Why does 930 wastegate have two air-lines?

beepbeep · 08-02-2001, 08:52 AM

Wastegate on my 81' 930 allows boost to go up to 1.0 bar, which is a tad too much for my taste. I'm planning to replace it with either used one, or some aftermarket external WG.

Looking at it, i couldn't figure out what other "vent-line" is doing. I peeked into factory manuals, but it just mentions "control-line" (on the bottom) and "venting-line" (attached on the top of WG).

What's this other "venting" line for?

Cheers!

rattlsnak · rattlsnak's Garage

Hey, the side line is the pressure line or control line, the one coming out of the top is the vent line. Inside the wastegate housing, there is a spring and a diaphram. When the cars goes into boost, the boost pressure in the control line overcomes the spring pressure and pushes the air on the other side of the diaphram outside. When it comes back off of boost and into a vacuum, the spring pushes the wastegate back and air must come back into the vent line to create the difference in pressure. NOW,... if you are getting 1.0 bar boost,.. (which is what most people want) but want less, it sounds like someone has installed a stiffer spring. All you have to do is reinstall the factory .7 spring to bring it back to stock. Wastegate assemblies are NOT cheap! (i have a spring if you need it) Or install an HKS or Andial adjustable boost control.

beepbeep · 08-02-2001, 09:41 AM

Ho-hum...so i could actually open up my WG assy and replace the spring?

Actually, i don't know exactly if my WG has 1-bar spring or if diaphragm is bad. It happened few times that i run into overboost-switch, which shouldn't happend if it was only boosting 1-bar. Also , this happened when releasing throttle from full boost so it might be air-recirculating valve that is faulty instead. (people say that there is one, under CIS-maze, i'll take a look)

Well, i'll try to open WG and take a look first.

rattlsnak · rattlsnak's Garage

The diaphram sure could be bad if it goes into overboost. (which means the wastegate is not opening) BE CAREFUL when opening up your housing. That spring is SERIOUSLY loaded in there. If you take the six nuts off with the slightest bit of cocking it will break the aluminum housing. It takes two people with a vise and huge channel locks to put it back together. However i dont know what you mean when you say ran into the overboost switch, especially on decel. You can check the diaphram by blowing regulated compressed air into the supply line and see if it moves.

AMCPorsche924Powerhaus! · 08-09-2001, 08:43 PM

The overboost switch could also be bad in addition to your 1 bar mystery (maybe a spring, or a diaphragm problem, is it consistently 1 bar? or does it change, if it isn't consistent its not a spring), on the 931 they use a lot of 911 parts, not sure if it is the same switch, but a lot of people have problems with the switches cutting out early on the porsche 931s, they go and buy a new one to fix the problem and it is no better, sometimes you get a good switch that cuts out at 1.5 bar, sometimes one that is inconsistent and annoying on foggy days or other weather changes.

beepbeep · 08-10-2001, 02:06 AM

Well i replaced control-line to WG and replaced fuel-filter and i get steady 1-bar boost acording to in-dash pressure indicator. So far so good. Unfortunately, now i have problems with leaking gasket between heatchamber and head. My (ex) mechanic had overtorqued bolts and stripped the threads in the head. Bollocks!

Now it needs to be drilled-out and helicoiled and all that...