Radiator cap

[fordracing65]

What pound radiator cap are you using for your ford fe big blocks. 7lb or 16lb. Those seem to be the 2 they offer most.

[patrickt]

Originally Posted by fordracing65

What pound radiator cap are you using for your ford fe big blocks. 7lb or 16lb. Those seem to be the 2 they offer most.

RS-62, 13psi. Seven is too soft; Sixteen is too hard; Thirteen is just right.

[cycleguy55]

Not FE-engined, but I'm running a pair of caps - a 16 PSI one on an inline filler mounted high and a 13 PSI one on the radiator.

[streetrod1927]

7lb in standard FE surge tank and 7lb in additional overflow tank.

[fordracing65]

Originally Posted by streetrod1927

7lb in standard FE surge tank and 7lb in additional overflow tank.

I thought 7 was correct. I wonder why so many run 13 or 16 pounds

[mooreken]

For every pound of pressure the boiling point is raised 3 degrees. So if your car runs hot a 25 pound cap can keep the water from boiling up to near 250 degrees.
As long as the water does not boil there are no hot spots in the motor to cause trouble. Granted I don't recommend that high a temp under normal conditions, but it does give some motor safety if it does run hot.

Just don't try to remove the cap while hot....very evil things happen.

[dcdoug]

13lb on my FE.

[fordracing65]

Sounds good. Thanks.

[cycleguy55]

Originally Posted by mooreken

For every pound of pressure the boiling point is raised 3 degrees. So if your car runs hot a 25 pound cap can keep the water from boiling up to near 250 degrees.
As long as the water does not boil there are no hot spots in the motor to cause trouble. Granted I don't recommend that high a temp under normal conditions, but it does give some motor safety if it does run hot.

Just don't try to remove the cap while hot....very evil things happen.

Ethylene glycol also raises the boiling point, but is less efficient at drawing heat from the engine. OTOH, glycol also reduces the vapor pressure.

[Roger Bolick]

1. Ford caps (see amkproducts.com) during the early 60's were 13 or 14 lb caps. The pretty expansion tank will "bulge" at higher pressures...

2. Roughly 3 degrees raise in boiling temperature for each PSI.

3. AT SEA LEVEL (roughly) water boils at 212 F, 50/50 antifreeze/distilled water at 233 F.

Using advanced math, a 13 lb cap with near 50/50 mixture boils at ~270 F.

(ignored is the 5,000 ft elevation boiling temperature of water ~203 F)
(ignored is the 29,000 ft elevation boiling temperature of water ~162)(Mount Everest!!)
(totally ignored is the actual relationship to barometric pressure not altitude)

PRACTICAL EXPERIENCE:
A completely stock, all original foreign car was spec'ed as a 7 lb cap through 1972.
Factory changed little else, but switched to a 13 lb cap 1973+.
I managed to obtain an ORIGINAL 7 lb cap... for an event in the Southwest US, i.e. 95+ F outside.
Car ran great, no heating issues, TILL stopped quickly after a long interstate drive...
Sitting nearby about 15-20 minutes later, the car started having "gas"... and I don't mean the liquid kind!
It was obvious, burping, belching, shooting water/steam out the overflow pipe.
The performance lasted ~10 minutes.
Consistent performance on hot days until I changed to a 13 lb cap.

HEAT SOAK is real and will cause hard starting (mixture) and slow turning (starter), AND "gas", 5-30 minutes after a hot run.


NOTE: ideally ~40% antifreeze to optimize the cooling process, lower freeze temperature, and higher boiling temperature.

[DanEC]

I've noticed that bulge on mine after shutting off the engine. Not sure without going out to the garage what cap is on it.

[fordracing65]

I talked to Shelby engines today and they recommend a 16 pound cap for a Kirkham expansion tank. Kirkham said to use a 7 pound cap. ????????

[fordracing65]

Originally Posted by Roger Bolick

1. Ford caps (see amkproducts.com) during the early 60's were 13 or 14 lb caps. The pretty expansion tank will "bulge" at higher pressures...

2. Roughly 3 degrees raise in boiling temperature for each PSI.

3. AT SEA LEVEL (roughly) water boils at 212 F, 50/50 antifreeze/distilled water at 233 F.

Using advanced math, a 13 lb cap with near 50/50 mixture boils at ~270 F.

(ignored is the 5,000 ft elevation boiling temperature of water ~203 F)
(ignored is the 29,000 ft elevation boiling temperature of water ~162)(Mount Everest!!)
(totally ignored is the actual relationship to barometric pressure not altitude)

PRACTICAL EXPERIENCE:
A completely stock, all original foreign car was spec'ed as a 7 lb cap through 1972.
Factory changed little else, but switched to a 13 lb cap 1973+.
I managed to obtain an ORIGINAL 7 lb cap... for an event in the Southwest US, i.e. 95+ F outside.
Car ran great, no heating issues, TILL stopped quickly after a long interstate drive...
Sitting nearby about 15-20 minutes later, the car started having "gas"... and I don't mean the liquid kind!
It was obvious, burping, belching, shooting water/steam out the overflow pipe.
The performance lasted ~10 minutes.
Consistent performance on hot days until I changed to a 13 lb cap.

HEAT SOAK is real and will cause hard starting (mixture) and slow turning (starter), AND "gas", 5-30 minutes after a hot run.


NOTE: ideally ~40% antifreeze to optimize the cooling process, lower freeze temperature, and higher boiling temperature.

Great info Roger. Thanks.

[Roger Bolick]

CONCLUSION: a 7 lb cap is marginal if you consider the entire US.
(foreign manf changed US cars 7 lb -> 13 lb... Ford used 13 lb)

The Southwest has hot air temperature, but that's NOT all. The SUN blasts all day and the tarmac reaches extreme temperatures (try barefoot, not certain of the actual temperature, but HOT comes to mind!).

A car driven for hours, parked outside experiences engine heat soak and tarmac heat radiation... and maybe paint color heating.

Technical Details:
180 degree thermostat (carb'ed car)
Antifreeze % was ~10% used mostly as a leak detector in new build.
Cast Iron block and head. (Aluminum head dissipate heat quicker?)
Dark color car (black is seriously hotter than white)