Hood Scoop Opinions....

[jl222]

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.

Dean
I've noticed that the most modern aero prop engines use the same intake as the one pictured above, they are big engines but use a lot less cfm than you would believe because most only turn at 2800 rpm.

      JL222

  Scroll up to green aero prop engine

[Rex Schimmer]

jdincau,
You are correct that NACA Paper No. 5120 stated on page 19 "Submerged inlets do not appear to have desirable pressure recovery characteristics for use in supplying air to oil coolers, radiator, or carburetors of conventional reciprocating engines." Regretfully you forgot to read the remainder of the conclusion "The required diffusion of the air and the range of inlet-velocity ratios is to great to give desirable characteristics at all flight conditions."  and further it states "In conclusion, it should be stated that the submerged entrances have a definite advantage over other inlet types for certain inlet and air-flow requirements. The design of such inlets is more critical than that of other types because of the effects of the boundary-layer thickness and local velocity fields."

What this is saying is that for an air inlet that is efficient at all speeds the submerged duct inlet is not the one of choice but for specific speeds and locations it is superior to other designs. So it gets back to the fact that a landspeed car will have a specific target speed and if the submerged duct is properly designed for that speed and also place in an area of the body that has attached flow and a thin boundary layer it will be a very acceptable air inlet, again as the submerged duct on the Stainless' Bockscar so well proves.

Tzoom, thanks for posting the link, I have downloaded for future reference.

Rex

[Sumner]

.............So it gets back to the fact that a landspeed car will have a specific target speed and if the submerged duct is properly designed for that speed and also place in an area of the body that has attached flow and a thin boundary layer it will be a very acceptable air inlet..............

Too many if's for me unless I had access to a wind tunnel or was copying the Bockscar (Stainless), but you will probably make it work  .  What if your target speeds are almost 100 mph apart running different size engines and also NA and blown?

In my case I wanted somewhat of a fin/tail on the top of the car and hopefully the scoop won't add much to the frontal area if most of the air is going to the engine and now I have my fin/tail,

Sum

[Bville701]

What would someone suggest for someone with a modified roadster that wanted to run both NA and blown?

I like this post. I'm learning lots.   

[interested bystander]

Probly more that 2 cents worth on the subject.

There's a lot of data out there. YOU just need to  resarch it on the net -  F-1, ALMS , CFD examples, and so forth . You can draw your own conclusions. In spite of this being a GREAT website  for technical advice, you're gonna get as many opinions as the BLIND MEN AND THE ELEPHANT legend about air inlets.

The easy answer just follow Jimmy Sixe's statement a few posts back. I.E.:

Build a system so the intake (Carb throat(s), velocity stacks, blower inlet, etc.) sees dead -  high presure air!. Simple.

Re Rex's comments:  he's right about not emulating drag race scoops. At least on engines in front of the driver in CLASS racing it's hard to optimize because of the eleven inch above the carburetor top and the scoop top rule. That makes it a real challenge - I'm sure they try REAL hard at the top level to work around that rule.

K&N do have a developoment effort going on, by the way, on that subject.

The old Chevy Power book by Bill Jenkins made a statement that the scoop entry only needs to feed enough air to allow the engine to leave the starting line or words to that effect. Air speed down track will take care of the rest.
Sorta like Nish's tiny opening. Food for thought.

Think exhaust port short turns also.

[jl222]

What would someone suggest for someone with a modified roadster that wanted to run both NA and blown?

I like this post. I'm learning lots.   

How about a picture of the car, it would depend on what you have to work with.
I would learn as much as I could from the prostock guys that I could, lots of money spent there.
Carroll Smith shows how scoops and naca ducts should be built in one of his books Tune To Win, basecally shows tubulent flow unless scoop gets wider from opening and is about 1 1/2'' wider at carb or injector opening on all sides [unblown engines]

   Good Luck JL222

[MAZDA1807]

We ran a big pro stock scoop in the beginning with the Mazda and it proved to be non efficient.  We found this out by mistakingly leaving the plug in the scoop and going faster.  After that we ran a smaller opening in the scoop.
                                                                     peter

[Bville701]

What would someone suggest for someone with a modified roadster that wanted to run both NA and blown?

I like this post. I'm learning lots.   

How about a picture of the car, it would depend on what you have to work with.
I would learn as much as I could from the prostock guys that I could, lots of money spent there.
Carroll Smith shows how scoops and naca ducts should be built in one of his books Tune To Win, basecally shows tubulent flow unless scoop gets wider from opening and is about 1 1/2'' wider at carb or injector opening on all sides [unblown engines]

   Good Luck JL222

Here is a photo of the car.

[jl222]

We ran a big pro stock scoop in the beginning with the Mazda and it proved to be non efficient.  We found this out by mistakingly leaving the plug in the scoop and going faster.  After that we ran a smaller opening in the scoop.
                                                                     peter

How did the engine get air?

   JL222

[Seldom Seen Slim]

JL, in the "drawing" of the prop engine (in green) -- isn't that a turboprop  That is, turbine spinning a propeller through gears?  In which case the motor is likely spinning way more than 2800 rpm, and I don't know how much air a turbine engine uses vs. a piston engine, so maybe the comparison isn't valid.

[jb2]

Hey all,
involved somewhat from a distance with the Byrnes car.  It is in process of changing from lakester to liner so the location of the scoop in relationship to wheel plane is inconsequential.  Also, being a work in progress, it will have body work surrounding the intake. 

Jim

[tortoise]

Re scoop blending into roof line:

Freiburger wrote up the wind tunnel testing on his Camaro in HOT ROD  a few years ago and said that if the windshield is at least 45 degrees from the vertical nothing is gained by putting something in front of it to increase this angle. If he's right, than a scoop like on the Stude moves the CP forward a bunch (bad thing) without decreasing drag.

[Sumner]

Re scoop blending into roof line:

Freiburger wrote up the wind tunnel testing on his Camaro in HOT ROD  a few years ago and said that if the windshield is at least 45 degrees from the vertical nothing is gained by putting something in front of it to increase this angle. If he's right, than a scoop like on the Stude moves the CP forward a bunch (bad thing) without decreasing drag.

Your right about the CP and the reason we didn't go with another design that would have picked the air up down by the nose.  Still the scoop is not just getting air it is covering up an 8-71 blower and all the crap associated with it, so I think it is better than having all of that out in the air along with the "bug catcher" intake.  I read the Hot Rod article and regardless if I could have a windshield that sloped back at 10 degrees and it would fit the rules I'd do it.

If, when we go turbos the look will change.  Would you mind doing us a rendering of what you would do in our case with the present blower?  We are always open to suggestions,

Sum

[Sumner]

What would someone suggest for someone with a modified roadster that wanted to run both NA and blown?

I like this post. I'm learning lots.   

I'll go first, so everyone fire away........



........... of course I don't remember the present intake track or any of that, so this might not be practical at all,

Sum

[Rick Byrnes]

As I wrote in the earlier note, the car is now a streamliner.  However, as a lakester that entire component is bolted to the engine, not the body, and the discussions in impound may have been interesting.  It wouldn't be the first time I got kicked out of impound

But, everything is different now.
The body lines around the scoop will be changed to enhance flow on that side.

Rex, yes. and he made his living for many years in the auto industry providing council to the stylists to "try" to improve aerodynamics of CARS.  Very educated, very experienced in theory as well as hands on in Lockheed wind tunnel.
If you re read my note, relative to his statements its about correct design, and I readily acknowledge that the NACA duct that Stainless uses is very good, and well researched.

The design of the "mail box" that you see in the photos is just what JD described and was not intended to be run without body changes.  Years ago we called that type of plenum a "no-flow" manifold because it has virtually no velocity.  Just a couple columns of dead air above each inlet bell.  The box can easily be made taller, but since I am changing the body to provide more clearance around the inlet I did not think that necessary.
If we see in CFD that the air requires management around the inlet, that can be done.

I do have an alternate design that eliminates the side HDRA scoop, but it needs to be replaced by another on top but the engine looses 6 horse power.  For the first iteration as a liner I have not been prepared to do a NACA duct because of available area under the skin.  The development of a "better" air inlet can and will be an "Evolution" kind of thing.  Right now, I just want to get back to doing some testing with the car, then back to the salt.
First, body shape, size and all the other macro items need to be completed.

I do understand the advantage of a "no drag" air inlet but already have enough gremlins hiding everywhere.  I understand the elimination of the HDRA scoop is worth more than 6 HP, but again, only so many things at one time.

I have so many changes that it is practically a NEW car.

Oh, I already have 4 NACA ducts on the car, so I am a believer.