powdrhound

This is for the smart guys here. I am looking to supply Motec with a post throttle body pressure/vaccum input (manifold pressure) using a Bosch pressure sensor. The easiest way would be to "T" off one of the plastic vacuum/pressure lines that attach to the plenum. I'm assuming of the 3 lines that attach to the plenum via the 3 nipples/ports (left of the throttle body), the left and right port will provide both boost and vacuum. The left port goes to the diverter switching valve while the right port goes to the fuel pressure regulator. The center port appears to have a check valve which prevents boost from reaching the lines down stream but allows vacuum. Can anyone confirm. Thanks

pfbz

Your diagram is a bit different than this one from the service manual and seems to be missing a check valve.... A second check valve is on the line that runs to the diverter. Both of them are the little black and white check valve ( 964-110-950-01). They are installed in OPPOSITE orientations.

pfbz

The check valve allows air to flow from white to black and resists pressure black to white.

Check valve #1 has the black side of the check valve towards the plenum, so it allows vacuum to be pulled but prevents pressure.

Check valve #2 to the diverter has the white side toward the plenum, so it allows pressure to the diverter controller but prevents vacuum.



Below: Check valve #1, allows vacuum only past check valve.




Below: Check valve #2, white side to plenum, allows pressure only to diverter switch. I think this is to make sure the diverters are well seated and hold boost pressure when NOT commanded to divert. The diverter three way electronic switch also gets vacuum from the plenum from the four-way connector after check valve #1, switches vacuum to top of diverts when commanded to divert.

pfbz

...if the engine is in the car (or you want the motec sensor T location to be more serviceable with engine in), teeing into the line that goes to the diverter just before check valve #2 might pretty accessible, and as long as you t before the check valve, will have manifold pressure and vacuum. FPR is also a boost/vacuum line but more of a PITA to get to engine-in.

Third-Reef

The FPR line is the one you would want to use as it is purely a sense line. No valves or actuators in it that might cause pressure lag or spike. It should accurately read manifold pressure, you would want to tee in as close to the manifold as possible and keep the volume of the sensor line low but conductance reasonable.

pfbz

Hadn't really thought about the diverter switch causing spikes, but makes sense.

I just hate getting to the FPR vacuum/pressure line. PITA access.

John probably doesn't have the air pump in the way so it's probably easy on his car! Or AC lines, compressor, fuel cooler...

powdrhound

Thanks guys! ^^ Pretty much what I thought but wanted a little confirmation. Thanks for the pics Paul! I was planning on using the line that goes to the FPR as I figured that the diverters will cause momentary pressure spikes in that line. My engine is reassembled and on the engine stand right now so getting to the FPR line is no problem. My engine is very simplified as it does not have an A/C compressor, PS pump, an air pump, fuel cooler, or any of the hydraulic and A/C lines in the engine bay. As such, getting to the FPR and the pressure line is a piece of cake as there is a ton of room to reach around various parts of the engine. To drop the engine is super easy as you simply need to disconnect the fuel lines, water hoses, and harness connector. Getting the engine and gearbox out is about a 2hr job with this.

s65e90

Use the FPR or get a dedicated vac block at this point.

s65e90

I have my Autometer boost gauge teed off the FPR line for many years w/ no issues. Make sure to secure w/ zip ties.

powdrhound

OK, so one more question guys. Instead of cutting and teeing off the FPR line, I can also install a small threaded nipple into the silver plug shown in the center of the picture below. I am not using that port which goes to the top of the "gun turret" of the plastic plenum. This port us normally used for the suction pump used by the brake booster but since I am using the cam driven 997GT3 vacuum pump I have this blocked off. I think there might be a check valve built in to the top of the plenum which prevents pressure from going into this port but not 100% sure. I'm thinking there is really no need for a check valve there however as under boost the pressure is equalized pre and post throttle body. Thanks!

pfbz

I'm sure someone will chime in with a definitive answer, but looking at the replacement IPD 996 Turbo plenum, doesn't look like there could be in integral check valve in the replacement, so I'm guessing no check valve on the Porsche plastic plenum either...



I think the hose coming out of the other side of the sucking jet has the integrated check valve you might be thinking of right where the "B" is pointing to...

s65e90

Should work fine. Nothing wrong with either. Based on your car/use I'd get a dedicated vacuum block.

powdrhound

I think installing a vacuum block would just complicate the situation when all I'm looking to do is get a single boost source. I'm trying to keep it simple.

powdrhound

Thanks Paul! I just pulled the sucking jet pump (part E in your diagram above) out of the parts bin and there is actually a built in check valve incorporated directly in it. The allows air to be drawn though it from the brake booster side into the plenum but not in reverse. Air can flow freely in both directions through the part of the jet pump that is connected via the smaller hose to the Y-pipe. Based on this, I believe there is not another check valve present in the "gun turret" of the plenum related to this port as it would not make sense to have two check valves present.

Looks like I will just install a nipple into the aluminum plug and that will be my pressure source.

s65e90

A vac block is as simple as it gets. You've already eliminated most of the lines. Easier yet, just tap the FPR. But my prior response also stands, that line off the plenum works fine, it's just a tree branch (like a manifold) for the vac lines.