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Hood scoop/ pressure question

Shocks, Springs, Brakes, Frame, Body Work, etc

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Hood scoop/ pressure question

Post by sgcuda » Sat Jul 08, 2017 4:35 pm

Running a dual 850 setup on a tunnel ram 500 cid Mopar. Planning on running low 8's. I put an old school snorkel on top that has 2 1" tall half moon baffles, 1 in front of each carb. Snorkle opening is 2 x 13. I have been told that a couple of pressure relief holes on the backside of the snorkel would help the airflow into the carbs. I have seen this done on street engines, but never on a full drag race car. Opinions?

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Re: Hood scoop/ pressure question

Post by Greenlight » Thu Jul 27, 2017 3:47 pm

A properly designed air scoop will always perform the best when it is sealed and the pressure inside is the highest possible. A properly designed air inlet system converts high velocity low pressure air to low velocity high pressure air (a very small supercharger). Proper sizing of the inlet and shape of the scoop, will determine whether or not the air scoop provides an overall ET gain.

An improperly designed air scoop MAY perform better with reduced pressure (i.e. add holes in the back).
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Re: Hood scoop/ pressure question

Post by panic » Fri Jul 28, 2017 4:24 pm

The pressure/speed conversion is simply the X-area of the entry vs. the X-area directly above the air-horn, and enough distance from A to B and volume for the conversion to take place.
Remember that terminal speed determines entry size: faster = smaller.
There's probably a ratio for both, but I don't have them. The SCTA site does: http://www.landracing.com/forum/index.p ... 718.0.html
Ideally, the air entry would resemble the dynamic cone retraction mask used on the SR71, which restricts the area at speed while retaining a streamlined shape.

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Re: Hood scoop/ pressure question

Post by BirdMan » Wed Aug 16, 2017 4:14 pm

My son and I used small clear tubing attached to hood at various locations and heights. We had several at a time and the tubing faced forward on the hood and inside the car made vertical manometers with colored alcohol/water mix. We taped the tubing onto a board/cardboard and after positioning/attaching the board we marked 0 (where the level was at rest, like the flow bench ones) and drove the car on the interstate at varying speeds and with/against the wind to determine the location for the scoop opening of our handmade thin alum scoops, my car still has it on it and will be on it when I convert to street driving but with an air filter somewhere to get clean air.
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Post by dwilliams » Sat Sep 02, 2017 1:14 pm

I did the same thing to find the highest-pressure spot to put a NACA duct on the hood of my RX7. I just used water and food coloring, but alcohol would be less dense and therefore give a larger change of column height for the same pressure.

1/8" clear vinyl tubing is what I used.

While you have all the manometer lines taped to the car, it can be instructive to move them around and check the pressure under the engine bay, ahead of the front spoiler, ahead of the radiator, in the engine compartment, in the wheel wells, and in the front and back of the front fenders, and write the numbers down for future reference in case you ever want to add brake ducts or need to cut some exhaust vents to cure an overheating problem.

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