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Your ignorance...
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399 Posts
Discussion Starter · #1 ·
I finally got my boost gause installed. Hooray I can see what the hell is going on! Here's the deal. I have an MBC, and I posted a while ago about it maybe not working right. With it turned all the way down, I will hit about 8psi and it is very stable- it goes up to 8 and stays there. When I turn it up maybe two screws in, I will shoot up to 11psi and thats where I hit cut, consistently every time just as I pass 11 I cut. I can turn it down gradually until I dont cut, but here's whee it gets weird. As I accelerate, it rises to 10psi, then as I continue to accelerate it drops slowly down to about 8-9 psi. If I had a leak, wouldnt it leak at 8 as much as at 10? Why else would I be losing boost above 8? wtf I am confused.
 

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Street Sleeper
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1,114 Posts
boost spike, can happen with MBC's because they can be slow to react so you go over the set point then fall back down to it as the controller stablises.
 

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Project:: Reaper
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1,401 Posts
yes boost spike. my car was fine on the 60-1 i have at 9psi but then i would turn it up and itd spike to 12 and then become stable around 10psi. now i have it turned to 15 and it doesn't spike it just hits 15 and stays.
 

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Destroyer of Turbos
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4,461 Posts
its also normal for the stock ct26 to run out of breath in the upper revs and drop some. the fix for this is to get the exhaust wheel clipped some! this however does increase lag.
 

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Your ignorance...
Joined
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399 Posts
Discussion Starter · #6 ·
about it spiking- that makes more sense than what I was thinking; that it was dropping off. As in, it is really set at 9psi. so that is where it is stable, but it spikes to 11. Not that it is set at 11, but drops down or leaks to 9. My piping isnt the greatest, so it is possible, but I dunno, it isnt bad that I know of. Have to make up a leak detector amd see about that.

HOWEVER- barring a leak, how do i fix the spiking issue? I dont think my turbo is incapable of 11psi. I actually put a weaker spring in the mbc, the other one was too hard, it would 'do nothing' or 'overboost always' there wasnt really any adjustment inbetween. I cant justify an ebc yet, I need other mods first.

Also, my other and last question: Is 11psi safe on my setup. Newb question, perhaps, but so be it. Stock motor, stock turbo, stock fuel, intake w/ cone filter, 2.75" dp with 2.5" exhaust. From what I have read, 11psi is pretty good for only those mods. The cut is raised with more air in/out, and more fuel. My question (okay I lied, this is my last question) is this: what determines where cut is? The ecu, I am sure, but how does it know where to cut? I dont want an FCD and just blow things up, I know it cuts to save the motor from detonation/lean out, but in what way does it know where to cut, it's not like it is a static 8psi via a pressure sensor or something.
 
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