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LanEvo Reader
2/14/16 6:35 a.m.

I'm doing some work on the 190E 2.3-16 racecar. Race engine is going in: higher compression; more aggressive cams; twin Webers in place of CIS/KJet.

The car has functional aero from the factory and I'm fitting a factory Evo I kit: deeper airdam; adjustable splitter; fender flares; larger rear wing. I've been doing some reading on aero.

What do you guys think of hood vents/extractors to reduce drag, improve flow through radiator, and reduce under-hood temps?

I've seen different configurations: single, central vent behind the radiator (Shelby GT500; Lancer Evo 8/9). Twin vents on the sides of the hood (E46 M3 GTR). A combination of the two (V12 Vantage).

I was thinking about putting a GT500 vent on my hood. There's plenty of space behind the radiator with the 4-banger engine. I could even fab up a scoop/tray to direct hot air from the rear of the rad up toward the hood vent.

Worth the trouble? Total waste of time/energy?

Raze UltraDork
2/14/16 7:05 a.m.

Worth it if done right, be careful putting a vent too close to the windshield due to higher pressures which can totally hose you at higher speeds.

We vented the Merkur and it actually cools down to around 190 while on track vs the old 210...

kevlarcorolla HalfDork
2/14/16 7:58 a.m.

Its worth it imho,as mentioned its best just behind the rad in the low pressure zone so air is drawn thru the rad and out.High underhood pressures can mean to much resistance to the air trying to go thru the rad so it takes easier routes around it.Higher pressure area near base of the windshield will likely pull air,potencially same problem with vents on the sides of the hood like the E46 GTR depending on shape of the car.

Plus it reduces front end lift at speed so win win

Keith Tanner
Keith Tanner MegaDork
2/14/16 10:01 a.m.

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

Nick (LUCAS) Comstock
Nick (LUCAS) Comstock UltimaDork
2/14/16 11:16 a.m.

Some of the basics can be read about here.

PeteD New Reader
2/15/16 5:39 a.m.
Nick (LUCAS) Comstock wrote: Some of the basics can be read about here.

There are some really good tech/DIY articles there ... Thanks for the link!

-- Pete

Rodan New Reader
2/15/16 9:18 a.m.

Take a look here: Singular motorsports

Their Miata hood vents have been getting good results, and they're now making them for Subarus. Something of theirs may work for you.

engiekev New Reader
10/17/16 11:10 a.m.

Heard about these guys at a track day:


SVreX MegaDork
10/17/16 11:37 a.m.

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.

SVreX MegaDork
10/17/16 11:46 a.m.
Nick (LUCAS) Comstock wrote: Some of the basics can be read about here.

Good article.

Keith Tanner
Keith Tanner MegaDork
10/17/16 5:24 p.m.
SVreX wrote: 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.

I've done that with some fender mods. Results were not conclusive - it didn't tell me if airflow was going where I wanted it to, only that the airflow wasn't enough to bring the high and low pressures to the same point.

It's fairly simple. Air wants to go from high pressure to low pressure. So you let it. In theory, the high pressure underhood area will drop, but what if you're keeping the same pressure differential but simply pumping a bunch of air through your vents? Gets you the airflow over the radiator you wanted.

SVreX MegaDork
10/18/16 6:16 a.m.

In reply to Keith Tanner:

You can't "pump air through a vent" if there is no pressure differential. No pressure differential = no movement (unless you are moving at Warp 5).

jpnovak Reader
10/18/16 8:41 a.m.

Hood Extractors do work. I put one on my Suby powered 914 Challenge car. Above 30 mph you could feel the heat coming out of the hood. I somewhat cheated and found an aerodynamic analysis for the body style. I then place the hood extractor vent behind the area of high pressure on the hood. Then created a small wickerbill on the leading edge. The inlet was low on the bumper above a slight lip - obviously high pressure.

As mentioned before, make sure there is a pressure differential. A poorly placed exit will make things worse.

An additional benefit of the hood extractor is not just cooling but somewhat functional aerodynamics. Hot air is less dense. You don't want it under the car. Vent up top to help prevent body lift. You can also vent to the sides. This is where the dual hood vent shines. Exit hot air in front of the windscreen and then use a curved wind screen to divert the air spill-over to the sides. This provides clean air for the rear spoiler.

There are some guidelines that are easy to follow but the details of execution make all the difference.

DaveEstey PowerDork
10/18/16 9:13 a.m.

Great timing. I have a LeMons race in the RX7 coming up and could use some aero data to inform the work on my hillclimb RX7. I also have a magnehelic gauge handy.

Keith Tanner
Keith Tanner MegaDork
10/18/16 10:02 a.m.
SVreX wrote: In reply to Keith Tanner: You can't "pump air through a vent" if there is no pressure differential. No pressure differential = no movement (unless you are moving at Warp 5).

I don't think you'll ever get to no pressure differential at all. You might see a decreased differential, but we're talking about very significant amounts of air here. A big hood vent is a couple of square feet, but it's installed in a much larger surface that's driving the local pressures.

SVreX MegaDork
10/18/16 10:26 a.m.

In reply to Keith Tanner:

Well THAT'S not very science-y!

You are contradicting yourself. Your PDF showing pressure zones (which is VERY good, and VERY science-y) shows a positive pressure at the cowl, and negative pressures on most of the rest of the hood. Most of us understand the cowl is not a good place for the vent exhaust (but that would be a hood position that could show no pressure differential at all, therefore have no benefit).

But it does not show under hood pressures, nor does it show the differences once vents are installed.

The obvious spot for a vent on an NA hood according to that PDF is the point on the grid showing -1.75. However, without the under hood pressure, it's a bit of a guess.

For example, what if the under hood pressure at that point is -1.25, but the under hood pressure under the point identified as -1.25 is +.5? The differential in the first location would be .5, but the second location would be .75. The optimum location would not be where it is generally believed to be, but at the point where there is the greatest differential.

And under hood ducting could change everything. An under hood duct routing air directly from the radiator to an extractor located in the center of the hood (.5 and 0) might be significantly better than a hood vent if it is pulling air from in front of the grill where the pressure is 4 or 5. (Note- these are made up numbers).

I am just observing that there is some really great information out there (including yours), but it is incomplete without the under hood information or testing after the changes are completed.

SVreX MegaDork
10/18/16 10:35 a.m.

My opinion is that hood vents will never exceed the performance benefit of an extractor hood with proper ducting (combined with a splitter/ under tray). Especially considering the under hood pressures are typically created by poor airflow management coming through the grill in the first place (into the dirty air engine bay).

But I have never tested it.

engiekev New Reader
10/18/16 11:10 a.m.

Where is a good (cheap, this is GRM!) source for an efficient extractor vent?

These seem like a good design: http://garagestar.com/blog/?product=garage-star-universal-drop-vent-heat-extractor

Keith Tanner
Keith Tanner MegaDork
10/18/16 11:17 a.m.

Define the difference between an extractor hood and hood vents?

I've always gone under the assumption that underhood pressures are fairly consistent, as there's very little airflow down there. If you want to go measure some, be my guest :)

Ducting the rad to the vents would be great, as it would minimize restriction to airflow. I suspect it would be a relatively small change from simply venting underhood pressure IF all of your underhood inlet air was directed to come through the rad already. That's pretty unrealistic, but theoretically possible. More importantly, ducting the rad directly may not be possible due to other items in the engine bay - so while hood vents may not perform as well as a fully ducted extraction setup, the latter may only be a theoretical construct and the hood vents are what you can actually DO.

My fender testing did measure pressure differential directly. Opening up a vent behind the wheel did not change the pressure differential.

It's worth noting that an Exocet cools very efficiently compared to a full body Miata. The difference? Big open sides on the engine bay, so there's very little pressure buildup. They're not in the "right" place, but they're so freakin' big that it doesn't matter. Meanwhile, the Catfish has real cooling problems unless the hood is vented as there is no escape path for the high pressure air.

Keith Tanner
Keith Tanner MegaDork
10/18/16 11:20 a.m.

We did some back-to-back testing on a car with Big Freakin' Louvers a year or two ago. No change in the cooling problems when swapping between the vented and the non-vented hood. Which means that this particular car was suffering somewhere else. We've seen hood vents work on other cars, though. It all depends on the problem.

I put a set on the Targa Miata primarily for the front downforce benefits.

DaveEstey PowerDork
10/18/16 11:21 a.m.

Cheap? make your own. It's just metal.

SVreX MegaDork
10/18/16 11:24 a.m.

In reply to engiekev:

The most important thing about an extractor is not the vent- it's what happens UNDER the hood that matters. The ducting.

The vent is just a hole. The ducting routes the air.

You could build a very efficient extractor vent out of cardboard and duct tape (as long as there is no risk of fire). It needs to:

  • Capture the air coming off the back of the radiator (sealed box)

  • Route it to the top of the car

  • With as smooth a path as possible

  • Without constrictions or restrictions

That usually means mounting the radiator at a 45* angle or so.

NOTE: the grill inlet should also be sealed to the radiator. The point is to force air through the radiator without a path around it and the shortest path out the back.

The radiator itself is a restriction. Air would rather flow around the radiator than through it.

SVreX MegaDork
10/18/16 11:27 a.m.

BTW, I have already mocked up an extractor duct for a Miata. There is PLENTY of space to remount the radiator on a 45* angle to enable good airflow.

I figure if it can be done on a Miata, it can be done on most cars.

Keith Tanner
Keith Tanner MegaDork
10/18/16 11:33 a.m.

I've seen a V mount on a Miata, but only once. A coolant reroute would also help a lot by disappearing the thermostat neck. You're moving the AC condenser too, right?

SVreX MegaDork
10/18/16 11:39 a.m.

In reply to Keith Tanner:

Don't have AC on my racer.

But yeah. Intercooler too when appropriate. Oil coolers, etc. Anything that has air flowing through it.

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