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....--- Notes from John Burk &Tom Burkland ---
.......................--- On Air Inlet ("Scoop") Size ---
......... For a Spreedsheet that uses the formulas below
...........................................(CLICK HERE )
Important Note: Remember you are the final one resposible for what you use for an Air Inlet size. The following are thoughts on this subject you make the final decission on this.
John Burk presented the following two posts on Landracing.com in regards as how you figure the scoop inlet opening size for a motor:
For calculating the size of the scoop inlet you need to find how many inches you go in two turns of the engine . Say you have a 300 ci 200 mph roadster with a 3.5:1 rear and 30" tires . In two turns of the engine the wheels turn .57 times (2 divided by 3.5) . The tires are 94.2" around (30 x 3.14). So in two turns of the engine the roadster moves 53.7" (94.2 x .57) . The 300 ci x 53.7" long bar of air that got scooped up in 2 turns of the engine has a cross-section of 5.6 sq. in. (300 ci divided by 53.7") . The opening in the scoop should be 2.67" dia. or anything with 5.6 sq. in. This is for an unblown engine and for a trans with a 1:1 high gear . We'll forget about details like tire growth and engine volumetric efficiency but we should add 5% (5.6 + 5% = 5.88 sq. in.) for the supercharge effect we've gained at 200 mph .
(I then asked how you figure the drop of air speed in the inlet (scoop) if the cross-section of the inlet increases towards the motor)
Sum, If you want the air to slow from 200 mph to 40 mph (1/5 the speed) the area needs to increase 5 times . If the inlet is 5 sq.in. it should increase to 25 sq. in. If the inlet is 2 1/2" round (4.9 sq.in.) 5 times that is 24.53 sq.in. and the dia of that is 5.59".
The area of a circle is dia.x dia.x .785 The dia of an area is the area divided by .785 and the square root of that (divide before you hit the square root button).
By the way I think the inlet needs to be radiused for the lower gears when all the air doesn't come from straight ahead .
(The following are Tom's thoughts on the inlet along with another formula for figuring inlet size. His and John's give about the same numbers, but be careful as John's gives you the area of the opening and Tom's gives you the diameter of a round opening and you will then have to figure the area if you aren't using a round opening.)
First, a few thoughts on the intake as related to engine performance. I prefer round openings for their better entrance area to edge flow disturbance ratio. That being said you've also seen many examples of my inlets that are not round so don't feel this is an absolute requirement. The equation to calculate the capture area at the forward end of the inlet is:
Diameter (inches) = square root of (cid x gear ratio / tire diameter (inches)x 4.937)
This equation assumes volumetric efficiency of 100%, if you have manifold pressure the displacement will need to be multiplied by the pressure ratio (this is if you are running a supercharged or turbo motor under boost--Sum) or if your volumetric efficiency is lower than 100%, reduce the displacement proportionally. Your manifold pressure (gage pressure) will roughly double the effective engine displacement (this is in reference to running 14.7 lbs of boost), depending on actual density altitude at the track. This could also be viewed as a volumetric efficiency of 200%. The diameter derived from this equation is the inside diameter of the nozzle. If you want a nozzle other than round the capture area should match the capture area of the circle. Give a little thought to what happens to these inlet dimensions if the variables change at the track.These thoughts show up in the Nish streamliner (inlet) duct with its replaceable front nozzle sections to cover the many engine sizes and gear ratios they use.
The inlet's effect on the total aerodynamics of the car can be summed up as follows, reduce drag coefficient with the shape as much as possible, reduce wetted surface area (or increase it as little as possible), and attempt to move the CP as far aft as possible for your rear wheel drive application.
The other significant factor in higher powered cars is the lower speeds being typically run with lower overall gear ratios (transmission reduction, if used) so the torque output of the engines are intentionally reduced to prevent excessive wheel spin. Our streamliner, which may be a little on the extreme side of the power-to-weight ratio equation as far as Bonneville cars go, runs a 2.60 low gear in the transmission and on really good salt will spin all four tires above 12% throttle opening. At these low throttle openings the inlet size is still not the controlling factor in air flow. The inlet sizes that are correct at the higher speed ranges obviously would be severe limiters to total air flow at low speeds if you really wanted to produce peak power at these speeds. Terry Nish’s inlet uses a set of bypass check valves to allow the engine to draw additional air at the lower speeds and reach maximum power output. You may want to consider something like this for the small engine in the lakester since it may run at much higher throttle openings at slower speeds and shorter distances down the track. Constant flow fuel injectors with pressure reference in the plenum/diffuser will tend to hold the air-fuel ratio relatively consistent across the speed range regardless of how well the inlet achieves its pressure recovery. The other common “fix” for the part throttle, high rpm, low speed rich condition you discovered is a secondary bypass in the fuel system that is closed at high throttle opening by the barrel valve.
Tom Burkland
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