Landracing Forum

Tech Information => Technical Discussion => Topic started by: 836dstr on January 24, 2007, 05:53:05 PM

Title: Plenum Design
Post by: 836dstr on January 24, 2007, 05:53:05 PM
Once more into the breach!

This is a continuation of the topic on "air box design". The method of getting the air into the engine compartment is now resolved.

I remember a discussion of hood scoop design where it was suggested that the intake air entering the plenum should be slowed down by increasing the area of the plenum.

The basic design I have for the air intake system has the intake opening of about 7 Sq.In. (a round duct merging into a flatter cross-section with half round sides. The initial duct will be about 8" long tapering over the next 10" to an area of 11 Sq.In. about 9 1/2" in front of the center of the Carb. The taper contiues to the centerline of the Carb where the area is 27 Sq.In.. At that point the taper ends and the  sides are perpendicular to the forward/aft centerline of the Carb. The Plenum continues 10" past the center of the Carb with about a 5 & 1/2" radius at the rear corners and the area increases to about 30 Sq.In.

This creates expansion ratios of:

     1.57  @ 9" before Carb C/L
     3.0   @ begining of Carb airhorn
     3.82  @ Carb C/L
     4.29  @ rear of plenum

1. Is this enough to counter reversion of air coming off the rear of the plenum?

2. Would having some openning/s in the rear of the plenum to vent air help?

3. Would rounded edges of the plenum out perform vertical edges?

The operating parameters would be in the range of 150 to 190 MPH car speed.


Also, the dimensions of the plenum are only restricted by height to hood underside and the firewall. The general shape, taper and widths are all open to better ideas.

Any questions or suggestions would be greatly appreciated!

Tom
Title: Re: Plenum Design
Post by: JackD on January 24, 2007, 08:37:03 PM
It sounds like time to build a manometer.
Title: Re: Plenum Design
Post by: 836dstr on January 25, 2007, 12:37:53 PM
Jack,

Just trying to get a good start on a basic design that can then be tweaked using pressure readings. I'm trying not to reinvent the wheel but learn from those with more experience.

Unfortunately it's hard to get meaningful baseline data and make changes when you only get 1 or maybe 2 runs at El Mirage at any given meet (except the 2 day events).

Well, I hope to hear some more comments, specially about venting the back of the plenum.

Tom
Title: Re: Plenum Design
Post by: JackD on January 25, 2007, 01:23:26 PM
You can learn a lot at freeway speeds with a simulator.
Kitchen table cardboard mini models can also get you started.
Using a shop vac to supply the blow and the suck at the same time.
Oh, and the homemade manometer.
Title: Re: Plenum Design
Post by: 836dstr on January 25, 2007, 10:54:00 PM
Jack,

You got me thinking (always a good thing). I could make a clear mock-up of my design, take the top out of the wifes Corvette and at freeway speed create some type of smoke generator in front of the mockup and see how the smoke reacts.

I will probably need one of my budies to drive the car rather than my wife!

I can already see the article in the local paper after getting pulled over and trying to explain.

Worth a try. Might be better to use the shopvac & smoke.

Tom
Title: Re: Plenum Design
Post by: Harold Bettes on January 26, 2007, 12:44:42 AM
Remember that if you do make a model (models are always good), that you need to consider the airspeed vs the sizing of the model. Its an L1V1 = L2V2 thing. For a smoke tunnel, no numbers, just seeing what is attached and unattached.  :wink:

There are all sorts of dyes and markers that can be used, but if your scoop design is clear, tufts of plain old yarn are very hard to beat. Taped in the middle of each piece allows some great flow indications. That allows you to have that Superman Xray vision view. :-D

However all that said, it seems that 7in^2 is a bit small for the acceleration up to speed, but I am assuming some things about the power level and I did not go back and read the whole thread. :?
Now I will go back and read thoroughly and see how much shoe leather I just ate. :|

Regards to All,
HB2

Title: Re: Plenum Design
Post by: JackD on January 26, 2007, 01:13:17 AM
You see ?
A wealth of information , you just gotta count the money and spend it wisely.
Title: Re: Plenum Design
Post by: Seldom Seen Slim on January 26, 2007, 08:55:55 AM
Oh, Tom, forgive me, but the proofreader in me insists that I've gotta do this.  To quote you:

"I will probably need one of my budies to drive the car rather than my wife!"

You do realize, don't you, that an exact interpretation of your words implies that your buddies drive (?!) your wife but will change for this experiment -- and will drive the car, instead of her.

I know that we talk about content being more important in this forum than proper spelling and such -- but perhaps you'd have been better to state:

"Instead of having my wife drive the car I'll have one of my buddies be the driver."
Title: Re: Plenum Design
Post by: JackD on January 26, 2007, 10:10:42 AM
Well sure, if that is what he meant. :wink:
Title: Re: Plenum Design
Post by: 836dstr on January 26, 2007, 12:09:32 PM
HB2, thanks for info!

S3, point taken. In business writings I always found it easier to do the editing than the writing. As the writer you know what you want to say and are influenced by that perspective. As an editor you try to figure out what is said , and meant.

The English language is so much fun. Spell check let's a lot of stuff slide by.

Tom
Title: Re: Plenum Design
Post by: tortoise on January 26, 2007, 12:46:11 PM
Spell check let's a lot of stuff slide by.
. . . such as superfluous apostrophes.
Title: Re: Plenum Design
Post by: Seldom Seen Slim on January 26, 2007, 02:17:57 PM
See my post a few weeks ago -- never ever use a comma or an apostrophe and you will be wrong far less often than if you continue to (mis)use them as do many folks.  I had picked on Tom enough.  I wouldn't have said anything -- but having someone else drive his wife was just too much to let slide. . .
Title: Re: Plenum Design
Post by: tortoise on January 26, 2007, 04:15:57 PM
I wouldn't have said anything
Cant follow your own advice, I see.
Quote
-- but having someone else drive his wife was just too much to let slide. . .
Speaking of exact interpretations, are you a slim person who is seldom seen, or a person seldom seen to be slim?
Title: Re: Plenum Design
Post by: 836dstr on January 26, 2007, 05:32:28 PM
Good thing my wife's name is not Daisy. Different story.

Back to the topic please. Some good ideas are being posted.

Tom
Title: Re: Plenum Design
Post by: hotrod on January 28, 2007, 10:09:08 PM
Quote
However all that said, it seems that 7in^2 is a bit small for the acceleration up to speed, but I am assuming some things about the power level and I did not go back and read the whole thread.

A solution to that is to use a spring loaded trap door that will be sucked open if you develop a significant negative pressure in the scoop but at speed will be forced closed by ram pressure.

Larry
Title: Re: Plenum Design
Post by: 836dstr on January 29, 2007, 01:13:50 PM
Larry,

Thanks for the idea. All the ducting to the plenum will be behind the radiator(non-functional & thin) and under the hood. It would be easy to add a flap with very light springs that would would close as speed increases.

Tom
Title: Re: Plenum Design
Post by: Sumner on January 29, 2007, 03:50:49 PM
Tom, I don't know if you read them or not, but I have some thoughts and sizing suggestions on air inlets on my site.

You can find them ( HERE (http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html) ).

I also today added a spreadsheet to my site that uses the formulas in those suggestions to figure an air inlet size ( HERE (http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html) ).

I would remember that when you read anything on my site you should always double check with a second source or so to see if you feel comfortable with the information before you use it.  This might be a good idea for anything you read. 

Hope this stuff helps a little,

Sum
Title: Re: Plenum Design
Post by: 836dstr on January 29, 2007, 05:09:08 PM
Thank you Sum !

I'll have a look.

Tom