Check Your Radiator Caps Now!

This is a good question. Doesn't more PSI equal more stress? My original cap was 0.9 bar.

 

On an old beat up driver, the rad cap went. So I replaced it. Next day the upper hose went, replaced it. Than the rad blew, recored it. Next went heater core went. Cut hoses and bypassed it. Next went thermostate. Moral of storey - should have just left the faulty cap on.
Bottom line, replacing an item means the next weakest part goes. If specs are for 0.9, better for a working 0.9 to go than a higher rated cap. There is tolerance built into system, but only if your system is fully up to date. I can see at high altitude, ie 5000 feet asl or higher, but otherwise mfg recommendation is what I would go with

 

Thank you !

 

Guys a bought a 0.9 bar cap. I got a bit confused and concerned over 16 PSI being too much so I sprung for the $25 and got an original Italian cap. Keeping the Stant as a spare.

 

Stant make a 13 PSI cap with or without a safety lever that will fit.

 

It will break soon enough.

 

A previous owner of my '89 328 had coolant overflow problems from the expansion tank. Ferrari of Denver apparently finally solved the problem by installing a BMW radiator/expansion tank cap. The only number on it is a "140", which from what you say, means 1.4 bar.

 

Ok...we finally got to it. As I mentioned on my thread, there seemed to be no concern of putting a stonger cap on. In reality it will put unnecessary stress on all components of your cooling system...case closed...give me a 13 pounder

 

I'm not sure that the case is closed. One thing I can't figure out is whether a cap rated for higher pressure might be advantageous for operation at high altitude (such as here in Colorado). My understanding of how the caps work is that the spring in the cap determines the pressure at which the cap will partially open and thereby release some internal pressure. (This spring strength is what determines the pressure rating of the cap). If so, then it doesn't seem like it should matter what the outside ambient air pressure is. On the other hand, I've come across info saying that modified, high-pressure caps are commonly used for the Pikes Peak high altitude races. This implies that the pressure differential between internal system pressure and ambient air pressure plays a role. Does altitude make a difference?

 

maybe some.

I remember reading somewhere that when I was up at Copper mountain (about 10,000' at center village) that atmospheric pressure was about 10 psi, vs 14. If the spring works off a differential, it will open 4 psi sooner at Copper center villiage than here in Houston. Probably significant enough to compensate for. For most cars - probably not significant enough.

I agree with the guys that say - if you make one component too strong, you will expose the weakness of the system. I think of other cars where - if you re-work the head (top end) and do nothing to the bottom end - you are likely to have a problem.

I have a Stant cap I bought on Amazon, but I may just spring for the actual Italian cap. Originality matters.

 

Thanks - I'd missed this earlier re. the altitude question.

 

I think I'm going your route.

 

0.9 bar or 13 psi is set so that the pressure releases when the water temperature reaches the maximum recommended for the 308. Rather than continuing to get hotter the water boils. A higher pressure cap will mean that the car can overheat before boiling. In any event you need to watch the temperature gauge!

The cooling system is a closed system until the threshold pressure is reached. The release pressure is set mechanically by the spring in the cap so the altitude has no effect.

 

Here's what I got from NAPA cross reference from Stant #10331

I don't think that's right.
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I believe you are incorrect. There is about a 2.5 net psi drop at 5000 feet with the same cap. I am in a meeting currently and killing time and will go over this in more detail later. It has to do with atmospheric pressure and gage pressure. Yes gage pressure is constant as the spring controls the gage (or gauge) pressure, but only this. There are two other pressures also at play

 

The 1698 is 16 PSI without safety lever.

 

The whole point is to keep the system from ever boiling over and still allow for expansion of the fluid as the engine heats up to normal temps.

The boiling point of 50/50 water/ethylene glycol antifreeze is around 223F at normal atmospheric pressure at sea level. If your engine never got hotter than that.... you could leave it open without issues. Every PSI one way or the other would change the boiling point about 3 degrees F. So with a .9 BAR or 13PSI cap the boiling point would go up to around 260F, 1.1 Bar or 15PSI would get you to just shy of 270F. But just increasing the pressure and therefore the safety margin between operational temps and the boiling point doesn't buy you anything except more load on the system components. It wont run cooler or hotter. The operational temperature is determined by the thermostat, radiator size, cooling fan flow and the heat load the engine is putting on the system... not by the operating pressure.

Going up in altitude to say 7,000 ft does reduce the absolute pressure by 11 psi and so the boiling point by about 32 degrees. With no pressure in the system (no cap at all) the boiling point is around 190F and your radiator is boiling over at normal operating temps. With a .9 BAR cap, boiling point is back up around 230F and you should have plenty of margin on a normally operating system with a thermostat set at 180F

So changing the cap pressure buys you some margin at the expense of load on your hoses etc.

Upping the pressure in your system by 3 psi would get you a whole ten degrees of boiling point margin. If you are not racing or driving a lot in hot or high altitudes... there isnt much reason to up your cap to 1.1 Bar.

Just to clear the air, racers are often limited to water only when on tracks (ethylene glycol is slick and hard to clean up , not something you want on a track) so they often run 20 PSI systems... but they also design the whole system to take it.

 

Ferrari went to 1.1 in the 90's.

Everyone here is still working off information printed in the 80's and was long since updated.

 

Or as in other Ferrari updates they are correcting a mistake. It was a Ferrari decision to use 1.1 and it was a good one.


Updates have included changing fuse amperages. It does not mean the circuit will not handle the load, it just means that under normal usage the needs of some components are underestimated.

I have a wheelbarrow full of books from Ferrari full of such changes.

 

You must have misread your source. AT 7000' the pressure is reduce about 3.4 psi TO 11.3 psi. Boiling point of a 50/50 mix drops about 10 *F to 213 *F.

 

Made a hell of a lot of difference in a 308 QV.

Not to mention the fact that 3lbs can mean all the difference in the world to prevent cavitation that has been responsible for destroying motors.


And we don't run 20lb caps on race cars to keep water off the track.

 

And when we do it is quite often with standard hoses and water pumps. We don't go to the parts counter and ask for the "Special" water pump with the magic seal.

 

Oh, ok, thanks for clearing that up, in that case, I might get me a NAPA #1698

 

To finish off what I had suggested before, aside from the issues of water vs coolant mix (ie the ability to carry and dissipate energy) or cavitation from vapour, the question was more on whether the the cooling system loses pressure on a closed cooling system and if a higher cap rating is required at high altitudes.
Maximum absolute pressure is what counts = absolute pressure outside the cooling system + the force of the spring in the cap. The force acting on the cap = atmospheric pressure outside x area + spring force. So if atmospheric pressure drops, the force on the spring also drops.
Agreed that gauge pressure of the spring is a constant, but atmospheric pressure is not as the spring only controls the gauge pressure. At sea level, the absolute outside pressure is > absolute pressure inside the radiator (cooling system), which is why the same liquid boils at a higher temperature than at a higher altitude.
The pressure setting of the rad cap sets the overpressure inside the rad, it does not set the absolute pressure inside the rad. Based on charts, the average drop in pressure is just under 3 degrees per 5,000 feet increase in elevation.
Most of the time, this will not noticed if a system is in good working condition. But in some vehicles, to obtain the same 13 psi at sea level, the pressure on the rad cap needs to be increased to 15 or 16 psi to obtain the same 13 psi at sea level. Above 7,000 feet, unless you are running fuel injection or a form of compression of air (super or turbo charging), other things will start to suffer on a normal carb car. I also cant see anyone driving their car, stopping before going through a high altitude area, waiting for the engine to cool down, than changing to a 16psi cap, and than on the way down, stopping again to wait for engine to cool and than changing back to 13 psi cap. I would just run it with the 13 psi all the way.

 

Oops that's what happens when you type with a sandwich in your hand and a phone in your ear. Youre absolutely right... and that 213F is only if you dive around with the cap off. With a .9 bar cap its still 250F or so.