Radiator cap psi?

[kidglok]

Going to run water with rmi once the temps stay above freezing here in the northeast. Ive been doing some research on this and cannot draw any sort of conclusion on the subject. Some real race cars use 29lb caps, but if my car never sees above 190* is there any reason not to run a zero pressure cap? Ive pushed a little coolant into the overflow when i topped off the radiator, obviously when liquid gets hot it expands and has to go somewhere. I've also pushed headgaskets into the coolant jacket which blew anitfreeze under the car doing 120mph at the track and almost kissed the wall. Some also say that you need pressure in the coolant system to prevent caviation, but too much will cause it. I've also heard that pressure is required when running water to prevent vapor pockets around the cylinders which (at the very least) can cause detonation. Im using a 16lb cap right now.

Anyone got the straight poop on this?

 

[earlbrown]

Some of those real race cars have the cap mounted between the pump and the radiator core. That subjects the cap to system pressure plus whatever restriction the flues cause in flow plus velocity of the coolant. Google search:"Psi T".

Higher pressures that ambient helps keep localized boiling and cavitation at bay. Imagine the coolant is under a 12PSI cap hauling ass through the inside of the head. As it's going over the combustion chamber there's a flash of metal sticking up into the stream. There will be an area in front of the flash that's higher than system pressure... on the backside it will be a low pressure area (think airplane wing's low pressure area on the trailing edge). If the pressure drops enough you could cause boiling at less that 212*. When it comes to heat transfer faster is better and pressure helps the cooling system do it's job

On our Buick's the cap is after the radiator core so it's after the pressure drop from the core. (that's why we can run without a thermostat in the summer and not puke out all the coolant like all the SBC wives tales)

 

[kidglok]

Some of those real race cars have the cap mounted between the pump and the radiator core. That subjects the cap to system pressure plus whatever restriction the flues cause in flow plus velocity of the coolant. Google search:"Psi T".

Higher pressures that ambient helps keep localized boiling and cavitation at bay. Imagine the coolant is under a 12PSI cap hauling ass through the inside of the head. As it's going over the combustion chamber there's a flash of metal sticking up into the stream. There will be an area in front of the flash that's higher than system pressure... on the backside it will be a low pressure area (think airplane wing's low pressure area on the trailing edge). If the pressure drops enough you could cause boiling at less that 212*. When it comes to heat transfer faster is better and pressure helps the cooling system do it's job

On our Buick's the cap is after the radiator core so it's after the pressure drop from the core. (that's why we can run without a thermostat in the summer and not puke out all the coolant like all the SBC wives tales)

So how much is too much would be the question?

 

[earlbrown]

the correct answer is "it depends"!!

Anything more than you need would be stressing the components for no reason... anything less than you need could cause boiling and cavitation that might start a domino effect that leads to bad stuff.


Clear as mud?


I can't think of any reason to run 29psi and 190* in our cars on purpose.

 

[kidglok]

the correct answer is "it depends"!!

Anything more than you need would be stressing the components for no reason... anything less than you need could cause boiling and cavitation that might start a domino effect that leads to bad stuff.

Clear as mud?

I can't think of any reason to run 29psi and 190* in our cars on purpose.

Haha like everything else my friend! I'll start off with a 7psi cap and see how she likes it.

 

[earlbrown]

What's stock on our cars? 12? Honestly I've never looked at mine. I'm running the same cap that was on my car when I bought it in '98. For all I know it could be original! lol

I did notice when I started ported timing covers the flow mods on the water side did make make my temps change faster. Back when I wrote that porting article I was still running a clogged up stock radiator so the cap was probably seeing 10psi less that what the pump was putting out

 

[TurboBob]

blowing out coolant when the head lifts is a real issue. I think its probably the reason the ZPlus car crashed a couple years ago in NC.

Years ago I worked with some guys in the FWD drag series, and the Ecotec was over-pressurizing the coolant on a pass because the head was lifting. One of the fixes was to run the coolant system unpressurized, and have a swirlpot in the system to let the gasses escape instead of blowing the radiator hoses off (or splitting them or the rad.) They then just ran it with the heads lifting every pass (7 seconds.....)

Its probably not practical to do on a GN, but planning ahead is prudent when you want to go that fast....

Anyway, on a drag car, I think running low or zero pressure is probably fine.

Bob