I am trying to find out if reverse bonnet scoops are an affective way of drawing out warm air from the engine bay, or if infact they are more of a gimmick? Does/has anybody used one???

 

A basic knowledge of car aerodynamics in general would suggest that no, a reversed scoop would have no extraction effect beyond that which a simple hole would - the area around the scuttle panel is always a high pressure zone, hence why fresh air intakes on just about every car ever made are right there. And many air intakes are placed in the back half of the bonnet by manufacturers including of course, Subaru. If you look at cars which do have extraction bonnets e.g. Lotus, Mitsubishi, the extraction vents are at the front of the bonnet just behind the front edge, which traditionally is a low pressure area and perfect for genuine extraction.

I personally think they can't work much better than a plain hole and in fact may not do anything at all except upset the natural airflow through the engine bay and out along the sides of the gearbox. But I have no data one way or the other and haven't tested it myself so can't say for sure.

Probably the one good thing is there's a kind of neatness to reversing the scoop if using an FMIC and you still do need some kind of an outlet for turbo heat at slow speed and standstill.

 

You can see if your engines on fire.

 

Thanks for your advice silent night, interesting point about high/low pressure points on bonnet. How do you know where these are, or is this just common knowledge???? But, as you say, the lancers have them towards the front and I'm sure they didn't cut corners when it came to R+D! I am sure somebody must have had some temp sensors under the bonnet before and after! Fair point micky, suppose you don't even need to get out the car to put it out, just point and spray out the window!

 

Next mod for Pavlo then?



M

 

Well I've done a bit of reading around the subject shall we say, as I had a Clio 16v before which I'd turbocharged and heat management was a big issue, hence I couldn't let in a few fancy scoops and hope for the best. I needed function over form.

A lot of car aerodynamics is kind of counter-intuitive until you learn about it. Like the fact that a small opening widening out to a larger intercooler will get you more airflow through it than if you just have it right up front all on show. Or that in a tight engine bay, trying to get cold air to a filter cone with a big bit of hose ducted from the front bumper is a complete waste of time because you're trying to force air to flow up into a high pressure zone when it would much prefer to just avoid your intake altogether and go around.

The way to test them is to build yourself a U-tube water manometer with a load of transparent hose, some coloured water and a long piece of wood. And a dual carriageway and notebook. You then can start logging pressure readings at different points across the front of the car to build a picture of what's flowing where. For example if you read the same pressure in front of a FMIC as behind it, then there is effectively no airflow through it. The wider apart the locations of the readings are, the less you can rely on them as a way of revealing airflow i.e. high pressure in front of the radiator and low pressure behind the gearbox does not necessarily mean airflow goes from one to the other. It may take an entirely different route.

In general, the whole front bumper/grille area is high pressure. Some air flows around but if you put holes in the right places you can get air to 'push' through it. The underneath of the engine bay tends to be low pressure and the front part of the bonnet as air gets pushed over the top. The rear half of the bonnet starts to become high pressure again before hitting the windscreen. Obviously something like an old Beetle with a vertical windscreen is a great example. You sometimes get another low pressure zone just behind the leading edge of the roof and over the rear window. You can make a high pressure area by adding say a spoiler to the rear end of a car or a splitter to the front.

As I say these are only generalities, but if you want to extract air from a bonnet, do it at the front, not the rear.

 

Very interesting! In your opinion then, running a cold air duct from the foglight area up to just in front of the filter is pretty pointless then? So what do you think is the best way to get cold air to your after market air filter then?? And if you are going to run a front mount, what's the best thing to do with the scoop?

 

Surprised no-one's weighed in with an opposing viewpoint yet. Still, IMHO, cold air ducts into most engine bays are pointless unless you have it hooked up to a closed airbox, in which case it's a great idea. No surprise that that's exactly what manufacturers have been doing for decades then, because it works.

If you're ducting up to an open induction cone, then either don't bother and let engine bay throughflow do the job (as I do on mine and I have no charge temp problems at all), OR shroud off the whole induction cone area from engine bay heat in which case a sealed duct coming up to it would work. But then, if you do that, aren't you just spending a lot of time and money in building what the manufacturer gave you as standard - i.e. an airbox?

My opinion is if you need an induction kit either because of airflow requirements or an FMIC installation, then fit it and live with it. It will do its job fine and unless you do all your driving at 5mph or less the difference in charge temp will be negligible.

I have data from previous work in this area (not a Scoob) to show that after intercooling, apart from at standstill, there was a maximum of 5 degrees C difference between a cone in a packed engine bay and the same cone repositioned down in the wing. Simply shrouding off my chargecooler rad properly rather than letting air flow around it gave me a consistent 5-10 degree charge temp lowering.

I would expect a reversed bonnet scoop would work very well at standstill and low speed to keep the engine bay cool, plus keep water out. At speed I don't know what it would do and until I try it myself I couldn't say. Suffice to say I thought it enough of an unknown to leave my forward facing scoop on even though I have an FMIC. And I am big on heat management, so it's not through laziness - all my intercooler pipes and exhaust from headers to downpipe are heatwrapped.

 

As mentioned before, the scoop helps airflow around the gearbox area, if you've upped the power then the gearbox needs all the help it can get!

 

Now now....

 

I've got an early WRX ra as a track day car.
Last October I tried some temperature tests to the induction kit. With normal scoops / vents in place the temperature sat above 30deg C. (Outside temperature was only 8deg that day). I fitted an induction bonnet scoop and an air intake to the fog lamp cover. Temperatures dropped to 12deg.

I've now fitted a front mounted intercooler to the car and also a reverse bonnet scoop. I haven't done any temperature test to this yet, but it does put out a good heat haze so it is letting out a lot of underbonnet heat.

I will try some temperature tests soon, but I'm pretty sure already both are working very well.

Dave

 

defo defo escapes from them! might not be a huge difference but a diference it still is, the pressures casued by driving would be not great enough to surpress the radiant heat from escaping in my opinion

 

What's the cheapest and easiest way to measure under bonnet temperatures then?

 

You can get digital thermometer units from Maplins.

 

the classic rally cars had the outside vents un coverd and enlarged, i assume thats pretty much the best way to remove heat from the engine bay if needed (dont think they had TMIC) ?