Hood Scoop Opinions....

[jimmy six]

There are some specific demensions that should be used. I discussed this with Dave Dahlgren about 5 years ago to put on my roadster. The owner of the enging did not want me to cut up the hood or side panel so I did not do it. It was for an inline 325" and it's demensions were for 5000 rpm. Dave helped me with the demensions of the lower plate buy using the butterfly size. Same for stepping it so the front cylinders would not be siphoned. It sounds funny but the theory is to make pressurised "dead" air with a naturally aspirated engine. Good Luck

[Rick Byrnes]

"Dyno"

I had thought about running a NACA duct on my Roadster but in talking to some guys very knowledgible in aerodynamics I learned that they are only effective on long flat surfaces like on Stainless' car.







I recently had a lengthly discussion with my aero guy over the hard points on the liner.  His take on NACA ducts are that even for a 3 litre engine it would have to be very large, and "properly designed" which means the correct section changes, length and air foil of the floor.  It seems that Stainless has a very proven design, and the boundry layer must not be too thick at that point on the car.

He advises me to stick with the simple scoop that I had done after eleminating the big top scoop.  Since with the liner, engine placement has not changed the basic scoop and air box will remain.  Streamlined dramatically, but still the same design intent.

Many changes.  Rear suspension 90% done.  Front started.  Spindles and hubs being machined now, and body design begins in two weeks after digitizing the entire chassis with the saved body work.  (from foot box to 4 feet behind main hoop.)
I cant afford the most swoopy body, and I WILL NOT do an extreme laydown car, so we will be quite conventional in design.
I once heard some one say, "there is a good reason the Russian and American space efforts looked alike."  (thanks buddy, you continue to be a voice of reason)

I will follow this thread with great interest.
Thanks Mike for starting it.

[Sumner]

We made the scoop opening on Hooleys stude........



...... using the formulas on my site that I got elsewhere..........

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

........... and oversized it about 10-15%.  John did an excellent job of making it and I think it is beautiful and really makes the car unique myself.  It has worked fine, but is the only one we have used since going to blown gas and so have nothing to compare it to.

I'm not in favor of a scoop that isn't at least 6 inches away from surrounding body work and would like to see it out there in clean air either in front of or above the body and that is what I'm doing with the lakester.....

 

(Work on it starts here:  http://purplesagetradingpost.com/sumner/bvillecar-3/construction%20page-116.html)

I considered a NACA type duct but was afraid I might not get it right, obviously Stainless and crew have.  I looked at Mike's on Act Attack and thought about that, but he told me (hope I get this right Mike) that he wouldn't do one on an un-blown motor and since I hope to run both blown and un-blown I decided to stick with a more conventional intake.  Considering that I personally (no proven data) believe that the intake track isn't as critically on a blown application where the blower can suck and somewhat compensate for bad design vs. a NA motor.

I will have individual inlets to put on my intake in front of what is shown in the picture above.  Actually the smaller motors will need larger ones since the car will be traveling slower and the column of air that the motor picks up in 2 revolutions of the crank will be less.  Mike with the stude we aren't able to use more than 30-40% throttle in 1st and 2nd due to HP vs. traction, so I don't think having the more restrictive opening has hurt us at slower speeds, but your results might vary.  On my lakester with a small motor and a lot of weight for traction I might need more air at slower speeds and might have to do something different than I have (additional inlet openings like some have done that close at higher speeds).

Here is a scoop I designed and built also with the formulas on my site..........



........... with construction details here........

http://purplesagetradingpost.com/sumner/temp-pictures/temp-web-nc-1.html

............ great subject and one that is hard to document the results.

c ya,

Sum

[Dean Los Angeles]

The key phrase here is computational flow dynamics (CFD)

TORNADO is a multiple-block, implicit, finite-difference 2-D Navier-Stokes code, developed through an on-going collaboration with the University of Toronto Institute for Aerospace Studies. Turbulence models implemented in the code include the Baldwin-Barth, Spalart-Allmaras, and Mentor's SST model. The implementation of matrix dissipation improves the accuracy of drag prediction, and the multigrid technique provides significant convergence acceleration. In the CFD 96 code validation exercise, TORNADO was shown to be competitive with some of the most respected 2-0 aerodynamics codes in the world.

Did ya get all that? If you want to spend the money . . .


Where you put the air inlet, (and it isn't necessarily a "hood scoop" depending on the vehicle,) matters a great deal. If you look at a hood scoop, that is a handy place right above the inlet to the engine. It isn't necessarily the optimal location.

Clean, non-turbulent air from a high-pressure area is what you would like.

The round inlet pictured on one of the cars above isn't the optimal shape.

As the inlet gets wider the base also gets wider and adds drag. By changing the round shape to the one pictured above the inlet area gets larger without making as much drag at the base.

The Hooley Stude is a good example of not only making an inlet but improving the aero on the car while they were at it.

Gratuitous jet inlet photo thrown in for no reason:



You can bet that the F1 teams are using CFD for the multiple inlets.


Jason Line's pro stock.  As successful as they are at 200 mph I'm going to guess there was some wind tunnel work done.

[Rex Schimmer]

Dean,
The scoop designs that you have shown are for drag racing which have completely different requirements than landspeed racing. Their needs are to have unrestricted air inlet available at low speeds when they are developing maximum horsepower and they have very little concern for the aerodynamic drag created by the scoop at their maximum speed. The requirements for landspeed racing is to have a scoop that is sized to match the engines inlet requirements for the projected maximum speed of the car/motorcycle and be most efficient at that speed. If the scoop is to large then the excess air that is not taken into the engine is spilled around the inlet and generate large,high drag vortexes that take horse power to drag along with the car. The other requirements  that a properly designed inlet needs to provide is expansion of the inlet tract  volume to slow down the air velocity to regain pressure (Bernoulli still rules here) and it needs to guide the incoming air into the engine inlets without causing internal vortex generation.
 You are right about the shape of the inlet on Rick Byrnes lakester not being "correct". In my view the actual inlet is very correct in shape, with a very well rounded surface going into the inlet the problem that I have with the scoop is that it is to close to the car body and nothing is done to make its shape low drag it is a nice big brick with a hole in it mounted on the side of Rick's car. I am sure that if you ran this car in the wind tunnel you would see a large low pressure vortex being formed on the top and the bottom of scoop and trail down the intersection of the scoop to the body.

Rick, regarding your "aero guys"comments about NACA ducts I would ask if he has ever designed one? NACA ducts can be very efficient, as proven by Stainless and his lakester,  and very low drag but they operate under the same requirements of a scoop that projects from the car bodies surface, they need to expand into a large volume to slow the air down and recover pressure and a  properly designed NACA duct must placed in the correct area of the body to be efficient and if properly done they can be every bit as efficient as a projecting scoop.

Rex

[4-barrel Mike]

I saw this one on the web, and wondered...



...but it won't be really fast 

Mike

[jdincau]

     I have the NACA report on submerged inlets, NACA ACR No. 5120(AKA NACA ducts) on page 18 it says;
"Submerged inlets do not appear to have desirable pressure recovery charicteristics for use in supplying air to oil coolers, radiators or carburetors of conventional reciprocating engines. "
     If you really want to read all 57 pages I will PM it to the head guy here and he can put it up for all to read

[McRat]

So if I'm reading this right:

You want the inlet area to equal the column of air at the MPH you want to go based on the air consumption of the engine?

So if I eat 140lb/min of air, and I want to go 200mph, I need 15.3 sqin of inlet?  About a 4.4" dia circle, or 3.9" square.

If you miss your guess, which direction do you want the error in?  Bigger or smaller?

[Tzoom]

         If you really want to read all 57 pages I will PM it to the head guy here and he can put it up for all to read

Or you can go here:  http://www.n91cz.com/Interesting_Technical_Reports/NACA-ACR-5120.pdf

[JohnLevie]

There is a high pressure area under the windshield, on the cowl of most stock bodied cars, right? Hence the invention of the cowl intake...did it work?  How will using the cowl as an intake work on a comp coupe?  Nascars intake from the cowl, I believe.  They approach or exceed 200 on super speedways.  What about using a hood scoop to smooth out the transition of hood to windshield.  Hooley's Stude scoop goes clear to the roof.  I am trying to figure this out myself, without a windtunnel or an aeronautical degree. 

[Sumner]

.....The Hooley Stude is a good example of not only making an inlet but improving the aero on the car while they were at it............

That was the idea.  If we are going to have a scoop use it to the max aero advantage you can get from it.  We are blown so there is no height requirement that has to be meet.  I feel that the frontal area of the scoop has a better Cd than the windshield that it coves up, but can't prove it.  Now the scoop I designed for Phil's car could only be so high due to the fact he is running NA.  I still did what I could to improve the Cd with it in front of the windshield.

So if I'm reading this right:

You want the inlet area to equal the column of air at the MPH you want to go based on the air consumption of the engine?

So if I eat 140lb/min of air, and I want to go 200mph, I need 15.3 sqin of inlet?  About a 4.4" dia circle, or 3.9" square.

If you miss your guess, which direction do you want the error in?  Bigger or smaller?

Did you go to the formulas on my page I linked to and read this stuff.........

http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html

.......... I'd figure a little larger, but remember that smaller will just cost you some HP.  The size is going to depend on tire size and gearing as you are trying to capture the air in front of the scoop in the distance a car will travel in two revolutions of the motor.  That is the air that it needs to fill the cylinders in those 2 revolutions since all the cylinders would have fired on a 4 stroke.  Remember that you will need more air if blown than NA in those 2 revolutions.

When you figure this the scoop frontal area will look awful small and all your buddies will tell you it won't work, but remember it is like Rex said, we can't use the HP off the starting line in most cases.

c ya,

Sum

[Sumner]

............  Hooley's Stude scoop goes clear to the roof..........

Yep  and so would Phil's...........



................... if the rules permited, or the roof would of come down more  ,

Sum

[WZ JUNK]

I remember the first year we showed up with the new scoop on Hooley's car.  Dan came by to check the height of the scoop.  Someone had commented to him that the scoop was to tall and not legal.  I know that we determined that in comp coupe there is not a rule that would make this scoop illegal but I think there is in some of the other classes.  You might keep this in mind when you are designing a scoop.

When I made the scoop on Hooley's car, I made a template of the roof of the car and I carried that arc forward to the inlet.  My thought was to have a smooth continuous path for the air flowing over the scoop and the top of the car.  The scoop just covers the blower assembly.  I tried to keep it as tight as possible to minimize the amount it would interfear with the windshield area.  It really looks to big but it is as small as I could make it.

I made the internal part of the scoop and the flexible adapter to the Enderle injector first.  Then I made the external part of the scoop around the inlet.  I started from the inside and worked out.

John

[Sumner]

...............When I made the scoop on Hooley's car, I made a template of the roof of the car and I carried that arc forward to the inlet..........

Here is a link to John making the scoop.........



http://purplesagetradingpost.com/sumner/hooley/hooley-construction-2005-1.html

Sum

[Stainless1]

Rick B, the scoop on the lakester picture looks like it extends past the inner plane of the tires....  We tried one (an outie) like that on the side of the car years ago just to see if there was a pressure difference in the box.  Race officials told us it violated the lakester rules... 

And guys there are still a lot of NACA ducts being put on modern airplanes for intercoolers and oil coolers....