Radiator overflow tank

[Brave Salmon]

My 351w seems to expand enough that the overflow tank overflows onto the ground. When the engine cools down, it pulls all the coolant out of the tank, every drop. Anyone run into this problem and how to fix it? Surely the tank should be large enough to allow for proper expansion without losing fluid.

[scottiec]

Are you using the overflow provided by FF? I know for the roadsters that it is too small. Not sure if it is the same overflow for the 33. Wouldn’t surprise me if it was.

[Brave Salmon]

Yep, stock cylinder 2" x 14" and approx 44 oz computed of capacity. How much larger should I go and what options are you aware of? I'm thinking that I should at least go up to 150% or 65-70 oz.

[NAZ]

Before making any complicated or expensive changes check your radiator cap. Unless the system is overfilled with no room for expansion (not likely on the hot rod with that laid back radiator) or the cooling system is overheating you shouldn't have coolant blowing past the cap. In the old days cars didn't have overflow tanks and as long as the system didn't overheat all was well with the world. If you can't test the cap get a new one or better yet, a known good cap and see if the problem goes away.

[Brave Salmon]

Great, now you've blown up my total knowledge base on cooling systems. I thought the idea was that the radiator stayed full and when hot, the coolant expanded past the cap into the tank. That's why my oem cars have tanks labeled hot/cold. When the coolant cools down, the cap allows the radiator to suck in coolant from the tank thereby keeping radiator full at all times? I will swap out the cap with a known good one from my family car and see if either has any changes. Thanks for giving me something else to analyze; I really needed it. Take care, NAZ

[mike223]

I thought the idea was that the radiator stayed full and when hot, the coolant expanded past the cap into the tank. <snip> When the coolant cools down, the cap allows the radiator to suck in coolant from the tank thereby keeping radiator full at all times?

That is exactly how it works, but to some extent that depends on how full of liquid the system is (and how hot it gets, etc).

What NAZ is saying (I think) is that with the laid back radiator you're always going to have some air trapped in the top of the radiator - that air will compress as the liquid expands (reducing the amount of liquid sent to the overflow when hot).

As opposed to if you had a bleed valve at the top of the radiator, and kept the air bled out - in that case you would have more liquid sent to the overflow (less air = less room for expansion).


For the record, I have a 393w in a Mk4 (laid back radiator) with a bleed valve at the top of the radiator, and I try to keep the radiator full of liquid (air bled out).

I'm using the same FFR overflow tank which tends to run empty cold, and 3-4 inches liquid hot (180 degree tstat / 16psi cap).


But over the years a number of people (various engines) have had trouble with the FFR overflow tank being too small - those people seem to go to one with one about twice the size (and after that they mostly seem to be happy - unless they had other problems in the first place).


Hope that all makes sense.

[Brave Salmon]

Ok, so it looks like my options are to live with it since the motor is not running hot or replace the tank with a larger version. It's hard to get out of the oem mode where little things like this are engineered to work rather than cobbled together to sort of function good enough.

Thanks for the help. Guess I need to get my Christmas list updated..

[greenbaygreg]

Yep, stock cylinder 2" x 14" and approx 44 oz computed of capacity. How much larger should I go and what options are you aware of? I'm thinking that I should at least go up to 150% or 65-70 oz.

I had same problem. Ordered the larger overflow tank from Jegs and no problems since.

[NAZ]

Back in the day (think the nifty fifties) cars had vertical flow radiators just like the 33 Hot Rod comes with. We used the upper tank as an expansion tank (on some cars like Fords the upper tank was not the highest point in the system so a separate expansion tank was used). When coolant is heated it expands and the system pressure will increase. How much expansion and pressure increase depends on the operating temperature and the amount of air space available to compress and allow for the liquid expansion. If you fill your hot rod radiator and try to use it like a newer car coolant system then you can expect some of the coolant to blow past the radiator cap.

You will need a larger recovery tank if you insist on overfilling your radiator. So you have to ask yourself what that gets you. Does adding extra coolant to the radiator really help cool your car if it simply ends up in a collection tank once the engine comes up to temperature? And how big of a recovery tank do you need? If you're good with math, Dow chemical has some info you can use to calculate the expansion of glycol based coolants and get a good idea of how big a tank you need. It will vary by system capacity and operating temperature so there is no one size fits all answer.

I plan to use my top tank as an expansion tank just like I did with my old fashion cars from the fifties. And my T-Stat is old fashion too as it opens at 160F not 190F or more like emission choked cars of today. We'll see how old school works once I get on the road.

[mike223]

It will vary by system capacity and operating temperature so there is no one size fits all answer.

I completely agree with everything else you wrote.


But it has surprised me over the years how true the "no one size fits all" aspect turns out to be.

- In FFR cars with exceedingly similar fluid capacities / radiators / build plans / etc.


I don't really understand that aspect - I've made every effort to overfill mine (repeatedly), but the small recovery tank continues to work perfectly for me.

But not for all.

lol, maybe I'm not overfilling it good enough...


And I'm a crossflow roadster, so that gets off into apples + oranges.

[JRL16]

Had the same issue with mine. Ordered a larger one that had the bonus of a sight glass tube on the side. I believe it's a Canton product available from Jegs or Summit. Wish I could look for you to be sure but car is at painter. Good luck.