Cooling

There are LET enthusiasts all around the globe. From Australia and South Africa, to Romania and Sweden. Hot and cold climates --- but they all agree on one thing: these cars do tend to overheat.

The cooling system is only just adequate for the donor engine, the C20XE, but the turbo produces 54 horses more, and the super-hot turbocharger in front of the bay doesn't help at all. To make matters worse, the radiator size took a bit of a hit, so that the stock intercooler could fit up front (kinda). As the icing on the cake, the super-hot turbo bearing dumps most of its heatload on the coolant, which is then promptly emptied into the (plastic!) expansion tank.

Cut-out of a turbo waterjacket:

If the car also has aircon, then another massive radiator sits up front (the condenser).

We're not finished yet: Any turbo worth its salt needs a proper intercooler, and that will be front mounted. Yet another heat exchanger preheating the air BEFORE it enters the engine bay.

There's an oil cooler as well, integral part of the lubrication circuit

A tuned turbo will produce 50-100% more power under load, and all the heat that goes with it.

No wonder people get desperate in the summer, with coolant temps hovering over 100C. Switch the engine off and it can easily exceed 110C for a short while. Nothing good can happen at these temps.

 

Is there an ideal coolant temperature?

It depends on how the car is used.
On a track day, you'd like the coolant temp to be around 90C all long (whether it stays there is another matter!)
On normal start/stop driving where part-throttle response is more important, mid-nineties is better.
Emissions are lowest around 100C, that's why manufacturers try to keep temps around there.

But the Octane requirements of the engine increase with coolant temperature. For an 11C increase of the water jacket (say from 80C to 91C) there needs to be a raise of 2 AKI points, if we are to keep the safety margins intact. That could be 2 RON and 2 MON points, or 4 RON points. Quite a lot, eh? Look at the Ignition and Fuelling sections for more.

On the other hand, a cold engine (below 70C) should never be pushed hard. Metals need to expand, fluids need to warm up. Let them assume battle stations before applying full throttle!

These two thermostats are on the two sides of the spectrum. On the left is a pretty 'cool' one, 72/85 (Meaning that it cracks open at 72C and is fully open at 85C). In the winter the engine will struggle to reach 80C and will not exceed it on motorway journeys. Too cold for the engine's own good. On the right is a standard one for the aircon LET, 92/102. Much better. Remember that the engine's efficient operating range is 90-100C

To the left is a pattern thermostat. The build quality is nothing like the German on the right. The bleeding screw has been omitted to save costs (not that it's needed anyway...)

If you find yourself 'needing' a cooler thermostat, then you need to clean up the whole cooling system. A thermostat cooler than 92/102 should only be needed in tropical climates. Otherwise it is a botch, masking the inadequacy of the radiator, expired coolant, neglected cooling passages (full of scale) or even water pump. Don't fall for the "easy fix" of a cooler thermostat, far from being a cure --- it's not even an aspirin.

 

Coolant

Manufacturers claim (and punters believe) that antifreeze is good for ya. 50/50 mix all year long they say, keeps the engine happy. Hey, it's got anti-boil additives, so a strong antifreeze mixture will resist boiling - isn't that a good thing?

Well, no. While the anti-boil additives might raise the boiling temp by a few degrees, all that antifreeze (instead of water) will be far less effective in absorbing the heat from the engine and dumping it on the radiator.

Eh? why do they recommend it then? Surely they don't want people's engines to fail prematurely? Exactly that is the reason they recommend it - they know that the average punter isn't too bothered to check anyway (unless it breaks down) so the easy-to-remember 50/50 ratio is convenient for all-round the year. They don't want people running weak coolant mixtures and claiming on warranties for their cracked engine blocks in the winter.

But for optimum performance you need a different mixture, depending on the ambient temperature. Antifreeze has one major role: to stop the coolant from freezing. It also contains anti-corrosion and lubrication agents. The truth is that water is the best cooling medium around. That's distilled water, or at least de-ionised. The heat-transfer properties of water are superb. Antifreeze can't match it, so running a 50/50 mix when you can avoid it is plain silly. There is a minimum mixture strength and that is 25%. It's not wise to go lower, because the anticorrosion and lubricant additives will be diluted too much. Even so, running 25%antifreeze 75%distilled water makes a big difference in the summer. At the end of the summer, just drain off 2 litres of coolant (2.5 if you're in the highlands) and then topup with pure antifreeze. That would bring it back to a safe 50/50.

It must be stressed that all coolant should be drained and replaced every year or two, because it eventually turns acidic. There are chemical reactions happening there, it's not like a glass of water left covered in a dark room. The coolant reacts with the metals around it and produces contaminants that appear as 'furring' on the inside of the coolant passages. A 0.3mm such layer can cut down heat transfer by a third. Hard to clean it, too - best to avoid it in the first place!

A warning for those with pets: antifreeze is toxic but sweet. It smells and tastes sweet (apparently) so pets go there and lick it. Then they die.

Methanol-based antifreeze was nasty even to smell (it's banned now via an EU directive)

 

Checking the coolant with a Voltmeter

Eh? you're kidding right? Coolant testers use boyancy, the small eye-drop things with the floating balls.

Actually a digital multimeter can be very useful in checking the condition of the coolant. Water is not a very good insulator, that's why regulations don't allow installations of electric sockets in bathrooms.

Coolant is a mixture of (hopefully) clean and deionised water, along with all sorts of additives that help make it quite a conductor. The older the coolant, the more acidic and conductive it becomes - that's not good.

Set the DMM to measure low voltages (below 1 DC hopefully!) Stick one of the probes of the DMM in the coolant, the other somewhere on an exposed metal part of the block or the inlet manifold, whichever is closer. Even with the engine switched off you may be surprised to see a voltage.

If it's below 0.1V then it's fine.
Between 0.1 and 0.3V it's not perfect, but you should be prepared to change the coolant at some point.
0.3V - 0.5V is not good.
Over 0.5V then you have a problem and should flush the lot immediately!

 

Coolant flow

Liquids flow in strange ways, sometimes counterintuitive.

You think it may be better if you run the water pump a bit faster, and sometimes this is indeed a good idea, as it circulates the coolant quicker. Other times this is counterbalanced by eddies in the wrong places, *reducing* flow and losing pistons (one reason turbocharging the Vauxhall V6 is tricky)

Ah, but then you think it may be better to run the pump a bit slower, giving more time for the coolant to do its stuff, and sometimes this is indeed a smart move. In other cases you get localised boiling and hot spots - not good at all.

You then have a stroke of genius and decide to increase the diameter of the waterways to promote flow -- and you end up actually decreasing pressure, depriving remote spots of their cooling. This is a common issue with aftermarket 'performance' cylinder head gaskets, where the coolant holes have been enlarged to promote flow. It's like the manufacturer was negligent or absent-minded and made the gasket passages smaller than the block/head waterways. The aftermarket 'guru' fixes this oversight, pressure in the head goes down, and remote chambers in the head lose their coolant circulation altogether. The head goes west, and something else takes the blame (usually the turbo!)

The motto is THINK before forcing an 'improvement' in the coolant circuit - it may have far-reaching repercussions you never thought of.

 

WaterWetter

Does it make the water any wetter? Yes. Well, at least it appears so, it reduces surface tension.

Does that make a massive difference? No. If you just add it to your 'recommended' 50/50 mixture you'd be hard pressed to feel any difference.

Where it really excels is on the track, because it contains all the needed additives (apart from antifreeze!). So during the track day you can run just 100% distilled water and WaterWetter. This does make a difference, but it's not the wetting effect at play, but the lack of antifreeze. Nice though.

Warning: WaterWetter might react with some additives in the antifreeze and create a 'curry froth' in the expansion tank. It looks like the head has gone porous, but it's a false alarm. Best to change the coolant though, and avoid mixing WW with that particular brand of antifreeze.

 

Water pump

This is a weak point of these engines, as it tends to suddenly deteriorate, leak and eventually die - sometimes taking the engine with it. It's therefore good practice to have it changed every time the cambelt is up for change (30-35Kmiles), since most of that work has to be done anyway.

Both of the above pumps are on sale, the one to the right is better (it will not cause cavitation).

 

Radiator

When a FMIC has been fitted, the old intercooler should be disposed of. There is no point in it being there restricting airflow just because the coolant rad leans on it. The rad mounts can be extended by small brackets, allowing the rad to be positioned a bit more centrally and leaving some space for extra cooling air to get into the bay.

These are close-up shots of the new rad mount extensions:

An even better move would be to replace the radiator with a bigger one, that would use up this extra space for a good cause. The coolant hoses might need to be extended in this case.

An intermediate solution is to use an aftermarket rad, direct replacement of the existing one, but with a thicker core. It wouldn't cost much, and it would bring the following benefits:

• new radiator is totally clean outside, the fins are straight and the paint is intact
• new radiator is totally clean inside, all the muck, scale and electrolytic garbage are thrown away. No rad cleaning can ever achieve that.
• new radiator core can hold another 0.5 litre of coolant. More coolant in the system means less chances for it to hit critical temperatures.

 

Can I run the turbocharger without the coolant?

Funnily enough, yes.

It's not recommended of course, and the extra heat load will be dumped on the oil. People have ran the LET turbocharger on engine blocks without coolant pipes. The turbo obviously doesn't live as long (around half the lifespan, which is not too bad really) and the oil changes have to be frequent and only use fully synthetic (which is a good idea anyway!)

 

Engine-bay Airflow

This is a very important issue, which some people neglect, or take for granted.

Fresh air enters through the front intakes, moves through radiator fins, pipes, hot components etc, then exits via the vents at the back of the bonnet, as well as underneath the engine. At speed these 'exit routes' are well defined and part of the original design.

Vents like these help the hot air from the engine bay exit quicker, increasing the efficiency of the cooling system and the intercooler core as well.

In contrast, these are scoops: in this Mustang they seal against the V8's air intakes underneath, creating a ram-air effect. If such scoops are introduced on a conventional engine bay, the airflow will be disturbed and probably made worse. Scoops create positive pressure, while behind the radiators we want negative pressure.

For extra air intakes on the bonnet, the best way (aerodynamically) is to fit a NACA duct:

Ill-placed vents and scoops can disrupt the airflow towards the alternator or the ignition coil, and create eddies that reduce the efficiency of all rads under the bonnet.

Rule of Thumb: if in doubt, leave it alone.

 

Here's how the cooling system works in general

Cooling tips from US hotrods. Even more here. Some are more relevant than others...(local copies here and here)

Useful link if you're looking for a pattern or modified radiator

 

On to Ignition ...