RPM Range for the Idle/Progression to Main Circuit Transition? | FerrariChat
Buy

RPM Range for the Idle/Progression to Main Circuit Transition?

Discussion in 'Technical Q&A' started by newto308, Apr 22, 2013.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, Skimlinks, and others.

  1. newto308

    newto308 Karting

    Mar 20, 2007
    92
    McLean, Virginia
    Full Name:
    Chris
    #1 newto308, Apr 22, 2013
    I realize there are many variables in this equation, but with that being said, is there a fairly standard RPM range where the idle circuit/progression holes transition to the mains on the Weber DCNF?

    I got my first 100 miles on my rebuilt engine and just drained the break-in oil. I can start running up to 4500 RPM now. I'd like to get some understanding of where this transition should occur so I can begin to formulate Jet,E-Tube, A/C and ignition timing issues.

    Any thoughts appreciated
     
  2. Motob

    Motob Formula 3
    Professional Ferrari Technician

    Nov 11, 2003
    2,387
    Frederick, Maryland
    Full Name:
    Brian Brown
    #2 Motob, Apr 23, 2013
    The transition is dependent on both RPM and throttle opening, so there is no set RPM range were the transition occurs. Carburation adjustments are the last thing that you do. Ignition timing must be sorted out before adjusting carburators.
     
  3. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
    Full Name:
    Philip
    #3 pma1010, Apr 23, 2013
    Chris, I had a 308 and changed carbs (44s) and cams (to P6) and ignition systems. I ran a wide and AF meter to help understand optimal jetting.

    The 76 factory WSM provided rpm ignition curves and these were a good place to start. While you can tune the shape of the advance curve, keeping the total advance to the maximum Ferrari recommened at 5000 rpm was important. Our race twin cam engine didn't go much above 38 degrees BTDC which as I recall was the Ferrari max too.

    On jetting recommendations, the idle, main and AC jets interrelate, and as we tended to fatten up the main and AC jets for bigger venturis (34-36), the idle jet could be reduced to provide crisper response. IIRC, the transition to mains starts around 1800 rpm and they are all the way in by 3000 rpm.

    I am not sure what modifications you've done, but with lumpy cams, I set the idle jets and bleed screws to make the car run. While we modified the ports to enable hooking up manometers for accurate balancing, in reality there was so much reversion below about 2000 rpm that they were pretty approximate at best. I don't recall the jetting we used for WOT running (I'd see 11.x A/F ratio on top straight at Road America) but would generally travel with a box of jets and see what worked best. The wideband provided a wealth of information.

    My advice would be to use Ferrari's standard jetting as a starting point if your motor is basically stock. After the ignition, get the float heights right, then look at jetting. If you've modified and Mike at Pierce is still around, he knows Webers intimately and should be able to provide you with good directional advice.

    Philip
     

Share This Page