How does the radiator/expansion tank system work?

[leigh_m_gardner]

I need to relocate my radiator expansion tank (actually relocate and replace with something that doesnt look like it came from tupaware) and *ideally* I would like to sling it below the passenger foot peg rail. I have a Predator 3-1 on the right side, and therefore there is a gap on the left side normally plugged by the stock exhaust, so am looking at slinging the expansion tank on the left hand foot rail.

The expansion tank is this one:



So questions :-

(a) How does the radiator/expansion tank system work?

and

(b) slinging the expansion tank horizontally on the right passenger foot rail - will it be ok?

NB: I cant and dont want to put it in the old air filter hole under the seat.

 

[atomsplitter]

The expansion tank receives excess coolant as the engine heats up and the coolant expands. Water (50% of your mix) decreases density as temperature increases. The coolant system is pressurized by the radiator cap to keep the coolant subcooled (pressure dependent). As the coolant expands it is relieved to the small tank in front of the throttle bodies. When the system cools down the contracting coolant is drawn back into the radiator by the vacuum created by coolant volume contraction. Locating the coolant bottle above the radiator provides a hydrodynamically balanced system (pressure pushes the coolant in, vacuum and gravity draw it back). Moving the coolant tank to the air box the tank remains on a level with the radiator, but may not have sufficient draw to replace all the coolant back into the system. Locating the coolant tank below the radiator would require greater vacuum than the system is likely to make based on volume contraction alone (why you need some elevation head). Therefore the best alternative would be to make a similar volume tank and locate it on top of the engine (above the radiator but away from the intakes). You can locate it below the engine if you want, but install an auxiliary pump to return the fluid to the radiator after cooldown and depressurization (a whole lot more work and expense).

For those interested the cap maintains about 14 psig on the system which equates to 250 degrees saturation temperature at that pressure. The system kicks the air fan on at 207 degrees so the system operates 43 degrees subcooled. Just in case you were interested.

 

[leigh_m_gardner]

Thanks for the full responses :eek3:does anyone know the capacity of the stock expansion tank? As an alternative, I might consider mounting a front 'oil' radiator to act as the expansion tank, but I can only find one with a capacity of 150cc (0.15ltr). Any downside to using a oil rad as an expansion tank?

 

[Blastedbones]

Personally why go to all the effort? the OE plastic tank fits ( works) tucked outa sight behind the left side panel... Unless you're using that space already???as for an oil rad, so long as it's uncorroded, & has at least the capacity of the OE expansion tank, I can't think of any reason why it wouldn't do just fine so long as there's good flow & no airlocking, & your breathers work properly.

 

[leigh_m_gardner]

Personally why go to all the effort? the OE plastic tank fits ( works) tucked outa sight behind the left side panel... Unless you're using that space already???as for an oil rad, so long as it's uncorroded, & has at least the capacity of the OE expansion tank, I can't think of any reason why it wouldn't do just fine so long as there's good flow & no airlocking, & your breathers work properly.

there isnt any room in the side panel (got my stebel going in there) so its down to some other expansion tank. i dont know what the capacity of the stock expansion tank is - do you or does anyone know? - if its ok to use a oil rad for a groovy looking expansion tank, then it would be a new one.

 

[Blastedbones]

If you can fill it from the lowest point & use a breather at the top, & the flow is good ( test on an old one from a breakers yard?) then it will work. You will need a capacity of about a litre to ensure breathing room for min & max volumes. Fill the thing to the top, set the motor running , let the excess overflow as it heats up & that should clear any air bubbles. Ideally you wan't the chamber to settle to about half full when cold, @around half a litre sitting in the tank. Your only problems with a rad is that its a concertenerd tube & might air lock? that's why a bottle is a better vessel. Experimental ground there mate:smile:

 

[Sidecar Flip]

Techinically speaking the expansion tank allows for no headspace in the radiator, that is, the radiator is completely full all the time as the headspace is in the auxilliary tank. You could, in theory, dispense with the tank entirely but then that would reduce your coolant capacity the amount of headspace required by the expansion of coolant at running temperature. In olden days, no vehicle had a coolant tank, just an overflow hose and the headspace served as the expansion tank, but then radiators were purpose built with adequate headspace just for that reason. The R3 radiator isn't.

You need something with volume capacity equal to the existing tank. An oil cooler is not the way to go. A SIG bottle with TIG'd nipples would work, plus it has a screw down stopper that can be vented with a ball check. Whatever you use, it has to be vented to break the vacuum/pressure differential created by the heating/cooling coolant or the weakest point in the system will fail, most likely a hose clamp or a flexible hose.

 

[Blastedbones]

Although a fake oil cooler, ( a metal tank that just looks like an oil cooler) would do the trick:smile:

 

[atomsplitter]

OK Flip, looks like its time you got started on a coolant tank replacement project for us that require a bling alternative.