Best way to setup Miata Hood Vents

Hey guys looking to get some quick opinions on how to do hood vents on my NA Miata.

Current setup: I currently have done a K24 swap and because the hood is currently being adjusted to fit the engine, traditional hood vents don't work (i.e. the Singular Motorsports vents). I also have a ducted drivers side headlight cover from Flyin Miata to direct cool air to the intake filter that sits right behind it.

Goals for new setup: I want to increase airflow through the radiator, decrease under hood pressure and temperature, and maximize cool air to the intake. Also minimize cutting into the main structural parts of the hood (mainly the 2 supports that start at the back corners and meet at the middle by the latch)  

Possible Solution:  I'm thinking about purchasing the following: http://vraptorspeedworks.com/mazda-miata-na-hood-louvers-rear-opening/  

I've looked at the hood pressure map done by Flyin Miata and even though these aren't in the optimal place they look to be in an decent spot to promote air flow out of the engine bay.  It's still a little hard to tell because of the resolution of the data. I was also thinking about just installing the passenger side one so that air flow from the radiator (which doesn't have a fan installed on the pass side) would primarily travel through the "hot" side of the engine bay near the headers.  This would mean the drivers side would be pressured up with cooler air from the ducted headlight cover.  I could also build an enclosure for the intake filter, although it would be great if I didn't have to do that.  

Thoughts on how to setup vents/louvers to optimize air flow under the hood?

We recently tackled this very subject in the magazine, and here's some video that might help, too.  

https://www.facebook.com/plugins/video.php?href=https%3A%2F%2Fwww.facebook.com%2Ffacebook%2Fvideos%2F10155088092558140%2F&width=500&show_text=false&height=281

David S. Wallens said:

We recently tackled this very subject in the magazine, and here's some video that might help, too.

Thanks for directing to the video.  Some good information in there.  Might need to do some of my own testing to ultimately tell what air flow is doing in specific areas under the hood. 

If you give the underhood air a nice low pressure escape route, it'll go there. Even if those louvers aren't in the ideal spot, they'll still move air. Then your underhood airflow will change - so don't stress that too much. It's more important to find the low pressure zone on top of the hood.

I'd box in that intake. Depending on how you have it set up, the baffle for the FM turbo systems may work for you.

Keith Tanner said:

If you give the underhood air a nice low pressure escape route, it'll go there. Even if those louvers aren't in the ideal spot, they'll still move air. Then your underhood airflow will change - so don't stress that too much. It's more important to find the low pressure zone on top of the hood.

I'd box in that intake. Depending on how you have it set up, the baffle for the FM turbo systems may work for you.

Good point on the air flow changing once the holes are punched.   I guess with that being said I'll place the vents a decent spot and focus on boxing in the intake.

Also, good call on the baffle for the FM Turbo System, that might actually work.  It'll take some adjustment on both the baffle and intake filter location, but that seems like the best solution.

I was talking to a friend at the track about underhood cooling.  He has put a couple shims underneath the chassis side of his hood hinges.  Lets air flow out the edge near the windshield. 

That only works if the high pressure area at the base of the windshield is lower pressure than the underhood area. It's about the worst possible place to put a hood vent.

JBasham said:

I was talking to a friend at the track about underhood cooling.  He has put a couple shims underneath the chassis side of his hood hinges.  Lets air flow out the edge near the windshield. 

That really only works at near stationary speeds or if you want high pressure air for the airbox/filter (look at cowl induction examples).

At high speeds, the pressure at the base of the windshield is pretty high and you'd be pumping a ton of air under the car, which could cause some handling quirks due to a reduction in downforce at the center and rear of the car.

Agreed with Kieth and Stefan on the back edge of the hood being a bad place to open a gap. Mostly at the center, due to the higher pressure at the center of the windshield. The outside back corners however do have a lower pressure than most of the rest of the hood, so if you can basically bend the outboard back corners up to allow a leak, then you can get some pretty good outflow there. The escaping heat and possibly fluids (if you have a leak) will try to get into the cabin however. 

The absolute lowest pressure on the hood is going to be near the front edge at the center. Look for the area with the highest curvature (tightest radius). That is the ansewr without using pressure taps to locate the low pressure spot.

Depends on your feeling on looks for a street car and hood vents. Might keep a spare track hood if you care about that. For a turbo Miata I'd like to try a radiator duct hood exit front center and a smaller hood vent right over the brake master cylinder area.

Thanks I can save myself the time involved in figuring out how to shim mine now.  I was aware that the back edge was often the highest pressure zone on a hood but I spaced out on that.

Speaking of, anybody have any experience measuring hood pressures and engine bay pressures with one of these digital manometers instead of an analog tool? 

Digital air flow meter

In reply to JBasham :

That digital tool is the equivalent to the analog gauges mentioned earlier. It will work just fine. For pressure taps on a surface you can make do with a tube taped to the surface for starters. If you want to really get the STATIC pressures you are looking for you will need to drill thru the surface with a tube mounted to the back side of the panel, or make a small circular sensor. This type of sensor is washer shaped and has a small hole on the face and another hole or tube connection coming out of the side. This lets you get the sensing port aligned with the body panel and fair in the edges to reduce errors in the measurement. I looked for an online picture but couldn't find a good one.

You can make a simple manometer from clear plastic tubing.  Put a U shape on a board.  Add some ATF as a visual. Then a length of tubing to reach where you want.   Tape the open end of the tubing to that point.  The fluid will move indicating pressure.     I made one years ago.   Don't know what happened to it.

stafford1500 said:

In reply to JBasham :

That digital tool is the equivalent to the analog gauges mentioned earlier. It will work just fine. For pressure taps on a surface you can make do with a tube taped to the surface for starters. If you want to really get the STATIC pressures you are looking for you will need to drill thru the surface with a tube mounted to the back side of the panel, or make a small circular sensor. This type of sensor is washer shaped and has a small hole on the face and another hole or tube connection coming out of the side. This lets you get the sensing port aligned with the body panel and fair in the edges to reduce errors in the measurement. I looked for an online picture but couldn't find a good one.

Thank you!  If I'm just doing it with a tube instead of a probe or oval sensor, do I orient the tubing so it's parallel to the direction of air flow or perpendicular?  I know the probes measure perpendicular (through the little holes drilled in the side).