radiator cap

What is best way to determine what pressure radiator cap you need?

If its street oriented, 16 psi, road course, Ide go with 32 psi, drag race, 7 psi, and I pop mine off when not running. Ie no residual pressure when car is resting.

Thanks. Guess I should have put in specifics. 90 foxbody, 552 w/glide. dragrace only. 3 inch thick aluminum rad, dual fans. think 7 lbs still good?

Yup, not going to build a lot of heat, and you don't want residual pressure to push coolant into cylinders and such. Ive got copper head gaskets, and even with RTV etc... they like to drip a little when cold. So no pressure on the system when not needed.

Thanks Kim. 7 it is!!

I have seen cup plugs push out with a high pressure cap along with other problems. You only need enough to keep it from boiling if your cooling system is sub-par. Then I would work on the cooling system.

The higher the cap pressure the higher the boiling point! 16lb and higher I would think would be used for screw in block intake and head plugs .I wouldn't use more than 7lb to 10lb in your application or I would think you would be in a lot of hot water!  Just my opinion

IDT-572 wrote:I have seen cup plugs push out with a high pressure cap along with other problems. You only need enough to keep it from boiling if your cooling system is sub-par. Then I would work on the cooling system.

Not the cap pressure that pushes out core (freeze) plugs.  Cap pressure just holds residual pressure on radiator, and upper hose, to prevent steam flash of hot water.  Core plugs push out because of poor installation and some really killer water pump systems with too little coolant flow.  Now water does not compress, same as brake fluid, so on the solid water filled side, everything after the water pump and before the restrictor or thermostat its difficult to assign a force but the water pressure back there can be damn sizeable when you have unrestricted flow into a cast impeller water pump, a sealed system and that water pump spinning at 7000 or more RPM.  The pressed blade impellers bend and distort so they typically push a little less water at high RPM.  I would wager if we put a gauge in a block to determine the hydraulic pressure of the water inside blocks, we would easily see in excess of 100lbs.  Once it comes through the thermostat though, its whatever the radiator cap says....

No air space and a 30 lb cap pushed them out, could have been faulty core plug install but it happened when the cap was changed. I saw this twice on two different round track engines. They had the in line top filler hose an it was higher than the radiator.

I was just letting him know that the high pressure can cause problems in the system. And core plugs tend to send water under the rear wheels..............

Yup, agree, and it aint fun from what I've seen. I had a hard enough time with dry tracks... Will Phoenix wasn't such a hot track to start with, slicker than greased steel most of the year till the ground it.

Lots of the low buck roundy round guys pin the core plugs, drilling and installing roll pins into the block. Most aren't stressing the block area bad enough that its a concern.

I prefure screw in plugs. I tried taking a small lower jacket drain screw in plug out afte a few heat cycles. I think with the engine pulled and an impact it might come out. I damn sure know them big ole 2.5" plugs aren't going to fall out. Not unless the block breaks around them.

The cap pressure is the max pressure before it burps, it doesn't mean you are reaching that pressure. Most new Ford vehicles come with a 16psi cap from the factory. I run one on my car with no issues, I'm usually hitting the return road at 200*F max in the summer heat as I try to hit the burnout box at 140F or higher. With straight water a higher boiling point is a good thing because you don't have the coolant there to increase it some for you.

With bottom feed overflow systems that don't leak, the system stays at 16psi above atmospheric, cold or hot. Or 7 or 32 depending on the cap rating. When the system exceeds that pressure the cap releases coolant to the puke bottle, when pressure decrease below 16 psi, the cap allow the system to pull back coolant. System stays full, unless the coolant bottle leaks, or simply bleeds to ground. Then system will run empty.