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Old 02-05-2014, 03:09 PM   #1
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Default DIY Extraction Hood Questions

I'm thinking about doing a DIY extraction hood. The pressure numbers here support the idea, and I figure it'd be a good functional mod that I can work on in the basement as my tinkering-itch is getting BAD this wisconsin winter.



I'm planning to do something similar to this one from m.n, except I'll be painting the mesh either black or dark green to match the paint to make it less "rice"



I'm also thinking I'll cut the holes on both sides, and put mesh in all 6 holes, but cover the back 4 holes so that the only *actual* vents are the ones almost directly behind the radiator in a really good spot to be "real" extraction vents.

I thought about only cutting the front ones, but I think it would end up looking like a big pig/cow snout, and I like all 6 holes cosmetically a little more...so I can just block off the rear ones. Or I could do some testing and see which holes uncovered gives me the best IATs/coolant temps/etc.

What do you think? Both sides or just one side? Or neither side?

I did some digging in old threads and found that--much to my surprise--the general population of MT.N is pretty accepting of DIY vents (as long as they're functional) so I feel like I can probably ask this without putting my flame suit on (although I'll keep it close at hand... :P)

Pardon my crude MSpaint skills, but this might help demonstrate what I'm thinking. Keep in mind, teh car is emerald mica, and I'll either be painting the mesh black or emerald mica to match.

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Old 02-05-2014, 04:19 PM   #2
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I guess I suck at posting actual questions--I just kinda rambled... my bad. Anyways my questions are:

1) Do you think I should put vents on just one side or both sides? (I'm leaning towards both)

2) Do you think I should block the rear holes? (I'll perform actual tests to determine this, but wondering on opinions as well.. the rearmost holes are almost definitely approaching a pressure-neutral or high-pressure zone)

3) Is cutting your hood apart nearly as scary as it seems? Cuz it seems really scary..
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Old 02-05-2014, 05:18 PM   #3
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Little K-Kustoms of your own?

Guess I'd ask what you are trying to accomplish. Cooling? I'd personally go with a larger radiator or better ducting. Not sure I'd want water pouring in there as well. Looks? Well, it's not my cup-o-tea but that's just an opinion and if YOU like it, go for it.

For SURE you don't want cut outs like our drifter friend (lets do it all thread)- for me it would be scary and (for me) hard to do right enough for a part of the car that's super visible.
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Old 02-05-2014, 05:40 PM   #4
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The problem with those cuts is water. I did my own hood a while ago and placed it such that any water that enters drips into a non damaging area. I just cut the stock hood, bent it in and sealed the edges with JB welded sheet metal. Been on the car for years without issues. Placement is a little different since the NA pressure zones are a different.



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Old 02-05-2014, 05:43 PM   #5
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Cutting into my hood was not all that scary, but then again, I didn't have good paint at the time. I did just one side (over the header) and then added a large, central vent behind the radiator. I live in S. Florida and it rains hard here all the time, no issues whatsoever. When I drive in the rain, I can see water shoot out of there. Just tape off the whole area and draw out the places you want to cut. I used a unibit to get the rounded corners and then connected the lines using an angle grinder.


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Old 02-05-2014, 06:27 PM   #6
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As far as water goes, that's another reason for only actually opening up the front ones. The rear holes would be over a lot more electronic stuff I feel like.

I need to spend some quality time with my car to see where exactly the forward vents would open up to, but I'm pretty sure it's basically RIGHT behind the radiator, which would be ideal.

I'm only going to do it if it looks like I can route the water to a "non damaging area" (I was thinking more along the lines of a sheet metal vent cover like you see on the side of houses, I could trim it down and mount it under the hood to route the water forwardand inwards, which I think would dump it in the big empty space behind the radiator.

If that doesn't look doable, I'll just keep some sort of cover that I can quickly throw over the holes (from inside life the others would be blocked) it's going to rain. Seeing as how I'd have to stop to put the top up anyways, this doesn't really seem that unreasonable to me.

And to answer the original question, mostly to pull heat off the engine, but also cosmetic. I think they look nice, and especially in calmer colors it would be a bit classier, and less rice (they won't be chrome).
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Old 02-05-2014, 08:02 PM   #7
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If you have access to a welder and feel like a little body work, you can try something like I did:

https://www.miataturbo.net/build-thr...t-66942/page2/
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Old 02-05-2014, 09:48 PM   #8
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That looks nice. This is what I'm talking about for deflecting the water, btw. A vent like this, shave it down, and mount it under the hood to deflect the water one way or another.

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Old 02-05-2014, 11:08 PM   #9
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Looks like I was right, the front holes are RIGHT behind the radiator--which is *perfect* placement for extraction vents. I imagine the radiator/radiator fans are probably okay with water? I would guess they get splashed more than enough driving in the rain. Just need to rig up some sheet metal under there to dump the water forward and maybe inwards to keep it away from the headlights and I'd be good.
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Old 02-06-2014, 01:12 AM   #10
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Don't forget about keeping the alternator/PS belt dry too. I did a setup similar to midtenn's, but I did it the quick and dirty way by cutting the entire rectangle out and then bolting a piece of aluminum angle to the front edge. I drove the car to the track with that hood and got monsooned on on the drive up. I was surprised the alt belt made the trip, because it was a constant cycle of belt slip, squeal for 5-10 seconds, lights start dimming, belt grabs. Repeat every minute or two.

That hood went in the trash after that weekend. My next attempt will be almost exactly like midtenn's. I might also add a louver/cutout above the header.

How did you get the data in the first picture?
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Old 02-06-2014, 03:19 AM   #11
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Mine drips onto the belts when the car is stationary. However, when moving there is enough air passing through to keep the rain out. The entire indented section actually stays dry.
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Old 02-06-2014, 09:17 AM   #12
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Yeah, I imagine there won't be a ton of water down there while moving unless it's a terrible downpour, in which case like I said I'll just have a blockoff I can slide in place from below.

The "rain channel" really just needs to move water away while stationary, and I think if I just dump it forward from there it'll all end up pretty much on the radiator, which would be just fine by me.

The data came from here: DIY vented hood mod - MX-5 Miata Forum Post 34.
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