HOW MUCH TO CUT???

[1212FBGS]

Well Dick, a lot of air blowin around here, and no real answer. We can give ya a couple of good formulas if your good with math. I was gonna look on some NASCAR web sites today for a hole size but got 2 busy. good luck and let us know what ya found out.
kent

[dwarner]

"Unless the openining is around a foot above the hood surface, you are going after low pressure air, as ..."

2005 Rulebook, page 39, section 4.R

DW

[Bob Drury]

Dan, the point I was trying to make is that a forward facing opening,  low, as in abutting the hood surface, is probably less efficient than a rear facing opening.  At least, with a rear facing opening, oversizing is not a issue.  Thanx for the headsup anyway.   Now I am wondering how a aerodynamic pro stock type bubble would work with side mounted NACA duct openings.  It would sure look cool, anyway.  The obvious best would be Kents method, to pull off the front, but its not very practical in most cases.

[Rex Schimmer]

Probably no one has done more research into engine air scoops than the F1 teams,(subsonic that is) especially now that they all have their $60 million wind tunnels. Gleaning what little info you can from some of the data that appears in the mags that cover F1 I understand that inside an F1 scoop there are a lot of guide vanes, expansion areas, etc to try to get the air to slow down and also remain or become attached and hopefully something close to laminar. Old Dan Bernoulli's theories work best with laminar air flow, but he basically says that the pressure of the air at speed is inversely proportional to the velocity of the air speed square. So this tells us that we need to take the high speed air coming in the inlet and slow it down to regain some pressure. We would also like it to be directed, guided by vanes let's say, such that it does not become turbulent. This is also the reason that you need to place the scoop above the body work and in areas that are not turbulent, such as way above the body in air that has not been affected by the car body or way out front for the same reason. If the scoop is close to the body surface you need to be above the boundry layer as this is air that has lower velocity because of its realtionship to the body.

NACA ducts are a somewhat different challenge, as they work best in areas of the body to which the air is still attached and they work by generating two turbulent votices along each side of the duct that draw the boundry layer into the duct and accelerate it so you again need an expanding area to recover the pressure from the air velocity. From what I read the sizing on a NACA duct is very critical so putting in a real big one will reduce its efficiency and ability to recover pressure.

So let's all go over to Frog land and see if Renault will let us have a couple of days on their wind tunnel to work out some air scoops.

It's only money!

Rex

[John Burk]

Corrections  (1) Dave is right  , I should have added 5% to the scoop inlet area for 200 mph . (2) Math error on giving the dia for 7.56 sq in . The correct area for the 200 mph 400 cu in car would be 7.94 sq in (7.56 + 5%) or 3.179" dia . Exceptions are , very efficient engines add for VE and if the air slows getting to the scoop factor that in . Within reason too big on the scoop inlet is better than too small . The "supercharge" effect of a perfect scoop at 100 mph is 1.2%    150- 2.7%  200-4.9%  250-7.8%   300-10.9%   350-15.2%    400-19.4%

[Tom Bryant]

Well guys, in my opinion the jury is still out on scoop opening sizes. I have tried several different sizes and designs with varying results. I run a 302 cu in motor and we are inching toward 250 MPH. At present I run a Harwood scoop that has about 35 sq. inches of opening. The car performs well and the scoop pressures are running 0.0 to 1.0 inches of water less than the pressure on the front of the car. The EGTs are normally within 50 degrees of each other, so I don't think that turbulence is a problem.  At top rpm is the greatest difference. The engine still wants more air than is being delivered by the scoop. In 2001, I ran an opening of 5 sq. inches and the scoop reading reached the lower limit of the pressure (-9.6" H2O) measuring device by 6000 rpm and returned to 0.0 when the engine was shut down. At the Lakes in  November 2001, on the 3rd run, I cut the nose of that scoop off, giving me about 25 sq. inches of opening starting at hood level, and we got the first positive numbers (0.6" H2O at the top of high gear).

Having said all this, I do not argue with the engineering of the opening. But, there are just too many variables involved to make a flat statement about scoop sizes. I don't think that you want to argue that we are not posting some good numbers with 35 sq. inches of opening. Anyway, I am not through experimenting. I still want to see a pound or two of positive pressure. I believe that one of our problems is getting clean air to the scoop, I have a plan for next year to help correct that. The picture of the Stude is a good example of putting the opening in clean air. Our opening is only about 2 inches off the hood.

The bottom line is...you have to start somewhere. What works for me may not work for you. I have found that to be true throughout my racing career. There is a proverb that I keep in my mind "A person who walks in another's tracks leaves no footprints".

[Bob Drury]

Tom, thanks for your two cents worth.  I wish more guys would post results of their trial and error efforts.  As you so elequently stated, Nothing is for certain with a LSR vehicle.  It is nice that you have taken the steps of actually gleaning results. with recorded backup info.  Thanx again, it is great to have you on this site.

[John Burk]

Tom , I agree with Bob , those are valuable numbers you give . At 250 mph the scoop pressure would be 31" of water or 7.8% of atmospheric pressure if it was in perfect air. Wouldn't it be nice to click a button and see what a scoop that was the maximum 11" tall or out near the nose would do . There are 2 ways to get pressure from moving air , let the scoop create the pressure or get it from a spot where the car's body has created the pressure . On a car with a low Cd there is only a tiny point on the nose with full impact pressure and only a few sq in with 80% of the impact pressure . An 11" tall scoop doesn't need to add much frontal area to the car . It could be 2" wide with enough length to flow well but with a wide intake on toplike in the picture Sumner posted .

[1212FBGS]

the very center front of the car can be a stagnation point. the duct must protrude forward quite a ways to pick up good pressure. if the car has a decent grill and proper flow through the radiator the stagnation bubble can be a lot less and the duct shorter. over the hood ducts sometimes are way out of a positive airstream due to body shape deflection. sometimes these ducts have to be quite large just to catch what air is left after it is sent over the hood. I think stickin out the front would give the best results. thats what's cool about our sport, ya see a lot of people's idea of what they think will work.

[panic]

[1212FBGS]

on the earlie road race air boxes and Shawn Gann's prostock bike we added a reed valve close to the carbs. when the bike wasn't moving the reed would flap open with negative pressure. when the bike was at speed the valve would be shut.

[Tom Bryant]

I think that we all agree that a scoop opening that will feed adequate air to the engine in high gear is not large enough through the gears. I had a discussion with Terry Nish about his streamliner scoop a couple of years ago. The way Terry addressed this was to make a rubber flap in the floor of his scoop which opened to allow more air if needed, remaining closed when the opening provided needed air. Clever, I thought.

I plan to entend my scoop forward a foot or two and make a smaller opening. By extending the scoop on the same level as it is now, the slant of the hood will give it a few more inches above the hood. Hopefully this will put into clean air. The extention will be tapered from the current width to a 5 sq in opening. This will allow for cutting off the extion and expanding the opening until I get an opening that is optimal. That's the plan, we'll see what happens.

[John Burk]

Some thoughts on the theory of using the inertia of moving air to supercharge an engine . The pressure comes from the air decelerating inside the scoop . With an oversize inlet some of the air slows down and gives up it's energy before it gets inside the scoop . Panic's multiple small scoop idea with solenoid operated butterflys is perfect in all gears . A stagnation point is probably a high pressure area and should be a good place for an air inlet . Merry Christmas everybody

[Sumner]

The car performs well and the scoop pressures are running 0.0 to 1.0 inches of water less than the pressure on the front of the car. The EGTs are normally within 50 degrees of each other, so I don't think that turbulence is a problem.  At top rpm is the greatest difference. The engine still wants more air than is being delivered by the scoop. In 2001, I ran an opening of 5 sq. inches and the scoop reading reached the lower limit of the pressure (-9.6" H2O) measuring device by 6000 rpm and returned to 0.0 when the engine was shut down. At the Lakes in  November 2001, on the 3rd run, I cut the nose of that scoop off, giving me about 25 sq. inches of opening starting at hood level, and we got the first positive numbers (0.6" H2O at the top of high gear).

Tom that is interesting information.  Would you mind telling me/us how you are collecting that data.  We have an Innovative Wide band O2 sensor and the control box for it lets us data log up to 5 other inputs besides the air/fuel (they have to be 0-5 volt inputs).  Last year we data logged throttle position and this year we hope to add rpm, boost and EGT.  It would be interesting to also monitor the inlet pressures like you are doing.

Thanks for the info,

Sum

[dwarner]

Our driver says we don't need to data log throttle position. He is always flat, never lifts.

DW