New Version of an Old Mod

[shotgunslade]

Just installed Dayle's Polished B Tube Intake pipe that I had bought a long time ago. It got dissed on the network for allowing excessive heat gain to the intake air.



I wrapped Dayle's pipe in foam pipe insulation tape with aluminum backing. Looks pretty good and it is a much much slicker air pathway than the OEM pipe. Also the aluminum tape surface has a very low emissivity (0.1 compared with the plastic pipe 0.9) so that it reflects most of the radiant heat emitted by all those very hot surfaces in the engine bay. With two layers of 1/8" foam and the aluminum radiant barrier, I'm sure the thermal resistance of the new pipe will be greater than that of the OEM plastic pipe, resulting in cooler intake air.



The OEM pipe is a disaster area for fluid flow. It has a 3" diameter and starts with a short radius bend that goes directly into a corrugated section. The corroguated section has a free diameter of only 2 3/4".

[/IMG]http://www.subaru-svx.net/photos/files/shotgunslade/28924.jpg[/IMG]

I am sure these corrugations would generate turbulent vortices that would effectively reduce the free flow area of the tube to 2" or less, based on the pitch and depth of the corrugations. The turbulance generated by that corrugated elbow in the OEM pipe would not only generate resistance, but also seriously erode the convective boundary layer on the inside tube surface, greatly increasing convective heat transfer to the air. Flow velocity at 6400 rpm would be between 1000-1200 fpm in the smooth straight area of the turbe, It could be double that in the center of the corrugated area. All this pressure drop decreases the density of the intake air, resulting in a lower mass flow of oxygen to burn the fuel.

Finally, the turbulent flow regime developed by the corrugations at the inlet to the throttle body would likely be highly unstable. Minor changes in pressure generated by throttle movements would result in significant reconfigurations of the vortex geometry and the pressure drop through that area. Changes in throttle position would therefore start a process of reconfiguring the flow regime resulting in some hesitation or fluttering after abrupt throttle position changes.

All that is theory, and, unfortunately, my butt dyno isn't sensitve enough to tell the difference. I think I notice a slightly smoother engine response to abrupt accelerator changes, however. But, it's probably my imagination. Anyway, it was a fun way to spend a Sunday.

[ensteele]

Very interesting. You will at least be able to start a conversation when you open your hood at a meet.

[UberRoo]

Ever hear of turbulators? In many instances, having a certain amount of turbulence can actually reduce overall flow resistance. The term, 'turbulator' is often used [in context] to describe a particular device used in certain applications, but it's actually a fairly non-specific term, applying to anything that purposely introduces turbulence into fluid flow. It's difficult to successfully utilize turbulators consistently - it's still a bit of a black art - but they do have their place. Golf balls for example, behave very poorly without their dimples. Wakeboards and trick skis, while quite slippery when pointed forwards, have a very high resistance when turned against the direction of travel. Turbulators are used to combat this problem. Salesmen call them "Phasers" and usually don't even know what they do, but engineers know what they are. Aerodynamically (or hydrodynamically,) aircraft and boats sometimes use turbulators. Sometimes they help. Sometimes they don't.

The truth is that even engineers and aerodynamicists have a hard time predicting the results of turbulators. Clean surfaces are a pretty safe bet. Turbulent surfaces can yield small improvements, but more often result in large losses instead. I wouldn't put it past Subaru to have actually studied the matter when designing the intake, but I suspect that even if they did, in the end they probably decided it didn't make any difference one way or the other.

[shotgunslade]

Yes, I have heard of turbulators. Their two primary uses are to detach a laminar flow from a surface to reduce drag from surface friction and to increase heat transfer by disrupting the boundary layer. Your golf ball is a good example, because a sphere is so lousy aerodynamically that attached flow really increases drag. Turbulators are also used on airfoils and boat keels to reduce drag by separating the flow from the surface. Spoilers and Kamm-effect tails on racing cars do the same thing by separating the flow across the car at the tail. The turbulators themselves impose drag, which sometimes is less than the drag imposed by a laminar flow across the surface, or by having the attached flow bend around the trailing edge of an object like a golf ball. Don't think either of those two effects are beneficial here, given the configuration of the itake path. If you were concerned about an abrupt transition at the outlet of this tube into the throttle body assembly, a little lip or circular spoiler would be enough to detach the flow from the surface of the pipe at its termination. Certainly increased heat transfer isn't beneficial. Think the corrugations exist just to make it easier to bend the tube for installation. It wouldn't make much difference except at high rpm's when the velocity in the tube gets quite high. But, since Ecutune has given us such a good bump at mid-range, it's high rev's that are stumping the NA crowd. This certainly can't hurt.

If we weren't so busy, I'd do a quick CFD on the flow in the tube. It doesn't look much like what we normally do, but since I'm responsible for CFD here, I still might be able to sneak it in. If I do, I'll post it.

[SilverSpear]

You did 10% of the Mod i am doing to my intake right now. But instead i will use an Amiante tape to cover the metal intake and overall wrapped with rubber.... You'll see, when i am done...