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billjam billjam is offline
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Join Date: Aug 2007
Location: Perth, Western Australia
Posts: 2,001
I had a similar problem recently with a newly-installed mechanical boost gauge which was not reading as it should.

I ditched my clock and installed a gauge in the clock hole. It was connected via a 4mm (ID) hose to a fitting I drilled and tapped into the LH end of the manifold (where the smog pump used to be). From this fitting, the hose runs through the car to the gauge via an Innovate LMA-3 under the RH seat.
I considered tapping into the brake booster line at the booster, but decided to run a separate line to get what I thought would be accurate boost to the LMA-3.
The gauge was reading vacuum OK, but as boost came on, the needle moved up to zero then moved back into the vacuum range until boost got towards 0.5 bar (on the electric tacho gauge). As boost continued to rise to about 1 bar on the electric gauge, the needle would move eventually up into the boost range, but never to more than about 0.5 bar.
The electric gauge in the tacho "feels" correct and I tested the mechanical gauge out of the car with vacuum and boost using a similar length of hose and it works just fine.

The only thing I can think of is that as the air is flowing past the hole that I drilled in the manifold, some kind of venturi effect is affecting the pressure at this port.
I have since re-positioned my connection point to a tee fitting in the 12mm hose to the booster. There is a plastic connector in this hose about 8" out from the engine, so I replaced this with a home-made copper tee and connected my 4mm hose here (just visible right in the fat hose at the LH edge of the photo).

Unfortunately, I can't tell you if it fixed the problem just yet because my fuel distributor is out of town and my suspension parts are in transit from PP, but it seems that there is more to connecting up a boost gauge than meets the eye.

I'd also be interested to know the "ideal" tapping point for vacuum and boost.


1988 Carrera - 3.6 engine (with ITBs, COPs, MS3X) and a whole set of turbo body panels waiting in the attic.
Day job ...
Memories: '68 912, '72 911T, '80 911SC, '84 911, '85 930, '86 930, '87 911
Old 01-21-2009, 02:53 PM
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