Learn me: hood vents/extractors

Definition: An "extractor" (without ducting) is a large vent, usually installed in a less than optimum location for underhood venting, which gives the car's owner a great sense of satisfaction at having spent a lot of money for a very cool looking piece that has questionable performance characteristics.

How's that?

I can't find any links, but I am pretty sure the term "extractor hood" originally came from the idea that it extracted heat from the coolant via the improved flow across the heat exchanger, not that it extracted heat from under the hood.

Will lifting the rear of the hood have a similar effect?

No, it's actually much worse for flow. You're opening the hood at a high-pressure zone which reduces the pressure differential.

In other words, the exact opposite of what you want to do.

In reply to z31maniac:

Sorta true. The center is higher pressure, but the outboard back corners should be lower pressure where the hood rolls off to the fenders, so you get a loss of underhood low pressure and a lateral flow from the center of the hood to the outboard edges. You will likely vent some underhood heat, but not neccesarily increase radiator cooling. Just getting the hood to vent at the back corners would be useful, but keep the center sealed somehow.

I did that on my ZX2/SR. Don't know if it had any effect.

stafford1500 wrote: In reply to z31maniac: Sorta true. The center is higher pressure, but the outboard back corners should be lower pressure where the hood rolls off to the fenders, so you get a loss of underhood low pressure and a lateral flow from the center of the hood to the outboard edges. You will likely vent some underhood heat, but not neccesarily increase radiator cooling. Just getting the hood to vent at the back corners would be useful, but keep the center sealed somehow.

I think I learn something every time you post. At least it seems that way. Post away!

In reply to STM317:

Thanks, I will take that as a compliment.
Now if my kid would understand that I know something the world would be all right.

To carry the engine bay cooling info on a little more, dropping the front of the hood under the front fascia/nose edge will draw a good bit of heat out of the engine bay and MAY give a little more downforce.

That makes sense. What about while the car is sitting will having the rear raised let air flow up and out the top seeing as the hood slants down. I want to do more drift events with my car and air flow is well useless with drifting and I have no plans for speed with my car.

Would that get hot air out of the engine bay while its sitting between runs?

Skervey wrote: That makes sense. What about while the car is sitting will having the rear raised let air flow up and out the top seeing as the hood slants down. I want to do more drift events with my car and air flow is well useless with drifting and I have no plans for speed with my car. Would that get hot air out of the engine bay while its sitting between runs?

That should let some heat out at low speeds, but definitely try to seal the middle section of the back edge and let what little airflow over the hood pull the heat out at the back corners.

Skervey wrote: That makes sense. What about while the car is sitting will having the rear raised let air flow up and out the top seeing as the hood slants down. I want to do more drift events with my car and air flow is well useless with drifting and I have no plans for speed with my car. Would that get hot air out of the engine bay while its sitting between runs?

Test it and see. Are you having heat problems now? If you aren't I wouldn't worry about it.

Or you can rely on guesses from us.

Skervey wrote: That makes sense. What about while the car is sitting will having the rear raised let air flow up and out the top seeing as the hood slants down. I want to do more drift events with my car and air flow is well useless with drifting and I have no plans for speed with my car. Would that get hot air out of the engine bay while its sitting between runs?

You could always do louvers or something similar if you're primarily concerned with letting heat out while sitting. Heat rises, and wants to take the easiest path, so give it the easiest path possible and punch some holes over the warmest parts.

Ya id rather not cut the hood at least for now and the way the hinges are on the FC id have to come up with some spacer that would work. A standard spacer wont work.

Just wanted to see if there were really good advantages to it. If there were I would have.

Thanks for the guesses haha

SVreX said:

Yes, I believe in extractors.

I am amused sometimes by the negative pressure/ positive pressure discussions. Once you cut a hole and start moving air through the hood, all the pressure zones change. Very few people seem to re-measure the pressure zones AFTER the installation of an extractor.

I've never seen a good before and after study done.

Actually Jaguar did a lot when they got ahold of a Ferrari GTO.  While the GTO made more power than the XKE the added drag of the GTO negated it.  

Others spoke about high pressure area’s in front of the windshield Jaguar actually measured pressure coming out of some vents stalled at others and going in  all in the same row of Louvers.   I know I have that article at home in my collection someplace. I’ll see if I can find it and figure out how to post it. It’ll probably wind up as pictures because I know how to do that now.  

1 - a front facing hood scoop isn't necessarily better than a rear facing one and may be worse, aerodynamically

2 - everyone always wants to put a vent/scoop on too close to the windshield - right behind radiator level is optimal in most cases

3 - don't forget to consider venting the inner fenders as well or instead - that can work quite well

4 - 'tuft' tests trump theory. If you stick bits of wool on the bonnet and they suck into rather than blow out of your new scoop, whatever your theory said was wrong.

Come to think of it I never got around to tuft testing my old MG racer after I switched to vintage and added a (rather low) windscreen, more for looks than effect, to see if the venting still worked. On that car, with a nominal 1-2" 'cheater' screen to get by the rules, the very best way to vent air was to slightly raise the back of the bonnet by adding small spacers on the hood pin pedestals.

Went from this  

to this:

Keith Tanner said:

Hood vents are one of the best aero additions you can make. The best way to place them is to check the pressures across the hood using a magnehelic gauge. It's pretty interesting stuff. You want the vents at the points of lowest pressure.

Here's a sample with real-life measurements - all three generations of Miata. https://www.flyinmiata.com/support/instructions/misc/louvers.pdf

a *very late* reply... but seems to me like the NC hood would benefit from a screen right where the 'Nike swoosh" is around the hood's center contour. Based on your numbers. And maybe some venting right behind the radiator. I haven't gone to look at the support structure underneath to look at practicalicity of that placement though. 

I tried them on my E36 V8 track car. 

I used a magnehelic gauge to test and find low pressure areas on the forward part of the hood. 

West Marine sells these vents for $10 Vents.   

My radiator cooling was already fine, even before the vents.  It had been a little iffy at first, but I put an aluminum undertray from the bumper and back about 18", and that fixed that.

I was mostly seeing if I could get intake air temps down.  I can measure the data reported by the air intake temp sensor that feeds the ECU.  Turned out, with the vents open or the vents closed, I was getting close to ambient air temp.

I had high oil temps with this motor at high RPM use, thanks to ring blowby I''m pretty sure.  My electric-fan/oil cooler brought them down to tolerable, and the vents didn't get things any lower.

I have them blocked off for now because they let rain in otherwise.  I'll be getting around to a splitter and a rear wing eventually, and I'll do some more testing then to see if it makes any obvious differences in anything.  Mean time they look very racecar.

I'm not sure what the etiquette is for  bringing back dormant threads but this one is worth whatever ridicule could potentially be dished on me. Great reading and links!

And I have a question related to airflow through the radiator which seems to be the point of an extractor or vent. Wouldn't a front air dam lower the pressure under a car enough to allow the radiator air a lower pressure path out?

Also, I have a cowl induction hood that I do not use for air intake but is currently open at the base of the windshield. My induction air intake is behind a headlight in the drivers side. Should I close off the opening in the rear of the hood?

67LS1 said:

And I have a question related to airflow through the radiator which seems to be the point of an extractor or vent. Wouldn't a front air dam lower the pressure under a car enough to allow the radiator air a lower pressure path out?

Also, I have a cowl induction hood that I do not use for air intake but is currently open at the base of the windshield. My induction air intake is behind a headlight in the drivers side. Should I close off the opening in the rear of the hood?

As long as you're not a canoe; I think it's generally accepted.  In fact, I think it's semi-tolerated even if it is a canoe... since we've sometimes gotten closure on things.

re: airdam
yes, an airdam should create enough of a pressure differential behind a radiator to encourage exit flow from the radiator.  This is, effectively, how the majority of OEM systems work.  In part, because, it creates a large volume to dispel heat from the radiator/engine in stop/go traffic.  afaik/iirc, the trade off is two-fold: 1) you could be making more downforce by exiting the flow out the top of the hood; 2) you're dumping the radiator exhaust in an uncontrolled manner under the car, which could increase drag and decrease other underbody flows (diffusers/etc).

re: cowl induction hood
if you're not using the intake, then it'd probably be beneficial to close off the flow into the hood from there.  Although, keep an eye on coolant/oil temps, and be able to unblock it if the additional air from the cowl is somehow providing some cooling enhancement.  imho.

Keith Tanner said:

Hood vents are one of the best aero additions you can make. The best way to place them is to check the pressures across the hood using a magnehelic gauge. It's pretty interesting stuff. You want the vents at the points of lowest pressure.

Here's a sample with real-life measurements - all three generations of Miata. https://www.flyinmiata.com/support/instructions/misc/louvers.pdf

How did you use the gauge for testing - drill a bunch of little points for sampling tubes in the hood?

In reply to MadScientistMatt :

We marked out a grid on the hood surface and measured the pressure just above the surface. The assumption was that the fairly stagnant underhood air meant very little pressure variation compared to the upper surface, so the upper surface is what mattered.

You can see a replica of the complex equipment at the 4:20 point in my video - it should start playing there. It shows up again at 30:35 or so.

sleepyhead the buffalo said:

 

re: cowl induction hood
if you're not using the intake, then it'd probably be beneficial to close off the flow into the hood from there.  Although, keep an eye on coolant/oil temps, and be able to unblock it if the additional air from the cowl is somehow providing some cooling enhancement.  imho.

The mid 60's Chevelle in question has a fixed panel at the base of the windshield so the rear edge of the hood isn't very close to the base of the windshield. Raising the rear of the hood on some cars built like that showed cooling improvements. I remember a thread on Corner Carvers among vintage Mustang racers who found improved engine cooling when the rear of the hood was raised. So the open cowl hood may actually be helpful, I'd do some tuft testing.

67LS1 said:

I'm not sure what the etiquette is for  bringing back dormant threads but this one is worth whatever ridicule could potentially dished on me. Great reading and links!

And I have a question related to airflow through the radiator which seems to be the point of an extractor or vent. Wouldn't a front air dam lower the pressure under a car enough to allow the radiator air a lower pressure path out?

You are correct.  My old Neon had overheating problems for a while after a 2.4 engine swap, even with a bigger radiator, until I found out they had a small air dam from the factory on the core support that my car was missing.  I'd have to pull off the highway and let it cool occasionally if I was driving more than a few miles.  I created an air dam out of garden edging and it worked like a charm.

In reply to eastsideTim :

IIRC there was a GM car like that, possibly a later F-body (Camerobird). The little air dam was critical to pulling air out from underneath.

In reply to Keith Tanner :

C4 and c5 vettes as well as later 4th gen f body 

Keith Tanner said:

In reply to eastsideTim :

IIRC there was a GM car like that, possibly a later F-body (Camerobird). The little air dam was critical to pulling air out from underneath.

That sounds right.  I'm going to do a little experimenting with one on my S10 once it is back on the road.  V8 swaps on them tend to use all the cooling capacity they have, and GM threw multiple tricks at stock square body S10s that look like total hacks nowadays (pieces of foam jammed between the radiator and core support, thin plastic strips to cover other gaps on the core support, plastic to separate air from coming into the radiator from the grill from air coming in from below part of the air dam).  I suspect if the air dam was on the core support, rather than the bottom of the bumper, some of this would not be necessary.