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Old 06-24-2010, 08:04 AM   #1
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Default What do you know about cooling ducting?

I only know what I have read thus far and that said: A cooling duct opening can be shrunk down to 1/4 the size of the face of the heat exchanger and the ducting is more efficient when the opening is like this /_\ vs. \_/ or |_| (the heat exchanger being the line on the bottom and the top would be where air is coming in through). What else should I know before I fab my ducts tonight?

Last edited by astroboy; 06-24-2010 at 10:03 AM.
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Old 06-24-2010, 12:03 PM   #2
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Pictures of ducting would be great. I've sifted this forum for them and have always come up just short...... Story of my life......
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Old 06-24-2010, 12:40 PM   #3
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I will have photo's of my construction of said ducting tonight or tomorrow.
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Old 06-24-2010, 02:38 PM   #4
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Transition from the opening to the exchanger should be curved and not straight- that's what allows the air to follow the surface of the transition and expand cleanly vs. dropping off and coming back around. Good info here- check out "curves have it"
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Old 06-24-2010, 02:40 PM   #5
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Quote:
Originally Posted by astroboy View Post
more efficient when the opening is like this /_\
I may just be a dumb noob in this particular application.. but that makes absolutely no sense at all. Sub-sonic fluids speed up in a necking situation and slow down in an expansion... does slow air make more sense in your ducting situation??
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Old 06-24-2010, 03:23 PM   #6
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Warning: beer physics follows, and I have not had a beer yet today.

Having the ducting like this /_\ gives the high-pressure area on the nose of the car somewhere to go - through the heat exchanger. Expansion inside the duct promotes that "through" flow. The walls of the duct keep the airflow from them piling up on the restriction of the exchanger and possibly spilling around it to the sides. The overall effect of this duct shape is to have less air piling up on the nose of the car and spilling over the hood and down the sides, and more going through the duct --> heat exchangers.

Having the ducting like this \_/ causes a flow restriction. Yes, there is a high pressure zone built up inside the duct. Yes, airflow will go through the exchanger. But the high pressure zone on the nose of the car sees an even higher pressure zone inside the duct, so a lot of air spills over the hood and down the sides of the car. Net airflow loss through the heat exchanger.

If you are feeding air to a jet engine, you want \_/ because the engine is going to actively pull that air through and take care of the high-pressure zone inside the duct for you - it needs, and will use, all the air it can get. But for passively redirecting airflow based on pressure differentials, /_\ is the way to go. No, the radiator fans don't count as jet engines. They do promote airflow, but they aren't jet engines.
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Old 06-24-2010, 03:36 PM   #7
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+1 on good info. I would have never imagined it worked that way. I will put the idea into service with my brake ducting plans! ...of course I'll take some pics while I'm at it.
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Old 06-24-2010, 04:12 PM   #8
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Quote:
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Warning: beer physics follows, and I have not had a beer yet today.
Wow, that beer is absolutely AMAZING! Where do I get some!?
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Old 06-24-2010, 05:17 PM   #9
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M2cupcar, that's a great link. Thanks!

Mobius, don't think I could have put it better myself.
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Old 06-24-2010, 05:56 PM   #10
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I will have photo's of my construction of said ducting tonight or tomorrow.
What material? I've seen crazy carpets used before on budget setups.
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Old 06-24-2010, 06:04 PM   #11
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30 gauge aluminum and pop rivets until I have the time to do dzeus fittings
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Old 06-24-2010, 09:45 PM   #12
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We definitely need pics after you make this
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Old 06-25-2010, 01:07 AM   #13
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cut your ducting template out of cardboard, then trace on to ABS plastic. The ABS plastic won't go through your heat exchangers when you hit something/someone/go off the track. It's relatively cheap in a roll or sheet.
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Old 06-25-2010, 02:42 AM   #14
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She's all done. I'll upload the pics in a few hours.
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Old 06-25-2010, 11:34 AM   #15
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Done, the opening is about 20" smaller than it should be but I got a nice curve on the roof. It was infinitely more difficult doing it on an ott setup and I pray to powers that be that I NEVER have to pull the intercooler off/out. Stayed in place and didn't see any change in temps (up or down) on the meter...gotta check with megatune later. I may pull it apart and widen the mouth. On with the pics!

Started like this:









Then it was like this:




And then it was like this:




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Old 06-25-2010, 03:39 PM   #16
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Good work, now seal off the mouth totally. That looks great though.

I have very janky plastic ducting on my car. I put a temp-strip on the rad and apparently never went over 190*f at the track in 95* a couple weeks ago. The fans never come on and don't have to at the track.
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Old 06-25-2010, 03:45 PM   #17
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Thanks, I need to finish sealing it off like you said but am not sure the best way to go about it. And suggestions guys?
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Old 06-25-2010, 04:36 PM   #18
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I've sealed gaps in ducting with rubber hose, sliced open lengthwise, and wrapped over the end of the duct. This works well if the gap is tight, like 3/16" or less. You can span larger gaps up to about 1/2-ish inches with high-temp aluminum tape, the kind used to seal HVAC ducting (not the same as cloth reinforced duct tape). If there is no flex b/t the duct and the exchanger this works really well, but if there is a lot of movement then the fiber reinforced tape works best, but looks ghetto under the hood.
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Old 06-25-2010, 09:28 PM   #19
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Quote:
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I may just be a dumb noob in this particular application.. but that makes absolutely no sense at all. Sub-sonic fluids speed up in a necking situation and slow down in an expansion... does slow air make more sense in your ducting situation??
Slow is just what you want at the radiator face. The flow velocity through the radiator is very slow, particularly compared to the forward speed of the car. If you have a 2 square foot radiator, and a 2 square foot inlet, the inlet gathers (and essentially stops) two square feet of oncoming air at however fast you are going. So lots of drag. If you put a diffuser on it (this: /_\), you only stop as much air as will pass through the radiator. So much less drag. The pressure available to push the air through the radiator is the same in either case. You just have to match the inlet area so it "ducts in" only the amount of air you need. It is complicated by the fact that the inlet area needed is speed sensitive - the faster you go, the smaller the inlet hole you need.
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Old 06-26-2010, 11:59 PM   #20
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Sealed all that **** up with some waterproof aluminum tape. I'll post pics in the morning.
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