Rad cap pressure

[sujitroy]

Is there any harm is using a 13 psi rad cap than a 20 psi rad cap? My Stag doesn't have an issue other than it slightly leaks water from the water pump. The temp gauge behaves itself. I periodically top up the Stagweber header tank. The problem is the current rad cap doesn't have ears to grab on to making it difficult to undo. I have many 13 psi rad caps which do have the ears. The car is a Mk1 with a Mk1 rad with a Stagweber header tank. I'm aware going from a 13 psi to 20 psi could cause hoses to leak, but I'm assuming going the other way should be OK for my Stag.
Sujit

[Phil S]

Not a problem as long as the overall height from the cap to the base seal are the same as some caps are different heights. The pressure raises the boiling point in the system by around 1 degree per psi

[Andrew S]

13lbs every time for me. Why put the inlet manifold gaskets under extra pressure. Probably why so many have water in the V. LOL
I believe that having a higher pressure cap was to increase the boiling point temperature level. Might be wrong.
If your car is running cool, then use a 13ib cap, I have for years and I know a respected Stag specialist who has recommended this for years now

[Goldstar]

As Andrew says that was the original spec on the early (up to LD 10195) cars 20 psi post that. I’m guessing that coincides with the change from cap on the rad to cap on the expansion bottle?

[jbuckl]

Is there any harm is using a 13 psi rad cap than a 20 psi rad cap? My Stag doesn't have an issue other than it slightly leaks water from the water pump. The temp gauge behaves itself. I periodically top up the Stagweber header tank. The problem is the current rad cap doesn't have ears to grab on to making it difficult to undo. I have many 13 psi rad caps which do have the ears. The car is a Mk1 with a Mk1 rad with a Stagweber header tank. I'm aware going from a 13 psi to 20 psi could cause hoses to leak, but I'm assuming going the other way should be OK for my Stag.
Sujit

It will lower the coolants boiling point. As per phil’s reply. Using a 13psi cap is similar to using a band-aid that’s smaller than the cut.

[Wheelz]

It will lower the coolants boiling point. As per phil’s reply. Using a 13psi cap is similar to using a band-aid that’s smaller than the cut.

[stag8manod]

Hi, I cap run a 16lb cap on my custom header tank, no problems.
Cheers Ian A

[wilf]

My engine rarely if ever goes above 90 - 95C according to my gauge (which I have "calibrated"), so in theory I could run with no pressure cap at all as it never actually reaches boiling point.

However - I susepct the reason for the Stag engine having/needing pressure in its cooling system is to deter spot boiling within the system, proabably around No 8 cylinder/cylinder head. But as with so many things, it is quite possible that the 20psi figure was chosen based on little if any evidence/research, so with that in mind, a 13psi cap could be just as effective.

[DMT]

Am I missing something here. If the car runs at less than 13psi then using either cap will be the same (assuming both caps seal effectively) so fit a 13psi cap, put a vessel under the cap overflow and if it ends up with coolant in go back to the 20 one.

[Goldstar]

Am I missing something here. If the car runs at less than 13psi then using either cap will be the same (assuming both caps seal effectively) so fit a 13psi cap, put a vessel under the cap overflow and if it ends up with coolant in go back to the 20 one.

Seems logical to me

[marshman]

My engine rarely if ever goes above 90 - 95C according to my gauge (which I have "calibrated"), so in theory I could run with no pressure cap at all as it never actually reaches boiling point.

However - I susepct the reason for the Stag engine having/needing pressure in its cooling system is to deter spot boiling within the system, proabably around No 8 cylinder/cylinder head. But as with so many things, it is quite possible that the 20psi figure was chosen based on little if any evidence/research, so with that in mind, a 13psi cap could be just as effective.

Seems like a very sensible / plausible explanation/reason for a high pressure cap.

If time and money were no object it would be interesting to fit multiple temperature sensors actually in the water flow areas all around the heads to form a much better picture of what is going on, but doubt that will ever happen.

I find it interesting how the head gasket has obviously been "engineered" to modify/restrict the water flow around various parts of the head, with some water channels blocked completely, presumably to try to get even flow and temperatures along the head. The Dolly Sprint is the same - as I suspect is the Dolly 1850, the head gasket restricting water flow to the front cylinders much more than the rear.

I have one of those "rare" Stags where the temp gauge sits just left of centre all the time (even in 30 deg + temps sitting in traffic - standard MK2 set up with expansion bottle and only the viscous fan. I removed the kenlowe fitted by a previous owner when I saw how much of the radiator core it obscured - it must have an effect on the air flow through the rad and the rads ability to cool at speed. Since doing the that the temp has been very stable, before it had a tendency to rise sitting traffic or at motorway speeds.

[sujitroy]

Thanks all.

[jbuckl]

Retain a 20psi cap. Ensure the system is sealed.