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I started discussing this in a fender vents thread but figured it should get its own topic. My previous comments:
With the SC400's ginormous coolant reservoir and upper hoses obstructing much of the space between the fan and engine, there's only room for a fairly narrow hood vent. That would probably take on a bit of a Big Bird look, like the cowl extraction hoods that don't fit the body's lines.
The part I just came up with which could make it work is a wider opening on top, funneling down into a narrower duct behind the fan. Additionally, that taper would provide a good means of addressing my water drainage concern, since all water would be directed into the middle where it would clear everything pretty easily. (I'm still leery of the steep driveway I park on, which would bring water back into the drive belt and pulleys. Maybe a center drain hose could extend down to clear them, but that would have to be anchored somehow to avoid getting caught in the belt [edit: and fan]!) There's enough width there to pull half to two-thirds [horizontally] of the fan's air out, with the wild card being how far down to extend the duct to grab as much air vertically as possible. A proper vent would extend all the way to the bottom of the radiator, but it would also have enough room for a shallow angle.
It would have metal mesh over the cutout, perhaps sandwiched between the hood and under-supports. The duct itself would be a fiberglass mold, fastened to the bottom of the hood's bracing. That non-permanent (i.e., not molded into the hood itself) approach also allows the funnel to be opened up wider if modifying the radiator/reservoir system down the road. I'm still working out details in my head, but it seems like it has potential.
A pre-dark break in the rain afforded a few minutes to mock-up what I was describing. I didn't fully develop the upper funnel, but picture it curving out to fill the horizontal side flaps. I also didn't take measurements vertically to figure out exactly where it would be positioned to contact the bottom side of the hood, and to make sure the arc as it swings up doesn't affect clearance (the 2nd pic shows the fan gap). The cardboard is 7.5" wide at the lowest point. It'll probably need to go down closer to 7" to not brush the smaller hose on the pass. side when raised and lowered with the hood.
I refined the design a bit more and did a test fit today. It's oh so close, with one seam just brushing against the radiator cap when lowered almost all the way.
My thinking is to do two cutouts in the sheet metal, leaving the center brace covered from above. Hopefully, I can separate out the sheet metal from the bracing (hard, black glue holds them together) and insert some dark metal mesh between the layers to give it a more finished look. I haven't mocked that up from above yet, but I visualize it taking on a dual nostril GTO look.
I know there's a lot of air tunnel science behind high end venting, so if anyone has any suggestions for improvements, feel free to chime in. Things like whether the necessarily steep angle is counter-productive to smooth air flow, if it still does a good job without covering the bulk of the fan (my design covers ~25%), if a reversed hood scoop would be more aerodynamic than mesh over the cutouts, if adding the duct would interfere with the engine's direct cooling in any way, etc.
I probably won't tackle this until spring when I can count on a longer stretch of dry weather, so there's plenty of time still to further refine it ... or go a totally different direction. The hood vent idea is Step 1 of my larger weight reduction goal of doing a fiberglass insert to the hood, keeping the bracing intact to support it.
Also, part of my motivation for adding a hood vent is to potentially go away from the hydraulic fan down the road, either if it fails or if I want to eliminate front end weight and complexity. Electric fans sound good, but they seem to run pretty hot if trying to come relatively close to the cooling capacity of the stock setup. Having the vent would ensure a lower spec'd e-fan could run cool. Being able to put a more standard radiator/reservoir setup in place of the SC400's super proprietary one would be the icing on the cake, but probably way off in the future since that stuff is all in good shape on mine.
Very good idea here. I would imagine though that the way everything is setup in the bay, if the vents are purely for heat release then they should be facing open towards the windshield, this way, the fans push hot air outward and the air dragging over the car pulls it out regardless if a fan is being used or not, thats just the thermodynamics of it I'm pretty sure...this would make it less of a "hood scoop" and more of a vent. Also, if you just want to design a scoop, I would have to say somehow strengthen the hood latch or add hood pins because a scoop might add some serious resistance at high speeds. With a vent opening facing the windshield, this isn't an issue. I hope what I'm trying to say makes sense...
edit: I suppose a scoop would work great as well by increasing the CFM of whatever fan setup is in place. I'd think the biggest hurdle would be design in terms of it flowing with the lines of the SC. Interested in seeing what you create regardless.
I would imagine though that the way everything is setup in the bay, if the vents are purely for heat release then they should be facing open towards the windshield, this way, the fans push hot air outward and the air dragging over the car pulls it out regardless if a fan is being used or not, thats just the thermodynamics of it I'm pretty sure...this would make it less of a "hood scoop" and more of a vent. Also, if you just want to design a scoop, I would have to say somehow strengthen the hood latch or add hood pins because a scoop might add some serious resistance at high speeds. With a vent opening facing the windshield, this isn't an issue. I hope what I'm trying to say makes sense...
I kinda think you're viewing it backward... The vent is angled back toward the windshield, pulling hot air away from the fan in the direction of air flow over the hood. The scoop suggestion was to have it reversed so the opening faces the windshield, basically just being a cover for the vent cutout below. So, basically we're saying the same thing, just you thought I was saying the opposite.
Hmm, maybe we're using the terms "vent" and "scoop" differently? Most vents I've seen are basically an extension of the hood, sloped down toward the bottom of the radiator. That's what my design is, just bolted to the under-hood braces instead of being part of the overall hood. I could see how it looks a bit like a scoop placed upside down, though.
edit: I suppose a scoop would work great as well by increasing the CFM of whatever fan setup is in place. I'd think the biggest hurdle would be design in terms of it flowing with the lines of the SC. Interested in seeing what you create regardless.
Yeah, I would definitely have to do some cardboard mock-ups of a reversed scoop to see if it fits the car at all. It could look awkward that close to the nose.
Forgot to mention... My intermediate mock-up (not pictured) had the horizontal lip closely matching the v-shape of the under-hood bracing. I had to modify it to move the whole thing 4" back and open up the width of the upper vent (thus the shallower angle up top), which resulted in the lip not quite following the braces. I'll fiddle with that a bit more once all the other lines are where I want them.
I also realized after taking today's test fit pics that I should make the horizontal lip a bit less horizontal, since it meets up with the sloped front of the hood's underside. The way it is now makes the vent a bit more vertical than it probably needs to be.
you don't need to do any mounting or anything. cut the vents and leave the part where you want it connected to the hood uncut. bend it down and fiberglass magic the rest. that's what i'll be doing to my hood. just one hole right by the radiator and fan.
you don't need to do any mounting or anything. cut the vents and leave the part where you want it connected to the hood uncut. bend it down and fiberglass magic the rest. that's what i'll be doing to my hood. just one hole right by the radiator and fan.
Yeah, I know, I've seen the DIYs for that method. I'm just not a fan of it. I want something that looks like it could have come from the factory, rather than something that was obviously done in the driveway. That method has several shortcomings that I've been intentionally designing around improving on, such as:
- have to cut out the under-hood bracing
- can't extend the vent down as far because it's limited by the length of the cutout
- likewise, you're left with more of a gaping hole vertically because no overlap up top
- can't get as much "scoop volume" with a flat panel down into the narrow space available on the SC400
- likelihood of uneven sheet metal bends
- edit: one and done. I can always open my design up later if changing the radiator/fan/reservoir setup. The other method would have to patch the cuts to the hood and try to blend the sheet metal bends.
- edit 2: water runoff can't really be routed where you want it to go
Here's what I just wrapped up for v3, complete with some sexy curves in the upper transition (3D measuring is tough without computer modeling!), a tiny bit of slope to match the underside of the hood, and hopefully a perfect fit for the 95 degree under-hood bracing wedge. On this version, I made everything more symmetric by making each cut a mirror of the other side, instead of piecing them together from separate cuts. I also jotted down measurements for the main pieces, so others that want to do likewise can recreate most of it with minimal trial and error.
I expect a perfect fit tomorrow! Then again, I expect to be disappointed.
Edit: All the pics are from the front of the vent, looking into the bowl shape of the forward facing curve. It's crazy how hard it is to tell three dimensional directions apart forward vs away in two dimensional pictures.
I think I got it! Version 4 was discarded halfway through, as I liked the shape of v3 better. So, I took the changes made for v4 and applied them to v3, creating v3b, to fill out the angled under-hood braces (what the vent will mount to) and match the slope of the hood. It's now narrower at the bottom for coolant reservoir clearance, and wider at the top to match the braces.
The only thing left to do fit-wise is place tape sticky-side up throughout that area of the engine bay and make sure none of it is picked up by the vent after dropping the hood all the way. I went as low as the point of latching and didn't feel any contact, but it's tough to be 100% sure with how heavy the hood is and how light the vent is.
The downside to abandoning v4 is, all my careful measurements and step-by-step photos are worthless. I have the basic shapes measured out, but then there's a lot of subsequent trimming and extending to make it all fit together properly.
Without further ado, the part everyone is waiting for ... more pictures!
^ The front ear flaps hadn't been tidied up yet, as I wanted to see where it lined up on the under-hood braces first.
^ Trying to capture the 3D depth perspective...
^ Plenty of clearance by the coolant reservoir now. Will be increase a bit more by using the inside of the template as the mold.
^ Here you can see the amount of fan clearance in front and hose/coil/engine clearance behind.
^ And the finished product, all tidied up! Again, that's looking at it from the front.
^ One more, showing how the flaps will line up with the under-hood bracing.
If someone were daring enough to try a hood vent project in conjunction with stripping out the hydraulic fan, I really think you could go fan-less and do away with the electric fan shortcomings. A generic radiator and small reservoir alongside the power steering one, plus no hydro fan reservoir by the battery, would open up at least 75% of the width of the radiator for a vent. No fan means it can extend forward quite a bit more, and without the hydro fans plumbing halfway down, the vent could extend much lower. Both of which work together to keep the angle from being too steep. The challenge is picking a radiator that routes the upper and lower hoses as far out of the vent's way as possible.
I did some experimental hood lightening last week and figured it was time to finally fiberglass the vent and put it all together. This was my first real fiberglass project, having done one test project earlier this summer that came out so-so.
I need to trim the edges a bit more and sand it down, but it's looking pretty good after a quick cleanup with scissors. Definitely better than I feared it would turn out! It weighs in at a fairly feathery 1 lb 4 oz, which I'm sure is less than the sheet metal it'll replace up above.
Now for the scary part: trying to get it perfectly lined up for doing the cutout...
Anyone have a favorite process for finishing fiberglass? I've read that a gel coat is advisable, plus a polyurethane paint. The tip I found somewhere online to use Johnson's floor wax as a substitute for very expensive mold release wax (same active ingredient) worked great.
^^ Nice first fiberglass piece, t2d2! I can't quite place where this will visually sit but I'm interested to see where your vent project goes.
A while back I thought of adding a single offset NACA duct to my hood (probably on the passenger side right above the turbos on 2JZ's) because I liked the way Nissan did it to the hoods of R34 Skyline GT-R's (mounted on the driver's side in that application) but I could never find out if it truly helps cool the turbos in those cars or not.
Thanks, Kahn. Did you look through the earlier photos in the thread? I designed it to mount to the under-hood bracing, following the main V-shape. But, with a spare hood, I decided to experiment with bracing removal and a flat surface to mount to.
The scoop of the vent drops in between the coolant reservoir and upper radiator hose to the sides, and radiator/engine front and back. It's a pretty tight fit, which required a few revisions of the design. Which in turn means that I have to pretty precisely place the cutout or all of that was for naught! The shape of the vent was also designed to channel water to the middle, away from the alternator and battery. I don't have the main engine splash shield installed (been thinking of making just a side small shield below the alternator that won't sag in the middle like the stock one), so water will be free to drain down below.
NACA ducts are hugely popular in the Miata community, and there is lots of documentation on improved under-hood temps (and cabin temps for window applications), but I don't know how much that varies by car, if at all.
dude this is awesome, keep going with this, you could produce these regularly and sell them as well, all people will have to do is cut the hood hole, i'm your first customer if i don't do this myself lol. great job on the fiberglass as well, did you just cover the cardboard in it?
dude this is awesome, keep going with this, you could produce these regularly and sell them as well, all people will have to do is cut the hood hole, i'm your first customer if i don't do this myself lol. great job on the fiberglass as well, did you just cover the cardboard in it?
If I were to build one for others, or if I were to attempt a second one for myself, I would need to improve the mold. (And become much more efficient at fiberglass work!) I used the inside of the cardboard mold, as opposed to fiberglassing over the outside of it. I figured that would give me the best control of the visible portion from above.
What I didn't account for is how horribly the duct tape bubbled up and became a wrinkled, sticky mess on the back side of the fiberglass. Makes sense, seeing as that's what duct tape does when it gets hot... I was able to clean the indentations up a bit, but once mounted it probably won't be too noticeable. Doing it again, I would see if the same cardboard mold could be done with no tape on the inside. That way, the only marks would be extra fiberglass material at the seams which could be sanded down smooth, rather than bubbled tape indentations which I didn't have much luck filling in after the fact. Otherwise, I would need to figure out how to make a foam mold or something.
Also, the top mounting lip didn't end up perfectly level when the cardboard sagged under the weight and heat of the fiberglass. Hopefully, that doesn't present problems with mounting it flush against the hood. I'm hoping to use parts of the under-hood bracing to wedge it in there and then use windshield sealant or something to seal it in. Again, if trying a second vent, I would make sure to brace the flaps around the top lip from below, making it harder for them to sag.
I've got it painted black now and am going to try to get some cutout work done on the hood today, now that the rain has passed.
You can sort of see how the side lips aren't sitting flush. Hopefully, I can address that with whatever adhesive I use, without the hood bracing putting too much pressure on the fiberglass. You can also see the bubbled tape marks on the back of the vent. I used a charcoal gray there and a deep black on the front (visible from above), which actually sort of helps differentiate the inner vs outer curve challenge in the thread's original pics.
I got the back v-shape edges cut out. Tomorrow, I'll figure out where I want the foremost edge of the opening to be and finish up the cut lines to that point. I took a few measurements from the final test fit above and transferred those over to the hood being cut. I'd be lying if I said I'm supremely confident that I'll end up with perfect placement, given how many places there are to go wrong with something this complex and irregular. Just getting the cut lines was tough enough, since it required tracing the back edge of the lip and determining how far forward to come from that.
12-01-14, 10:53 AM
designing a hood vent
