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OPENING IN BACK OF HOOD SCOOP? #2637060
03/25/19 06:18 PM
03/25/19 06:18 PM
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USA
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540DUSTER Offline OP
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I have seen cars with a scoop with holes and even open in the back,whats the thinking for doing this?

Re: OPENING IN BACK OF HOOD SCOOP? [Re: 540DUSTER] #2637085
03/25/19 06:53 PM
03/25/19 06:53 PM
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Fulton County, PA
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CMcAllister Offline
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Trim the back of the scoop to fit the windshield nice. Fab an interior panel to seal to the carb plate or directly to the top of the carb. I wouldn't put any holes in it or leave it open. Defeats the purpose of the scoop or worse with air flowing over the top of the carb..


If the results don't match the theory, change the theory.
Re: OPENING IN BACK OF HOOD SCOOP? [Re: CMcAllister] #2637233
03/26/19 07:26 AM
03/26/19 07:26 AM
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Locomotion Offline
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A lot of weird things can happen inside of a hood scoop (turbulence). I have a friend who ran a '65 Plymouth with a factory style Hemi scoop. The single 4bbl carb w/pan was sealed to the hood. The car ran faster and much more consistently when he cut some sections of the seal out for venting. Just gotta experiment.

Re: OPENING IN BACK OF HOOD SCOOP? [Re: Locomotion] #2637302
03/26/19 11:06 AM
03/26/19 11:06 AM
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I did this to resolve turbulence issues.

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Re: OPENING IN BACK OF HOOD SCOOP? [Re: BloFish] #2637317
03/26/19 11:36 AM
03/26/19 11:36 AM
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Fulton County, PA
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CMcAllister Offline
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Those old scoops that sit flat on the hood look cool, but they don't work very well. The Chrysler developed snorkel or boundary layer bleed off scoop is still the basic design today. The size and shape of the pressurized area above the top of the venturis is what matters. Look at the shape of some of the interior panels offered by the high end shops selling scoops.


If the results don't match the theory, change the theory.
Re: OPENING IN BACK OF HOOD SCOOP? [Re: 540DUSTER] #2637343
03/26/19 12:44 PM
03/26/19 12:44 PM
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New York
polyspheric Offline
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The opening must be smaller in cross-sectional than the area of the interior behind it. The faster the car the smaller the opening.
The air horns should be in still air with as much velocity converted to pressure as possible. "or worse with air flowing over the top of the carb" is true.


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Re: OPENING IN BACK OF HOOD SCOOP? [Re: polyspheric] #2637353
03/26/19 01:05 PM
03/26/19 01:05 PM
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The idea is to create an area of positive pressure feeding the engine. Having an area inside the scoop, sealed to the carb without a lot of turbulence, is the trick. I'm not an aerodynamic expert, and I haven't hired one. But I do try to pay attention to what the guys who have are doing. If you think turbulence is an issue, experimenting with air cleaners, stub stacks or some other way of straightening the airflow may pay off.


If the results don't match the theory, change the theory.
Re: OPENING IN BACK OF HOOD SCOOP? [Re: CMcAllister] #2637383
03/26/19 02:34 PM
03/26/19 02:34 PM
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IIRC Dom solved some afr stability issues by drilling some holes in the back of his scoop.

Re: OPENING IN BACK OF HOOD SCOOP? [Re: 540DUSTER] #2637479
03/26/19 08:07 PM
03/26/19 08:07 PM
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A39Coronet Offline
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The thought is the hood scoop captures way more air at speed than the engine can consume, so instead of causing drag because your essentially dragging around a giant ball of air, the air can pass through and not cause an issue.

With a 5" hemi scoop, my carb to hood air pan slowed me down .015. I have ABA slips to prove it.


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Re: OPENING IN BACK OF HOOD SCOOP? [Re: 540DUSTER] #2637484
03/26/19 08:16 PM
03/26/19 08:16 PM
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New York
polyspheric Offline
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There are formulas on http://www.landracing.com/forum/index.php/board,31.0.html to calculate the hood scoop opening size based on top speed etc.


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Re: OPENING IN BACK OF HOOD SCOOP? [Re: A39Coronet] #2637811
03/27/19 05:56 PM
03/27/19 05:56 PM
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Originally Posted by A39Coronet

With a 5" hemi scoop, my carb to hood air pan slowed me down .015. I have ABA slips to prove it.

Do you race with an air filter, or is the carb exposed directly to the incoming air stream?

What type of carb, and what MPH are you running?

I'm asking because your results are just about the complete opposite of mine when testing with and without the air pan that seals the carb to the T/A-type hood scoop.

Last edited by BradH; 03/27/19 05:57 PM.
Re: OPENING IN BACK OF HOOD SCOOP? [Re: BradH] #2637818
03/27/19 06:19 PM
03/27/19 06:19 PM
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When my 5" Hemi scoop was sealed to the hood it picked up, (Eddy carbs).
Doug

Re: OPENING IN BACK OF HOOD SCOOP? [Re: BradH] #2638594
03/29/19 07:31 PM
03/29/19 07:31 PM
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A39Coronet Offline
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Originally Posted by BradH
Originally Posted by A39Coronet

With a 5" hemi scoop, my carb to hood air pan slowed me down .015. I have ABA slips to prove it.

Do you race with an air filter, or is the carb exposed directly to the incoming air stream?

What type of carb, and what MPH are you running?

I'm asking because your results are just about the complete opposite of mine when testing with and without the air pan that seals the carb to the T/A-type hood scoop.


No filter, 1050 dominator, 128mph. Was having issues with the hood trying to lift off the car because of the positive pressure in the scoop.

Ran numbers using cfm demand at 6800 vs. volume of air for scoop and determined the seeled scoop was causing drag. Removed the pan and picked up, and hood lift issue ceased.


Follow my G3 Hemi Barracuda build on Youtube
https://www.youtube.com/watch?v=XSDAWczXoZw&list=PLTus_wQu8POADHEeJNJp2nr4NMHEyB9EK

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Re: OPENING IN BACK OF HOOD SCOOP? [Re: A39Coronet] #3178369
09/26/23 01:25 PM
09/26/23 01:25 PM
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maybe a spring loaded flap would work well, full effect up to a certain speed and then gradual bleed off when the spring is overcome?


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Re: OPENING IN BACK OF HOOD SCOOP? [Re: polyspheric] #3178393
09/26/23 02:43 PM
09/26/23 02:43 PM
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Lake Villa Il
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Originally Posted by polyspheric
There are formulas on http://www.landracing.com/forum/index.php/board,31.0.html to calculate the hood scoop opening size based on top speed etc.


Maybe something to take into consideration but LSR scoop will be smaller than a drag race scoop. (doesn't matter if the LSR car is restricted at the beginning of a pass)


69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700
Re: OPENING IN BACK OF HOOD SCOOP? [Re: INTMD8] #3178399
09/26/23 03:05 PM
09/26/23 03:05 PM
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Seems to me that venting the hood elsewhere - say, along the sides halfway or so back - and leaving the scoop sealed could be optimum.
Also helps rid underhood heat, and vents the pressure buildup.
The hood lift thing happened on my Duster once it got fairly quick (about 120 MPH); adding a front/center hood pin fixed that.
Always wanted to vent its hood, but it was thin & flimsy fiberglass; I had to run supports along the inner ledges of its fenders to prevent the hood sagging.

Re: OPENING IN BACK OF HOOD SCOOP? [Re: topside] #3178463
09/26/23 07:59 PM
09/26/23 07:59 PM
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Positive pressure in the scoop is a good thing. Same reason why a blower/turbo car makes power!


Putting holes in the back of the scoop is a bandaid, and defeats the purpose of the scoop grabbing that cold air all together.

Re: OPENING IN BACK OF HOOD SCOOP? [Re: BloFish] #3178489
09/26/23 10:15 PM
09/26/23 10:15 PM
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Originally Posted by BloFish
I did this to resolve turbulence issues.

I cannot contest your hole drilling benefited your car. But how and what it did exactly to achieve that and whether it might actually on another's car do the same is unproven until tried and/or tested.
What actually might be happening in your case it might be allowing high pressure air at base of windshield to flow into the rear of scoop and then slowing the air entering the front and thereby reducing turbulence AND increasing ambient air pressure the carb sees, only as a guess.


Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
Re: OPENING IN BACK OF HOOD SCOOP? [Re: jcc] #3178490
09/26/23 10:31 PM
09/26/23 10:31 PM
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Fulton County, PA
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CMcAllister Offline
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If your car picks up by putting holes in the scoop, the interior of the scoop is not finished correctly.

The idea is to increase the pressure of the air available to the carb or throttle body. How can it be pressurized after knocking holes in it? Turbulence is the problem. Interior panels done correctly can benefit that.


If the results don't match the theory, change the theory.
Re: OPENING IN BACK OF HOOD SCOOP? [Re: CMcAllister] #3178506
09/26/23 11:41 PM
09/26/23 11:41 PM
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At speed there is often a high-pressure air bubble at the base of the windshield. That is the basis of Nascar intakes and the typical open rear Vette type scoops. If the scoop shown here has benefitted from rear holes being added, it might be from the additional air from the rear and it's slowing down front entering air, which could mean less turbulent air also at a combined higher pressure. We also do not know for certain if the front air scoop entrance is below the boundary air separation or maybe only partly above it, which impacts the positive contribution of any possible rear entry air.


Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
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