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Old 09-25-2008, 12:38 AM
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Too small scoop opening?

How small opening can I get away with on a ram air scoop?
I would like to make a hood inspired of the hood on Chip Foose's P-32.
And I want the scoop to be functional and the only air source for the carb.
The engine is a Chevy small block and the car is a '34 Ford.


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Old 09-25-2008, 06:29 PM
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I'm gonna step way out on a limb here and try to help you. I remember back in the 70's, Chevrolet used air cleaners on their 350's that had a long snorkel tube on them. If I had to guess (and if you trust my memory ), the diameter of the opening was about 7cm. All the air taken in by the motor passed through that opening. To figure the area would be: 0.7854 X 7 X 7, or 38.48 square cm. Now, you'll have to realize that these motors were of the smog variety and were probably all done by 4000 r's, so if you were to figure a 350 at 5000, you'd multiply 38.48 X 1.25 and get 48.1 square cm. Figuring for 6000 r's would be 38.48 X 1.5 and you'd get 57.72 square cm.

Now, looking at the photo you posted, I would guesstimate the opening at 50 cm wide by 15 cm tall and would identify it as an elliptical segment of an ellipse having a width of 50 cm and a height of 30 cm. To find the area of HALF an ellipse, multiply the width times the height times 0.7854 times 0.5 So, 50 X 30 X .7854 X 0.5 = 589.05 square cm.

I know I'm flying by the seat of my pants, but maybe I've provided you something to work with.
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Old 09-26-2008, 07:27 AM
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You have.
Thank you!

Althoug I dont quite understand you numbers on that hood photo. The scoop opening is the small opening on top of the hood, and that opening can not be as wide as 50 cm (or 19,69").

But, as you also mentioned, judging from a snorkel on a stock 70's air clearer, a scoop opening don't have to be very large.

I've also heard (from motorcycle technology) that ram air intakes should be small at the opening and then widen out and be large arund the air filter area, as this will increase the speed on the air coming in.

I will keep you updated!
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Old 09-26-2008, 09:11 AM
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A scoop like pictured will likely not have a ram air effect, it is too low on the hood. There is the boundary layer affect.
The carb will draw through it though, and it will be outside air.
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Old 09-26-2008, 12:45 PM
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Yes I know. In lack of a better word I called it ram air.

What I want is outside air in large enough amount. Since overheating is such an usual problem on early thirties hot rods, the under hood air can not be very good for performance.

And besides that, I like the different look on that hood very much!
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Old 09-26-2008, 10:12 PM
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Well I'm with you Staleg. I think the hood looks slick. -and per techinspector1's impressive calculations, it should provide plenty of air.
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Old 09-27-2008, 08:18 AM
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Actually if you do a bit of research you will find that a low hood mounted scoop is useless as far as ram effect. All those fancy scoops used in the 60's were nothing more than looks and unless a scoop is extended well above the hood (I remember reading somewhere that it should be a minimum of 14" but that would depend on hood shape) it can actually be counter-productive due to a pressure drop from the air flow over the hood. Since the air does not flow on the surface of the hood but separates at the front and flows several inches above it there can actually be a pressure DROP at the front of the scoop due to "Bernoulli's principle" thus instead of the scoop being functional in producing ram air it can have the opposite of the desired effect. The auto engineers of the 60's were well aware that the scoops used on such cars as Mach one Mustangs and the like were just for show and GM even went so far as to address this problem with the "Cowl induction" design for taking air from the BACK of the hood! Most low mounted hood scoops will do a much better job of creating LOWER pressure at the scoop opening than they would at creating a ram effect.
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Old 09-27-2008, 11:03 AM
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Good information. But have another look at that P-32 hood. The scoop is at the very front edge of it.
Isn't there a fair chanse that it will chatch the air stream before it leaves the hood surface?

But I admit this is details. And have you ever tried to raise your hand up thru an open sun roof while driving at highway speed? The wind hits your hand right over the roof surface, not like 14" higher up.

Last edited by staleg; 09-27-2008 at 11:13 AM.
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Old 09-27-2008, 03:30 PM
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There is quite a bit of difference in the air flowing up over the windshield and the roof than than the grill/hood. Also depending on the roof/windshield design the flow can be higher on different cars. There is no question that a low mounted hood scoop is worthless as far as ram function and this is a known fact not just speculation. There may be some advantage having the scoop mounted near the front but I doubt there would be much, if any, ram effect unless the scoop is right at the front edge and angled down slightly into the air flow. If you really want a true ram effect try using an intake mounted in the grill but if you are feeding this ram air directly to the carburetor you will need to enclose the whole unit, bowl vents and all, and not just try to ram air down the carb intake.
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Old 09-27-2008, 06:49 PM
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That scoop in the photos will provide pressurized air into the engine, the front of the vehicle is a high pressure area. The top mounted hood scoops are in an area of low pressure hence why they don't work.

It will be a small pressure effect but it will be better than a hood mounted scoop. I wouldn't worry about getting the size correct, the engine will draw what it needs.

Worry instead about getting the look right.
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Old 09-27-2008, 08:37 PM
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I presume this is for a street car, with a radiator and radiator fan that draws air through the radiator and discharges into the engine bay. (typical street car setup). What's the airflow (in cfm) of your fan? I'll bet it far exceeds that required by your carb. So it won't be starved for air. Design the scoop any way you want, the one in that pic looks awesome!

If you're going to duct the scoop directly to the carb, take the carb opening size (measure the arae of your throats - in square inches) and multiply that times two. Your scoop's inlet opening size and duct/tunnel leading to the carb(measured in square inches) should be no less than twice the square inches of the total carb throat area.
Hope this helps.

Antny

Last edited by AntnyL; 09-28-2008 at 01:15 PM.
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Old 09-27-2008, 08:47 PM
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Quote:
Originally Posted by oldred
If you really want a true ram effect try using an intake mounted in the grill
That sounds sorta like the way the old Thunderbolts did it, right?
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Old 09-27-2008, 08:48 PM
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Quote:
Originally Posted by staleg
You have.
Thank you!

Althoug I dont quite understand you numbers on that hood photo. The scoop opening is the small opening on top of the hood, and that opening can not be as wide as 50 cm (or 19,69").

But, as you also mentioned, judging from a snorkel on a stock 70's air clearer, a scoop opening don't have to be very large.

I've also heard (from motorcycle technology) that ram air intakes should be small at the opening and then widen out and be large arund the air filter area, as this will increase the speed on the air coming in.

I will keep you updated!
Yeah, I messed up by working with centimeters. A guy should stick to what he knows (inches).
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Old 09-28-2008, 06:04 AM
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The scoop in the photo would produce a ram air effect, if it is the leading edge into the air stream. If this is on a typical rod, then you have a roughly 3" radiator shell extending out in front that disturbs air flow.
There is not set amount of distance of boundary layer above a surface, there are too many variables.
Look at today's Pro Stock cars, you can bet that the wind tunnel has determined where their scoop opening needs to be for their application.
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Old 12-12-2008, 01:37 AM
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Decided to make an update on this.
Here is how I made my scoop and cold air intake:







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