boost shut off
05-02-2010, 04:59 PM
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boost shut off
I just put in a msd6al btm control. It seems to work fine but I still have the over boost shut off working, it shuts off just before 12 pounds. I took the old ignition box out so why is the boost shut off still working. ???? thanks cullen
05-03-2010, 01:02 PM
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Cullen, I'm not sure I'm on the same wavelength here with your question. The MSD boost timing module has nothing to do with your boost settings; all it does is to retard timing in response to boost, enabling you to run more timing advance before boost builds and to safely retard the timing on boost.
When you say "over boost shutoff", I presume you're referring to the fuel pumps shutting down when you hit 12 PSI or higher of boost. Two things to look at: (1) do you know that your boost gauge is accurate when it reads 12 psi (about .8 bar), or could it actually be much higher than that? (2) the overboost cutoff pressure switch - mounted to the secondary air intake manifold (the boost recirculation manifold) - may be getting old with age and tripping sooner than it should. It's a simple pressure switch with a single white wire feeding it. When the switch trips, it interupts the ground source which in turn trips the overboost relay and shuts down the pumps until the boost drops. Usually those switchs trip at around 1.2 to 1.4 bar, depending on their age. You want to keep that protection circuit working, for obvious reasons.
When you say "over boost shutoff", I presume you're referring to the fuel pumps shutting down when you hit 12 PSI or higher of boost. Two things to look at: (1) do you know that your boost gauge is accurate when it reads 12 psi (about .8 bar), or could it actually be much higher than that? (2) the overboost cutoff pressure switch - mounted to the secondary air intake manifold (the boost recirculation manifold) - may be getting old with age and tripping sooner than it should. It's a simple pressure switch with a single white wire feeding it. When the switch trips, it interupts the ground source which in turn trips the overboost relay and shuts down the pumps until the boost drops. Usually those switchs trip at around 1.2 to 1.4 bar, depending on their age. You want to keep that protection circuit working, for obvious reasons.
05-03-2010, 08:27 PM
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thanks mark. I was not sure how the over boost worked. I was not sure if it was a switch or what. at the same time I put in the msd I also added a manuel boost control, knock sensor and exuast temp gauge. I talk to the porsche dealer and he said that my year the over boost switch came on at 12 pounds. troy. It could be my waste gate I don't no all the ins and out. 76 turbos have a external waste gate, we took the top hose off and used the bottom hose from the waste gate for the boost control. thats what I read to do. any help would be great thanks cullen. psI don't no if the moters stock I no they raced it.
05-04-2010, 02:05 PM
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thanks mark. I was not sure how the over boost worked. I was not sure if it was a switch or what. at the same time I put in the msd I also added a manuel boost control, knock sensor and exuast temp gauge. I talk to the porsche dealer and he said that my year the over boost switch came on at 12 pounds. troy. It could be my waste gate I don't no all the ins and out. 76 turbos have a external waste gate, we took the top hose off and used the bottom hose from the waste gate for the boost control. thats what I read to do. any help would be great thanks cullen. psI don't no if the moters stock I no they raced it.
Wastegates: The function is to vent excess exhaust pressure to control the level of boost. There is an internal spring that's set to open the wastegate at a predetermined boost level (from the boost signal line that attaches to the bottom of the WG housing...the top hose nipple is just vented to atmosphere). You can change the spring tension by installing different springs, and thus change the point that the WG actuates. Or you can delay the boost signal reaching the WG by any of a number of boost controllers. And oh....all 930's have external wastgates to my knowledge.
Overboost protection: These circuits are generally comprised of a pressure switch that interrupts a ground connection, in conjunction with a relay (at the relay/fuse tray in the engine compartment). The switch is supposed to trip when boost exceeds the safe limit for your car, and the relay then shuts off the fuel pumps.
If your Porsche dealer said the boost switch should trip at 12 Troy pounds (the same as 9.87 Avoirdupois pounds...or about .7 bar) then he is incorrect. I don't know for sure, but your 1976 930 maybe be designed for a maximum boost of .7 bar (which might be what he was referring to), whereas the later cars are set to deliver .8 bar.
In a .8 bar boosted car, a new boost cutout switch should trip at around 1.2 to 1.4 bar I believe....in that range somewhere.
Your symptoms: I need to be clear on what's happening. Are you seeing a sudden loss of power when you hit 12 psi (like the engine completely shutting down)? If that's what's happening, it is very dramatic...you go from strong acceleration to a violent deacelleration until you take your foot off the gas - the boost drops to a safe level - and the car starts back up. Those symptoms mean that your overboost protection has kicked in and shut down the fuel pumps. That could happen if you have fautly protection components, or a malfunctioning wastegate.
Lots of stuff to think about. Before you start playing with boost controllers and advanced ignition timing control via MSD boost timing modules, it's best to know that your car is operating as designed in stock configuration. Then, change one thing at a time to fully understand the impact.
05-04-2010, 06:16 PM
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mark. yes the car engine just shuts down and restarts when the boost goes down and car runs fine. and yes it does throw you forward. everything worked fine before I put the new stuff in. how do you tell if its something with the waste gate. cullen
05-04-2010, 07:07 PM
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The common denominator is the "new stuff" just installed. The first thing I would do would be to disconnect the boost controller you installed and reconnect the line from the WG back to it's original position on the intercooler or wherever it was connected pre-throttle body. If that cures the problem of overboosting, then the issue is with the boost controller not adjusted right or malfunctioning.
You can bench test the WG by applying known air pressure to the bottom nipple. You can do it with the engine running if you like, and listen for the exhaust to escape as the valve opens. Or remove the WG from the car and watch the action of the valve when applying air pressure (with a .8 bar spring, you should expect to need about 15 psi or so in order for the WG valve to open when tested off the car). And, you should be able to do a similar test with the manual boost control to confirm at which point it opens, and set it for a low opening point well below your WG normal setting, then trial-and-error slowly adjust it to achieve the extra boost you desire. Make sense?
A bad wastegate that won't open at all or opens too slowly/late will cause high boost spikes, obviously. The usual culprit is a torn rubber diaphram in the housing or a badly worn valve stem and/or seat allowing air pressure to escape.
Hey, at least you know your overboost protection circuits are working....I think...though it seems to be kicking in too early in my opinion.
You can bench test the WG by applying known air pressure to the bottom nipple. You can do it with the engine running if you like, and listen for the exhaust to escape as the valve opens. Or remove the WG from the car and watch the action of the valve when applying air pressure (with a .8 bar spring, you should expect to need about 15 psi or so in order for the WG valve to open when tested off the car). And, you should be able to do a similar test with the manual boost control to confirm at which point it opens, and set it for a low opening point well below your WG normal setting, then trial-and-error slowly adjust it to achieve the extra boost you desire. Make sense?
A bad wastegate that won't open at all or opens too slowly/late will cause high boost spikes, obviously. The usual culprit is a torn rubber diaphram in the housing or a badly worn valve stem and/or seat allowing air pressure to escape.
Hey, at least you know your overboost protection circuits are working....I think...though it seems to be kicking in too early in my opinion.
