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I'm chasing down a low boost problem on my 87 951. After installing a proper boost gauge, I could see it only produces about 6-7 psi. I tried shimming the wastegate with no change. Finally I made a wastegate blockoff plate to bypass the WG and then I could get it to boost over 11 psi. So a small pile of cash later, I installed a new Tial 38mm WG. But it still will not boost over 6-7 psi, maybe 8 at best. I have a manual boost controller installed, but I'm not sure I've done it correctly. It is tee'd into the banjo bolt line, with the line going to the side of the WG tee'd in before the MBC. At first the nipple on top of the WG was just vented to atmosphere, but then questioning that, I plumbed it back into the MBC:
Does this seem right right? I plan to switch over to an in-cabin MBC, but I want to prove this is working before that. No matter which way I adjust the MBC, there is no appreciable change in boost.
Now I suspect the cycling valve and am considering bypassing or deleting it. Is this possible without removing the intake manifold? Any help would be much appreciated!
there must be a leak some where, most likely at the venturi or cycling valve. I would check those or delete like youre planning to, much less clutter and future leaks to worry about.
Also your MBC should be in series with the cycling valve not in parallel like it is now.
The two lines running back over the intake are into and out of the wastegate.
So the MBC should be tapped into the line that goes from the CV to the wastegate? That makes sense. I'll try that and see if it changes the boost before I start tearing into it further. Thanks!
The factory boost for 951 is around 11 psi and will drop to around 7.5 psi at redline. The OEM WGs usually have worn out springs and rarely hold more than 7 - 8 psi (had the same issue). However, when you blocked the WG with a plate, you should get 17 - 18 psi at around 3000 - 3500 rpm instantly in higher gears and at that moment the DME should cut off the fuel to protect the engine from over-boost. Thus it seems, that you might have some vacuum leaks, exhaust leaks or some issues with the turbo. I would investigate the potential issues in that order. Even relatively small vacuum leak will not allow you reach more than 7 - 8 psi.
Thanks for the replies. I just finished getting the car going again after removing the intake manifold and deleting the cycling valve. The valve was removed and the port at the j-boot plugged. Now the MBC is simply routed inline from the banjo bolt to the side port in the Tial wastegate. The top port is vented to atmosphere, so running in single port mode.
The boost characteristics are virtually unchanged. I tried the lines into and out the MBC both ways and had the controller turned way and way out with both line routings. Still no change.
While I had the intake out, I inspected the cold side of the turbo and it seems fine; no broken vanes and it spins easily. When I installed the wastegate, I removed the cat and checked it for flow and it seemed fine as well. I've also been pretty meticulous about the vacuum lines while I worked on it, replacing a few, but I have not found any dry or cracked lines yet.
Getting a little frustrated, but determined. Any more suggestions would be most appreciated. Cheers!
Can one raise the boost correctly without having some upgraded injectors? ECU work done? I have two modified turbos but I assumed all these things went together.
Can one raise the boost correctly without having some upgraded injectors? ECU work done? I have two modified turbos but I assumed all these things went together.
It's not my intention run higher than stock boost with the MBC at this point, but rather to work my way progressively toward a well modified engine. The MBC is probably too early in the process. If need be, I'll install a new cycling valve and set aside the controller for now, but I still don't think it will solve my low boost issue, since it has now been taken out of the equation with no change.
It's not my intention run higher than stock boost with the MBC at this point, but rather to work my way progressively toward a well modified engine. The MBC is probably too early in the process. If need be, I'll install a new cycling valve and set aside the controller for now, but I still don't think it will solve my low boost issue, since it has now been taken out of the equation with no change.
Have you tried driving it without any boost control, MBC or cycling valve? You need to be VERY careful not to excessively overboost (although I think you still have a stock ECU which will shut it down). This would tell you if the system is capable of producing more than 6 psi boost. If so, then it has something to do with your controller. If you still only get 6 psi, you may want to look for exhaust leaks pre turbo/wastegate.
03-18-2016, 01:46 PM
Just another low boost issue...
