Cooling - "Keeping the 'monster' cool..."
How do you keep a racing motor cool? Here are some of the do's and don'ts found out at much trouble by your friendly neighbourhood mechanics, Steve & Jez. |
We started off with the original radiator, as supplied with
the car. This radiator had seen some service but was basically whole
and the tops of the tubes weren't completely filled with gunk. This
was connected to the motor, a single thermo fan bunged on the back and
the whole thing started up. Funnily enough the thing was running
warm just at idle. Hmmm..... time for a rethink.
Go the twin thermos! These things don't pump out a lot of air but they've got to be good for something. Still running warm. Hmmm... more thinking. Drop a smaller pulley on the crankshaft to stop the water pump cavitating. Hmmm.... helped but not enough. Thinkkkkk... Time to move the thermos to the front of the radiator, install a radiator shroud and bolt the stock fan back on (so we lose 5HP, we may break less drive shafts!). Add to this, buying a new oil cooler and moving it out from behind the radiator into a fresh air flow. Finally, some success! |
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The stock fan/shroud must move about 10 times the
amount of air the thermos do, but we left them on anyway. I'm sure
the new oil cooler helps as well. Saw something mentioned somewhere that
rotaries are cooled almost as much by the oil as the water.
Note that the shroud is a custom fibreglass unit (as the radiator offset is different from that on a standard Mazda) and the external thermoelectric switch. This works a treat, just wind the pot down a bit to account for the external mounting, and you'll never have problems with leaky pipes again! Couple of things to note:
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Note the oil cooler - AU$700. Damn that's expensive. Note that the
location/orientation of the
oil cooler in the Commodore is better
than this installation.
We went the whole hog and put in some Earl's connectors (to get the right angles to miss the bonnet) and some braided hose (which is an easier/cheaper option than getting a couple of hoses custom made). Note that the Earl's are quite susceptible to stretching, and as a result don't like being over-tightened. Note the smashed light. Damn trail bikes! Remember to protect your radiator, oil cooler and lights from dirt/rocks thrown up by other cars!!! I'm sure I mentioned this before. |
Cooling Systems
Well it's about time I updated this topic... Cooling an engine is a bit of a black-art, and there are many factors that can affect how well a cooling system will work in a car. Hopefully I've managed to cover a few of the basic ones below:
Basics
Cooling occurs when hot water/gas from an engine is pushed through a radiator from which heat is radiated away by thermal conduction/convection via a cooler air stream. Cooling efficiency is proportional to the difference in temperature between the air stream flowing through the radiator and the radiator fins themselves. The end result of this, is that it's more difficult to keep a car from overheating on a hot day than a cold one. It's also apparent that the best cooling circuit would have extremely cold coolant entering the engine, but engines need to operate within a fairly tight temperature range
Cross-flow or Vertical?
Vertically cored radiators do allow a small amount of thermal syphoning to occur without a coolant pump. This flow is completely inadequate to cool a modern engine and should be disregarded. Aside from that, I can't think of any good reasons to use a vertically cored radiator. Cross-flow radiators have exactly the same cooling capacity, plus they allow the pressure cap to be positioned on the outlet of the radiator. This can be important, as there is a pressure drop across the radiator (due to the flow restriction caused by the tubes) and a cap positioned on the radiator inlet may experience larger than normal pressures caused by high engine RPM, resulting in unwanted coolant release. Placing the cap on the radiator outlet mitigates this risk. The coolant is also less turbulent on the outlet side of a cross-flow allowing for a better air trap.
Copper or Aluminium?
So do you buy a copper or aluminium radiator? Well copper's thermal conductivity is about twice that of aluminium's, however it weighs more and is harder to work (ie. higher cost). But the real issue between one or the other is related to how the coolant transfers heat to the radiator fins. It turns out (unsurprisingly) that the greater the internal surface area of the radiator, the better the heat transfer. Now obviously this doesn't take into account the temperature gradiant or thermal conductivity, however it's pretty simple to see that a radiator that spreads the coolant out thinly is better than a big hollow drum. So good radiators require thin/wide tubes, and pressure causes wide tubes to balloon if their walls aren't structurally sound, which means thicker walls, meaning more weight, and aluminium is a heck of a lot lighter than copper. So that's why everyone runs an aluminium radiator: wide thick-walled tubes creating a very large surface area with relatively low weight (at least compared to copper).
Coolant Flow
So it's not number of cores that count, but their cross-sectional profile (flatter is better). Of course, any particular engine will require a certain flow of coolant (at a set pressure) to remove waste heat (1/3rd drive, 1/3rd exhaust, 1/3rd oil/water is a fairly rough split). If the radiator creates too much resistance to this flow, then heat can't get out of the engine fast enough, and it will overheat. A better pump or a less restrictive radiator should help.
It should be noted that a standard pump is designed to operate most efficiently within a given RPM range on any particular motor. Pushing the engine consistently higher into the rev. range may result in greater pump cavitation and subsequent overheating. A few options exist to fix this type of problem:
- Change pulley diameters to under-drive the impeller
- Re-blade the pump to improve pump efficiency
- Increase the operating pressure of the cooling system
- Replacement with an external electric pump
Thermostats
These things increase pressure in the block (allowing more heat to be extracted from the metal) as well as regulating engine coolant temperature (because engines dislike running cold as much as they dislike running hot). All-in-all, they are a really good thing and shouldn't be removed under any circumstances. Things to look for are good flow and the right operating temperature.
Coolant
Water has the highest specific heat of any semi-obtainable liquid that a sane regulating body will let you put in a car. So you can actually use distilled water for an exceptional cooling capacity. The problem with this is the lack of additives that perform other functions in the engine, quite unrelated to cooling. A couple of note are lubrication of pump seals, and certain anti-corrosive additives. Choose a notable brand that can be mixed/obtained reliably, doesn't foam (bubbles are bad, mmmkay) and stick to it. Mixing coolants is also bad (even if they are both green).
For the technically minded; a 50/50 mix of Ethylene Glycol and water (by volume) provides a specific heat capacity of roughly 0.78 to 0.87 (dependent on temperature); much lower than pure water at 1.0.
Pressure Caps
These units retain pressure within the cooling system, raising the boiling point of the coolant and increasing the temperature differential between the coolant and ambient air, increasing radiator efficiency. Higher pressures are generally better, but the rest of the cooling system needs to be capable of working at the pressure as well. Recovery system caps are a neat idea, but a good air-trap in a race system will work just as well (it's just a bit more maintenance). WARNING!!! You cannot use a big enough rag when removing one of these things from a hot pressurised cooling system. Cooling system burns are nasty (if fact all burns are nasty), but this type of accident is easily avoidable, so don't take the damn cap off in the first place if the engine's had a chance to warm up.
Forced Air
Mechanical fans (those directly connected to the engine), tend to provide a far greater air flow than a realistically priced electric fan. The obvious limitations to a mechanical fan are being linked (either directly or indirectly) to engine RPM (not so desirable), and limited placement (due to drive considerations). Conversely, electric fans can be bolted directly to the radiator (good), and can be switched on/off as desired (also good); however the high CFM versions are expensive and require a decent power supply. At the very least, you should install a decent shroud to force all air passing through the fan to come from the radiator (or visa versa).
