My question to someone out there is how does a 930 produce manifold VACUUM. The turbocharger, by definition, produces a POSITIVE manifold PRESSURE, not a NEGATIVE VACUUM. This is verified by the boost gauge - mine never goes below zero (0), and on full boost goes up to 1.0. Normally aspirated cars would show a vacuum (negative pressure) which is what ordinarily works the vacuum brake booster. So where does the vacuum come from in a turbocharged engine??
My modified '79 930 currently has a very hard brake pedal - similar to a n/a car with a bad booster, or when the engine is shut off and vacuum is no longer available.
Any help??
RevRon

 

I'm no expert but my understanding is when the throttle body is closed there is vacuum present when the motor is decelerating. There is a large canister on the brake assembly that store the vacuum. There is also a check valve in the line the runs to the brake booster so only vacuum is received.
Your boost gauge is reading pressure above the throttle plate. I believe the 944 turbos receive their boost gauge signal below the throttle plate thus showing boost and vacuum.

Tinker

 

Tinker,
Your observation is correct, a closed throttle will produce very high manifold vacuum. I think that is the answer to my question. The turbo pressure is all on the up side of the throttle plate, so when the throttle is closed the engine produces manifold vacuum like any reciprocating engine. And there is enough residual vacuum in the canister to begin braking assist during the transition from open to closed throttle. My boost gauge is not original, so I don't know where they plumbed it in.
Thanks for stimulating my thought process. Now tell me why the brake pedal should be so hard. Just kidding, I'll have to see if the booster is bad, or maybe somebody disconnected the hose, or it has some incredibly hard, racing pads, or . . .
Thanks again,
RevRon

 

Check all your hoses and I bet you have a crack in the hose that runs out of the tunnel and makes a 90 degree turn up to the trans / engine area.
I've experiences the same problem.

Tinker

 

+1
The vac line off the intake manifold goes down from the engine bay on the driver's side of the tranny and connects to a hardline there at the shift coupler that runs through the tunnel. That hard line pops up again under the front bonnet just beside the brake booster where another smaller vac line runs from it to the booster's check valve. There you should also find that your boost gauge is plumbed right off that same check valve...

Good luck & let us know what you find!
RC

 

Tinker, CaptainCalf,
Thanks. Try to find time in the next few days to check out the system. I have seen the end of some of these hoses but have not had time to dig under everything to check them out. Might be a good time to just replace thirty-something year old hoses hoses. I suspect a reinforced hose is required to keep it from collapsing under vacuum.
Thanks again,
Ron

 

Captain Calf,
If the boost gauge is plumbed off the check valve at the brake booster it has to be reading manifold "vacuum" AND pressure when on boost. This was where my question started - how would a boosted engine build vacuum for a vacuum assisted brakes. I am going to check the car out before I make any more suppositions, hopefully tomorrow.
Thanks again,
Ron