How to improve our V6 Miata’s cooling? Hood vents.

After a Lucky Dog endurance race at Carolina Motorsports Park, it was clear that our Miata’s cooling system needed some attention.

[What you learn from your first time racing a new car]

Why? Well, blame us: We swapped a LFX V6 under the hood with a V8 Roadsters swap kit, meaning we were now cooling 300 horsepower with a radiator …

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Whether or not they are needed, a (well-installed) set of hood vents looks rad on just about every car.

I did the SuperMiata 3 vent set on my turbo NA and installed them from the underside.  It's MUCH more of a PITA to do with all the webbing underneath the hood skin that needs to be trimmed back but I like the way it looked afterward.  It makes a huge difference in keeping it's cool on hot days, I don't love that rain can get in and the noise from the engine bay is much more noticeable now.

Miata hoods are a dime a dozen, y'all could have saved track day time by having the louvers installed already and simply swapping hoods in the pits for the back to back comparison.

That's a good idea for a stock car, but a Miata hood won't fit on ours--it interferes with the swap. This is a fiberglass swap hood from Treasure Coast Miata. 

https://grassrootsmotorsports.com/project-cars/1996-mazda-miata/does-lfx-v6-swap-fit-under-hood-na-miata-project-e/

Colin Wood said:

Whether or not they are needed, a (well-installed) set of hood vents looks rad on just about every car.

The problem is that they are so frequently installed for looks, not function.  Usually the best place is up front, right behind the radiator level, but people seem to prefer them further back and in many cases far enough back to get into the high pressure region in front of the windshield and then they actually take air in rather than venting it out.

In reply to wspohn :

I hear on the looks. I feel like hood vents fall into a similar category as a rear wing. If it's not making your car faster/handle better/etc., it tends to look a little out of place.

KyAllroad said:

I did the SuperMiata 3 vent set on my turbo NA and installed them from the underside.  It's MUCH more of a PITA to do with all the webbing underneath the hood skin that needs to be trimmed back but I like the way it looked afterward.  It makes a huge difference in keeping it's cool on hot days, I don't love that rain can get in and the noise from the engine bay is much more noticeable now.

Miata hoods are a dime a dozen, y'all could have saved track day time by having the louvers installed already and simply swapping hoods in the pits for the back to back comparison.

I was looking at photos of a first-gen BRZ with some of these earlier and I would do the same as you - mount them from underneath.  They look too square and clunky on most modern cars.  That said I can't argue with their functionality, and the manufacturing looks really simple and clever. I'm assuming they are either laser cut or water jetted from a single sheet and then the flaps are bent to their final position.

Some hood vents do look better mounted from underneath and the same goes for some hood "pins" like Aerocatch latches.  The less on the surface of the hood, the better.  For pure function, however, does it really matter?  I don't think it does.  I like the fact that the Miata hood vents are black, which contrasts nicely with the red paint, just like the wheels, etc.  Sweet little track car that.

Vents on the hood look sexy but the hood tends to be an area of positive pressure

You want air in through the grille and out underneath or through the wheel openings.

An extractor vent in the fender with a duct if needed works well.

You can also make the front edge of the wheel arch protrude further than the rear edge to create a low pressure area to draw the under hood air out.

On the Fieros some used to put small vents (c. 3" across) right behind the radiator level on the hood - very effective.

ShawnG said:

Vents on the hood look sexy but the hood tends to be an area of positive pressure

You want air in through the grille and out underneath or through the wheel openings.

An extractor vent in the fender with a duct if needed works well.

You can also make the front edge of the wheel arch protrude further than the rear edge to create a low pressure area to draw the under hood air out.

Depends on where on the hood, it varies quite a bit. Also, what matters is if the pressure on the upper surface is lower than the pressure in the engine bay, because it's that pressure differential that will drive airflow.  I think we have a pretty solid body of knowledge that venting out the top of the hood can work. 

DavyZ said:

Some hood vents do look better mounted from underneath and the same goes for some hood "pins" like Aerocatch latches.  The less on the surface of the hood, the better.  For pure function, however, does it really matter?  I don't think it does.  I like the fact that the Miata hood vents are black, which contrasts nicely with the red paint, just like the wheels, etc.  Sweet little track car that.

Aerocatches work better when mounted on the top side of the hood, as then the whole flange of the latch is holding the hood in place. If you mount them under, all you've got is the rivets/screws in tension.

Keith Tanner said:

ShawnG said:

Vents on the hood look sexy but the hood tends to be an area of positive pressure

You want air in through the grille and out underneath or through the wheel openings.

An extractor vent in the fender with a duct if needed works well.

You can also make the front edge of the wheel arch protrude further than the rear edge to create a low pressure area to draw the under hood air out.

Depends on where on the hood, it varies quite a bit. Also, what matters is if the pressure on the upper surface is lower than the pressure in the engine bay, because it's that pressure differential that will drive airflow.  I think we have a pretty solid body of knowledge that venting out the top of the hood can work. 

Just by themselves the individual flaps create a low pressure area behind them by virtue of sticking up into the airstream a bit. All that's needed is for there to be a pressure differential in the area you want flow in a specific direction. High ---> Low. 

There's a couple paragraphs in a book by Frank Costin that talks about venting inboard brakes into the cockpit because of the relative pressure differential. I think it was on a BRM.  The driver ended up with a black face at the end of the race but the brakes stayed cool.  

DavyZ said:

Some hood vents do look better mounted from underneath and the same goes for some hood "pins" like Aerocatch latches.  The less on the surface of the hood, the better.  For pure function, however, does it really matter?  I don't think it does.  I like the fact that the Miata hood vents are black, which contrasts nicely with the red paint, just like the wheels, etc.  Sweet little track car that.

For pure function it doesn't matter. But to some people (Like me, an industrial designer) it's criminal to leave things ugly if I can help it. I can usually help it.  

I'm curious.  Where, in general, would a hood vent perform best for cooling.  My assumption from reading and watching various wind tunnel tests is in a low pressure area that's behind the radiator and as far forward as possible.  Would love votes and facts and vehicle specifics.  I have 3 vehicles that I'm contemplating hood vents, a Cobra replica where I think the lowest pressure area would in the hood just behind the radiator - so across the very front of the hood, a 1987 Nissan Z31 and  the current choice is the area right behind the radiator across the forward area of the hood.  My last one is an F150 Tremor hood where I would open up and make functional the existing non-functional "vents". 

I did a video on hood vents a while back.

Here's a pressure map we did of an NC Miata a while back. From what I can tell, it's fairly typical - a V-shaped low pressure area that starts a bit behind the nose. You'll notice that the vents on Tom's car sit in that V.

The units don't matter here, they show relative pressure compared to the 0 of the front center location. Lower numbers are lower pressure, higher numbers are higher pressure.

I approve this miata hood vent discussion

Thank you guys..  I will actually do a hood mapping on the carrs we need to vent..  Best DIY trick I've seen in a while..  !!!  THANKS  !!!

In reply to calteg :

Nice sticker–thank you. 

Nice Miata, too. :) 

Do these vents do anything to reduce lift on the nose of the car and improve front traction?

They should. 

calteg said:

I approve this miata hood vent discussion

I'm not the biggest fan of those particular vents.  Here's some video of them in action with tufts:

my assessment of that video, when I first saw it is that the way the front gurneys are mounted, they're creating two vortices; one each between the center vent and the outer vent.  These two vortices aren't particularly strong, and/or the flow out of the back of the side vents (or the presence of the windshield) means those vortices tend to collapse sinusoidally over time (you have to look at this slowly to 'see' it).  This whole situation is a bit exacerbated by the fact that the side vents are angled 'out' across the hood, and the flow over the hood (despite the curves / based on the tufts) is more 'straight back', which is contributing to the flow instability "imo".

In reply to Keith Tanner :

Is there any way to do something like that pressure chart at home with no specialty tools? I will be venting my mirage over next winter and starting to think ahead on it. 

In reply to bmw88rider :

All we used was a magnehelic gauge and a stick. The gauges cost about $30-40 on eBay. I suspect you might be able to make one by being clever with water and a tube but the mayhem might make you wish you'd paid $30 :)

Tufting the louver vanes is an easy way to check which way the air is flowing.