling and ran at lower temperatures where everything was less efficient. The cap opening on a sealed system does not immediately lead to removal of the excess heat by venting steam. First, it generates bubbles around the local hot-spots causing potential head damage due to localised complete failure of cooling. That's much more immediate an issue than the delayed effect on oil temperature.

You appear to be advocating "releasing the steam" as an early warning. I'm insisting that it's already too late when that happens, and postponing the event is therefore better.

[johny]

well the cap opening is a good warning in the case that the driver hasnt noticed the rising engine temperature (it happens!) and its very effective in ensuring action is taken (stopping). Of course it depends on the driver but from a purely engineering point of view I think its better this happens earlier at the temperature Triumph recommended... 

[Pete Lewis]

its equally as important that the cap seal depth is correct or the cap wont fit or the seal is not going to seat inside the filler neck

( there are 2 cap depths around )

on the semi sealed the upper cap seal must be fitted to enable coolant expelled is sucked back from the overflow  and replace the expelled coolant when cooling 

in a non sealed overflow you dont need this upper seal but essential on the semi sealed 

part numbers give the correct clues 

pete

[1969Mk3Spitfire]

Reading the Rimmer Parts & Accessories catalogue it says that Mk3 from FD 48653 had a more efficient 16 inch radiator with 13 psi cap.  In the parts listings, it shows that the 16 inch, 7 psi radiator is no longer available.

My car is FD36xxx but it has a new radiator with 13 psi cap.  I almost certainly have a 51 year old heat exchanger in the heater box.  I have a, probably unfounded, fear of putting too much pressure through the heater inducing a leak.  Having spent an uncomfortable couple of days lying on my back, contorted into the footwells to remove and refit the box, I'm not keen to repeat the exercise.

What's the affect of using a 7 psi cap on the "more efficient" radiator, anyone know?

 

[Pete Lewis]

nothing  the pressure just increases the boiling point of coolant   7 lb will be fine 

if you have a reasonalbe cooling system and you can keep the header topped up without spillage  most will run without a cap quite happily

this doesnt work well on low header radiators 

Pete

 

[Iain T]

After a run on a hot day I noticed coolant coming out of the rad cap and not going into the overflow bottle. On inspection I found the cap when closed had very little pressure on the top seal and probably the bottom. I changed the cap and solved the problem. Another case of variations in aftermarket parts! My old cap is stainless steel and looks to be better made but it doesn't work.... 

Iain