Technical Radiator Cap PSI - Now Plugged Radiator

Service manual for my '56 Vette says they used a 7lb. cap up until 1960, then a 13lb. Same thing for the full size? Why the change? OK to retrofit a 13lb.?

 

Are you still using the original radiator in your Vette HRP

 

the early radiators tanks are not designed to hold more than 7 psi, so it's good to use the recommended cap, unless you've installed a radiator with a smaller tank.

 

Probably not, pressure just increases the boiling point, use a 7 lb cap for a rad built for a 7 lb cap.

 

My 56 was a 7. I changed the radiator 7 yrs ago so I knew the radiator could take more but I left it because of the heater core. Last December I added an AC/heater unit so I moved it up to 13/14. I also noticed when the engine ran for awhile at 205 the coolant recovery container was almost to the full at 7 psi. Down 8 oz with the higher value cap.

 

It will increase the boils point of the coolant by a bit.

there is mixed reviews on using a higher pressure cap and possible damage to the rad/ heater core etc.

ive had plenty of older cars with low pressure cooling systems that I’ve stuck what ever cap I had lying around on it with no ill effects.

I would try and find a 7lbs cap if you can but a higher pressure cap should not hurt.

the cap will still vent excess pressure and barf coolant on the floor.

 

a friend gave me a nice radiator from his 56 chevy, the tank was ballooned, because he used a higher pressure cap than it should have had (which was 7 psi).

I learned that lesson vicariously.

 

tried running the LaSalle at zero, it just boiled away...4 psi kept it sort of happy.

Pressure is a good thing, if you can keep it in the system.

 

For every one pound of cap pressure the boiling point in the radiator goes up by 3 degrees. So with a 5 lb radiator cap, instead of boiling at 212 it will boil at 227 degrees.

 

The way I heard it, could be wrong, the engine temperature is not uniform throughout. So there are localised hot spots, much hotter than the temperature gauge indication particularly around the piston cylinder jackets. A pressurised system keeps things happy, the alternative is the possibility of coolant boiling away from those specific areas and maybe causing trouble. I don't believe it has to be a lot of pressure, like modern OT engines, but there needs to be some.

Overtemperature or overheating permanently removes the spring tension or temper out of piston rings for example.

 

FYI, Pantera's used to run a 22 lb pressure cap. Ford sent a letter saying if customers complain that their Pantera runs 235 in traffic and you verify the condition (it is not above 235), the fix was a 1/4 ohm resistor in series with the gauge.

 

Thanks guys. Mine has a 7lb. and what looks like an original radiator. It's not boiling over but the gauge reads high. My infrared gun shows only about 140 at the aluminum valve covers but 220 at the radiator top. I think I have a mechanical gauge I can swap in for a test.

 

a different pressure rating cap won't help.

Might do some more checking of the temps. The end of the head is a good place to measure, or the intake near the thermostat housing.

If the radiator is partly plugged, it won't cool very well. They used a core that was made like a heater core in most Chevy radiators back then, not the tube and fin design we're more familiar with these days...and the old style core is hard to repair. Pictures help, if you can.

 

We re cored our 57 ford radiator to take 13lb , but forgot that the heater core would not accept that , cost darn near as much the core for the heater. !!!

 

I chased an overheating problem for a long while, turned out to be the cap. I always run the higher pressure cap now.

 

Right, a higher pressure cap will keep it from boiling at higher temperatures, but the temperature won't change.

Most if not all engines that are topical here are probably full of scale and sediment in the coolant passages by now unless special effort was made somewhere along the way to remove it, and that's even if all those previous owners kept antifreeze w/ corrosion inhibitors in it. A lot of people don't.

 

Higher than came from the factory shouldn't be necessary, I wouldn't think.

 

This may or may not be relevant. We run a dualpass aluminum Affco radiator in our super stock dirt car. We test caps and buy the high test available from circle track suppliers which test at 27-28 psi. In this summer heat it’s common to run 250-260 in a race with no yellow flags when the temp will go down. We don’t loose coolant which is only distilled water and 1bottle of water wetter. We do not ballon tanks since dumping Chinese knockoff radiators.
If your system is built for it high pressure is not a problem. More will be gained in a SBC by lines from the rear of the intake to the thermostat housing under the thermostat.

 

My problem was boiling over, and when it boiled off enough coolant it would over heat.

 

Quick question: Do you have a coolant recovery tank?

 

The main difference is that the 56 had what GM called a cellular core aka honeycomb radiator in 56 like the older GM rigs had that doesn't handle more than 7 lbs. Most like my 48's stock radiator ran 4.
Scroll to the written page 7 pages (gmheritagecenter.com)
Scrolling down the page a 57 also has a cellular core and takes a 7 lb cap pages (gmheritagecenter.com)

58 again uses the cellular core and takes a 7 pound cap scroll to page 7 pages (gmheritagecenter.com)

59 the same thing again page 7 pages (gmheritagecenter.com)

60 still used the cellular core and 7 lb cap

61 went to an aluminum cross flow radiator. again printed page 7 pages (gmheritagecenter.com)

57 full size was the last full size car to use the cellular/honeycomb radiator and 7 lb caps pages (gmheritagecenter.com) it's down the page.

58 full size used a tube type radiator with a 13 lb cap pages (gmheritagecenter.com) scroll to P57

 

As has been said, cap rating won't make the engine run cooler (unless it's boiling over of course). So why did Chevy feel it necessary to go to a higher pressure cap? Higher operating temps from Increased displacement, horsepower, added accessories?

 

I did use a recovery bottle on the dragster, where it was required. It was over filling my little tank. I also had the problem on my 57 Chevy. Did not use recovery tank, but know I should have.

 

Good question. Wasn't just Chevy, everybody else did too. Might be interesting to know the timeline. Ford went to a then unheard of 7,500 mile oil change interval around 1963 (or near enough), in part because they changed to a 192° F. thermostat, and their Rotunda filter. Less crankcase & sludge and contamination with higher operating temps. SAE research showed much lower piston & cylinder wear rates with higher engine temperatures.

Maybe, they needed higher radiator pressures to offset higher engine temps? Just spitballin'. There's also some heat transfer jiggery-pokery that I don't understand, it allows for better cooling when there is a greater temperature differential between the engine and radiator.

 

...but don't go too cool. You want it a little over the boiling point of water, to get rid of condensation and fuel that gets in there.

 

"Boiling over" is the definition of overheating. If the coolant boiled, i.e. it turned to steam, it overheated.

 

Sounds logical. I checked my service manual, no thermostat specs listed.

 

chevy parts book shows four different numbers, to fit all of them. 151, 160, 170 degree (no specifics given), and 180 degree "for max heat".

I'd run a 160 in it, and not worry about the thermostat rating, as long as it's a good functioning one. You can test it on the kitchen stove in a pot of water, if you have a thermometer of some type.

 

I found a mechanical gauge in my stash, going to swap that in for a test against the stock electric gauge. May be chasing a problem that's not even there. Also plan to shoot the infrared gun at the sending unit location (manifold) and see how that compares.

 

The original electric gauge doesn't have any numbers on it, does it? It's an indicator...not a measuring instrument. But once you know where "too hot" is on it, you can use it to know when the engine is too hot. On my late 50s chevys, it's always been near the upper dot.