Air Box Design

[836dstr]

Help !

I am planning on changing from a hood mounted scoop on a Street Roadster to a air duct thru the upper front of a '32 grille shell into a air box.

 I will have some options closing out the current hood opening from a flat (or stock hood surface) to some type of "bubble" to get the optimum distance above the carburator neck.

Here are some specifics:
1. '32 grille shell to a '27 body with some taper front to back.

2. 301 C.I. Chevy using a Victor Jr. manifold with 750 Demon 4 bbl.
Anticipated performance 7,500 RPM @ around 185 MPH.

3. Air duct openning 6.6 Cu. In. I plan to make it an elongated "D" shape with the flat side down. It would be 5" wide and 1 & 1/2' tall @ the center with a .60" radius @ each end.

4. The air duct would extend 5 or 6 inches in front of the grille shell and would have a constant cross section for another 6 inches to the front hood line. I currently have oblong plate 17" long by 11 inches wide attached to a air horn that has a radius necking down from 7" dia. to 5".

5. The air horn is 1 & 3/4" tall. The distance of the plate to hood tapers from 2" in front to 2 & 1/2" rear for an average total height of 4".

6. I plan on building the air box to taper back from the end of the air duct from 5" to 11" wide @ the front of the current carb plate. This transition for the air box would be 11" long and maintain the width for 17" to the back of the plate. The air box would be sealed to the underside of the hood.

After all that, my question is, "Is the average height of 4" of the plenum above the carb to the hood adequate?

Most of this will be fiberglass construction so it will be easy to add a radius at the bottom or top of the air box to eliminate 90 degree corners if that would help. Well, I guess that's another question.

Any questions or comments would be greatly appreciated !

Tom

[Stan Back]

I dunknow!

But make sure you don't eliminate any of the Radiator Area with the duct.

Stan

[roygoodwin]

"Air duct openning 6.6 Cu. In." -- I assume that's really 6.6 Sq in ?   I think you'd want a radiused opening at the open end rather than just the sharp edge of the sheet metal/fiberglass.

I'm having trouble visualizing the air box where it's under the hood.  especially the "The distance of the plate to hood tapers from 2" in front to 2 & 1/2" rear for an average total height of 4"  I guess you're adding the 1 & 3/4" tall air horn to the 2"/2 & 1/2"   to get to 4" average ?    Maybe a sketch ?  

roy

[836dstr]

Stan,

The air duct would be above the grille insert, and the current radiator fills the whole grill shell openning and would not violate the area or contour of the grill shell for the purpose of streamlining. I checked with the SCTA Roadster Committee Chairman.

Roy,

You're correct, I meant 6.6 Sq. Inches. At the vertical centerline of the carb the height from gasket surface of the carb plus the air horn (bell) to the underside of the hood is a total of 4". A rounded or "bull nose" entry to the air duct would be easy enough to fabricate. Would this be better than a knife edge?

I made a feeble attempt on the photo of the car to erase the scoop and add the air duct using the limited photo editing feature on my PC. This was my first attempt. A sketch might be possible. Maybe I'll have to ask the kids how to do it. I hate doing that !

Thanks for the comments !

Tom

[Stan Back]

Tom --

Ye're good for all I can see.  I was just trying to see if you knew about the Radiator thing before you got in trouble.

Your sketch is better than I could produce, but it looked like it would be in part of the radiator.

Good luck on that soft record!

Stan[/i]

[836dstr]

Stan,

Soft? Yeah right ! Like trying to push a 4 wheeled brick to 185.

Actually the main focus is to get clean air to the carb. Being 6'2" and running a '27 body really leaves me and the cage hanging out in the air. Unfortunately can't change either body.

Tom

[hotrod]

A rounded or "bull nose" entry to the air duct would be easy enough to fabricate. Would this be better than a knife edge?

Yes, a sharp edged duct will only be efficient if the local air flow is exactly in line with the axis of the duct. As soon as you get any sort of off axis air entry you will develop a vena contracta (flow separates from the edges of the entry and the effective area is smaller than the physical area).

Think of the old hot rodder velocity stacks --- they all had a bit of a trumpet mouth for this reason. Usually they recommend an inlet radius of about 1/4 the radius of the duct, but any radius helps.

http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

Larry

[836dstr]

WOW Larry,

A simple question just became more complex. Thanks for the link. I am going to have to digest this info for the air duct lip design. I'm glad I have 6 months until the next El Mirage meet to get it figured out and fabricated.

Thanks again !

Roy was "on spot" as the Brits would say.

Tom

[dwarner]

"The air duct would be above the grille insert, and the current radiator fills the whole grill shell openning and would not violate the area or contour of the grill shell for the purpose of streamlining."

If the radiator fills the whole grill shell and the '32 insert fills the grill shell how can the proposed air duct be within the grill shell and not effect the area specifided by the rules?

DW

[JackD]

If a new  performance only rule prohibits a former common practice then I trust
 that all previous records set without the performance restriction will be retired.
Let us look and see.

"TRUST BUT VERIFY."

[jimmy six]

I have the same concern as Dan. To me the only way I feel a air intake could be used as described would be if I could look through the intake opening and see the radiator tubes which by the rules must fill the shell.

As I look at a car in impounds for a record to "cert" the body I look underneath and for obvious infractions of the class. I do not feel at this point the impound technicians do not need the protest arena to accomplish this. If we see something, we stop that process on the car and get clarification. That's why some records are not immediate after we measure an engine.

This is a tough spot. Most who work in this area have more that just a passing knowledge of rules for every class of vehicle. Dan is instrumental in writing the book every year and over years has excellent knowledge.
I have 2 cars and they have been able to run in 5 body classes over 30 years so I feel fairly competent in those areas. Others in the Special Construction work also in impounds and have that knowledge. For the most part we are very lucky.

It appears 836dstr is not trying to circumvent the rules and by talking with the rules chairman of his class is apparently wanting to make sure what he is doing will be legal. I believe the entire committee should be contacted and perhaps they have.

As far as I can remember, we have never seen an air intake protruding thru a grille shell in Street Roadster before so this may be new territory. To me it's thinking outside the box which is ingenuity at it's best. But the ideas still need to meet the rules for the class...J.D.

[836dstr]

J.D.

On page 50 of the Rule Book regarding Street Roadsters it states:
"The radiator must fill the shell opening". Then it goes on to talk about using only a stock grille shell insert. It doesn't state front or rear opening, but I would think the intent is to fill the front opening.

By chance I took a picture of the Stuhaan/Kranz 848 car at El Mirage last Sunday which has an air duct almost the same as what I'm considering (photo attached).

Tom

[836dstr]

JD,

Well I tried to attach the picture of the front of the 848 showing the duct coming out of the top of the '27 grille shell but got a message that the attachment size was too big. I don't understand that, but maybe since I've already attached a picture I've exceeded some file size limitation.

Oh well, back to the point, the 848 STR has a metal air duct welded to a stock '26-'27 grill shell with a radius at the base. It could be 'glass. I didn't tap it to check.

I guess I could E-mail you the picture if you would like.

Back to my main question, is the 4" air gap from the carb gasket face to the underside of the hood OK?

Tom

[836dstr]

JD,

On page 59 of the 2006 Bonneville Program you can see the air duct on the 848.

Tom

[Stan Back]

Tom --

I take it all back.

I'm certainly not an official, nor an inspector, but . . .

Just because someone else is doing it doesn't make it legal.

What I read into it is there's no room at the top of the front radiator/insert/grille shell to put an air intake.  You're either recontouring (or eliminating part of) the shell from stock with the intake (which I think would be illegal) or taking up radiator area.

I'd be very careful going down that road.

Stan