Rads and Caps !

[68spitfire]

Hi again,

 

Whilst I have been doing my coolant change I have noticed something that some enlightened person may be able to offer a perl of wisdom on.

 

I have a 1968 Mk3 Spitfire in standard tune.

 

The difference being that since I bought the car (1989) it has always been fitted with a Mk1 full width (22" ") radiator and expansion tank over the carbs.

 

Now looking in the Haynes manual it states that the MK1 with wide radiator requires a 7lb radiator cap.

 

The Mk3 as original would have had a narrower radiator with a 13lb cap.

 

 

Here's the difference. I appear to have a 13lb cap on my expansion tank, if the 13 stamped on the top is what it means.

 

Now as I have the MK1 Radiator should I change this to a 7lb cap ?

 

Does cap blow off pressure dictate how hot/cold the engine runs? as I read somewhere that the radiators were changed to narrower ones due to overcooling.

 

Any thoughts?

[Guest]

7lb to 13lb is quite a hike and will put significant pressure on the radiator core if a cooling problem occurs; as you say the lower the cap pressure the easier it is for the cap to allow excess pressure to be dealt with, i.e. the coolant.

 

Additionally IIRC the higher the rad cap pressure the higher the boiling point of the water will become - although I will be grateful if someone can confirm that as I am not 100% certain.

 

Some owners put a cap on that is a pound or two higher but a jump of 6lbs is too much and of course the system was not designed to take such an increase.

 

Hope that helps ??

 

Regards.

 

Richard.

[Casper]

 

 

Additionally IIRC the higher the rad cap pressure the higher the boiling point of the water will become - although I will be grateful if someone can confirm that as I am not 100% certain.

True but the temperature of the coolant as a whole is controlled by the thermostat.  Any nucleated boiling in the passageways of the head (where it's hottest and the cooling is required) will be affected by the pressure.

My understanding was that the caps were increased to 13 lbs as 1. this was beneficial, and 2. the construction of the hoses had changed and they were able to take the pressure.  

Radiators designed for the 7lb cap will not normally take the 13 lb caps as the tabs are larger.   But yours can.

I run a 13 lb cap on a wide radiator on my 1200 (I had to open up the fitting to allow the wide tabs.

BTW, I believe the cooling ability of a narrow rad is notionally the same as a wide one because the they are 3 row, compared to two row on the wide.  They were of course cheaper, which is the main reason for the change.  

Over-cooling will not occur.  Cooling is controlled by the thermostat, not the cooling ability of the radiator.

Awaits flames

 

C.

[68spitfire]

Thanks for the replies.

 

Well in the years I have run the car it has never boiled over so I will keep it as is then.

[Guest]

Thanks Casper - I'm obliged.

 

Regards.

 

Richard.

[Guest]

Absolutely - if it's not broke why fix.

 

A good point raised, though.

 

Reagrds.

 

Richard.

[Pete Lewis]

Casper if your waiting for flames

 

sumuts wrrrrong

 

no pressure Ha

 

pete