Increasing overdrive

Discussion in 'PSI Superchargers Tech Questions' started by doorslammer, Aug 9, 2012.

Page 1 of 2
  1. doorslammer

    doorslammer New Member

    Joined:
    Aug 9, 2012
    Messages:
    11
    Likes Received:
    0
    We have a 525 ci big chief wedge motor ,SSI 1471 makes about 30 lbs boost at 20%.Looking at making small increase to 28%. How much more fuel will we require. Can we just make a pill adjustment or do we need to add across the entire system. we have an enderle pump 14.5 gpm.
     
    #1
    doorslammer, Aug 9, 2012
  2. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    I show you would make 6.25% more boost which is 1.875 lbs increase. If you compute that you are using approx .36 GPM per one pound of boost right now (way safe) that would mean that you would need .675 GPM more of fuel. That means you need to close your mainjet down by about 11 sizes. Given that data you have supplied that is all approximate but should oput you in the ballpark.
     
    #2
    Mike Canter, Aug 10, 2012
  3. Blownalky

    Blownalky Top Sportsman

    Joined:
    May 9, 2005
    Messages:
    826
    Likes Received:
    8
    So what is the math that gets us the change from 20% to 28%? I'm sure I used to know this but how do we get a 6.7% change in air volume?
     
    #3
    Blownalky, Aug 10, 2012
  4. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    You dont. You get a 6.25% change
     
    #4
    Mike Canter, Aug 10, 2012
  5. Blownalky

    Blownalky Top Sportsman

    Joined:
    May 9, 2005
    Messages:
    826
    Likes Received:
    8
    I really must need coffie, even though I don't drink it LOL. Going from 20% over to 28% over is a 6.25% change, how? I know it's not 8% difference but how do you get to a 6.25% change in overdrive between 20% and 28%?
     
    #5
    Blownalky, Aug 11, 2012
  6. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    You have to work actual blower RPM difference and not percentage of OD difference. Are you trying to figure the percentage change of the percentage OD? Do this...... 20% OD and if the motor is at 9000 rpm means the blower is at 10800 rpm. Now if it is 28% OD at 9000 engine rpm then the blower is now at 11520 rpm. So is not the change in blower RPM from 10800 to 11520 a 6.25% change? Somebody correct me if I am wrong. This is the way I have always done it.
     
    #6
    Last edited: Aug 11, 2012
    Mike Canter, Aug 11, 2012
  7. Micetich

    Micetich Member

    Joined:
    Oct 5, 2006
    Messages:
    76
    Likes Received:
    0
    I have always used percent of overdrive change and works great. (1.28-1.20)/1.20=.067 or 6.7% Figure the amount of change and then divide by current od. If you are tuning to keep system pressure the same you may need to make a few more change than just a main.
     
    #7
    Micetich, Aug 12, 2012
  8. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    I don't think you can work it that way using percentages in the formula. I know people do it but it is not correct and not representative of the actual percentage of RPM change to the blower.

    Compute it again based on the actual RPM change seen by the blower and you will see that it is different. Which one do you think is more accurate?

    120% of 9000 engine RPM is 10800 blower rpm
    128% of 9000 engine RPM is 11520 blower rpm
    This percent difference of the two blower RPMs is 6.25%
    If you were making 30 lbs of boost at 120% and increased the blower by 6.25% then that is a 1.875 lb boost increase. At an average of .36 GPM of fuel per one pound of boost then that requires a .675 gpm of fuel increase. Depending on the fuel pressure and I used 150 psi then that is an 11 main jet size change.

    I agree that the difference between 6.75% change and 6.25% change is not much but it means the difference between a .675 gpm change and a .729 gpm change. That maybe close enough for some.
     
    #8
    Mike Canter, Aug 12, 2012
  9. Blownalky

    Blownalky Top Sportsman

    Joined:
    May 9, 2005
    Messages:
    826
    Likes Received:
    8
    Got what you are saying now.

    Going from 20% over to 28% over:
    Percentage of change overdrive: 128-120=8 8/128=0.0625 which is = 6.25%
    Percentage of change blower RPM: 11520-10800=720 720/11520=0.0625 which is = 6.25%

    So when you go to bigger overdrive numbers, this will change. Yeah, I know on a roots this number for blower speed to too much. Just using the number to illustrate the difference even though the change is still only 8% from one overdrive figure to another.

    Going from 40% over to 48% over:
    Percentage of change overdrive: 148-140=8 8/148=0.05405 which is = 5.41%
    Percentage of change blower RPM: 13320-12600=720 720/13320=0.05405 which is = 5.41%

    Which is different using the below formula:
    20% to 28%= 1.28-1.20/1.20=.067 which is 6.7%
    40% to 48%=1.48-1.40/1.40=.057 which is 5.7%
     
    #9
    Blownalky, Aug 12, 2012
  10. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    Either way will get you close but one closer than the other. If you know your now GPM per pound of boost before the change then you don't have to use .36. That is just a real safe number but you can use the real number if you have a flow meter.
     
    #10
    Mike Canter, Aug 12, 2012
  11. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    This is statistics as the government does it. It depends on which blower speed is the reference.
     
    #11
    Mike Canter, Aug 12, 2012
  12. caseyspradlin

    caseyspradlin Member

    Joined:
    Sep 9, 2008
    Messages:
    214
    Likes Received:
    0
    charts

    so the charts that are everywhere on the internet are just using the difference in pulley teeth correct? According to Mike, this is not my actual blower OD. So if I initially have a 66 tooth bottom and a 55 tooth top, chart says that is 20%, should any blower rpm or efficiency be figured in the make this a different OD?
     
    #12
    caseyspradlin, Aug 13, 2012
  13. aj481x

    aj481x Member

    Joined:
    Jan 27, 2004
    Messages:
    976
    Likes Received:
    18
    Blower efficency should be considered in these calculations,as you reach a point where you don't make anymore power no matter how fast you turn it.
     
    #13
    aj481x, Aug 13, 2012
  14. doorslammer

    doorslammer New Member

    Joined:
    Aug 9, 2012
    Messages:
    11
    Likes Received:
    0
    Thanks for the input Mike. Thats pretty close to what we thought but there was lots of discussion just like this thread. But just to confirm , when you say 11 jet sizes it would be from say a 130 to a 119?
     
    #14
    doorslammer, Aug 13, 2012
  15. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    Yes. 130 to 119
     
    #15
    Mike Canter, Aug 13, 2012
  16. Blownalky

    Blownalky Top Sportsman

    Joined:
    May 9, 2005
    Messages:
    826
    Likes Received:
    8
    This is always fun to review. :eek:
     
    #16
    Blownalky, Oct 7, 2015
  17. KeithDyer

    KeithDyer Member

    Joined:
    Jun 9, 2009
    Messages:
    47
    Likes Received:
    0
    Yes . . . , it certainly is!!!
     
    #17
    KeithDyer, Oct 7, 2015
  18. turbo69camaro

    turbo69camaro Member

    Joined:
    Nov 26, 2007
    Messages:
    250
    Likes Received:
    10
    lol i would think you would need flow/CFM dyno numbers on the blower to get a true amount of air flow increase per amount of O.D change in order to make a true fuel system change.
     
    #18
    turbo69camaro, Oct 7, 2015
  19. Mike Canter

    Mike Canter Top Dragster
    Staff Member

    Joined:
    Mar 10, 2004
    Messages:
    4,630
    Likes Received:
    189
    each blower is different and each motor is different. It is a lot easier while at the track to use a rule of thumb method and change the fuel then read the plugs. Changing the OD also changes the fuel distribution pattern to the cylinders so you may have to balance the fuel flow again. Yes, I am sure that if we all had a blower dyno we could predict the change more accuratly. But if you know what you are doing then it only takes one run to correct the fuel.
     
    #19
    Mike Canter, Oct 7, 2015
  20. Bjs344

    Bjs344 Member

    Joined:
    Sep 10, 2013
    Messages:
    358
    Likes Received:
    24
    I run 8 mm and use .004 per tooth, if I don't have any better data, and then read plugs. Sometimes I round down another . 005 or so depending on how confident I am. I also have moved 1 tooth per 400 ft while leaving the pill alone when trying to run the same number. Results have been hit or miss on that.
     
    #20
    Bjs344, Oct 7, 2015
Page 1 of 2

Share This Page





XenPorta 2 PRO © Jason Axelrod from 8WAYRUN.COM