20psi coolant pressure?

simmit · Post by **simmit** » Mon Nov 12, 2018 1:38 pm

The 2.9efi engine (Middlebridge) has coolant system pressure cap at 14psi. Most modern cars run at about 20psi (advantage being that higher boiling point temperature gives less chance of steam forming in localised hot spots).
Do we think the 2.9efi engine would be OK at 20psi?

Post by **scimjim** » Mon Nov 12, 2018 2:27 pm

The first question is - why? Do you have a problem with localised boiling?

50/50 antifreeze boils at 106 (ish) deg and pressurising it to 14psi brings it up to 125 deg - the average temp in the block is 90 deg, so what's the max local temps?

Overall, if it was designed at 14psi, I'd be reluctant to simply change the expansion tank cap to raise it to 20psi as you're adding pressure throughout (hoses, rad, etc).

Kccv23highliftcam · Post by **Kccv23highliftcam** » Mon Nov 12, 2018 10:13 pm

Hmm.

My 18yr old quattro never lost a drop until cx'd, on spec, the water pump along with a shedload of "stuff" because we were in there.
It had a pink stain on the bottom from the gland seal.
I've spent 2 yrs going around discovering new weeps, drips and at times spurts!
But I'm happy that it will never cook because the impeller has melted....

simmit · Post by **simmit** » Tue Nov 13, 2018 1:56 pm

scimjim wrote: ↑
Mon Nov 12, 2018 2:27 pm
The first question is - why?

I'm playing with radiators/ engine cooling at the moment and simply curious to know if current thinking can be applied to our older cars with good effect.

It seems that in localised areas within an engine the coolant temperature can rise well above that indicated by the dash. gauge and steam pockets can get locked in awkward places (steam is poor heat conductor so not good). Apparently some engines do have several passages to vent steam pockets back to reservoir tank.

OK, perhaps this is over the top for our purposes but if the 2.9efi can accept higher pressure without harm (thus less likely to generate steam) then why not do so?

I'm not knowledgeable on the subject so ask the question.

Post by **philhoward** » Tue Nov 13, 2018 2:06 pm

I ran the cooling system on my 5a at 20psi (1.4 bar) - original type radiator, all new hoses and a Vauxhall X30XE V6 engine and it was fine. I did (as a test) run the cooling system up to 135 degrees with no boiling or venting obvious. My thinking was that the Omega that the engine came from ran a 1.4 bar pressure cap (and its cooling fan doesn't run at high speed until well over 100 degrees as standard). The question was whether or not the rest of the system could cope - which it did. Under normal conditions, when on the road, the rad fan was controlled by the standard Otter switch so held the temperature at around 90 degrees or so.

I suspect casting quality has as much as anything else when it comes to forming steam pockets - the newer the engine, the smoother the casting, hence the lower the chance of getting localised boiling.

Post by **Roger Pennington** » Tue Nov 13, 2018 6:16 pm

simmit wrote: ↑
Tue Nov 13, 2018 1:56 pm
if the 2.9efi can accept higher pressure without harm (thus less likely to generate steam) then why not do so?

I doubt if running the *engine* at 20psi is really the issue. The place where any problems resulting from the higher pressure are more likely to show up, is going to be the extra stress on the hoses (and particularly the hose joints) and things like the radiator/heater matrix etc. The higher the pressure you run at, the more important it becomes that everything in the system is in peak condition.

If you really have good reason to think you've got a steam pocket problem, maybe a better route would be to go for waterless coolant, and tackle the problem at source?