Thread: Fors and against Bonnet scoops

Originally Posted by abaddonxi

I had an idea a while back that I've yet to do anything about.

Then run a hose down to a Y junction, one end goes down towards the gearbox and the other end in the engine bay.

I thought of a similar idea on my previous County - except I wasn't going to get the air from the tops of the mudguards.

I thought why not just have some air ducts to the front of the car under the bullbar? One could, if one preferred, even put the air inlets into the bull bar with the tubes mounting to the back.

Ron

There has been some recent interest in this subject via this recent thread started by feraldisco.

best position for GU bonnet scoop on D2

I have a 3.9 D1 with a cooling system that I am quite anal about and so runs as good as they get. Feraldisco's thread and this one came to my attention when I wanted to do something about reducing engine bay temps, particularly during heaving slow going in deep sand, when I can feel alternator windings, hoses and electric looms frying from the increase in engine bay temperatures.

I have a standard viscous fan/radiator setup (that has never let me down) and only non standard item is a low collant alarm.Engine bay is cluttered with second battery, isolator, locker compressor, snorkle connections, additional relays etc. I have a headers/extractors (coupled to a 2 1/2 performance exhaust). These add ons all reduce ventilation/increase engine bay heat.

After investing some time in the pressure differential subject I decided to invest some time in measuring actual engine bay temps.

Using a digital readout in the cab with a sensitive type K thermocouple probe variously located in the engine bay the results were quite surprising. I stress these are preliminary results as with spring (Qld) there has not been a chance to test with higher ambient or surrounding temp or as yet a large number of engine bay locations for the temp probe.

1. During slow suburban stop and start < 50K driving the engine bay temps varied up to 2X ambient temp.

2. When slowing coming to halt engine bay temp would spike in around 5 seconds to around 3 X ambient temp. Under load it was more than 3X ambient. Raising revs while in neutral or crawling under load except for the latter made no measurable difference to engine bay temperature and there was no measurable increase in engine temperature. This test seemed to indicating viscous fan was doing its job cooling the engine.

3. The closer probe got to radiator/viscous fan the temperature dropped in comparison to rear/upper area of engine bay.

4. If road speeds could be maintained above 40k's and particualrly 50 -60k's engine bay temps would drop to approximately 5 degrees above ambient temperature.

5. Engine bay temperature drops very quickly once road speed > 40k is reached. At slow speeds under load at the height of summer I hate to think what temp would be reached.

6. It takes hours for engine bay to get within 10 degrees of ambient temperature.

My conclusion:

a) A bonnet scoop would do nothing significant (either pointing forward or backwards) and at either high (produces low pressure under car / high pressure at rear of bonnet and resultant pressure differential to engine bay) or low road speeds (pressure under car and over bonnet starts to match engine bay and differential pressure/flow is low);

b) Viscous and or supplementary/replacement electric fan(s) in vicinity of radiator have no or little impact upon engine bay temperatures.

The only solution to cure high engine bay temps IMHO would be via booster fan and hood vent designed specifically to exhaust hot air from the rear and top of engine bay. However one significant constraint is this is being closer to windscreen/cabin this would introduce hot air to fresh air cabin vents (at bottom of screen) and a open bonnet vent could raise cabin noise to an uncomfortable level.