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Too Much Boost!

Ozzy10

New member
Joined
Sep 18, 2020
I've got a 1991 740 Turbo and have remove the old exhaust system. I have installed a 2.5 inch side exit (just before passenger rear tire) with a flowmaster fx and am now making 12-14 psi. On hard accelerations around those numbers and sometimes earlier, the car will stop accelerating abruptly. I believe this to be boost cut but was not sure as it sometimes cuts at around 9psi. The car is bone stock other than the exhaust and was making the factory 7psi or whatever before. I was expecting a minor rise in boost pressure but nothing like this. Any help would be appreciated as I am pretty new to this stuff.

thanks:)
 
The easy way to fix it is with a manual boost controller, or adjust the wastegate arm.

I ran 22 psi on my '91 740T, it had no boost cut ever, not sure why yours does.

Edit: you can overwhelm the MAF, but I have a hard time believing that is what is happening at those boost levels.
 
Okay will give that a try. I actually have already purchased an MBC with plans to turn up the boost and then figured I now wouldn't need it but its in the shop so I will definitely mess around with it. You weren't running 22psi stock motor were you? I was though there was a cut somewhere around there but I'll do some more research. Thanks for the advice!
 
Stock motor, 15G turbo, #46 injectors, 3" 960 MAF. I'm not saying it's ideal but it worked fine, air fuel ratios were good. Except a junkyard 15G will only last a couple years doing that, it was pretty toasted when I replaced it with a bigger turbo. Just was ballin on a budget back then, having fun.

Set the MBC to the stock level for now till you have supporting mods, it should get you back to where you were.
 
Ahh okay interesting. I will throw the MBC on tomorrow and see if that fixes it. Thanks again.
 
You can back probe a multimeter into the MAF signal----Approx 5V=Fuel Cut.
 
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I had a bad fuel pump that worked fine at stock boost levels, but when I added more boost it would cut out at higher RPM/Boost levels.
 
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