BOSTONCAMARO

champs,that is it....I did some research and found it as well, thanks for posting and thanks for everyone's help

in short, without pages of theory and diagrams and sh-t

overflow - is just that for it to puke

expansion - allows fluid and pressure to transfer..the radiator can recover fluid from expansion tanks

most cars today, their overflows, really more or less are expansion tanks

gofastvette

a play on words, some are both but the expansion has a cap

Mike Ward

I forgot to save this the last time I typed it out but it's in the archives somewhere. Corvette prior to 1973 had no coolant recovery tanks and were essentially no different than a Model T Ford in technology level.

Coolant expands as it warms to normal operating temperatures. Cooling systems are normally designed to take advantage of this and operate at a pressure of about 15 lbs or so, the prime benefit being increased boil over protection and reduced tendency of cavitation at the water pump impeller.

Since liquids are non-compressible, the expansion of the liquid has to be taken into account. Pre-73 Corvettes were originally designed with 'expansion tanks' to accommodate this. Radiators with a cap on them, usually the copper/brass type, had tanks attached on each side much like cheeks on a squirrel and the rad was considered full with 3-4" of air inside. Aluminum plate rads were not manufactured with side tanks or a rad cap. A separate tank was integrated into the system- see Alan's picture above. This pressurized vessel, sometimes called a surge tank, is kept half full or less to allow expansion of the coolant and also has a cap built in.

In either case, there must always be 'X' amount of air inside the system to allow for expansion. Excess coolant or conversely insufficient expansion space would cause the system to rise above the set point of the rad cap causing the release of coolant overboard, ie. it pukes on the ground. This most typically happens with an overfilled system after engine shut down as the coolant inside the engine becomes heat soaked. I've seen owners chase their tales for years by constantly refilling the expansion tanks only to have it puke again after shutdown, instead of just letting the system find it's own level.

Starting in the late '60s most OEMs including GM got smart and started using coolant recovery systems. It took till 1973 for this feature to appear on Corvettes.

A coolant recovery tank by design is much simpler. It is located on the non-pressurized side of the system and therefore can be made of much cheaper materials and of a simpler design- a plastic bottle. The pressurized expansion space inside the coolant system is eliminated by filling the rad 'to the top'. As the coolant expands and the system pressurizes to the level set by the rad cap, excess coolant is routed past the cap and captured by the recovery tank, much better than puking on the ground. After engine shutdown and heat soak, the coolant contracts inducing a partial vacuum. Instead of air being drawn in, the coolant sitting in the recovery tank is brought back into the rad. This ensures that the system is full 100% of the time. The owner simply monitors the level in the recovery tank and insures that it's at max cold or max hot, as the case might be.

All moderns cars have the coolant recovery tank type of system.

Steve2147

Expansion tank is located on the pressure side of things, coolant recovery tanks is outside the pressurized system.

The way systems with recovery tanks work is that the system is filled right to the top with no air space. As the coolant warms and expands it is forced past the pressure cap into the coolant recovery tank. As the engine cools and coolant shrinks it sucks the fluid back from the recovery container. The advantage is that the radiator is always completely full utilizing every sq inch of cooling area where other cross flow rads with filler caps lost the top couple of inches to air/expansion space.

Steve g

Steve2147

Wrong. Few cars today use coolant recovery systems anymore. Most use the plastic expansion tank with a screw on pressure cap.

The expansion air space in the cooling system only became an issue with the use of cross flow rads where the cooling area (core) extended to the very top of the rad and infringed on cooling capacity of the rad. It was never an issue with top tank rads.

The down side to recovery systems is that people frequently judged the level in their cooling system by the markings on the recovery tanks. However, when a system develops a large enough leak it will not draw fluid back from the container, sucking air at the leak instead. So the overflow container would show full when in fact, the system can be dangerously low on coolant.

Steve g

BOSTONCAMARO

I am guessing that one of the heater core hoses goes to the bottom of the expansion tank and the other runs to the water pump outlet that is on the passenger side? thanks again for the help!

68thumper

Mike...maybe you can shed some light on my configuration. My DeWitt radiator has a 20 lb. Moroso racing cap on it and a standard 12 lb. cap on the expansion tank. Do you know if those caps sound about right? Brent.

MelWff

If you mean a factory expansion tank they should be 15 pounds. The 20 lb cap will never do anything since the 12 pound will open first. With an expansion tank the cap on the radiator should be gutted and only provide a seal.

Mike Ward

Neither is correct, as is your set up. No C3 Corvette came with two radiator caps from the factory. It either had one on the rad or one on the expansion tank. You've either got the wrong rad or somebody added an expansion tank which would be superfluous.

Which year car and engine have you got? Do you have A/C?

LT-1 kid

maybe the op is talking about the rear defroster motor that sits in the left rear fender well,

corvetteload

The spelling! Actually the expansion tank is designed to catch and then release, when the overflow just catches. The exp tank will replenish back to the radiator after the expansion part of the process.

68thumper

Mike....I have a 68 with a 468 big block. The radiator is a DeWitt and the expansion tank is factory appearing. As MelWff noted above perhaps the cap on the radiator is gutted. I will have to check this....thanks.

68thumper

I was thinking maybe the 20lb. cap held the pressure up to 20lb. then if exceeded the expansion tank would hold another 13...?

Mike Ward

Do you know what the car had originally? The only '68 BBC cars that had an expansion tank were the L88s.

Nope- the 20 lb cap will hold all pressure until the system reaches 20lbs- which is way too high. It's designed to work at 15lbs.

Your choices at this time are many, but I'd re-plumb the system to use the expansion tank as a coolant recovery tank.

BOSTONCAMARO

I did get my install all done awhile ago. My car was an original 350 350hp car with manual transmission and NON ac car, but it did have the larger copper brass radiator. Although some on the forum says that the cars never came like that, there are a few forum members who had original cars which indeed came as mine did.

I have now converted it over to an aluminum factory style radiator, steel shroud, expansion tank set up.

MelWff

It's all one system both caps see the same pressure. I have the same "incorrect" set up. The difference as I mentioned is that the cap on the radiator is a gutted cap with only the rubber seal around the edge to prevent it from leaking. The aluminum expansion tank requires a special 15 pound cap to prevent electrolysis. Below is the correct cap for the aluminum expansion tank.

http://willcoxcorvette.com/product_i...oducts_id=2736

jstipes

For those cars without an expansion tank, did the original radiator still have a connection for an overflow hose? If so, where/how was this hose routed - did it just dump on the ground on overheating? Does anyone have any photos or descriptions of this hose and how it was routed?

Thanks.

Spacecoast

I am curious about this as well, since my hose just hangs down and was wondering if I am missing something.

jstipes

Spacecoast, Could you post a photo of where/how the hose is routed on your car?

BOSTONCAMARO

It dumps onto a splash pan at the bottom of the fender of what I understand, I still have my hose looped at the end, need to figure out a nice way to dump it, zip tie it off somewhere or something