HOW MUCH TO CUT???

[dick elliott]

When putting a rear opening hood scoop on a race car, how close to the windshield should the scoop be, and should the hole cut in the hood be as big as the scoop, or only as big as the air cleaner or stack? Thanks, DICK.

[dick elliott]

Come on now! Someone out there must have though about this before, seeing how 1/2 the cars out there, have a rear opening scoop. Thanks for any and all ideas.

[John Burk]

Dick , there's  more pressure at the base of the windshield so that's where you want the opening to be . If the sides of the scoop are vertical cut off lower rear corners so you'd have a triangular inlet mabe 15 si per side . If your windshield has a lot of slant there won't be much pressure there and a foreward opening scoop would be better . The opening si for a front facing scoop should be how many ci the engine uses in 1 minute divided by how far the car goes in 1 minute in high gear .

[1212FBGS]

can ya give us an example of your math?
thanks
kent

[John Burk]

A scoop needs to be sized for high gear because you're in that gear the longest and because too big is bad . The formula is air used divided by distance traveled . If the engine uses 26,666 cu in/sec (8000rpm divided x 60= 133 revs/sec x 200 cu in /rev) and the car goes 3528 in/sec (200 mph=294 f/s x 12) 26,666 cu in divided by 3528 in = 7.56 sq in = 2 7/16" dia . Sounds too small but if it's bigger turbulance will screw up the flow . Radius the inlet to help the flow in the lower gears . If you want make the inlet bigger and make a 2 7/16" dia reducer that sticks out and try it with and without . It'll pull stronger in high gear with the reducer in place . The plenum should be bigger , just the inlet should be small .

[1212FBGS]

cool, sounds good 2 me. Probably one of the more logical formulas I've seen. When i started designing motorcycle air boxes in the earlie 90 I came on a research paper from WW2 airplane air intake formulas and the conclusion they came up with in the 40's was 1.5 x intake valve size. So if ya take a sbc with 2.02 intakes then your 2 7/16 is pretty close a little small but pretty close. of course that's all top end pressure. I had to design around bottom and mid range air flow restrictions for bikes pulling off the corners so we went quite a bit bigger. except for tracks like Daytona and RD America, we had a  cover that riveted over the hole making it smaller. Modern bikes red line at 13 to 15k, too small of air intake and the bikes fall on there nose. likewise too big of air box and they never pressurize.
Dick, depending on the front end design of your car, make sure there is at least 5" if clearance between the bell mouth of the carb and the inside of your hood. if you want to run an under hood duct work, you can usually find a tremendous amount of air packed in front of the radiator. just mold your duct under the hood and run it forward to about 2" in front and over the top of the radiator. then seal off the sides of the duct so air doesn't bleed off and around the sides and still forces air through the radiator. this duct size can be about 1 1/2" tall and about 12" wide and won't cause any exterior body top end drag. the reason I sugest a bigger opening is so the car will pull off the line and make it to high gear. and putting the bigger duct within the existing body openings won't cause any aditional drag.
kr

[ddahlgren]

Ah the infamous stack of air theory. All it does not account for is the VE of the engine, the Cd of the scoop opening, the reynolds number related to the sccop opening profiles, the boundy layer effects of adjacent panels, the pressure recovery of the scoop design and a bunch of other things. Other than that it works just fine.
dave

[Bob Drury]

Yep, all them Nascar Boys, and Pro Stock boys are all idiots, cause not one of em follows that theory................lol

[1212FBGS]

so what is your answer on his question..... dave! this guy has been asking for help since May!
Bob... nascar boys were running bench seats til the mid 80's and they cant have big protruding ducts or scoops. They wouldn't look "stock". They outlawed the headlight ducts in the 70's. Prostockers set at the lights with the motor at redline, they "have" to use a different formula or the car wont launch, ya see mph is just a bonus for them, et is what there after. so whats your answer bob? Our prostock bikes run an air intake larger the a prostock car, big deal. this guy came on a LAND RACING website for advise. lets help this guy out. I say 1 1/2 by 12 out the front of the car but reading his question he has a windshield duct. anybody ever done it?? anybody ever measured a nascar duct opening??

[John Burk]

Bob , Prostock cars size their scoops for the lower gears because ther're primarily interested in et . Do NASCAR cars have hood scoops . Don't they just have round air cleaners .

[Sumner]

   

The first quote below is what John posted last year and below that is what Tom Burkland sent to me when we were figuring the scoop for Hooley' Stude.  They both give about the same answer.  I think I upped them about 10% in size.

So far it is working for us, except we were never able to get in a full throttle run due to the condition of the salt.  He did run 212 at a final 30% throttle opening on the last run.  The 30% was the largest throttle opening at any point on any of the runs.  Like Dave says there is a bunch of other things that are going on, but for most of us you have to have a starting point (better than doing nothing) and I think these formulas have worked for others.

Of Course none of this reflects on the base of the windshield location that the post was about.

c ya,  Sum

John:

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 crossection 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 .

Tom:

The equation to calculate the capture area at the forward end of the inlet is:

 

Diameter (inches) = square root of (cidxgear 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 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, 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.

Changes to the Stude for 2005

[ddahlgren]

The only way to check the effectiveness of a hood scoop is to instrument it completly. There is no easy answer at all. Most good race engines the VE is about 110% The best you might hope for on an effeciency for the scoop is probably 0.90. So i would start with an oversize of 20% right off the bat and more if it is close to other bodywork.
Dave

[Bob Drury]

John, I don't disagree with your front facing formula, however, you failed to mention scoop height above the hood.  Unless the openining is around a foot above the hood surface, you are going after low pressure air, as the flow doesn't follow the nose contour.  Just my thoughts............

[Bob Drury]

And, no, Nascar runs a airbox to the vent at the bottom of the windshield, and has for many years.

[Bob Drury]

John and Dave, I wonder if any one has done a wind tunnel test on a pro stock type hood bubble with and without the opening in the front?  The reason I ask, is because I wonder what the opening does to the c.d. versus using a rear opening with the same hood bubble.  Granted, you would lose the "supercharging " effect, but  we can only theorize on what that gain might be.  We are still dealing with atmospheric pressure here on a normally aspirated motor, and a blown motor shouldn't care.  It sure would be fun to have a wind tunnel.............