Air density effect on roots blown?

Discussion in 'PSI Superchargers Tech Questions' started by overkill69, Jul 7, 2010.

  1. overkill69

    overkill69 Member

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    My promod ran 173 mph in 1400ft air this spring, made 42 psi boost. Last weekend it would only make 166mph and 38 psi at 3400ft with high water grains.Is this a typical loss for a roots blown alky setup? The screw cars seemed to run just as hard as usual.

    Thanks,
    Mike
     
    #1
    overkill69, Jul 7, 2010
  2. blowntrifive

    blowntrifive Member

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    That is about the norm for that much change in density. High humidty will lower your boost numbers and your ET. I ran as much as 3 psi difference in boost in high humid conditions. hope that helps.
     
    #2
    blowntrifive, Jul 7, 2010
  3. Mike Canter

    Mike Canter Top Dragster
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    Density altitude effects everyone. How high was the water grains. Normally you don't start seeing big problems until it gets over 100 in water grains but it is always there.

    Are you correcting your fuel tuneup for changes in density altitude?
     
    #3
    Last edited: Jul 8, 2010
    Mike Canter, Jul 7, 2010
  4. overkill69

    overkill69 Member

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    I leaned it out all day and the last session I sped the blower up 3%. It shook on the hit so I didn't get any data. It runs high teens in good air and low 30's was all I could get in Topeka.
     
    #4
    overkill69, Jul 7, 2010
  5. jim phillips

    jim phillips ta/fc

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    you can only lean it out so much in bad air sometimes your stuck with it unless you got a bad ass blower that will make tons of air
     
    #5
    jim phillips, Jul 7, 2010
  6. suddendebt

    suddendebt Blown Altered

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    How much did you get to run in topeka before the rain set in?
     
    #6
    suddendebt, Jul 7, 2010
  7. overkill69

    overkill69 Member

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    Qualifying was done, got through most of the first round.We didn't plan on qualifying but we were so busy we never really got to see any runs.
     
    #7
    overkill69, Jul 8, 2010
  8. Mike Canter

    Mike Canter Top Dragster
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    Overkill how are you computing the change in fuel based on Density Altitude changes. Are you using some tuning software or what?
     
    #8
    Mike Canter, Jul 8, 2010
  9. overkill69

    overkill69 Member

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    Mike, i'm just guessing at it. I've got a little data from some past runs. Seems like the car used to have a good size tuning window as far as cad left on the plug. Now that it's making more power/boost it seems like it needs all the cad burned off the end and turning frosty white. I've been told it's normal for a wedge with only 10.2-1.

    I leaned it .005 every pass at Topeka and didn't see any gain so I was almost thinking about richening it. That would've been a stupid plan.
    When I sped up the blower on the last hit I had to richen it again and I had no past data so I don't really know where I was at. I had to go conservative.

    I'm getting a flow meter on the car this week. It's time for some tuning software. Is there anything simple/economical. I know about Davenport.

    Thanks,
    Mike
     
    #9
    overkill69, Jul 8, 2010
  10. Mike Canter

    Mike Canter Top Dragster
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    You should be leaning it out 5 on the main jet for every 500 ft gain in density altitude. Like you said the problem is getting a baseline to work from which is also a problem with any tuning software. If you keep good records with tuneup and weather data then you should go to the best run you have ever made and use that main jet setting and Density Altitude as your baseline and compute the changes in DA from there and adjust that main jet from that setting by 5 on every 500 ft change or 10 for every 1000 ft change. You can adjust your baseline slowly richer or leaner to see if the car gets any faster and then establish that as your new baseline.

    So if you had a mainjet of 85 with a 1500 ft DA as a best run so far then at 3000 ft you would have a main jet of 100. If you increase the main jet by 10 again and it goes faster yet or the plugs look better then make a note that your new baseline is a 95 jet at 1500 ft and work the changes always from there. This is really all that Davenport's software is doing. We adjust the "Tuning Modifier" in the software until we calibrate it to the car then change the Tuning Modifier a small amount at a time until we get the fastest and safest setting.

    Some guys have a Excel Spreadsheet that does basically the same thing as Jetsize. Maybe somebody out there can email you one.

    Now if you change the boost then you have to adjust the baseline again. The norm is to add .3 GPM for every pound increase in boost and that computes out roughly to a .006 decrease change in the mainjet. Just remember that you always need to work from a baseline tuneup or you will be chasing your tail.
     
    #10
    Mike Canter, Jul 8, 2010
  11. blownhemi1

    blownhemi1 New Member

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    if anyone has the Excel spreadsheet I would love to take a look.
     
    #11
    blownhemi1, Jul 8, 2010
  12. outlaw02camaro

    outlaw02camaro New Member

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    mike

    on your general 500ft equals 5 on the main, does water grains or anything else a variable or is it pretty much da? thanks
     
    #12
    outlaw02camaro, Jul 8, 2010
  13. Greg Kelley

    Greg Kelley Member

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    My junk takes a big dump when it gets over 90 and the resulting 3000' DA's. I'm dealing with too tight of a converter for my low boost bracket racing setup, but I'm thinking air temp is my issue.

    I'm Procharger compared to you REAL blower guys, so don't know if that's a factor. I'm seeing air temps in excess of 250* (after the blower - EFI fuel injectors low in the manifold). I'm guessing with 70* ambient my temps are more like 180.

    I just added an air temp sensor the my Racepak, hope to understand what's going on.

    GK - http://www.motorsportsinnovations.com/Race-Team/procharger1.htm
     
    #13
    Greg Kelley, Jul 8, 2010
  14. Mike Canter

    Mike Canter Top Dragster
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    Water grains effect all who are racing so when they get above 100 then all are having problems. Yes, you can increase compression ratio or turn up the boost BUT most everyone has optimized their compression and max'd out their boost anyhow. As far as I know changing the AFR will not help in overcoming high amount of water in the air.
     
    #14
    Mike Canter, Jul 8, 2010
  15. overkill69

    overkill69 Member

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    Thanks Mike

    As always good info MikeC...I've been tuning pretty much to your plan, I just underestimated the effect of the DA on my setup. I've been creeping up on the higher OD for my blower ,just haven't had enough laps on a good track to get there. I've still got 2 degrees of total timing in reserve too.
    The next time we hit a good track will be Tulsa in October which is usually 1000ft DA.
     
    #15
    overkill69, Jul 9, 2010
  16. JP

    JP Member

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    Mike that is as good explanation as I have heard on how to work jetsize, I just want to add to it....

    After you have made your baseline data and are using the tuning modifier as a tool to adjust your setup for weather changes you have to keep the psi of the system constant so the same "fuel curve" is used.... in order to do just that..We have a single nozzle in the hat tha we make bigger or smaler in both the car and on jetsize that way we can make small and quick changes on race day..

    Also when big density changes occur (more than 3000) jetsize manual has the appropiate instruction to figure out your new jets and pressure opening points.

    Mike feel free to correct me as needed...

    Thanks
    JP
     
    #16
    JP, Jul 11, 2010

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