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Boost Pressure Switch

Hello. If I install a 1.0 bar spring in the wastegate should I uninstall Boost Pressure Safety Switch (mount wire to ground)? or should I leave it?

Regards john

Old 08-17-2010, 11:45 PM
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Just leave it as is. The threshold is over 1 bar, and a really good safety in case your wastegate fails
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1979 930: Garret GT35r turbo, EFI, carerra intake, Link EMS, custom GT2 cams, 98mm JE P/C, 964 crank (stroker), custom valves & ported (XtremeCylinderHeads) etc..etc..
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Old 08-18-2010, 02:53 AM
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Had a friend who de-activated it.

Believe it cuts the fuel pumps when boost pressure reaches 1.1Bar.

(He'd put a K27-7200 turbocharger and freer-flowing exhaust system on his '84 930, and think the boost pressure was momentarily reaching that pressure.)
Old 08-18-2010, 07:57 AM
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Quote:
Originally Posted by tj930 View Post
Had a friend who de-activated it.

Believe it cuts the fuel pumps when boost pressure reaches 1.1Bar.

(He'd put a K27-7200 turbocharger and freer-flowing exhaust system on his '84 930, and think the boost pressure was momentarily reaching that pressure.)
I believe a new sensor is supposed to cut off the pumps at between 1.2-1.4 bar. The older they get, the more sensitive they seem to get and the threshold drops.
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Old 08-18-2010, 09:24 AM
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I had disconected mine ( grounded to a body) untill I decide which way to go.
Sensor was cutting pumps off at cruising speed w/no boost at all.

atleast for now I can enjoy my car
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Old 08-18-2010, 12:44 PM
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. . .and leaning out under high boost isn't exactly what you want anyway. The switch could do more harm than good.
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Old 08-18-2010, 01:19 PM
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The PO disconected the boost switch on my car and the peak boost pressures broke the rings in two cylinders of my engine. The switch is there for a reason and it is very effective. Remove it at your own peril.

Mark
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Old 08-18-2010, 04:31 PM
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how can boost switch do harm??

Last edited by jeos; 08-19-2010 at 12:00 AM..
Old 08-18-2010, 11:57 PM
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how can boost switch do harm??
It does good, not harm, by instantly shutting down the engine when boost levels get too high and threaten detonation (because the fuel delivery lags behind with the amount of hot air being crammed into the engine). With the fuel instantly cut off, the engine instantly looses power and boost instantly drops. It doesn't create a sustained lean condition because the fuel isn't gradually depleted...instead, there is no fuel at all delivered to ignite.

I'm with everyone else on this....keep it installed just as insurance. Unless you're running at the ragged edge of boost intentionally, the thing will never come into play and you'll not even know it's there.
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Last edited by mark houghton; 08-19-2010 at 04:50 AM..
Old 08-19-2010, 04:48 AM
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A brief overboost didn't break your rings. . .
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» 1980 931 - Got boost? ♦ 1987 924S - Pro44 ♦ 1987 924S - Junkyard score «

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Old 08-19-2010, 06:10 AM
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Rasta,
I never said the boost was brief. More likely it was a sustained peak pressure and this occurred many times on 0-60 runs. It did not help that the vacuum line from the manifold to the EZ69 and the manifold to the waste gate were switched. And there was a 1.1 bar spring in the waste gate. These conditions were bad but the over boost sensor would have prevented the over boost condition that ultimately broke the rings, that was the point of my post.

I showed the rings to several engine builders, with universal consensus, over boost.

Mark
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Old 08-19-2010, 05:17 PM
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Seems like the detonation that accompanied overboost was more likely the cause of your busted rings, but in any event, the sudden lean condition caused by loss of fuel pressure (overboost cutoff) could do just as much engine damage. The real solution is to carefully maintain your wastegate diaphragm and control pressure line.

In any event, I hope your car is back on the road!
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Old 08-20-2010, 08:32 AM
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Rasta,
See Mark's post #9 above. When the fuel pumps shut down you don't get a sudden lean condition, you get a sudden shut down condition.

You list a 1980 924 Turbo in your signature block, I don't know if you car has the same overboost switch or not. When my switch opened under hard acceleration both of my hands came off the steering wheel, that's how hard the engine kill is. If you have never experienced that, you really cannot understand the function of that switch.

This was the real reason for my overboost situation. There was never any detonation, none. The control pressure line is perfect.



The car is back on the road and running perfectly.

Mark
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Old 08-21-2010, 07:02 PM
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Aha, didn't notice the year of your turbo. My car has CIS like the early 930s, and the overboost switch kills the fuel pump but not the ignition. . .this could lead to a few cylinders firing lean before the injectors close due to lack of fuel pressure.
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Old 08-23-2010, 10:06 AM
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Quote:
Originally Posted by lucittm View Post
Rasta,
See Mark's post #9 above. When the fuel pumps shut down you don't get a sudden lean condition, you get a sudden shut down condition.

When my switch opened under hard acceleration both of my hands came off the steering wheel, that's how hard the engine kill is.

Mark
This experience has been appropriately coined as the "face in dash syndrome". Lucky if you don't break some teeth when it happens. Violent, to say the least, usually resulting in choice expletives...WTF!?!?!...until you realize it was just the overboost sensor protecting your investment.

Yes, logic does suggest that with no fuel, you get a lean condition. But we're splitting hairs here...that condition lasts for a micro-second, during which time the engine shuts down and no air is being drawn in. Although the ignition is still firing the plugs (the OB sensor only shuts off the fuel in a CIS 930), there is no heat of combustion going on - thus no risk of damage.

Good discussion! I still say that Porsche put that protection there for a purpose, and with our aging cars (and aging WG's) I vote for keeping that protection active.
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Old 08-23-2010, 11:21 AM
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Quote:
Originally Posted by mark houghton View Post
But we're splitting hairs here...that condition lasts for a micro-second, during which time the engine shuts down and no air is being drawn in.
Since we are splitting hairs, air is being drawn in, due to the car being in gear, moving, and on WOT.



But, as I said above, the best protection is periodic refresh of the wastegate diaphragm and careful monitoring of the control line.
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Old 08-23-2010, 11:28 AM
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[QUOTE=Rasta Monsta;5521698]Since we are splitting hairs, air is being drawn in, due to the car being in gear, moving, and on WOT.



But, as I said above, the best protection is periodic refresh of the wastegate diaphragm and careful monitoring of the control line.[/QUOTE]

Yes, OK, I can agree on all your points. But (there's always a but, and this is where the true hair splitting resides) if you've ever experienced the shutdown, after screaming WTF!?! the very first human reaction is to take your foot off the gas and stuff in the clutch. So, the throttle body will be closed at that point. And with remaining residual fuel pressure in the fuel distributor and no (little) air flowing into the engine with the TB closed, the fuel metering arm will return to rest and the metering plate will close off any air flow upstream of the TB as well.

In summary, as you said: Keep everything in good condition to start with and don't worry the small stuff.

This would best be further debated over a couple of beers
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Old 08-23-2010, 12:24 PM
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Hey, so it would. . .didn't realize you were in the neighborhood. FWIW, a few of us Pelicans hook up in NW Portland every month to do just that. . .

http://forums.pelicanparts.com/pacific-northwest-us-wa-id-ut-hi-ak/500446-oregon-pdx-beers-burgers-9-30-a.html
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Old 08-23-2010, 12:27 PM
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All right. I keep it in active mode.


Regards john
Old 08-25-2010, 05:03 AM
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Hello to you. I am reading your thread with interest. I have a problem with my 930, 1983 mod. I have changed the cams, changed the turbo to a K27 and put in an wastegate spring of 0,9 or 1,0 bar. Lately my car is shutting abruptly down when the boost reach 0,8 bar. Especially when I put the pedal on the flor at about 2500 rpm. If I take it gradually up with not so much boost nothing happens, the car works fine. I do not know what to look for so do you have any tips I can start with?
Sorry for my english, I live in Norway....

Old 09-13-2010, 12:42 PM
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