Flathead radiator cap pressure?

O.k. Flathead guys, I need some help. On a flathead Ford what is the proper radiator cap pressure? I have heard stories about ruining the water pumps if the pressure is to high. Whats the deal, is this a myth or is a higher pressure cap o.k.
Thanks for your help..
Jimmy White

 

It would probably depend on the engine. Is it rebuilt? Old stock water pumps had carbon seals, the newer Speedway pumps use ceramic.
What's the condition of your radiator? I'm running a 12# cap on my car, but there pumps are new and the rad is also.

Flatman

 

Everything is new, Rebuilt engine, new radiator. And I am pretty sure new pumps. I was thinking I could run higher pressure, but I didnt know for sure. I have always heard the low pressure cap story, and didnt know if it was true or not. So did you have to run low pressure because of the carbon seals? If it has carbon seals, what pressure should you run?
Thanks for your advice I really appreciate it!!
Jimmy White

 

If it is all new rad and pumps just put a standard cap on it. If they are old pumps, then get some new pumps. Ha ha ha. Or you should probably run a 5-6 pound cap.

 

That was my thought!! This stuff is all new it should be o.k. I didnt want to chance ruining the water pumps, it is a customers car. That is why I checked here, I knew I would get some valid advice!!
Thanks
Jimmy White

 

the stock cap on the '47 is 4 lbs, i would say it would be safe to go to maybe 7-12 lbs. but i wouldn't push it to the 13-15 standard range (new cars are at 17).

 

I've got a 7 pound cap on my really old, tired, worn out flatty and it works just fine.

 

4 lb, or 7lb.

ROB

 

Depending on the year, most flatheads don't run pressurized systems. All the flatheads I've built run fine without pressure.

 

If everything is new,this is the best answer on this post. I'm 75 yrs old and never run a pressure cap on a flathead...ran a 36 21stud from
L A to New York City..no pressure cap..no overheat either.
P.S. be sure to run a thermostat...

 

And...the pressure limit is generally ascribed to the large size of the top tank, which does indeed balloon a bit under pressure.

 

Thanks everyone for the replies!! I have a few ideas of what I need to try now.
Jimmy White

 

i run a 7lb on my 53 239

 

I did a cooling system "revitalization" on my 59AB flathead in this tech post: http://www.jalopyjournal.com/forum/showthread.php?t=227231&highlight=notebooms+flathead+cooling

I run a 4lb cap, along with the other things performed above. Works great.

-scott noteboom

 

Jimmy - which radiator are you running? If it's a stock early radiator with a large upper tank, they can't handle much more than 3-4lbs. If it's a modern small header tank, you can probably go 7-12 or more.

The issue with water pumps is the early pumps with packed seals. Modern speedway pumps don't have this issue, so the radiator tank becomes your limiting factor.

 

I've been running 7 lbs on the customer cars we do and my own. Walker radiators and either the new Speedway pumps or our 348-409 pump kit.

 

The extra 'head room' allowed by pressurising your system is valuable on a flathead in modern traffic (jams!) and if its a customer car, he probably wont appreciate it puking when he stops with no pressure cap. I did an 8BA upgrade recently and ran a 12 lb cap no probs. original tanks seemed fine with it.

 

I think I have a 14# could be a 13# but just a standard modern cap system works well. I dont have an overflow, really should get one, but dont want the SS one on the car.

 

Most of the original radiators won't even work with a pressure cap - the neck is all wrong - no place for the seal.

Big tank early units ('33/34 come quickly to mind, but many early cars had huge upper tanks) will balloon. Over time, they will crack & leak (if they don't permanently distort) due to this excessive flexing & subsequent work hardening.

 

One of the hardest things to get across to young guys is that once you fill a non pressurized flathead radiator, it is normal for the coolant level to expand and "over flow" once it is brought up to operating temperatures. When it cools down the coolant contracts and it looks like the level is down when it perfectly normal. When it heats up again it will expand back to the operating level and not over flow again unless you topped it off by adding more when it wasn't needed. We are so conditioned to fill it to the top on our modern cars that this seems alien and wrong.

If your flathead boils over and pukes coolant every single time you drive it, you have a cooling problem that needs to be addressed and a Bandaid repair of a higher pressure cap is not a repair but only a cover up of a deeper problem.

I'm not anti-pressure caps but it seems to me that you would want the least amount of pressure for your cooling system as in what it came with. If a 4 lb cap works well, why would you want a higher pressure cap?

We often think if the modern cars are running 16 lb caps, then that must be an improvement over the old technology and I should put one on my old car. I don't agree with that theory. I don't want my hot rod operating at 220 degrees so I have no need for a pressure cap that strong. A pressure cap does not make your car run cooler it just allows it to run at much higher temperatures with out puking than it was originally designed to operate at. 180 on the road and 200 in traffic is what I want. No pressure cap is needed for those temps.

 

Thanks guys.
I'm sending off my radiator for a rebuild in a few days.
The neck inside with a modern cap I thought I might need, is now gone from the plans.
I'll stick with Henry's cap from 1932.

Puhhh, that felt good.

Paul

 

Good point about the lower level on older systems tommy.
i wouldnt want to run it at 220 either, but on a 100 degree day, stuck in a traffic jam, you may not have a choice. and if it boils all the coolant out due to a low or no pressure cap, thats just one more reason for a damn SBC. god forbid.
I would think running a modern radiator and a pressure cap would be a fine bit of insurance against a side of the road experience that no one enjoys.
Agreed it shouldnt be a bandaid for a constantly hot running motor, but a bit of 'headroom' as the audiophiles call it , is always good.

 

Just one more tidbit of info. 1 pound of pressure raises the boiling point 3 degrees.

 

I think pressurizing the system to at least 4 lbs is good insurance.
Normal driving at 40-45mph does allow the coolant systems to maintain a working coolant level in a non pressurized system.
But from my experience (Various flatheads).. running at higher speeds will push out more coolant and before you know it... at the next rest stop you're adding water. And believe me...the coolant leaves the system before the engine temp begins to rise. Not vice versa.
Pressurizing the system also allows you to monitor other issues like engine exhausts entering the coolant system through a crack or normally a head gasket leak.
Older non-pressurized radiator necks do not have a provision to just add a 4 lb cap because the overflow tube enters below the cap seal. The only way around this is to change the neck style or install a overflow tube inline pressure valve.

 

The funny thing is I've had more overheating problems with my Chevy powered Walker radiator equipped roadster in traffic than my original Deuce radiator equipped Merc. flattie. The Chevy problem was a loose wire on the electric fan that was on the car when I got it.

The flattie has never given me a reason to worry about a pressure cap. That is why I don't understand the desire for high pressure caps. Your experience may differ.

I'm not anti pressure cap but below 212 they are not needed.

 

The problem I am having is that it is puking coolant all over, out of the over flow. I know it doesnt have a bad head gasket, or other problem. I figured it was just pushing past the low pressure cap. The engine is brand new (re-torqued heads after fire up) The radiator is brand new (not an old style). The cap that is on there is a 4 lbs cap. so I figured that if we put a little higher pressure cap on there it would help with the puking. At first I thought it is just "overflow" or seeking its own level, but after driving for 100 miles it was still doing it. I will try a higher pressure cap and see what happens.
Thanks everyone for the advice.
Jimmy White

 

Jimmy,
I bet ya a dollar to a donut you have a compression leak somewhere.
If it loses coolant that quickly, I dont feel an extra 3 lb cap will help here. Before going nuts you could have your coolant checked for exhaust contaminants.
I hope I am wrong here.....
try retorquing the heads once more after it cools down..

 

I actually quit using the Ford pressure cap (4 or 7, can't remember) on my '48 because it made me nervous; running the engine, I could here a FWOOMP and see the upper tank pop to a slightly bulged position, then FWOOMP back to relaxed, apparently flexing along the seam where it soldered to header plate, as engine cooled down. This is at the normal thermostat temp of 180, not overheating. Big, unbraced tanks hate pressure...inner braces would be easy to add in a rebuild. Car has original radiator, bearing a soldered on badge from a rodding out by British Army garage in Hamburg about 1952...
Ford made the pressure cap standard around '46, quickly dropped it (reason unknown) but kept it as service part.
Benefits: higher boiling point is not a benefit unless you like a scary normal operating range. If your flathead is heating up, pressurizing only means you'll be 100 yards farther down the road when Old Faithful blows and you coast to the shoulder. Iy you are running too hot, you need to fi**it.
Real benefit, as explained by Smokey, is that pressure apparently helps suppress the minor pockets of cavitation from flow and also minor steam pockets of very local overheating that all engines have. Hence, slightly better cooling and better resisatance to overheating at normal temps.

 

This has been my original thought, I hope its not. This is the second flathead powered car we have built at the shop, and the owner supplied the engine. The first flathead powered car had a similar problem but it was alot worse. I was kind of hoping the coolant was seeking its own "level". I am going to try some different things, and see what happens.
Thanks everyone for the advice!!
Jimmy White