AIR CUT-OFF VALVE / COMPRESSOR BYPASS VALVE
 

I was just reviewing the Paragon Products website, and I came across a part lising that I haven't seen mentioned in the discussion boards previously. This is the "compressor bypass valve" or "air cut-off valve." (993.110.337.50)

According to the vendor: "Chances are if you have a 944 Turbo, the Compressor Bypass Valve (or what Porsche calls the Air Cut-Off Valve) needs to be replaced. This valve holds boost pressure when you shift gears so there isn't excessive lag while your turbo spools back up. Test your valve by removing the top hose and see if the valve will hold suction, if it doesn't, you need a new valve.

Can anyone comment on this part (i.e. is it really a common failure mode) and what is the best way to determine if it needs to be replaced?

When I bought my 89 951, the bypass valve was busted. The most notable sign was a sound like a turkey gobble under the hood while under boost. The way to test it is to draw a vacuum on the bypass valve port and see if the valve stays open. A rip or tear in the valve (it has a rubber skirt) and it will not stay open under vacuum and boost pressure will leak out making the turkey sound (you achieve the same effect by blowing through a busted balloon :). The bypass valve allows air to circulate from right before the throttle body back into the front side of the turbo when there is vacuum in the intake manifold. Boost in the manifold helps keep the valve shut. You need the valve to operate properly so the turbo stays spooled while you are shifting gears. Someone makes a metal bypass valve with brass internals that is an exact replacement for the Bosch unit, but I don't remember where I saw it for sale.

Yes it is common for these to go bad. If you have a Mighty Vac you can check yours.

If you upgrade to MAF, Kokeln intake, turbo or any "air" upgrades, go ahead and replace it with 930 one or greater aftermarket one with new intercooler pipes.

Mine sounded off when I shifted gears like air escaping in burst, with split second intervals after putting on the Kokeln intake from Lindsey Racing. easy fix for $28-$35 bucks.

Yes, it is very common for the original 951.xxx.xxx.xx part to fail. The procedure to test it is as you have mentioned. See if you can blow/suck on any of the openings, if you can get air through any of the openings it is bad. Replace it with the 993.xxx.xxx.xx part that you list in your original message. This is the improved part with the brass valve that should last basically forever.

Dal.