Engine air inlets:

[Rex Schimmer]

I have been doing some searching in the few aero references that I have regarding air inlets to the engine and I can tell you the water appears to be muddy. Looking at the evolution of the Nish streamliner, they have pretty much stayed with the same size opening, it looks approx 3 inches in diameter, but they have really appeared to have increased the volume of the inlet tract. This change has added a pretty substantual amount of frontal area but obviously they considered it worth while to go with their 600+ motor that they are planning to run next year (worked pretty good with the 500 incher too!). As I see it they are trying to get the airspeed down from the car speed of 370 mph+ to something that the engine likes, by increasing the volume of the inlet tract. OK this means that the pressure inside the inlet should be going up and the motor will like that. Getting the inlet air speed to slow is not a problem but what is the inlet speed that the engine likes? Should it be some multiple of what the calculated maximum inlet port speed is? That is my guess but what is that multiple? Obviously all of this is dependent on the engine size, max rpm at max hp and the estimated or desired car speed. OK someone take all of this and tell me how it relates.
 
 I am not a real fan of the NACA duct. To get them to work really right they need to be in an area of attached flow, and usually this is toward the front of the car which means on a rear engine car the ducting gets long and usually imposes on the drivers area.
 
 If you have any good references on this let me know.
 
 Thanks,
 Rex

[Glen]

The Nish streamliner has a whole new top section for this coming year. Not sure of the opening size but thare are a lot of changes.

[Bob Drury]

Rex, you might talk to Marlo Treit.  When he last ran his lakester with a PSI blower on it he had a long oval shaped duct that ran from the injector hat forward over the top of the drivers roll cage.  I believe he told me later that it should have had a "alligator shape" to be more efficient. This would be the reverse of what a ground effects tunnel looks like under a Indy type car.

[Glen]

Rex
 I sent you a photo of the air intake on Marlo's lakester.
 Glen

[interested bystander]

Air inlets and aerodynamic things in general.  Mr Schimmer, from what I've seen on this site and others, posesses much wisdom of things aerodynamic.
 
 Look up Prof Katz's book(s) on Amazon and Van Valkenbergh's stuff (no math, but good desriptions of the phenomena).
 
 Don't be fooled by what you see in some racing categories in that their bizzare rules may dictate what one must apply aero-wise!

[John Burk]

Rex
   Air scoops are pretty simple . Think of a bar of air extending foreward from the air scoop . The size of the bar of air is the engine c.i. divided by how many inches the car travels in 2 turns of the engine in high gear . Thats how many sq. inches the opening of the scoop should be . Too small , you loose pressure . Too big , there's turbulance in the scoop opening and you loose pressure . At 200 mph with the right size scoop opening in clean air you get about 5% supercharge .

[John Beckett]

OK, so how many inches does a car travel at 250 MPH @ 8000 RPM?
 
 John

[Seldom Seen Slim]

Four thousand, four hundred inches/second equals 250 MPH.  Tire diameter, gear ratio, and color of driver's sun visor may vary to get you to 250, but it'll still be 4,400 inches/sec.

[ddahlgren]

Does everyone thing the so called bar of air scenario accounts to the losses at the inlet dur to the usual improper radius high reynolds numbers etc and actually work?
 Dave

[John Burk]

John
   For calculating the size of the scoop inlet you need to find how many inches you go in two turns of the engine . Say you have a 300 ci 200 mph roadster with a 3.5:1 rear and 30" tires . In two turns of the engine the wheels turn .57 times (2 divided by 3.5) . The tires are 94.2" around (30 x 3.14). So in two turns of the engine the roadster moves 53.7" (94.2 x .57) . The 300 ci x 53.7" long bar of air that got scooped up in 2 turns of the engine has a crossection of 5.6 sq.  in. (300 ci divided by 53.7") . The opening in the scoop should be 2.67" dia. or anything with 5.6 sq. in. This is for an unblown engine and for a trans with a 1:1 high gear . We'll forget about details like tire growth and engine volumetric efficiency but we should add 5% (5.6 + 5% = 5.88 sq. in.) for the supercharge effect we've gained at 200 mph .

[ddahlgren]

Ignore the VE what a great idea? Without that you don't have a clue how much air you need. Armchair physics sure is fun though..
 
 How about you run it on a dyno measure the cfm and set the inlet size for a reasonable mach number corrected for temperature and pressure at the point of the opening. Not as much fun as a bar of hot air but a lot closer to reality.
 
 Then you can either test it in a wind tunnel or at the track to see how much the rest of the car screwed up the air entering it. If it were this easy real race teams would not spend upwards of six digits to design an air box and inlet.
 Dave

[John Burk]

Dave
   There's a general misunderstanding of air scoop inlet sizing to the point that a correct one like Nish's catches everybodys eye . If the volumetric efficiency is ignored for the sake of simplicity the few percent of oversize that you get is meaningless compared to the error from a seat of the pants guess . I agree that the less disturbed the air is that the scoop grabs the better .

[Rex Schimmer]

Guys, it anin't the hole size that I am really interested in because if you figure the area required based upon the engine size, rpms at speed, volumetric efficiency etc. you still need to add something to that area because when the air enters the opening, and if the air is not disturbed,which I plan to make sure of in my lakester design, you still have a boundry layer at the inlet that effectively makes the inlet diameter smaller. So things like interior finish and the radius of the inlet itself seem important. What I am interested in is how big should I make the volume of the inlet tract, the Nish air inlet really gets big. As you increase volume you decrease velocity and you trade the velocity energy for pressure. Also what about internal turns vanes and any thing to eliminate any kind of turbulance. Lets hear your thoughts on these areas.
 Keep those ideas coming.
 Rex

[John Burk]

Rex
   You're right that the air needs to slow down once it gets past the opening . It's only the opening that needs to be acording to the formula . No doubt there's friction going in the opening but I don't think it's much . If the opening is 5 sq. in. and the air enters at 250 mph the plenum only needs to be 6.3" dia for the air to slow to 40 mph .

[Sumner]

This is a very interesting thread.  Thanks for the information John.
 
   

If the opening is 5 sq. in. and the air enters at 250 mph the plenum only needs to be 6.3" dia for the air to slow to 40 mph .  

How do you figure this?  I'd like to apply this to a different diameter.  Also what do we want the air speed to slow to?
 
  <img src="http://www.nishmotorsports.com/2004Slideshows/VAB2/4.jpg" alt=" - " />  
 
 The picture is of the Nish's car's newer scoop.  Now I'm not questioning what they are doing here, but wondering why the volume has to get that large behind the opening?  I didn't have a chance to see the car on the salt last August.  They have accomplished a lot with that car.
 
 Thanks for the additional information,
 
 Sum