MC Bypass Valve
Last updated 2007-09-18 11:07 PDT

MC Bypass Valve

This is the bypass valve installation fabricated by Graydon Stuckey (aka GDS Racing) for what was then his 1986 Audi 5000CS Turbo quattro with an MC engine. It replaces the Michelin man (intercooler-to-throttle-body) hose and requires an additional vacuum tap to activate. Graydon put the vacuum tap into the intake manifold about an inch behind the throttle body, allowing a very short hose and correspondingly short response times.

Note: On a CIS-equipped engine, the bypass off pipe should be routed back into the intake tract after the mass air flow meter, to avoid confusing it and thus causing an overly-rich mixture (air which would normally be mixed with fuel will be dumped overboard, but the fuel won't).

(Yes, I know the picture quality is pretty poor. I didn't want to wait for the roll of film to get finished, et cetera, so I just tossed these things onto the scanner bed and "took their pitcher" that way. I'll get a digital camera some day...)

This partially assembled top view also shows the hoses and the bypass valve itself. I had to reinforce the TB-end hose with an old tubolari to keep it from bursting at the more-or-less constant 17 psi boost that the engine makes. Since my old sew-up bike tires are good for 200 psi, I figured it would hold up to the comparatively tiny pressure of a turbo intake tract.

The vacuum line connection is clearly visible on the top of the valve. The bypass connection (about 1" diameter) is straight down off the bottom of the valve (see note above about CIS!).


The top view shows the bung (and my scratched to heck paint job), which is lined with a rubber spacer about 1/8" (3 mm) thick and running the whole length of the bung from the visible end to the inside of the housing.


The throttle body end of the housing shows the oblong hole which matches the non-round shape of the opening on the throttle body.

The intercooler end is round.

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