Just a couple random questions dealing particularly with air flow.

1. If you take a rubber cylinder say 3" in diameter and stretch it into an oval do you decrease the amout of air it can flow? What about different shapes such as a triangle or square, etc?

2. What degree of a bend would have to be present to significantly disrupt air flow, 90*, 135*, 180* etc?

 

1. A rubber cylinder... assuming it's not in a "stretched shape", would have a constant edge length. Therfore the largest cross-section possible is a perfect circle. Anything less would result in a smaller cross-section and worse flow.

I'd imagine this would be a general guide of common shapes you "could" make it into... but the greatest flow would maximize cross-section area and the angle between sides.

• circle
• oval
• octagon
• hexagon
• pentagon
• square
• rectangle
• triangle

For specific examples you'd need to dictate the major and minor axis of the oval, but within reason it should do better than the multi-edged shapes possible as the boudry layer of "fouled flow" would be minimal, while edges (especially tight angles) would develop stalled air and decrease the effective flow rates (or theoretically the usable cross-section).

2. degrees doesn't really matter... it's the rate you change the arc that messes things up. Three long, smooth, flowing loop-ta-loops (3 full circles, 1080*) could easily be better than two sharp 90* cringled-edge bends. For what it's worth, in an automotive scale (which I'm assuming this is for) I belive Corky Bell listed 90* as the problem area... but again... it's how much tubing & packaging room you have to play with that is gonna limti you. Ever been on a water-slide? Ever seen the water get all messed up in a sharp 90* turn? there you have it.

 

Awesome explaination Steve, thanks for the response. Since a 90* turn maybe the worse, is its effect shown in the stock LT1 intake tract in your opinion? i.e. is it a restriction?

 

Therfore the largest cross-section possible is a perfect circle. Anything less would result in a smaller cross-section and worse flow.-Steve in Seattle
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the 2.000" inch Circle fits inside the 2.000" inch square
with room to spare

Circle at 2.000" diameter = 3.1416 sq.inch area

Square at 2.000" length sides = 4.0 sq.inch area

air doesn't like anything greater than 7.5 degree angle changes
and anything around or greater than 45 deg valve axis-to-port axis is going to hurt flow significantly.

 

 

That's the way I read the original post too.... although the wording in the 96z post is "fuzzy" at best. He might want to clarify which he meant.

 

what Steve in Seattle might have meant to post
was ;

the Circle gives you the greatest amount of cross-sectional area for the surface area or boundary layer area.

just based upon that statement, it would mean the Circle is the most efficient shape !

a straight round pipe would give the best flow.

but in Cylinder Heads with valves, its always a compromise, and you have to "turn" the flow ....then the Circle is not the best shape to do that with, nor even the best shape to approach the turn at the valve .

 

Very true, my appologies for being too general, I was referring mostly to exhuast and induction systems that operate on "pipe flow" style concepts. By the time you get to 6" or shorter pipes with obsticals and sharp turns your dealing with much different issues. Straight pipes are nice and easy in comparison.

As for the cross-sectional area? yeah... I was referring to a constant perimeter/edge length... such as a 3" rubber cylinder , or exhaust tubing that you "could" shape into other cross-sections to fit available space. If you have a set of heads and the issue is porting-out material without worrying about the amount of surface area produced you're obviously under different constraints.

I imagine the air density also has a fairly large effect on flow patterns as well. Turbo's/blowers at WOT probably experience less flow loss (smaller boundry layer) from obsticles that cause issues for part-throttle or NA systems would... the nice part is that while the flow might not be as efficient, there would be less gas to pass anyway so it usually isn't an issue at all... beyond the theoretical discussion that is.

Adding pulse-tuning to the mix would make this even more interesting... but yes, for a 3" rubber cylinder, cross-section (after subtracting boundry layer losses) is your best best to max flow.

 

yes... but in the 3" rubber cylinder example you get:

Circle at 3.0" inner diameter = 7.07 sq.inch area, 9.42" circumference
Square at 3.0" inner length sides = 9.00 sq.inch area, 12" perimeter

Hopefully that 3" cylinder can stretch 27% without problems, and if it can... you can make a 3.8" circle (12" circumference) with a 11.46 square inch cross-section.

Depends on the frame of reference / application really to discuss this too much I guess.

 

Wow... that's getting deep into it. The concept of plenum fill rates impacting runner flow rates is WAY more involved than just a straight-pipe flow. My understanding is that the sharp angle/edges between runners and the intake track is intentional in an effort to disconnect the flow pattern in the plenum... sort of a blender idea so the intake runners can be pulse-tuned to a specific RPM band (from valve to plennum)... continuing flow directly from the intake track to each runner (like 8 long tubes from the MAF back) would give you HUGH runner lengths and volumes.

When it comes to heads/intakes, I prefer just to send my stuff to to a pro... like say Meaux Racing Heads

As for the 90* angle between runner orientation and throttle-body, it seems to be an industry standard for most EFI applications... I doubt that's a coincidence.

 

Thanks for the responses guys. I was leaning more towards induction and straigt pipe flow than heads, etc. Neverless thanks for all the responses. I wish I could have been more wordy or direct but my knowledge of physics isnt anything to be proud of.

 

Once you focus on "pipes" rather than the complexities of the intake runner, it is possible to generalize, and you can say "round" is better for any fixed value of perimeter, that the larger the number of degrees in a bend, the larger the pressure loss (although it is not directly proportional to the number of degrees - a 180-deg bend will not have twice the pressure loss of a 90-deg bend) and that the larger the "radius" of a bend, the lower the pressure drop for any fixed number of degrees in the bend. Avoid sudden contractions and enlargements. There are available tables of "K" factors that can be used to calculate pressure drop for any of the above, or to compare various configurations.

 

Which one of these would flow more all conditions being equal? In this instance exhaust.

(1.) 5 feet of straight 4" exhaust that narrows to 5 feet of 3" exhaust or (2.) 10 feet of straight 3" exhaust?

Does (1.) flow more due to its larger initial diameter or is the transition a restriction?

 

Thoughts:

10 feet of steel tubing allows a lot of exhaust gas cooling from one end to the other, especially with air flowing over it as you travel down the strip. So as the gas cools maybe 3-400 *F, the volume decreases and if the pipe cross-section doesn't decrease, the velocity decreases. No steady state conditions here, unfortunately.

If the 4" to 3" transition is gradual, say a 15* included angle, the flow probably won't see it as a restriction nor as something to cause another wave reflection.

Maybe a constantly decreasing cross section pipe could be used to keep the velocity constant, if that's a good thing. That's difficult to do with a round tube, but flat ovals like Cup exhausts could easily achieve that. It doesn't look like they do such a thing, but it wouldn't be too difficult.

How about using the 4 to 3 as a reverse step to tune the collector length? Larry Meaux might have ideas on that.

If the 10 footer has to bend over the axle, those bends offer lots of restriction, so maybe staying with the 4 inch or even using a 4 inch 180* bend and 2 90s over the axle with 3 inch on either side might minimize restriction. With the decreasing temp, modelling this would be a challenge, but a neat student engineering project.

I guess my answer to your original question is "It depends".