Main Jet Change Rule of Thumb

Discussion in 'PSI Superchargers Tech Questions' started by Critical Mass, Nov 5, 2013.

  1. Critical Mass

    Critical Mass Member

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    What is a good rule of thumb for a main jet change if you go up or down 2 teeth on an 8mm pulley change with a 14-71 roots blower on a 548 in3 chevy motor?

    I've heard different numbers and I think they are referring to 14mm stuff?
    Thanks in advance, Joe
     
    #1
    Critical Mass, Nov 5, 2013
  2. Soldierboy0098

    Soldierboy0098 Active Member

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    Main Jet

    I have heard and used a theory that seemed to work well, for every lb of boost gained add .33 gpm the engine so I would figure out how much boost you think you'll be gaining or loosing then look at the current main jet and make your calculations from there. I always go a little fat then slowly lean it back in just to be safe. Remember spark plug reading is your friend.

    Hope that helps a little.
    Trevor Sherwood
    727-364-4368
     
    #2
    Soldierboy0098, Nov 6, 2013
  3. Critical Mass

    Critical Mass Member

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    Thanks Trevor for your response.
    Joe
     
    #3
    Critical Mass, Nov 6, 2013
  4. skeen

    skeen Member

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    rule of thumb that I have seen & used myself is 5 thousands increase per tooth change, works VERY well & will get you VERY close, PROVIDED your tune up is in the ballpark at your starting point.
     
    #4
    skeen, Nov 8, 2013
  5. Mike Canter

    Mike Canter Top Dragster
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    Forgive me but I am confused. When you say .005" per tooth change what pulley and what if you change both pulleys to go up a small percentage overdrive. How do I compute change on that? Let's say I run a 14.5% OD on a 8 mm belt using a 55 top and a 63 bottom. I want to go to 20% and I can use 50 top and 60 bottom because I don't have 66 bottom to go with the 55 top.
     
    #5
    Last edited: Nov 8, 2013
    Mike Canter, Nov 8, 2013
  6. aj481x

    aj481x Member

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    Somebody just had to make this complicated. :D
     
    #6
    aj481x, Nov 8, 2013
  7. Critical Mass

    Critical Mass Member

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    Sorry my bad. Mike you are correct; I didn't provide enough information. I use a 70 tooth 8mm bottom pulley with the following top pullies and overdrives: 57/22.8; 56/25.0; 55/27.3; 54/29.6; 53/32.1; 52/34.6; 51/37.3; 50/40.0; 49/42.9; and a 48/45.8. As you can see with a 70 tooth pulley on the bottom I have a lot of overdrive options. I am looking for easy rule of thumb for jet changes if I change overdrive ratios.

    Also, if I change timing from 30 deg. to 32 deg. this would also effect my main jet choice?

    Any advice would be appreciated. Thanks, Joe
     
    #7
    Critical Mass, Nov 8, 2013
  8. Mike Canter

    Mike Canter Top Dragster
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    Changing the timing does not change the AFR. There is only one correct AFR. Changing the timing does change the temperature inside the combustion chamber so you will see the plug reading change which makes it real confusing. The great thing is that the best timing for most blower motor combinations is known by racers on this forum so you can put the motor at the best timing and then tune for best AFR by reading the plugs. Let me add that too much ignition advance can be made to be safer if more fuel is added but that does not make it correct but does make it confusing. Also by retarding the ignition can make the motor act like it is too rich.
     
    #8
    Mike Canter, Nov 8, 2013

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