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.