Thread: Will a bonnet scoop reduce heat in the engine bay?

Originally Posted by DiscoMick

Wouldn't having the bonnet popped be more about venting hot air from the engine bay, not getting more air in through a vent? ....

Technically, it'd be the same thing.
Forcing more air in, in effect forces more air out .. otherwise the engine bay would blow up(like a balloon).
If you get more air in, whether via a large front mounted scoop/vent/opening, or with the bonnet slightly open, that incoming air expels the hot air already in there.

All I know is that back in the early 90's when I was almost stuck in Uluru, and faced with 2500klms to get home to Melb, the half propped bonnet got me to over 60k/h without risking blowing stuff due to overheating.
Should be noted that the radiator isn't the only way to cool an engine!
Radiant heat(as in build up) will also heat an engine up.
eg. just like on a stinking hot day, on first start your engine will get to operating temp much quicker than it will on a freezing cold day.

Probably easiest way to think of front facing venting is that it's a psudeo air cooling effect on the metal parts that get hot on the engine.
And I'm guessing that the hotter the engine is, the more effective a front facing scoop will be as an air cooling effect.

as an example: in an ambient environment of say 40°C, if you engine coolant runs at 80°C, then the 40°C ambient air forced in will have less cooling effect, as it would if your engine normally runs at 100°C.
Even tho the ambient temp is a hot 40°C, and to us it feels hot(human body temp is about 36-37C) .. to a hot engine, 40°C is a cool breeze .. to a hotter engine(the 100°C one) it'd feel like a freezing effect!

@LRJim: if I ever get of my lazy @$$ one day, I may have a bit of underbonnet padding if you like.. in fairly good nik! I have a large sheet of heat/sound proofing which I've been meaning to fit up one day(but previously referred too laziness = it's been sitting for about 18months!)
I may contact you if I ever get around to it.
Did a trip into the Vic High Country last week(Tdi) and I found out on the way out there that my viscous fan has apparently stopped fanning!
Boxing day isn't the most accommodating time of year to source parts .. and I'm not going to bother people during that time of year .. so I had it in mind to keep an eagle eye on the temp gauge(s).
Aftermarket is VDO based, and from my experience quite accurate. Only issue was at speed, up a hill. very slight inclines, that would knock 10k/h off my speed wouldn't bother the temps, but climbing Mt Stirling saw a max 100°C, where it'd otherwise sit at 80-83°C.
I just 'managed it' as best as I could. If I had a couple of tech screws, I'd have locked the viscous unit up, if needed.
Anyhow, we finally got to some very steep climbs(16 mile Jeep track was the first) some very steep, first low, and I'm watching the temp gauge more so than the track up!
Didn't budge off 83°C!
Had me stumped, previously on a cooler (ambient) run up to Stirling, it'd hit 100°, now in the thick of some 'arduous' 4WDing, not a problem. First thing I thought was that the hub came good.
Nope! .. run up a normal smooth gravel road, had it back into the mid 90's, and the very steep smooth gravel up to Mt Hotham, had it at 100°C again .. so viscous still stuffed.

But for 5 days, climbing some very steep rocky tracks, that had brother in his twin locked D2(but no CDL) struggling, coolant in the 'overheating' Tdi, never hit more than 83°C!
Even now I still have absolutely no idea why in arduous slow going it ran cooler, with a knackered radiator fan.

After a lot of looking/reading I dont think a D1 or 2 bonnet is the right place for a scoop,, after the initial ramp the rest of the flat surface is a dead zone,,
I reckon on the sides like a reverse snorkle,, though the drivers side inner guard is chockers with electronics, so by default that leaves---
There is plenty of air pressure around the side mirror area judging by the rain forced up to the top of the front windows... Hmmm

Originally Posted by Pedro_The_Swift

There is plenty of air pressure around the side mirror area judging by the rain forced up to the top of the front windows... Hmmm

I thought about this place also, only concern is the heat so close to the windows. The amount of heat and wind that blows out under the D1, I wonder if it will blow into the windows.
I was even thinking a vent leading into the wheel arches, but i dont know how well it would work.
Only other thought is venting air through the inside of the guards and instal an external vent down near the kick panel.
Obviously the main flaws in all is water/mud getting in or lack of space....
Cheers Jim

There are numerous vents which can be bought and fitted into the sides of Defender's, Discovery etc, like the ones which are standard on Range Rovers. I reckon they would've a better idea. Hot air wants to move. Letting hot air vent out the sides should cause more air to be sucked in through the radiator, so you'd get both more cooling and more airflow through the engine bay.

Originally Posted by LRJim

Sooo... 12 months on has anyone worked out a practical spot for vents on a D1 V8?
Since taking the deteriorated factory heatmat out from under the bonnet you can no longer touch it, probably cook eggs on it.
I dont have an aftermarket gauge but i don't suspect the engine gets hot. Its warmed up once and i caught it in time, since then i know the factory gauge is reliable enough. You just need to watch it like a hawk and make sure it NEVER moves
So has anyone worked out an efficient place to install the vents? I like the idea of them on the fenders but i dont like the thought of the heat coming out near the windows.

Cheers Jim

I'm going to do some actual scientific testing soon on my RRC. Will be a couple of weeks but I'm going to do some runs at different speeds with a manometer set up where I can see it with the inlet and vent in different locations.

First up I want to see what actual pressures are under the bonnet, followed by what seems to relieve that pressure the best.

Not sure what i'll find but hoping for some interesting results.

I will be watching this with interest.. I do have a spare bonnet for the D1, so if you want/need to hack one up, give me a yell...

undebonnet GoPro Gav ??


no wait!
underbonnet Drone with camera!!!!!

No, will be a manometer mounted in the cabin with pipes running where I need them. Still haven't worked out the finer details - I need to get the RRC going properly first - I'll have a whole other engine and cooling system by the weekend so I'll be able to rat some parts off that and finish things off I hope.