Weight and Bonneville:

[Rex Schimmer]

Scoops:
If you make a air scoop that expands in area and volume to slow the inlet speed down you will recover pressure BUT you also need to make sure that the air is directed toward your engine inlets so you need internal gudie vanes and splitters to insure that the air that is coming in is made to go in the direction of the engine inlets. If your scoop has sharp corners and areas that the air can "pile up" in then it will not recover the pressure your are looking for to assist the engine. What is inside the scoop can be as important as its shape.

Weight:
There is obviously a "perfect" balance point where the weight of the car times the coefficient of friction of the tire on the salt is equal to the aero drag and if you can exactly meet that spot then you have the optimum weight for your race car. I would think that being on the "heavy" side of this point would be the best. At your cars maximum aero speed much more horse power is going to over come aero drag than accelerate the car. Look at the cars that run in the 350+ area, they are accelerating maybe 20 mph in the last mile, do the math how much HP does it take to accelerate a 4-5000 lb car 20 mph in a mile as compared to how much HP does it take to over come the aero drag! Not even a contest. Obvioiusly car weight has an affect during the initial acceleration but once you are over 200 the aero HP requirement becomes vastly larger than the acceleration hp.

[ddahlgren]

You may want to look at this NACA report:

http://naca.larc.nasa.gov/reports/1927/naca-report-247/naca-report-247.pdf

This link did not work for me. Might make interesting reading what is the full title..
Dave

[hotrod]

Try the UK mirror:

http://naca.central.cranfield.ac.uk/reports/1927/naca-report-247.pdf

NASA  "improved" their web site and broke most of those older links.

Full title is "Pressure of air on coming to rest from various speeds"

To find the NACA reports easily through google do a search that includes the key NACA, the year of the report, and the report number like this;

+NACA +1927 +247

Put that string (or the appropriate values for a different report) in google and you can quickly find any of the reports if you know the report number.


Or

Google on the full report title

Or

You can use the base link for the UK mirror which still has the old format that lists the reports by year of issue. Its a lot of fun to browse through them and a TON of useful info for high performance piston engine people, especially during the war years and shortly after 1939 - 1947.

Larry

[hotrod]

I thought some of you might find the following information useful.

Source:
Chrysler Corporation Direct Connection series booklet titled:
Mopar Chassis Carrol Shelby's Speed Secrets

part # P4349341
1984

The most important consideration with any of these hood scoop designs is that the carburetor
must be sealed to the scoop. If the scoop is sealed to the hood, than the carburetor can be sealed to the underside of the Hood by using foam rubber.

If the carburetor is not sealed to the hood, the engine will pull hot-air from the inside the
engine compartment and he hood scoop will be virtually useless.

For an all-out race car in Pro Stock, Modified Production or Formula Stock, the boundary layer bleed-off or "Bauman" scoop
(named for the Chrysler induction expert that developed it) should be used (PN P3690153).

This scoop should be located as shown in Figure 16-6. It is imperative that the carburetor be well sealed
to the hood scoop. It is also very important that carburetors be depressed the .50" - .75" below the scoop as shown in Figure 16-6.

The forced-air aspect is more difficult. The scoops shown in Figures 16-2, 16-3,16-4 and 16-5 along with the shaker and air grabber designs,
will provide forced air to the carburetor. Forward facing and a 30 square inch opening are the important considerations. More area helps low speed and
smaller helps high speed. This is why the Pro Stock "Bauman" scoop has smaller openings than the 1968 and 1969 style scoops.

To test a scoop to see if it is functioning properly, the car should be base-lined with the scoop sealed and working. Then the scoop opening
should be taped over and the seal between the hood and the carburetor base plate removed so that the under-hood
air can easily reach the carburetor inlet. The same carburetor base plate should be used. It should not be run without the base plate
even though this may be convenient. Without the scoop the car should go 1 to 1.5 mph slower than before with the
scoop functioning. Be careful using this simple test too literally.
Adding a scoop to the carburetor system leans out the carburetor approximately 3 jet sizes.

If this carburetor adjustment is made to make the carburetor work its best with the scoop, then when the scoop is
blocked and under-hood air is used, the carburetor will now be too rich.

For reference regarding the scoop opening of 30 in^2, at the time this book was written the fastest builds they described were 9.8 second cars which should have trapped at about 137-140 mph.

However the most important aerodynamic consideration is that the scoop not change its shape because of the air
going into it or the air passing around it. This can unseal the scoop or allow the scoop opening to deform, restricting the air flow.

Larry