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Old 04-12-2010, 05:40 PM   #1
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Default Gutting car, defogging Q

As I'm in the middle of gutting my car I'm ready to ditch the HVAC system too. As the car needs some streetability I want to be able to defog the windscreen if necessary. I canīt find heated windscreens for the miata so I thought about using a bilge blower inline with a hose leading from the air intake on the RHD in the cowl area to the piping already in the dash.
Would one of these move enough air to be able to defog the windscreen?
Specs:
Output-open flow @ 12 volts - 135 CFM. Amp draw @ 12 volts DC - 2.7. Fuse size - 4 amps. Dimensions (including mounting feet) - 5-1/32" L x 5-1/16" W x 3-5/8" H

Something like this:

Mounted like this:
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Old 04-12-2010, 05:51 PM   #2
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Why not hook the stock blower up to the defroster vents. Free minus some time, 4 speeds, already wired, already mounted, etc, etc. That's what I did but I also kept the heater. Want some scary under dash pictures?

Go to Keith's Targa build and click on the weight loss tag, the first few pictures are pretty informative, can't seem to find good pictures on the blower installation.
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Old 04-12-2010, 06:20 PM   #3
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Ditching that purely for weight savings. All the tubing will be removed, except for the defroster ones. The blower with some hose seemed simple enough to do.
Scary pics? Post 'm up
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Old 04-12-2010, 06:22 PM   #4
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Get a rear defroster from E30 convertibles. Its a blower with a built in heating element (all electric) and is very low profile. I'm finishing a racecar build for someone and the guy is using one of those as his front defroster since he doesn't have a heater core. Its on a 68' BMW 2002 FWIW.
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Old 04-12-2010, 07:34 PM   #5
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http://cgi.ebay.com/ebaymotors/ws/eB...d=370356078800

It's not exactly cheap, but looks like it would work well. Maybe fab something up or drive with a roll of paper towel or regular towel in case the window fogs lol. I have yet to see how bad it will be in my car as I've ripped the entire HVAC out as well...
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Old 04-12-2010, 07:49 PM   #6
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I've been wondering about a solution for this as well
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Old 04-12-2010, 08:08 PM   #7
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Quote:
Originally Posted by RyanLewo View Post
http://cgi.ebay.com/ebaymotors/ws/eB...d=370356078800

It's not exactly cheap, but looks like it would work well. Maybe fab something up or drive with a roll of paper towel or regular towel in case the window fogs lol. I have yet to see how bad it will be in my car as I've ripped the entire HVAC out as well...
IMO its the perfect solution. Should work great.
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Old 04-12-2010, 08:26 PM   #8
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I'll keep an eye out for an E30 unit. I'll probably get the bilge unit if nothing comes up. They're only about 20 bucks for a 3" blower. Fabbing the ducting seems simple enough.
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Old 04-14-2010, 01:24 AM   #9
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I know of many cars using blowers just like that one with ducting to the stock dash ducts for the defroster.
There is also an anti-fog film you can buy from places like BSR hutch for $60 a roll. You apply it like window tint to the inside of the windshield.
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Old 04-14-2010, 08:41 AM   #10
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Good thread, thanks. I have been wondering about this as well. So just the airflow does it? No heat needed? I was thinking about a depowered hairdryer and an inverter
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Old 04-14-2010, 09:23 AM   #11
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Quote:
Originally Posted by cjernigan View Post
I know of many cars using blowers just like that one with ducting to the stock dash ducts for the defroster.
There is also an anti-fog film you can buy from places like BSR hutch for $60 a roll. You apply it like window tint to the inside of the windshield.
yeah, read about that too. Either that or a slight coat of rain-x on the inside. I'll probably get the bilge fan because those E30 blowers are nowhere cheap enough for me now.
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Old 04-15-2010, 02:00 AM   #12
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rain-x or car wax on the inside can help sometimes as well
I've been told that when applying the rain-x it needs to be done so that you leave a visible film. If you buff it off completely it won't work on the inside like you want it to.
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Old 04-15-2010, 02:42 AM   #13
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FWIW, rubbing pure dawn dish soap on the inside of a paintball mask lense until clear is the best anti-fog I have found.
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Old 04-15-2010, 08:43 AM   #14
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Quote:
Originally Posted by adamhershner View Post
FWIW, rubbing pure dawn dish soap on the inside of a paintball mask lense until clear is the best anti-fog I have found.
The actual paintballer antifog spray works pretty damn well too. It's unnecessary in this situation because it's just a Rain-X type chemical formulated to not eat your plastic mask.


ZX:

I ran without a heater core for a while, and rainstorms were a beyatch. You need more than airflow. For the same reason your car runs the A/C system when you use defrost, that the air needs to be dryer. If you heat the air you lower the specific humidity. It's basically the reverse effect of dew point in weather terms, which is oddly how an evaporator in a refrigeration cycle remove moisture from the air. One method is more effective, but the other is simpler, cheaper and lighter.

I forget all the scientific parts of it, but hotter air can take more water vapor before it's saturated. If, and I say IF the air you are blowing across the windshield is not saturated, or at close to 100% relative humidity, it will clear it up some.

In reality you find that point very quickly inside a Miata in rain and cold, unless you're fond of not breathing for long periods of time. Mine would fog up, front back to front. I could keep going forward once the side windows fogged, but once the windshield turned into "hazy flashback frame" mode I had to pull over, wipe, and repeat. The heater core hose routing rectified itself pretty quickly last fall.

You may very well know all of this, but due to your question I decided it would be good to relate this for posterity.


******:

How much does one of those BMW units weigh?

Last edited by gospeed81; 04-15-2010 at 08:53 AM. Reason: Not every day I get to teach a NASA engineer something
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Old 04-15-2010, 09:06 AM   #15
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Maybe try this:
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Old 04-15-2010, 11:29 AM   #16
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Quote:
Originally Posted by gospeed81 View Post
You may very well know all of this, but due to your question I decided it would be good to relate this for posterity.
Yep I do but good idea to write it all down for posterity since it is a safety issue. I have had fogging problems with my motorcycle visor on the street and on the track and it sucks. Last year when driving this LS1 Miata I had uncontrollable fogging on the windshield on the highway (in the rain) that was really bad and could have led to an accident. On the track, on a rainy day, I would probably have to stay in the pits.

I was experimenting yesterday and today with my defroster (in my truck driving to work) to see the effect of just airflow versus heated air. It has been raining a lot here for days and so conditions are right for mild windshield fogging. Just airflow alone will clear up a fogged windshield a bit, but heated air is much more effective.

Most car systems work like you said. They are basically dehumidifiers. The air is cooled with the a/c, water condenses out, then it is reheated with the heater core, producing air that is much drier than when it originally entered the A/C system.

But anyway that is why I asked the question about the practical racing use of blowing air through the defroster vents with no heat source. All of this was in my mind, so I was skeptical that just using unheated air could work. But if it works, it works.

One could use the old-school method of pulling heated air in from the engine bay, or from a 'stove' around the exhaust pipe, but then there are safety issues with that too. I'll bet one of those bilge fans with a simple little low wattage DC electric heating element in the airflow would help quite a bit. Maybe a high current resistor with a big heat sink on it. Wire it to produce say 240W ([email protected]) of heat or so. Cheap, simple, light, compact. All I know is if I am out on the track, and my window fogs, I want to flip a switch and have that window clear RIGHT NOW.
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Old 04-15-2010, 11:41 AM   #17
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$30 buck heater/defroster from walmart. I remember this being 20 bux in the store for some reason. Plugs into the cig outlet.

http://www.walmart.com/ip/Koolatron-...eater/11710385
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Old 04-15-2010, 11:51 AM   #18
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^^^ That's what I want...a completely electric solution that isn't off a german car and costs $100 used on eBay.

I'm about to get a manifold that will require me to reroute the heater return anyhow, so if I do away with the heater core altogether it will simplify things.

But yeah, just air, in a Miata, in any decent rain doesn't work well. I consider this steady state conditions since I was at cruise for 75 miles.
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Old 04-15-2010, 12:40 PM   #19
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That is cool that it is a turn-key packaged unit, but the reviews on WalMart and Amazon are not favorable.
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Old 04-15-2010, 12:41 PM   #20
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Yeah I was just looking at those reviews as well
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