Re: [FWDLK] radiator cap

The radiator cap has no effect on operating temperature. The cap pressure rating only affects the temperature it will overflow at. The system boilover point is higher with a higher pressure cap. If you are not experiencing any boilover with your current 7 lb cap, your cooling system is not exceeding 7 lbs internal pressure. It will make NO difference on operating temperature if you use a 14 lb cap.

On the other hand, a higher pressure cap will add more internal pressure cycling stress to the radiator. But all our ForwardLook cars had heavier gage materials in the radiator than newer cars, so a system in good condition should have no problems.

The cooling system is very simple.

The entire system must be capable of shedding heat faster than the engine generates it. This includes the worst conditions: high temperature days, slow engine speeds (slow fan speed), fast engine speeds (faster heat generation), hill climbing, deteriorated radiator, worn water pump, low water level, thermostat flow capacity, etc. If the engine overcomes the capacity to dissipate the heat, heat builds up until it boils over.
The thermostat temperature rating controls when the radiator get water flow. It holds the water in the engine until it reaches the release temp and then flows water to the radiator. If the system is too cool, it closes down a little until the temp rises. If you want a different operating temp, replace the thermostat with a different temp rating. Use a high flow quality part. If the operating temp of the engine occasionally rises significantly above the usual normal operating temp on a hot day, something in the system is marginal. On a hotter day, it could boil over.
The radiator cap only controls the upper operating temperature limit of the system. When the temperature rises the operating pressure increases. When the pressure exceeds the cap limit, the cap releases the excess pressure and the whole system blows its top. Water boils at 212 F at normal atmospheric pressure, less at altitude. A 7 lb cap raises the boiling temp (I forget how much, but maybe 10 or 15 deg). A 14 lb cap raises it more. Antifreeze also increases the boiling point, but too much (over 50%) reduces the heat transfer rate efficiency.

Dave Homstad

56 Dodge D500

-----Original Message-----
From: Forward Look Mopar Discussion List [mailto:L-FORWARDLOOK@xxxxxxxxxxxxx]On Behalf Of John Teske
Sent: Wednesday, October 22, 2008 2:23 PM
To: L-FORWARDLOOK@xxxxxxxxxxxxx
Subject: Re: [FWDLK] radiator cap

     It is safe to use a 7 pound cap instead of a 13 or 14 pound cap. The only difference is that under high powered useage, your engine will generate more heat. The higher power cap will help keep the engine cooling temperature under 212 degrees (boiling). For normal driving, it won't make any difference.

     I have a 56 Fury with a 7 pound cap on the radiator and it runs just fine. I don't race it. I would use a 14 pound cap, but don't see the need for it. Also, the 14 pound cap will cause leakage in the cooling system that I don't want to deal with.

     John

-----Original Message-----
From: Forward Look Mopar Discussion List [mailto:L-FORWARDLOOK@xxxxxxxxxxxxx]On Behalf Of Jesus Jimenez
Sent: Wednesday, October 22, 2008 1:20 PM
To: L-FORWARDLOOK@xxxxxxxxxxxxx
Subject: [FWDLK] radiator cap

I am need to replace my radiator cap and the auto parts people gave me one rated at 13 psi and closest to the 55 Ply 6 cyl since these are not available. It used to have a 7 psi cap. My question is it safe to use the 13 psi cap? Is this what some of you use?

Thanks

55' ply belv 6 cyl