Effectiveness of underhood ducting.

How much better is this ....

than this?

Much, but it's only worth the effort if you're having cooling issues or EVERYTHING else is already sorted. Terry Fair goes over the cosntruction of the hood on his Mustang and the why's and how's behind in in this thread Vorshlag Mustang The whole thread is good reading about the process of making a car go fast, but the hood discussion should be relevant to what you're doing.

DaewooOfDeath wrote: How much better is this .... than this?

Depends on the hood, which isn't included.

If the hood closes near flush to the cool looking duct, air flow through the radiator is killed, and the engine overheats.

On the other hand, if the hood has a vacuum duct, air flow through the radiator while driving at highish speed could be very good. Quite possibly thoroughly overkill for the needs.

The stock radiator configuration tends to work just fine, as the slight underhood vacuum and pressure in front of the grill do a perfectly adequate job of shoving/pulling air through the radiator. Some skirting and such to help pull the air through can help radiator air flow even more, while helping handling as well.

So the quick answer is "depends" and it depends on what isn't shown.

Extemely effective at reducing cooling drag. Ever wonder why the P-51B/D was 30 mph faster than the Spitfire IX when both had essentially the same engine and making identical power? It wasn't the laminar flow wings, It was the difference in cooling drag. Long divergent ducting in - short convergent ducting out.

mazdeuce wrote: Much, but it's only worth the effort if you're having cooling issues or EVERYTHING else is already sorted. Terry Fair goes over the cosntruction of the hood on his Mustang and the why's and how's behind in in this thread Vorshlag Mustang The whole thread is good reading about the process of making a car go fast, but the hood discussion should be relevant to what you're doing.

I just had lunch with Terry last Thursday. They have a very interesting 60's Camaro in the shop right now getting some really nice ducting done.

The physics of the hood vented cooling are: The low pressure at the front of the hood helps pull the air thru the rad and reduces that low pressure on the top of the body. Together this reduces front lift/increases front downforce and provides better cooling flow without dumping hot air into an already hot engine bay. The downsides are the hot air can wind up entering the cabin if the windows happen to be open and any cooling fan is exposed to the elements and fingers.
The low pressure on the hood occurs close to the front, where the curvature is tightest and quickly transitions back to ambient then high pressure as it approaches the windshield.

I'm trying to tastefully integrate something like this with a vented hood. I'm hoping for small downforce, enough cooling efficiency I can mount an intercooler behind the radiator and parallel with the hood and a decrease in drag. I'm going to flatbottom the Hyundai, just like the Daewoo, but I'm thinking I can use all the non-radiator flow to cool the front brakes.

Any suggestions?

mazdeuce wrote: Much, but it's only worth the effort if you're having cooling issues or EVERYTHING else is already sorted. Terry Fair goes over the cosntruction of the hood on his Mustang and the why's and how's behind in in this thread Vorshlag Mustang The whole thread is good reading about the process of making a car go fast, but the hood discussion should be relevant to what you're doing.

Great link. Thanks. :)

Is there any reason this should be the last thing I do?

I'm with SteveO on this one - which I should be, given that he knows a lot more about it than I do But it's my experience that far too little attention is paid to post-cooler aero. It's easy to imagine ducting everything into the radiator, but nobody ever pays attention to dropping the pressure behind the rad. It can make a big difference. We designed and built our own extraction hood for our time attack car back in 2003 and that car cooled better than any other turbo track Miata I think I've ever seen.

You also get the front downforce improvement, which is why I put some massive louvers on the Targa Miata recently. It doesn't need the cooling help, but it does need more front end stick so I can play with the wing more.

There was a drift Miata years ago that had a V-mount IC and radiator. Drifters have problems with cooling because their airspeed and direction is all jacked up! It would be a challenge to fabricate but very effective.

It's a lot of fun to play with a magnehelic gauge on a hood. You can see some of our results here - page 2, the NC hood. https://www.flyinmiata.com/support/instructions/misc/louvers.pdf

A hood-vented radiator and ducting in front of the radiator (so that less air can escape to the sides) will boost the radiator stack's efficiency. That said, your intercooler will still be vastly less effective if mounted behind the radiator. It needs to be right up front, ahead of AC condensers, oil/trans/PS coolers, anything. You might want to consider an air-to-water cooler with a separate radiator (which is the best-performing IC setup, if also the heaviest and most expensive) if routing the IC hose is that much of a problem.

Wow, thanks Keith.

Your method is a lot more efficient than what I did on the last car - flapper valves. The idea was that the valves open whenever there's higher pressure under the hood than above and close when there isn't. This worked, but it was fugly!

You can see it a little bit here.

I'm surprised that the highest vacuum area is near the outside edges of the hood. I would have assumed the biggest vacuum would be dead center. Actually that's great news for what I want to do. I'll post pictures of what I'm planning tomorrow.

GameboyRMH wrote: A hood-vented radiator and ducting in front of the radiator (so that less air can escape to the sides) will boost the radiator stack's efficiency. That said, your intercooler will still be vastly less effective if mounted behind the radiator. It needs to be right up front, ahead of AC condensers, oil/trans/PS coolers, anything. You might want to consider an air-to-water cooler with a separate radiator (which is the best-performing IC setup, if also the heaviest and most expensive) if routing the IC hose is that much of a problem.

I know it's less efficient for all out power but, if I can get mega airflow enough to compensate, mounting the intercooler flat against the hood would get me some really short intercooler piping. Am I wrong to think the air behind the radiator isn't that much hotter?

I like the idea of the hood/radiator exhaust. instead of all the air going in the radiator and out the underside/wheel wells of the car inducing lift it goes up and out maybe even adding down force (if the exhaust angle is right?).

In practice/ playing around with daily driver, i had problems with overcooling (with a +turbo car that was known for poor cooling from the factory) at speed and with warming up, and overheating at low speeds in traffic. It did allow for a larger coroplast belly pan which cut down on a good amount of lift/drag (increased mpg) but also cut cooling to the oil pan and transmission (radiator ducting+ belly pan cut most of the air flow from the engine bay). I had fun playing around with it all. If you try it out track tempratures on all the fluids you can.

DaewooOfDeath wrote: I know it's less efficient for all out power but, if I can get mega airflow enough to compensate, mounting the intercooler flat against the hood would get me some really short intercooler piping. Am I wrong to think the air behind the radiator isn't that much hotter?

Air behind the radiator is a lot hotter. Put your hand behind it when the fans are on, it feels like a giant blowdryer, even if the other heat exchangers aren't doing anything. Airflow at highway speeds is in the same ballpark as having the fans on.

I'm thinking a better way to do it might be a V-mount setup. It will take some fabbing and eat up a lot of engine bay space, but this way your intercooler and your existing heat exchanger stack can both be "in the front" at the same time.

I found some relevant info here.

http://www.autospeed.com/cms/article.html?&title=Undertrays-Spoilers-Bonnet-Vents-Part-1&A=113176