How many lbs radiator pressure cap is optimal?

[DocDirk]

Recently I've been reading about cooling systems and some tech articles urge the use of the highest pound radiator cap that the radiator is rated for....up to 28-29 lbs if possible. Mine is a 4 core, 18 x 26 aluminum radiator of unknown origin (a Vern's West Coast special). I have a 16# cap on right now, but overheating is a problem... and I've read that once the cap vents the engine will not reduce in running temperature until turned off (?perhaps the reason that running on the freeway didn't reduce the temp of my engine running at 225 deg in town?)

Coolant is 70/30 water/glycol. Pure water with an inhibitor might cool better but then boils sooner no doubt?

What pound caps do you run on your cobras? Is maximal really optimal?

Is there any way to determine maximal / optimal lb pressure for the cap without knowing mfgr. source to inquire?

Thanks!

Dirk

[Excaliber]

Myself I like to run a low pressure cap to ease the overall strain on the system. Even if it's a new system, hoses, radiator, gaskets, heater core, etc. A 15-16 psi cap with straight water will raise the boiling point to something like 250 deg's. Now add a 50/50 mix of antifreeze and that raises it again. I would say boiling point would be pushing 270 deg, or more depending on the mix.

Well if 15 lb cap will do the job why would I want to go higher? Surely your going to do SOMETHING (pull over, shut off the engine, get a tow) before you let it overheat to 270 degrees!

225 you say? Not bad say I. Seem's like most folks around these parts think anything over 220 is bad news, I disagree. Would I like it lower? Sure, but I wouldn't loose any sleep over 225 either, which no doubt, is well below your current boiling point. So you can rule out that as a factor as to why it won't come back down. Something else going on...

It would likely only vent when you pull over and shut if off, and the motor is really hot. That's when the engine water temp could exceed the boiling point, build up enough pressure to blow some out. Hopefully to your CATCH CAN and the water is then sucked back in when the motor cools. If the motor 'blows off' at 15 psi, again, I think you've got bigger problems than the pressure cap causing it. Higher pressure cap will only mask the symptoms, not get to the cause.

[PANAVIA]

With the WCC you may want to think about a larger Radiator, some hood vents and revised plumbing to drop the temps.

220 is too hot.

Steve

[Excaliber]

I think the primary reason for a high pressure cap is to make sure your not blowing water out the overflow at high rpm, when pressures and turbulence can get pretty high. My 427 doesn't blow water at high rpm with a 15 psi cap so I'm OK, but you might not be. Of course when you do loose water, for whatever reason, THEN your going to overheat for sure! So the advice of a higher pressure cap is generally a safe call that covers all the bases, but not necessarily called for in all cases.

[vector1]

i talked to howe because i had a car show up with a ruined radiator (howe brand) and they recommended a 22-24 lb. cap. the radiator actually had the cooling tubes expanded so no air could flow through. i asked them how this could happen and he explained that when an air bubble is ran through them it is real easy to do. i should have asked how but he didn't elaborate. the radiator had a 16 lb. cap installed and the puke hose was ran to the top of the catch can which is prob why the head gasket was leaking and blowing the coolant and filling the system with air, because the coolant was being expelled and not recovered. by the way my jbl was doing the same thing because the catch can was also hooked up incorrectly by the po, blowing coolant and not recovering which led to a leaking head gasket.

the cooling system i am currently running on the jbl is using a 22# cap. the puke tube is running to the bottom of a catch can with the catch can filled 1/2 full. the top of the catch can is prob inline with the top coolant hose. i am running a stewart high flow 180 thermostat with three drilled holes purchased from speedway already drilled. there is no air in the system, none, nada, zilch. when the system heats up the coolant is pushed into the catch can and when it cools down the coolant is drawn back into the main system. after the system cools i can take the cap off and it is filled 100% with coolant. this is on an all aluminum engine so i would think if the pressure was too much it would show on this setup.

i think unless you have air in the system because of hydraulics the cap is going to let go no matter what. my top tube is inline with the top of the engine, so its either going to contain quite a bit of air in the top hose, engine, and radiator, or 100% coolant, i went with the latter.

i had an oem 195 thermostat with two holes installed and the temp would fluctuate too much, i like the stewart high flow 180 much better for holding temp and should work well for racing.

hth

[1985 CCX]

I use water and wetter with about 10% antifreeze. 16lb cap

[RICK LAKE]

DocDirk Doc you shouldn't need anything more than a 15# cap. What thermostat are you running in the motor? Run a 180 temp would be about the best for your area. Do you have a shroud on the coolant fan? You want to pull as much air through the rad to control and lower the coolant temp. Water pump, who's do you have? Here's the best one, are you running UNDER DRIVE PULLEYS???from March?? They don't spin the water pump over fast enough to cool the motor and control heat. If the car sat for a couple of months you might want to replace the thermostat first. Drill a couple of 1/8" holes in the edge of the thermostat to help remove air from the coolant system. Make sure that is not alot of debrie in the fins on the radiator. One more check is to make sure that the rad has no cool spots. This could mean that the tubes are not flowing. Anything more than 16# cap you are looking at possible loosing on a hose to this much pressure. How low has the motor been built and running. Do you hear and bubbling or gruging sound coming from the motor when hot? Before you start playing with anything, get a infrared temp gun and check to see what the coolant temps are around the motor and rad. It is VERY POSSIBLE that your coolant gauage could be 10-20 degrees off and there is no problem. Let's do the basics first before we go hunting for a nightmare. This has happened before to other guys here. Rick L. Ps you have PM.

[DocDirk]

Originally Posted by RICK LAKE

DocDirk Doc you shouldn't need anything more than a 15# cap. What thermostat are you running in the motor? Run a 180 temp would be about the best for your area. Do you have a shroud on the coolant fan? You want to pull as much air through the rad to control and lower the coolant temp. Water pump, who's do you have? Here's the best one, are you running UNDER DRIVE PULLEYS???from March?? They don't spin the water pump over fast enough to cool the motor and control heat. If the car sat for a couple of months you might want to replace the thermostat first. Drill a couple of 1/8" holes in the edge of the thermostat to help remove air from the coolant system. Make sure that is not alot of debrie in the fins on the radiator. One more check is to make sure that the rad has no cool spots. This could mean that the tubes are not flowing. Anything more than 16# cap you are looking at possible loosing on a hose to this much pressure. How low has the motor been built and running. Do you hear and bubbling or gruging sound coming from the motor when hot? Before you start playing with anything, get a infrared temp gun and check to see what the coolant temps are around the motor and rad. It is VERY POSSIBLE that your coolant gauage could be 10-20 degrees off and there is no problem. Let's do the basics first before we go hunting for a nightmare. This has happened before to other guys here. Rick L. Ps you have PM.

Rick,

My current radiator cap is 16#. I'm running a 180 top quality thermostat but have not drilled holes in it yet. I saw one from Speedway that has them already. My engine is relatively new, less than 15 hours on it. There is no debris on the radiator. I'll check for radiator cool spots. I shall get the infrared temp gun as I want to check across my tires for best inflation pressure too. No bubbling or other strange sounds from the motor when it's hot that I can tell (lumpy cam and loud exhaust excluded). I have relocated my temp gauge to the block because of a complicated setup with the surge tank line to the thermostat housing and I'm told by some that's fine and by others it's reading too low there. My radiator is shrouded to the fan but not in front from the opening of the car. The water pump is rated 20% over stock output and it's OVERdriven by a pulley smaller than the crank pulley. Up until the hot weather spell the car was running at a steady 185 with a 180 thermostat so I figured the temp gauge was probably accurate. Some have suggested I'm pushing too much water through the system. I'm soon going to reroute the fill hose exiting the surge tank away from the intake side of the radiator to the low pressure side of the water pump as it may currently be providing a way for the radiator to be partially bypassed through the surge tank:. I"m sure I'll have this all sorted out and enjoying trouble-free worry-free driving by 2012 Thanks for your thoughtful analysis!

Regards,

Dirk

[Bob In Ct]

I have read somewhere along the way that a pressure cap also keeps the coolant from cavitating (boiling at low pressure) around the water pump.

Bob

[Mdman352]

Doc,
I had a similiar problem. What i fouind was the lower radiator hose was collasping at higher rpm. the fix is, buy a hose with a spring in it, or get the spring out of a hose and insert it into your hose. this will prevent the hose from pinching off water flow. Rev the motor up and what what happens to the lower hose. Hope this helps.
David

[FWB]

the small air gap between body and top of the rad is not helping i noticed a big difference when i shrouded that. the air takes the path of least resistance.


Fred

[windy]

I am running a 20# cap and stay a consistant 190 degrees in S Florida driving.

[FUNFER2]

220 degrees, is not hot my friends. Unless it's 30 degrees out, then you do have problems.

With my 428 FE, it will creep up to 215 on a very hot day of 95-100 but, I do hibernate,........... on those days.