Air flow through hood scoop

[What'saCobra?]

Ditto here, Anthony.

i would add that if you run slightly colder plugs, flame-outs can even happen in the summer, if you are a little careless and let the plugs get a little too fouled or use too much accel pump before hot starting.

The use of nominal foam is just simply a fire hazard in any semi-serious application. You can live with it, from time to time, but you get flame burns on the hood interior which is a PIA to clean-up nicely.

And it is a little embarassing to have the smoke and strange start-up noises
at a show or on a nice quiet street.

No-choke carbs are, as Morris indicates, the worse-case offenders. But, i like them for their nice flow characteristics and live in the South year-round. Morris knows of which he speaks.

Of course, they remind me of the old Webers on the race cars, that used to belch and flame a whole lot. Real loud. The IDA"s were not particularly easy to start even when warm.

i still prefer that bark and smoke of a Pratt & Whitney (lthough they cut-out inverted), rather than the fuel-injected Lycoming, as nice as they are in aerobatic utility.

It's just old time religion...

[Ant]

Morris,

Thanks, I can see the benefits of the reverse scoop, and being wider than my JBL styled scoop, if I went to a scoop like yours, then I could go to a 16" filter, but I will stick to my 14" x 3.5" K&N, as mentioned I dont think it will need anymore than 925cfm, and I wont be oiling this race filter. I like the look of your scoop, but I would like to like my car to be like it was in the 1960's, but these big forward facing scoops are ugly!

I guess I can just run a mesh in place of the filter element for extreme events, but crap is still going to enter the engine!

Aussis Mike,

Nice job you are doing there!!!

[DocDirk]

This has been an interesting post to be sure! I've seen some hood air scoops which are open on the rear side as well as the front, mostly larger ones customized to accommodate a larger engine setup. I've wondered whether the flow of air thru the scoop would have a chance of setting up a vacuum effect at some speed, potentially robbing the air intake? It's been said to me as well that such a scoop is used to aid in reduction of engine and engine compartment temperatures. Anybody have experience or an opinion?

Thanks!

Dirk

[CHANMADD]

I have always used an Edelbrock 1000 cfm foam filter,I have burned the foam once due to a backfire but that only happens if the motor is out of tune.My scoop runs the air directly into the top of the filter and appears to have some ram effect .My motor does sit further back than normal so that my carb is right at the back of the scoop.

[Bill Wells]

I have the traditional hood scoops on my SPF, GT 350 clone and 57 TBird and am always amazed at how little water gets into the engine bay when driving at highway speeds in torrential rain . There is always water on the front lower frame rails and lower inside fender shields etc, as well as on top of the radiator shroud/surround- most of that is from the tires .

Interesting is that there is very little water, or water spots, on top of the engine, valve covers or air filter lid . It is almost like there is a blockout plate on the hood scoop opening or a natural created air dam . Go figure , as with the scoops I would expect monumental amounts of water . Bill

[Badger]

Lew Ledyard is the brightest guy on this thread. The front of the scoop is a LOW pressure area! Air exits the front of the scoop! The smart thing is to run the scoop reversed, with the opening near the windscreen where the pressure is higher. Doesn't matter what the original Cobra team guys did..those guys made a lot of mistakes but didn't change simply because it "looked" right. Peter Brock

[mr0077]

Badger, you're probably right (about the front of the scoop being a relative low pressure area - the base of the windshield is definitely a relative high pressure area), but someone should do a "tuft" test to check that out (tape 1 or 2 inch long pieces of yarn or string onto the leading edge of the scoop and watch it while driving to see which way air was going at the scoop). Obviously having a turkey pan with a hood seal would affect the results due to engine air flow, and I suspect that there are a whole lot of other variables that could affect it...whether the cooling fan was on, is the radiator sealed to the body, is the radiator upright, etc.

[Anthony]

Originally Posted by Badger

Lew Ledyard is the brightest guy on this thread. The front of the scoop is a LOW pressure area! Air exits the front of the scoop!

Originally Posted by mr0077

but someone should do a "tuft" test to check that out (tape 1 or 2 inch long pieces of yarn or string onto the leading edge of the scoop and watch it while driving to see which way air was going at the scoop). Obviously having a turkey pan with a hood seal would affect the results due to engine air flow, and I suspect that there are a whole lot of other variables that could affect it...whether the cooling fan was on, is the radiator sealed to the body, is the radiator upright, etc.

With my car, you can see streaks of oil on the underneath side of the scoop going foreward, out the front of the scoop. My take on it is that the air coming through the radiator opening (after warming up) pressurizes the engine compartment, of which a portion exits out the scoop. So, if you nail the throttle, if you don't have a good seal on your turkey pan, not only are you getting low pressure air, but it is warm air as well, both decreasing engine HP. If you seal the turket pan to the scoop, at least you will get cooler air.

[Lew Ledyard]

Originally Posted by Badger

Lew Ledyard is the brightest guy on this thread. Peter Brock

My wife would so beg to differ.

[Badger]

77..you're right, these are all good points, but the fact remains that the position of the scoop on the hood is still in a "transitional area" where the pressure is not positive enough to really make a performance difference. Sealing the underside will help, but simply reversing the position of the scoop on the hood makes a big difference, as the intake will now reside in a positive pressure area. As far as tuft testing goes... I did all that on the first Daytona Coupe (not exactly the same issue here) but was astounded to learn that the air intake at the base of the windscreen had created so much pressure that the incoming air was actually pushing the sealant foam away from the edges of the turkey pan! (I'd already determined, on the roadsters, that the air intake was in the wrong place, so I put the intake on the coupes back at the base of the 'screen) This "discovery" was important because if we hadn't learned this at the first test the engine probably would have leaned out at high speed and burned the pistons! The simple solution, as can be seen in early test photos of CSX 2287, was simply to drill a few holes in the front of the "blister" . The pressure was still so great that fuel dye from carb "standoff" can be seen flowing from the front holes! The fun thing in "reversing" the hood scoops on a roadster these days is going to a modern Cobra meet and having all the so called "experts" thinking you were so dumb in assembling your "kit" that you mounted the hood scoop "backwards" :0) Peter Brock

[Bill Wells]

Badger , not to hijack the thread, but most importantly - How is your recovery coming along ? Bill

[Ant]

I have modified my JBL hood scoop and made it even higher, to suit my Ram SC, and its on more of a slope which lessens the front height! Will post a picture soon.

I guess that the front facing scoops dont work that well, but they might if you can get the air away from them or exhaust as the aeroplanes, GT40 Radiator vents etc. I am building a car of the 1960's and thats what they did, I looked at a reverse scoop and I dont think it will do, but I think a nice scoop tray around the carb, that doesnt seal that well around its base will help separate the under hood pressure and my carb should have adequate air!

Jac Mac suggested to block the front off and try and take air from the rear of hood, while still retaining the original look, but I still have that ugly scoop. In the UK, with the race cobra some have the air filter protruding out of the top of the scoop!

[a427sc]

Ant Compare my ERA w/ large scoop (in front) vs a Contemporary (99's) behind.

[Ant]

Originally Posted by a427sc

Ant Compare my ERA w/ large scoop (in front) vs a Contemporary (99's) behind.

a427sc,
Your car looks very good and much better than the one behind!
With my choice of Yates headed race engine, manifold height dictates scoop height and I am another inch of so higher than your car! I could go EFI but I cant really afford that and dont see it as being any great improvement on the track, but it would solve some scoop height and fuel economy!

[mr0077]

Badger, that's some very cool background on the coupe testing/development, thanks for the info...I didn't realize you had tuft-tested and got those results...
On the Cobra, I would bet that AT BEST the air coming over the hood to the scoop has no positive pressure, and may actually have a lower pressure than the carb inlet area below it. If I get the time I may do a little tuft-testing to see what mine does, but if anyone beats me to it, please post here to let us know what they find. And I guess different configurations (different kit brands and intake setups) may get slight to large differences in results (sort of a "Your results may vary" kind of thing).
Like you said, just cause it looks cool or "looks right" doesn't mean it works right :-)

[G.R.]

With all this discussion of the airflow or lack there of through the hood scoops does anyone know why the 289 comp cars almost always have a drilled aluminum plate at the front of the scoop? I really never noticed the difference in air flow until I was tuning my Webers, at idle with the hood up I could get the engine idiling at about 900-1000 rpm...then I drove it and needed to adjust one of the carbs, leaveing the engine idling, now at about 700 rpm, as soon as I opened the hood the idle increased about 200 rpm. And I don't have a "drilled plate" in front of my scoop. I never really experienced a difference in idle when I was running a 4-bbl...hood open or closed...with the Webers there is a big difference.

Just curious as to why this was done an the early comp cars? Especially if an open scoop reduces airflow...one partially blocked would seem to reduce it more.

[Badger]

Thanks for asking Bill. The broken leg is healing well, but not fast enough! :0) Also with all the hardware "installed" I can't get through the metal detectors at the airport without causing the alarms to go off! I can hobble around now without crutches, but doubt I could scramble away fast enough from another errant race car! Badger

[G.R.]

Glad you are healing up so well Pete.

[Badger]

The reason the "oversize" hood scoop on that ERA racer of Don's worked better than the stock scoop is that the raised profile was getting above the slower moving boundary layer and allowing the air flowing over the hood to enter the scoop. Same reason Prostock drag racers run those hellaishly high forced induction scoops. Works good for the quarter mile, but not exactly the best solution for a road racer. Badger

[Michael C Henry]

My thing with inline dual four is I have a tunnel wedge intake wich makes it high. Even the standard S&H elements have to be trimmed down in height. I feel the air filters are the restricion and when going full tilt the resulting vacum causes the mixture to go to the way rich side. Any better filtering would nessitate taller elements bringing the air filters up through and above the original plane of the hood .. At this point a scoop would more of another hood for covering the air cleaner. In a normal car these engines sit inclined with the rear lower and the carb sit somewhat level. In a Cobra the engine sits almost level which causes the carb to be inclined with the rear being higher than front. What I'm looking for is a scoop that looks appropriate and is as high in the rear as the front with respect to the hood surface. not a wedge blending to the rear of the hood. Large enough to get a Cobra style air cleaner with an adequet heigth element . So far cowl induction scoop[, the replica of the twin supercharged cars scoop , and one of those areo pro stock scoop are the choices.