Hood scoop science

[Gary Perkinson]

Hey, everyone:
 
 I'm trying to decide between using a rear-facing cowl-style hood or a forward-facing hood scoop on my C/CGALT '80 Pinto. Does anyone have any informed opinions or, even better, empirical evidence as to which is better, in terms of both aerodynamics and intake airflow? I'm leaning toward a big rear-facing cowl (think of the BWT Motorsports 'Vette: http://www.ecta-lsr.com/cars/1197.jpg) because it just seems to make more aerodynamic sense, but I can't find much science out there on the topic.    
 
 Thanks!
 
 Gary

[Gary Perkinson]

Sorry, that link shouldn't have included the parentheses:
 
 http://www.ecta-lsr.com/cars/1197.jpg
 
 Gary

[ddahlgren]

What would make you think that the air would want to go into that scoop? What other form of motor racing has one like it?

[Bob Drury]

Dave, I hate to be a naysayer, but unless your scoop is well above the hood line (as in at least one foot), you would be better off to draw the air off the base of the windshield.  Look at how NASCAR does it.  The only two high pressure areas on a full bodied race car are the bumper and the base of the windshield.  When air comes over the grill, it jumps up about a foot over the nose, and then it flows parallel to the plane of the hood, but aboout a foot above it.  If you were to duct the scoop rearward to the base of the windshield, you would pull better air.  The other problem with tall scoops is aerodynamic drag.  The reason you see them on Drag Race Cars, is they don't need to push the air for five miles.  Bob

[Gary Perkinson]

Well, intuitively, I wouldn't think the air would want to go into that scoop--a forward-facing scoop does seem to make more sense. However, a lot of door-cars in drag racing (especially Mustangs) do use the rear-facing cowls to run in the 7's and faster, so something about it must be working....
 
 Gary

[Gary Perkinson]

I've been digging around the internet a bit and found a few tidbits here and there on cowl induction, though not a whole lot of "official" stuff. This first quote comes from the forums on a site called theoldone.com, from a guy who seems to work on or with a NASCAR team:
 
 <<Cowl induction is a great way to intake outside air of ambient temperature. There is a high pressure area at the base of the windshield, and you need only turn your "climate control system" to outside and cool to appreciate the amount of pressure available there. I believe that for most normally aspirated cars, it's the optimum place to draw atmospheric "fuel" from.>>
 
 And then there's this from another site:
 
 <<The way [cowl induction is] supposed to work, is that at a high rate of speed, the cold air pools at the back of the hood just below the windshield area. This constant [supply] of air is then sucked up under the hood and more replaces it. However it doesn't really work that well at low speeds, and most engines use Ram Air in order to solve the problem....There is good positive air pressure at the base of the windshield, but it will not help a lot unless you are going over 150 mph....[it's] most useful at high speeds, and [in] straight lines....
 
 In any case, any other cowl induction info would certainly be appreciated--
 
 Gary

[Bob Drury]

Gary,  The best thing I did when building my car (besides buying a rule book), was to walk around the pits at Speedweek and talk to people who had cars similar to what I was building.  You will get answers from soup to nuts on everything you ask, and then you will have to sort out the information for yourself.  One of the greatest thinds about LSR racers, is unlike most motorsports, they will give you most anything you ask for, including parts, as we are all part of a Fraternity that races the timers, not each other.  One word of caution, a lot of old timers are afraid to try new ideas, so you need to take some of their info with a grain of salt and a healthy dose of respect. Don't,however, be afraid to try out new idea's.  Thats what LSR is all about.        Bob

[John Burk]

Gary - A simple test with a car with a profile similar to your Pinto and two pieces of 3/8" clear flexible tubing would tell what spot has more pressure - Duct tape one tube to the windshield and experiment with the second , like 1 foot ahead of the hood - On the ends inside the car form loops with equal amounts of water and see in which the level is pushed higher at 50 mph .

[ddahlgren]

The base of the windshield I would go along with. It is the other 3/4 of the scoop that has the problem.

[Gary Perkinson]

John: Thanks--someone described a similar pressure test on another site, but your explanation makes more sense. I'll give it a try....
 
 Gary

[Gary Perkinson]

Dave:
 
 Yeah, the only thing I can think of with that big scoop on the pink Corvette is that it's designed to both take air in and direct the rest of it over the roof, but you're right--that scoop sure does look like it would hinder any pressure build-up at the base of the windshield....    
 
 Gary

[John Burk]

Gary - At bonneville perfect hood scoop at 150 mph would give a 3% supercharge - 200 mph 6% - At a steady 50 mph on the BQE full impact pressure would raise the water 5/8" in one side of a loop of tube.

[Bob Drury]

One of the problems with forward facing hood scoops is that without a wind tunnel, you really don/t know where you are.  A scoop opening can in fact do more with less (look at the scoop on the Nish streamliner for example). A large opening can and will cause a turbulence that can have a negative effect on air flow.  By pulling air off the windshield, you do not encounter this problem, and I have doubts about the supercharging effect on a normaly aspirated motor.  Just my own personal thoughts, no science involved.............Bob

[Gary Perkinson]

<<At a steady 50 mph on the BQE>>
 
 NOW you're really talking theory    
 
 Gary

[John Burk]

Gary - Figure what size column of air it'll take to feed your engine in top gear (engine ci divided by inches traveled in 2 engine revs), add 10% to be shure and that's what size hood scoop inlet you need . No ram air pressure in lower gears when there's little to be had and little scoop turbulance in top gear.