DD, There are a couple of brands that do 20 to 25 psi radiator caps.
Morosso and Stant.

On a race engine where the water pump has been geared down to prevent cavitation at high rpm, at low rpm (when following a pace car or during a pit stop) the mechanical pump doesn`t make the block pressure to prevent localised boiling around the combustion chambers/exhaust valve seats so the high pressure cap helps prevent this.

Its a very real situation, even on road cars that are pushed hard. without a good radiator cap the temp guage will still read low but the coolant around the combustion chambers could be boiling. pushing the engine close to detonation.
The temp guage only tells part of the story.

High pressure raises the boiling point. So even if it the temperature is far more than 100C, no boiling will occur.

This is applied to all modern autos, which generally now use 100C thermostat and pressure almost 1 bar.

Ok checked the L-series 100mm w/pump pulley against an early A12 one. Fits across 4 bolt holes but out of alignment 10mm. Looks by other photos that it may line up with a later model one.

If Mr. Racetech was able to make 20+psi of block pressure he wouldn’t need to worry about a radiator cap strong enough to hold it, he would need to worry about his AGB bursting and his radiator end takes blowing off.

Running 20+psi on anything but a dedicated race car is not safe, and not needed unless you have a drastically inadequate or poorly setup cooling system.

I think a lot of people forget that in the 70's people made massive power with great reliability out of the A-series on almost stock cooling systems in full race conditions.

The stock system may well be making 20psi block presure. Ive never measured an "A" series system. This is the presure between the waterpump and the thermostat or restrictor. You dont have to have high presure in the radiator to have high block presure. The presure the pump makes in the block is over and above the cap presure.

Eg. Cap presure 13psi, Pump presure 7 psi to the thermostat/restrictor= 20psi block presure....
The most the radiator sees is 13psi.

I was suggesting the high presure caps when using an electric pump instead of the standard pump on race applications.

Sikyne's on the money.

Many don't realise this is an important part of how the cooling system works. Block pressure created between the pump and thermostat (or restricter) is what greatly assists the coolant picking up heat (then taking it to the radiator to be dissipated).

This is why you should never run an engine with no thermostat - the water temp reading may come down but that's simply because its not collecting heat from the engine.

Being an HVAC engineer at one of the largest automotive OEMS, on most every car I have ever tested, the pressure in the block has never been 2-3psi higher than radiator with the thermostat open (obviously closed is a different story, but how often is it closed in race conditions?) If you think the block is magicaly presurised by the thermostat then how do cars that run pre pump thermostats also generate slightly elivated block pressures?


Your biggest problem with removing the thermostat besides for the mechanical clearances is actualy flow not pressure, to much flow = not enough time to absorb the heat, this is why systems are are open loop use restrictors to slow the flow in key area's.

Again, 95% of above info is totaly pointless in the A-series unless your aiming for 180+hp

how do you put a restrictor in?

It is usualy a "washer" that is put in place of the thermostat when using an electric waterpump, you change the ID of the "washer" to get your correct flow rate, best measured by checking actual flow as well as the differential temp between engine in and engine out at WOT. It is not uncommon to use restrictors in more than one place.

Rezlo, i agree that the info i am providing isnt applicable to all but the top HP engines. But please dont discount it.
Im not a HVAC engeneer, But after owning a radiator buisness for several years i have learned a bit.

We do agree on most things:)

I regards to your questions, systems with pre pump themostats? any increase in pressure will still be due to a restriction somewhere. it may well still be the thermostat. after all it is a loop. The system itself will have a slight restriction all the way round. Even an open thermostat is a restriction that will give a block pressure increase at any decent RPM.