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Discussion Starter #1
So I decided to finally swap out my 3.33" pulley on my S trim to put on a 3.12. I thought it would maybe see 1-2 psi change, but boy was I wrong!
I have an old 5.0 with 3bars on it and Cobra intake. My boost went from 10-12PSI on the 3.33 up to 18 or so with the 3.12.

I can only assume it went this high due to how restrictive my top end is? Does that make sense to anyone else? Or do I have a complete anomoly on my hands?
 

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That's kind of nuts. Maybe the old pulley was slipping?
 

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Discussion Starter #3
I had that thought initially too, but I doubt it. I show no indication of it ever slipping. No dust or erratic boost.
The new pulley though does feel "grippier" though. Almost like there are tiny serations on the ribs, but otherwise looks like a standard pulley. I did give the new pulley/Belt setup like another 10 ft lbs of tq when I set it, but I dont think it was related to slip.
I did put the old pulley back on just because though. I just think throwing that much extra air in those little head and intake just does not equate to enough hp for me to want to chance it right now. Was more curious as to why I saw such a big increase in boost levels
 

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Discussion Starter #6
What you are measuring is restriction to flow

You caused a larger restriction, hence the larger pressure

Pressure goes up, flow goes down
True. I figured that was the case, but wanted to make sure I was not nuts. Thanks for confirming
 

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Discussion Starter #7
How big is your crank pulley? 10-12 out of the 3.33" is kind of high in my opinion.
stock 6.87. Its a stock GT40/Cobra intake car. It doesnt flow, so boost will be higher. Plus it has a power pipe. If I threw a free flowing top end on it, boost would go down a bit.
 

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stock 6.87. Its a stock GT40/Cobra intake car. It doesnt flow, so boost will be higher. Plus it has a power pipe. If I threw a free flowing top end on it, boost would go down a bit.
Look at the compressor map to see what is going on

General statements like you just made might not be accurate
 

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Discussion Starter #9
Look at the compressor map to see what is going on

General statements like you just made might not be accurate
Might not be accurate how? Every car I have seen with a centrifugal has needed less boost to make higher numbers with free flowing top end. Everyone I see that swaps alum heads on, their boost goes down and power goes up.
Im making +500 at the motor on 3bars that have not been touched and cobra intake that has not been touched. There is quite a bit of restriction in there. Im sure that blower was being overworked though at the 3.12 pulley boost level it was making so I put the 3.33 back on. No sense in spinning it that hard to cram in all that air that it cannot do anything useful with. Again, I was only expecting a small bump in boost with the small pulley change. Was not expecting to see 6-8psi when other people and Vortech themselves said likely 1 up top, 2 if I was lucky
 

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Easy

Cfm

Pressure ratio

It’s how you size and select the compressor

You must remember, you are dealing with a non positive displacement device
 

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The smaller pulley is pushing you off the map. At what rpm are you seeing 18 psi? Your P/R is 2.2 (at 18 lbs), but at ~42,000 impeller rpm (which should be around 5,500 engine rpm), you should be around 1.9.

I think you're right - a better flowing top end will puts things closer to the sweet spot and make more power - not just from additional airflow, but also because of less heat (lower boost) and the blower will be closer to it's efficiency peak.

The 3.33 pulley gives you about 40,000 impeller rpm at 5,500 engine rpm, and p/r of 1.74 and that's inside of where it should be.
 
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