He didn't tap into the reservoir line. He didn't know and was asking. He understands now and has it tapped in the proper location. As for the Sunpro gauges...

 

on a turbo the vacuum line to the brake booster has a check valve in it. so the booster can only see's vacuum. so for your boost gauge the good in dash ones have a replacement check valve with an additional 1/4 inch line for the gauge. just pop out the clock snake the hose to the back of the brake booster. pull the old check valve off the connect the new one. only takes a few minutes.

 

OK guys, I think I'm on board with this now. Makes sense, now that I've had a good nights sleep.
My external boost gauge is electronic, and wired into a sensor on the intake manifold...then wired and snaked laboriously up to the dash (a real pain in the a _ _ I might add). It never occured to me to use the brake boost line. I think I'll be looking to buy a manually operated boost gauge now, since it would be more accurate than the electronic version. Except I don't like the in-clock position of most of these gauges (I like keeping the clock so I can time my 1/4 mile runs). Something small and obscure is more my liking.

 

Also, the origonal boost gauge gets it's reading from a sending unit threaded into the intermediate manifold/blow off valve housing or the rear tank of the intercooler which is above the throttle body.

The brake booster vacuum hose connects to the intake manifold which is below the throttle body so connecting your manual boost gauge to that will give boost readings that are more accurate and in line with what the engine is actually seeing.

If you have both gauges it's kind of interesting to accelerate at part throttle and for example... you may see around .8 bar boost in the intercooler on the factory gauge, but you'll only see around .1 or so on the manual gauge.

Thats because the throttle is only open a little ways and the cylinders are swallowing most of the air being forced by it so the boost pressure in the intake manifold is lower than what the turbo is making upstream of the throttle in the intercooler.
If you go full throttle than the pressure above and below the throttle body becomes almost the same... depending on the size and restriction of the throttle body.

Bernard, your car is so clean it looks like it just left the showroom...
When I look down around the brake booster under the trunk carpetting on my '87, or any other car thats been on the road a while, everything has a fine coating of old dust.
I've brushed around everything with assorted size paint brushes and blown most of it away with 120 psi compressed air, but it's not as clean as yours.
You have a really nice car man.

 

Just to add, the brake booster has a check valve (non return valve) so no boost pressure goes in to the brake booster. The booster can store 3 pedal pumps of assistance anyway with no vacuum coming from the engine. Try it out when you turn off the engine when parked, if you can't get at least 2 pushes you have a problem...

 

Not that I have to explain anything to you smart but the reason I used the Sunpro was it was the closest I could find locally to match my stock gauges.

 

ok i'll bite since the clock doesnt have a second hand How freaking slow is your car?

 

It doesn't? Then what is the white hand in the clock in the pic above?

 

Gotcha, Ken! I was wondering if someone would bite, since our clocks do not have a second hand. Too much serious focus going on here, time for some levity.
The white hand (to answer the question from Equality) i just there to set your elapsed time from your starting point. Basically worthless.

 

I see. I can't believe I never noticed that. I guess I'm too busy looking at other things than the clock.

 

Mine is left at 12 o'clock.

 

That's Porsche's version of the "Trip Computer". You set it on the Hour Hand. It's basically a Telltale.

Les

 

When I had a clock I did the same thing. I used it as a directional pointer so that when my car is spinning I knew what direction was straight ahead! Kinda like flying by Instruments.

Les

 

VDO sells a little vac/boost gage just like the one you have (FYI).
I ran my boost signal back to the engine. The brake booster may work just fine but I don't like splicing into things that are dedicated to other important functions - like brakes.

I would assume some signal bleed during brake operation. You guys don't trail brake out of corners while on boost?

 

Don't worry about teeing into the brake boost/vacuum line afterall, Who needs brakes anyway?
Brakes are for ****!