The Stovebolt Site Forums The Shop Area Engine Shop Radiator - to pressurize or not

Putting a rebuilt but stock 61-261 into a 37 chassis. The original 216 was non pressurized and I believe the 261 was pressurized to 5-7#??. Am using predaves adapter plate w/ older rebuilt water pump and original 216 fan. Anyone with words of wisdom for me? If I go nonpressure, I don't have to change the radiator neck and eliminate pressure stress on radiator and heater core but I wonder about boilover and engine performance?? All info is appreciated

you could try it and see how it works, pressure was a year thing more than a particular engine, 7# caps were standard after 55, but with weak rads I've run with the cap loose for no pressure with both 235s and 283s without any problems

Bill

While battleing a cold weather condensation issue on my mostly stock 49 216 I decided to up the thermostat from a 160 to a 180 and add a 4lb pressure cap and over the 4 months or so since the cooling system has been working great. (knock on wood).
I also added an overflow tank and it seems to work great also.

I did have to add alittle JB Weld to plug a small pin hole on the top radiator tank but otherwise the entire cooling system, including the heater core, seems to handle the 4lb pressure fine.

When the engine is cold the radiator will draw antifreeze from the overflow tank until the level in the tank is at the very top (no air gap). When running the radiator will vent enough coolant to the tank to where the coolant level in the radiator is approx 1"-2" lower. I can open the radiator cap during operation or right after shutdown and all I hear is air suction escaping. Per the stock gauge the temps never rises past the "0" in the 180 indicator.

The jury is still out come the heat of the summer but I've had no problems yet (again, knock on wood).
Dave

I would NOT pressurize much beyond 4 lbs for fear of blowing the radiator and /or the heater core. If your block and radiator are clean it will cool just fine even at 100 degrees F outside air temperature. Just my opinion. Jerry

I ran my truck with a 195° thermostat for 25 years with no pressure and never had a hiccup. Even going through death valley with temps only in the high 90's with a GVW of 5600 pounds (yeah it was loaded) if that helps. However I did have a mercury with an overflow tank and somewhere it was leaking air and when it was cooling off it didn't draw the coolant back in. Eventually the overflow overflowed, but there was no water in the engine part. That is when I started actually looking in the radiator rather than the overflow tank. It did this for 10 years, and I never did see a drop of antifreeze anywhere outside of the car. I realize it was a mercury and not a chevy, but that part is the same. I don't run pressure in my truck, as the stuff is old, and it doesn't need it.

I see no reason to run a pressure cap.

0 psi was standard in 1937.
Is your cooling/heating system 1937 parts?

261s were used in trucks with 4-7 psi radiator caps, depending on the year.

Try the 261 with the no-pressure radiator cap.

If you get "boil over", you have serious problems. You should get expansion overflow until the coolant overflows enough to reach normal operating level in the radiator.

The only reason for pressurizing coolant is to allow it to go above 212 degrees without boiling. For every pound of pressure the boiling point rises 3 degrees, so a 15-lb. cap will raise the boiling point to 257 degrees. A 4-lb cap only raises the boil-over temp to 224 degrees, hardly worth the effort. The older radiators just don't have strong enough construction to handle over 4-7 lbs., max. Keep the core clean and flush the crud out of the engine block, and stick with a low-pressure cap. The manufacturers learned they could build cheap, inefficient radiators, if they just put enough pressure on 'em to keep the coolant from boiling. Most OEM electric cooling fan switches trigger the fan(s) at 220 degrees, so a bad radiator cap can cause a boilover before the fan ever comes on.
Jerry

My '37 was converted to a pressure radiator when I had the core replaced. I only ran a 4 psi cap and still had problems with it. The brass of the top tank is very thin and every heat cycle it went through you could hear it pop ( expand ). After a year of this the seam would start to leak so I would pull it out and reseal it. I did this about four times before switching to a new aluminum replacement. I never tried it with zero pressure. The new one came with a 21 psi cap !

Joe

I'm running with zero pressure and a catch bottle. It stays at 180 all the time except while idling at a stoplight. Then it creeps to about 190 and comes back down if I rev the motor or start moving.

If its a tube and fin radiator I would put a pressurized cap on it. If its a soldered honeycomb, then no. Back in the day ppl would check the coolant levels more frequently, so topping up was a common thing. Now a days most ppl don't know where the radiator cap is, or worse, add coolant to the system through it.

You can lose coolant just by steam loss. Unless you are in the habit of a thorough 'pre flight' you may learn you are to low on coolant far from a source of water.

Putting a pressurized cap seals the system and keep the water from steaming out yet allows it to vent if needed.

I say add a 4lb. cap. You don't NEED to, but the benefits out weight the risks.

How does the water (coolant?) "steam" out if the water/coolant does not reach boiling point?

Oh, right, forgot this was a physics forum.

Technically I meant evaporated, whereas you are thinking boiling.

Water BOILS at 212F (wait, since this is a physics forum I must add that the temp at which water boils will be affected by the pressure of the water, the atmosphere, the additives, if any, in the water the alignment of Uranus and the number of jelly beans in my left pock on the 5th Tuesday of every even numbered month. The formula used to include the relative position of Pluto, but since Pluto is not really a planet the equation was changed.) .

It will evaporate at a lower temp.

Tomorrow AM look at your hot (non-boiling, under 212F) cup of coffee. That. That is STEAM.

It looks suprisingly similiar to this.

Here we go again!

https://sp1.yimg.com/ib/th?id=HN.607991395823321337&pid=15.1

Jerry

as far as I know, the original reason pressurized caps were used is to prevent boilover when the engine was shut down due to heat soak back. Now they may actually run at over 212°. (I'm not so really up on the small details of modern engines, I have never had any even begin to fail in the last 20 years) I even run my mustang with no pressure (I'm afraid of that 40 year old radiator.) And I don't drink coffee, and water with anti freeze doesn't boil at 212°. Unless of course you aren't using antifreeze, in which case I don't want to talk to you. But what ever the case, these engines don't need them.

Is it silly to compare the evaporation in an open system, where the liquid is much warmer than the atmosphere (drinking coffee outside on a cold day), to the evaporation in an almost closed system where the liquid and atmosphere are at almost the same temperature (and that temperature is below the boiling point of the liquid).

in terms of physics farber, the steam from the gaseous phase of boiling water is invisible .... but is still evaporation [vaporization], which can happen at any temperature .... so mixing your metaphors doesn't increase your accuracy

if folks really watch their systems it'd be obvious the coolant loss is as brokenhead mentions, a bit of expansion loss at hot shutdown because the rad was overfilled ... there will be no 'evaporation' loss from the properly functioning system, unless you're taking the cap off when hot - vapor rises, doesn't descend

Bill

Besides raiseing the boiling point the pressure increases the cooling capacity of the system. It works good to use a 4-7 lb. pressure cap & a catch can. If your system wont handle that much pressure its time to fix something.

And the best reason to use a low pressure cap? So the coolant won't slop out at the first fast stop.

I just purchased a new radiator for my project after using Daves adapter plate to move the water pump. I also got a 4 pound pressure cap and an overflow cylinder. I would really appreciate what others here have to say about pressure vs none, overflow tanks, etc. Keep them replies comin!

There's another way to distinguish between water vapor and steam - stick your hand in it. Assuming an ambient air temperature of 70° F:
If it feels pleasantly damp, it's water vapor.
If the skin comes off, it's steam.
1 gram of steam contains 619 calories, which condenses to boiling water on your hand.

more malarkey - as someone who's spent the last 30+ years running a food processing plant, I can tell you that the 'vapor' coming off very hot water below 212* will NOT feel "pleasantly damp"

as noted above, steam is vapor, generated by the phase change of water that happens when boiling under pressure - the visible vapor from a rad is not steam

Bill

Deve, if you're using a new rad and 4# cap [could use 7# with a new rad] you won't have any overflow to catch as long as you do not overfill the rad and the system works as designed

Bill

Some real world results since I replied on pg 1.
Even though the cooling system on my 49 3100 w/216 was functioning fine pressurized via a 4lb cap, I had also purchased a new stock "0" pressure cap for just in case at the same time and decided to try it and run non-pressurized this weekdend.

I took the truck out for an 80 mile run on Sat., high seed, low speed red light idleing etc., and I did notice a slight increase in engine temp as registered on the gauge, but it was minimal.

Normal temp would register w/the pointer just about centered on the "8" of 180, for this cruise the pointer moved over to the "0" and only rose slightly above that at long idle times (red lights, fast food drive through etc) but it never overheated.

I'm going to run non-pressurized for as long as the cooling system keeps the engine at proper temps but with warmer temps approaching I've added the 4lb cap to my travel kit.

re. overflow tank. Even non-pressurized the coolant level in the tank remained the same when up to operating temps and when I checked it a couple hrs after shutdown the radiator had sucked it back. So coolant overflow operation remained the same.
I highly recommend a coolant overflow tank no matter if the system is pressurized or not.

Dave

I've never had an overflow tank before. All these years driving these things and I just accepted you added a little water now and then. Since I am putting so much effort into this build, I decided to add a 13" stainless overflow cylinder. No idea if it matters or not. I have been told all these years to run a 4 pound cap just as insurance. No idea what that means, but its worked well. I am not a scientist, just a driver and part time backyard mechanic.