what is the proper radiator cap pressure bbc.

quote:

Originally posted by WILLIAM HAMILTON:
what is the proper radiator cap pressure bbc.

a stock BBC back in the day was probably 7lbs. Now people run as high as 16lbs maybe more. Higher pressure, lets coolant get hotter before it boils over. basically lets engine run hotter before it over heats. Lots of variables to give you a definitive answer IMO.

Maybe the more correct question would be: What is the maximum recommended pressure of my radiator? An all aluminum unit is going to have much higher capacity that one with crimped on plastic end tanks.

Why run any pressure?

quote:

Originally posted by Phil Dees:
Why run any pressure?

Every pound of pressure raises the boiling point by approximately 3 degrees, but if you mean that its probably not needed I might agree with you - on alcohol, I don't think I've ever been over 190.

With regular head gaskets I use a 12 or a 15lb but with copper head gaskets I wouldn't go over 10.

quote:

Every pound of pressure raises the boiling point by approximately 3 degrees, but if you mean that its probably not needed I might agree with you - on alcohol, I don't think I've ever been over 190.

While your temp. gauge may never have gotten over 190*, I dare say the coolant temp around the aluminum combustion chambers has risen considerably higher. Pressure will minimize cavitation in this area. But, JMO, I could be wrong.

Good questions turns into a great topic.

You guys point out some key things.

The first is that the system "must" have pressure to perform correctly as in simple terms the pressure keeps the liquid against the metal surfaces as in the combustion chamber / valve seats / etc. where the initial heat is generated. Without pressure the water would boil in those areas creating air / bubbles / etc allowing for hot spots and detonation.

Yes you must be careful not to run to much pressure to not damage other parts of the cooling system / radiator / head gaskets / etc.

Correct when figuring the 3-4 degree factor per pound. But this certainly relies on making sure you aren't boiling the liquid in the system first. That's why having all air purged from you system is so critical.

Nascar Cup cars now run pressurized systems in upwards of 3x lbs in order to run water temps 260 plus without boiling the liquid.

... Carry On

1 more key element that I think is overlooked in most drag racers program is the type of cap they use and the overflow / recovery tank system.

There are 2 types of caps. 1 designed to return coolant to the cooling system during cool down and one that only allows coolant out during expansion. Make sure you have the correct one for your system.

If you just have an overflow can then you need the cap designed only to let fluid out, if not you will suck air back into your system instead of coolant during cool down.

If you have a recovery system then you need a cap that allows the coolant to be sucked back in during cool down or it wont happen and you will be stuck with air in your system after cool down because of the expanded coolant being pushed from the system when it was hot and the cap opened.

Hope this really helps people as I'm not sure many understand the concept.

If you are only using a overflow tank, you MUST fill your coolant every time your engine has cooled down in order to keep the air purged from the system and work properly.

or

Make sure you use a recovery type system like we do.

SL...

Pics for attention SCOTT...I have seen on EAGLE RACING ENGINE site in the tech section that the higher pressure cap makes cooling more efficient...it doesn’t explain why...he shows up to a 30lb cap...

Whomp I'd say that's a simple explanation. Bottom line keeps the coolant liquid against the metal surface which allows the heat to be extracted from that area pulling the heat out efficiently and allowing it to flow thru the cooling system.

If your overflow doesn't have a down tube like the top one, you must run the tube in the bottom to make sure it doesn't suck air when cooling off.

Your results may vary, but low or no pressure has worked well for me on gas and alcohol for many years.

Don't forget to factor in elevation, the higher the elevation,...the lower the boiling point of water. Example.....5280'......boils at 203 degrees.

quote:

Originally posted by DragRaceResults:
If your overflow doesn't have a down tube like the top one, you must run the tube in the bottom to make sure it doesn't suck air when cooling off.

Scott are you running yours like the top or bottom pic....mine is like the top but I’m pretty sure it as no line running down to the bottom on inside

quote:

Originally posted by Ron C.:
Don't forget to factor in elevation, the higher the elevation,...the lower the boiling point of water. Example.....5280'......boils at 203 degrees.

This has zero effect on a cooling system as it is a closed system and not subject to the effects of altitude. That only becomes an issue on unpressurized systems or open containers of water.

Whomp - that's most peoples mistake. Needs to be able to suck the coolant back in. We use ones we made like the top one.

Which cap are you using in the second picture?

quote:

We use ones we made like the top one.

That's me.

Well who makes one like the top one I got a old GOZA and it does not have a tube...

Good question. We made our own