Main change in realtion to OD change

Discussion in 'PSI Superchargers Tech Questions' started by isracing, May 30, 2009.

  1. isracing

    isracing New Member

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    Just putting some feelers out for some input on how going up in overdrive relates to how you should tune the fuel in accordance.

    For example, I have a 410" sbc with a HHR 10-71 and after a major cylinder head upgrade from last year, same OD on the blower, a little more fuel, the thing went slower. My new heads flow close to 20% more than the old heads, valves are bigger, compression maybe has a tad more at about 11:1. Running 30* timing and a 120 in the main last race the car didn't do anything, went to 34* and a 110 and it didn't pick up anything at all this is all at 7.7% over.

    Now I'm going to throw some blower at it, feed these heads a bit going up to 20% on the blower, and it was happy in the plug readings with the .110. Is there a calculation or rule of thumb on how much more fuel to put in when you go up in overdrive?

    I plan to set it at 32* timing, and probably go with a .080 to start off. What are your thoughts. If any other info is needed let me know I'll be happy to add details.

    This is in a 3200lb door car on a true 10.5" tire running 1/8 mile
     
    #1
    isracing, May 30, 2009
  2. Bob69

    Bob69 Member

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    please describe or send pictures of your happy plugs.

    Sometimes when you only change the head flow it doesnt mean that anymore air and fuel will enter, at slower overdrives the blower is the restiction.

    In saying that when you chuck in more blower with the higher flowing head it will suddenly wake up and the difference is measurable.

    Would be conservative on the timing, and put more fuel in it. Check your idle temp (exhaust) and be prepared to adjust barrell valve.
     
    #2
    Bob69, May 31, 2009
  3. Mike Canter

    Mike Canter Top Dragster
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    It is far easier to relate fuel requirement to the amount of boost and boost change. Safely one pound of boost increase requires a .3 GPM increase and that is about a .004 change in a main jet around the 100 size.

    You never said if you had EGTs and is so what they read as a peak at the end of a run.

    The other thing you might want to consider is reading this article
    http://www.dragstuff.com/techarticles/reading-alcohol-spark-plugs.html
     
    #3
    Mike Canter, May 31, 2009
  4. isracing

    isracing New Member

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    I do not have any EGT's or data logging equipment on the car right now so just going off plugs.
     
    #4
    isracing, May 31, 2009
  5. Mike Canter

    Mike Canter Top Dragster
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    So do you correct your main jet size for changes in Density Altitude?

    On your last run with new plugs how did your "happy" plugs look: still had all the coating on the base circle with the ground electrodes gray, base circle light gray, base circle dark gray to black? or what? Are all the plugs colored the same amount?
     
    #5
    Mike Canter, Jun 1, 2009
  6. isracing

    isracing New Member

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    Electrode cad plating burned off, down around the ring, and slightly curled over on the hottest plug. I don't have a weather station so I don't know what alt. I was running at so this is all is going just by plug readings only. As I race more and learn more I'm picking up more stuff, a weather station will be next, or I'll find someone at the track and log what the air is at that time.
     
    #6
    isracing, Jun 1, 2009
  7. Mike Canter

    Mike Canter Top Dragster
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    Weather is important for a blown alky application. If you don't compensate for changes in Density Altitude you can chase your tail on a tuneup or go into detononation. As a rule of thumb for every 500 ft increase in DA you need a .005 larger mainjet. For every 500 ft decrease you need a .005 smaller mainjet. If you don't do this then the car will only run well when the weather comes to you.

    You need to a log book and a cheat sheet and start off with a tuneup for a specific density altitude. Make your run and check your ET and finishline MPH then when at the same DA make another run with a 10 larger or a 10 smaller mainjet and see what happens. If the performance improves then you are going the right direction. Once you have the best performance with the reading you want on the plugs then that is your baseline. Now adjust that baseline according to changes in DA.

    If you don't do this then one day when the DA is at one reading the car may run like crap then another day with a different DA it runs great but without tracking the DA and tuneup it will drive you crazy.
     
    #7
    Mike Canter, Jun 1, 2009
  8. isracing

    isracing New Member

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    Now the smallest main pill I have is .080 basing on the fact that it has a .110 in it right now and going off what I have given for information, would it be advisable to get some smaller pills?
     
    #8
    isracing, Jun 1, 2009
  9. WIZBANG

    WIZBANG Member

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    I can send you a Gorr weather station for $ 600,,, Spike calibrated it and fresh batteries about 4 months ago !
     
    #9
    WIZBANG, Jun 1, 2009
  10. Mike Canter

    Mike Canter Top Dragster
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    Do you have a pump sizer loop? There is no way I can tell you what main pill to run. Again that depends on the DA and the motor. You need to compute the DA and tune the car while making corrections for the DA. A 80 is going to make that motor real rich from running a 110.
     
    #10
    Mike Canter, Jun 1, 2009

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