Overheating in DOHC Engines
Article by Guy Croft

Cooling system (rewritten & partly due to formula error)
From: Guy Croft
Email: croftengines@aol.com
Date: Fri June 16 00:59 PDT 2006

This article appeared in the Mirafiori forum June 16 and was in response to various ideas on the effects of unleaded gas in the dohc engine. The text is by Guy and is used with his permission

You do need a stat, but I like a low temp one, I suggest a 74-75 deg C type, (F = (9/5 * C)+32 = 167F) otherwise the flow round the head will be poor, ie: just come straight out the front. It acts as a restrictor you see. Moreover you damage the bores and rings on warm up without one. I can only speak about what I have tried and tested.

Don't imagine 74 deg C coolant is 'cold', it's plenty hot enough to put you in hospital and plenty hot enough for a race engine. You know that smell when everything is red hot? Getting the metal temp down will give more power and a much more reliable engine. 74-76 deg C was always my dyno run temp in every test I did, unless of course I was testing for power loss at higher temperatures – which I did some years back.

Remember for everything I write there will be other views as to what is best. No problem. I can only speak about what I myself have tried and tested. I cannot guarantee that if you copy my methods the friendship between you and ‘il motore’ will be idyllic - but it won’t go catastrophically wrong!

The cooling system effectiveness is a function of:- Pump efficiency (depends on impeller size, belt slip, rotational speed). Some people I know like electric pumps, all I will say they have all experienced a
high failure rate, usually in the middle of a rally stage.

- Coolant - water wetting in the head (use glycol or wetting agent to prevent nucleic boiling). If you want your cooling system to stay in pristine condition use only genuine Fiat Paraflu at 50% mix and change it each two years. That stuff is amazing; I have seen heads run for 25 years in MINT condition with it.

- Radiator efficiency - bigger is better usually if your engine has more than 20% more power than standard and in my experience an alloy one made up to suit the architecture of the car is best of all but it mustn't be so big it over chills the coolant. The hotter the ambient, the more cooling area it needs. The
radiator must be in clean condition; they silt up (and so do blocks). I have met more cheapskates when it comes to radiators than with any other engine ancillary. Make sure you have a pressurised system, coolant should never boil – and it’s the pressure and glycol (I think) in the system that stops it boiling at the usual 100 deg C at sea level. At higher altitudes water boils at a far lower temperature, hence the special pressurised boiling devices that climbers use, so it's even more critical if you drive in a high area/mountains. Once it boils there is no engine cooling at all, so stop driving, let it cool down before the head gasket blows.

- The behaviour of the auxiliary cooling electric fan or whatever the engine has to cool the rad in traffic and its thermostatic switch – if fitted. Personally I like an over-ride switch and a warning light to tell me its running. Over about 30mph it should NOT be running because the mass of air from the car motion is enough to cool the rad.

- Hose losses - short smooth hose or alloy tube is far better than long runs of crinkly stuff. The fibres in hose do fatigue so don’t trust old ones, and always use top grade hose clips. Don’t forget the heaterhoses will dump your coolant just as effectively as radiator hoses.

- Airflow into and OUT OF the rad. The whole underhood is a high-pressure zone, without venting in exactly the right places no rad will work effectively. Vents below the windshield - in the scuttle region eg, don't work because that's a high-pressure zone, that's why flies get stuck on your windshield.)
Remember there is heat coming off the header and brake system too, without venting there is no chance of the rad working effectively. An undertray below the engine bay helps.

- Oil cooling - oil helps to cool the engine and if the oil cooler suffers loss of airstream it will add to the general engine temp problem. I recommend 85 - 90 deg C oil temp going into the cooler.

Always instrument the engine to assess head temperature, no good measuring the rad temp – it’s the metal temp in the middle of the head you need to see. Never rely long term on additives to solve a leak, the longest I've ever known one last is about 6 months and then the failure was catastrophic.

An engine with retarded ignition or ex system back pressure will run much hotter than an optimised unit, plus hot intake air to the carb will exacerbate the overheating problem and upset the jetting, never mind robbing power due to charge density loss. If the head gasket is blowing anywhere, it’s going to run hot.