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Discussion Starter #1
I’m having some interesting boost numbers lately. Usually the car stays between 5-7psi at WOT throughout the gears. But lately, specifically after swapping a new alternator in and adjusting the spacing on the S/C idler pulley repairing a feed line leak. I have seen numbers as high as 11psi. Could there be an issue starting to occur?

91 5.0 with Gt40 intake. Stock cam and heads
Vortech S trim with Anderson power pipe and pro-m maf/cone
 

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Whats your pulley combo? 11 lbs sounds normal with a powerpipe and stock pullies.

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He needs to loosen up the S/C belt a bit to get back to his 5-6 psi.
 

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Discussion Starter #5
Lol. It’s not a belt tightness thing. Belt was always tight. It goes to 11 psi then drops down to 5-7. Sorry I should’ve clarified.
 

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The original post tells us your belt is probably slipping. You'll either make 5 psi or 7 psi but not 5-7 psi. If you are under boost and the needle of your boost gauge is moving around and jerking back and forth between two numbers that is the belt slipping. It should smoothly climb as rpm increases and once it gets to a certain point it will level off.
 

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Discussion Starter #8
It could be belt slip. Just very low boost to be doing that and on a stock pulley setup. The charger came with a 10 rib setup. But the s/c pulley is small, 2.50. I have a feeling it’d be around 15-20 psi. And I really don’t want to blow shyt up right now.
 

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It could be belt slip. Just very low boost to be doing that and on a stock pulley setup. The charger came with a 10 rib setup. But the s/c pulley is small, 2.50. I have a feeling it’d be around 15-20 psi. And I really don’t want to blow shyt up right now.
So what pullies are on it now? A 2.5" pulley would make a lot of boost but it will slip as well because the blower pulley is too small. The better way to increase boost is to increase the size of the crank pulley.
 

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3” s/c pulley and (I think) 6 1/4” crank pulley 6 Rib. I’m not sure if the 10 rib would slip or not.
It would be better to get a 10 rib 3" compressor pulley and run the 10 rib. That is for sure. The 2.5" blower pulley will make too much boost for you, probably 20 psi or so. You'd have trouble managing that without running E85.

That 6 rib setup is your issue though. If your boost gauge needle is jumping around under boost or the car makes more boost when it is cold it is slipping.
 
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