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Smaller Pulley = less boost???

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  • #1

    Smaller Pulley = less boost???

    Hey guys I swapped my 3.33" S Trim pulley for a 3.13" last night in hopes of hitting the magical 15#'s of boost. To my suprise I fell from a previous high of 12.8 back to 11.8-12.0. I think I have good belt wrap and it's already so tight I could play it like a guitar. I see no belt dust either. I'm beginning to wonder if it's belt slip or if I have a leaking hose. If I had a leak my AFR's would log really rich though right? My boost curve is really linear, just lower than before. Is there some sort of belt dressing I can apply just to verify if it's indeed belt slip? If it is I just can't see getting the belt any tighter.



    If I can verify the issue to be belt slip then I'll have a custom lower made so I can swap out the outer ring from a Gen 2 Lightning. 10" lower and 3.33" upper should put me just above 50,000 rpm by 5000 rpm on the tach. I would like to keep a 5000 RPM shift point.
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  • #2
    Belt slip is the only thing that makes sense. Belt dressing will let the belt slip quietly . Don't use it !
    The only difference between a tax man and a taxidermist is that the taxidermist leaves the skin. -- Mark Twain

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    • #3
      Mark the belt and the pulley and rev it up a few times, see if it's slipping.
      "When the people find that they can vote themselves money, that will herald the end of the republic." -Benjamin Franklin
      "A democracy will continue to exist up until the time that voters discover that they can vote themselves generous gifts from the public treasury." -Alexander Fraser Tytler

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      • #4
        x234 on belt slip.
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        • #5
          Originally posted by CJ View Post
          Mark the belt and the pulley and rev it up a few times, see if it's slipping.

          How's that going to work?
          Sent from my PC36100 using Tapatalk 2
          2015 F250 Platinum

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          • #6
            Originally posted by fordracing19 View Post
            How's that going to work?
            Sent from my PC36100 using Tapatalk 2
            Same way I did it with mine, if the belt is slipping the two marks will not align. If it is not slipping you can turn the crank and the two marks will line up. It's not rocket science. When I had the alternator pulley on mine to get 15# I had to get the belt so tight it ended up bending my blower bracket.
            "When the people find that they can vote themselves money, that will herald the end of the republic." -Benjamin Franklin
            "A democracy will continue to exist up until the time that voters discover that they can vote themselves generous gifts from the public treasury." -Alexander Fraser Tytler

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            • #7
              | <---mark on pulley
              | <---mark on belt

              I'd free rev it first, then check, then go romp on it on the street and check
              | = good
              | = good


              | =bad
              ----| =bad
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              • #8
                Originally posted by CJ View Post
                Same way I did it with mine, if the belt is slipping the two marks will not align. If it is not slipping you can turn the crank and the two marks will line up. It's not rocket science. When I had the alternator pulley on mine to get 15# I had to get the belt so tight it ended up bending my blower bracket.
                Already did that, so I made another onr out of .500" 7075 aluminum. No more bend. I'll try the mark.

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                • #9
                  I reconfigured my pulley tensioner to get a bit more belt wrap.

                  Doug

                  90 LX Coupe 5.0
                  90 7up Vert. 5.0

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                  • #10
                    Originally posted by BlueCoupeRedVert View Post
                    I reconfigured my pulley tensioner to get a bit more belt wrap.

                    That's also what I ended up doing, it finally solved my problem.
                    "When the people find that they can vote themselves money, that will herald the end of the republic." -Benjamin Franklin
                    "A democracy will continue to exist up until the time that voters discover that they can vote themselves generous gifts from the public treasury." -Alexander Fraser Tytler

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                    • #11
                      Originally posted by CJ View Post
                      Mark the belt and the pulley and rev it up a few times, see if it's slipping.
                      ^ this.

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                      • #12
                        Ok I had an interesting datalog today. I got on the truck in 1st and the tires broke loose from the get go. I guess the lack of load altered the shift point and the truck reved to 6000 RPM in the datalog. I looked at the logged boost and it said 15.3 #'s.

                        4500---------7.7#'s
                        5000---------9#'s
                        5500---------14.4#'s
                        6000---------15.3#'s (52400 rpms)

                        I compared it to the boost observed once it hit 2nd and saw about the exact same numbers only it shifted at 5100 rpm so I saw 9.8#'s. It seems there is a huge jump in boost between 5000 and 5500. Is this being caused by the effeciency of the compressor or just the fact that my heads and cam don't flow well beyond 5200 and thus the boost shoots way up? If it's impellar speed related then I can pulley the crank size up or the blower pulley down so that I see the 52000 RPMS at 5000-5200 instead of 6000. I just don't want to rev it to 6000 rpms but I like the way it pulls when the boost levels hit 15 #'s. I want it to pull like that at 5000 RPM. If all it takes is impellar speed then I would need a 2.75" blower pulley to reach that by 5000 RPM. That seems too small to grip, so a larger 9" crank pulley would be the right way to do it. Plus when it shifts from 1-2 the rpms fall to 3800 and that's only like 4 pounds of boost. I'd like to see 8-9 #'s at 3800 RPM, and 15 #'s at 5200. I think the 3.13" blower with a 9" crank would give me what I want. That would be an impellar speed of 51584 RPM by 5200 RPM.

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                        • #13
                          Belt had moved about 1/4" from the mark on the pulley.

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                          • #14
                            Originally posted by Diabolic View Post
                            Ok I had an interesting datalog today. I got on the truck in 1st and the tires broke loose from the get go. I guess the lack of load altered the shift point and the truck reved to 6000 RPM in the datalog. I looked at the logged boost and it said 15.3 #'s.

                            4500---------7.7#'s
                            5000---------9#'s
                            5500---------14.4#'s
                            6000---------15.3#'s (52400 rpms)

                            I compared it to the boost observed once it hit 2nd and saw about the exact same numbers only it shifted at 5100 rpm so I saw 9.8#'s. It seems there is a huge jump in boost between 5000 and 5500. Is this being caused by the effeciency of the compressor or just the fact that my heads and cam don't flow well beyond 5200 and thus the boost shoots way up? If it's impellar speed related then I can pulley the crank size up or the blower pulley down so that I see the 52000 RPMS at 5000-5200 instead of 6000. I just don't want to rev it to 6000 rpms but I like the way it pulls when the boost levels hit 15 #'s. I want it to pull like that at 5000 RPM. If all it takes is impellar speed then I would need a 2.75" blower pulley to reach that by 5000 RPM. That seems too small to grip, so a larger 9" crank pulley would be the right way to do it. Plus when it shifts from 1-2 the rpms fall to 3800 and that's only like 4 pounds of boost. I'd like to see 8-9 #'s at 3800 RPM, and 15 #'s at 5200. I think the 3.13" blower with a 9" crank would give me what I want. That would be an impellar speed of 51584 RPM by 5200 RPM.
                            In the lower gears it might not have time to build enough pressure (boost) before the shift.

                            Originally posted by Diabolic View Post
                            Belt had moved about 1/4" from the mark on the pulley.
                            Might want to see about some options with the tensioner...
                            Originally posted by PGreenCobra
                            I can't get over the fact that you get to go live the rest of your life, knowing that someone made a Halloween costume out of you. LMAO!!
                            Originally posted by Trip McNeely
                            Originally posted by dsrtuckteezy
                            dont downshift!!
                            Go do a whooly in front of a Peterbilt.

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                            • #15
                              Honestly I don't think I'm getting much slip. My biggest question is why the large jump in boost from 5000-5500 RPM. Is it going up so fast because the engine is getting restrictiive due to the small heads and small cam, or is it just in the perfect efficiency range of the S-Trim? At 5000 RPM it would be at 44,000 and at 5500 it would be at 48500.

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