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Dist Mechanical Advance

Discussion in 'Technical Q&A' started by Mike Florio, Jan 13, 2004.

  1. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    580
    NW Rural Nevada
    Full Name:
    Mike Florio
    Hi, Guys
    I finally finished rebuilding my Allen Distributor Machine and updating the electronics. It works great, and now I'm jumping into the distributors (Marelli type S 159 A dual point) from my carbed 1975 308/GT4. The advance curves were way off, non-linear and mismatched. I will be photographing and documenting all my work, and plan to submit my results as a tech article to this forum. I am determined to find out and document the effect of the springs and weights, and develop a procedure for correcting their operation

    I have a couple of questions for your collective wisdom

    (1) The workshop manual shows the advance curve for R1 as linear from 0 degrees at 600 RPM distributor speed to 13 degrees at 3400 RPM distributor speed with the static advance of 6 degrees BTDC (Max advance of 19 degrees)

    The emission control supplement shows 0 degrees at 600 RPM to 5 degrees at 900 RPM, then linear to 16.5 degrees at 3000 RPM with the static advance of 3 degrees ATDC (Max advance 13.5 degrees).

    Which one should I be shooting for on a motor with no emissions controls (exempt)?

    What should the maximum overall advance be? (Static timing + Centrifigual advance).

    (2) Lubrication of the advance mechanism. Mine had been totally greased up inside with pretty heavy grease. I cleaned it all out and then checked for recommendations. They ranged from light machine oil to Bosch distributor grease. What I've settled on is Bosch distributor grease on the posts, sliders and shaft. Light oil on the springs, pins and caps. Any comments?

    You're probably thinking "Why not just go to a modern single elecronic distributor set-up?" You're probably right, but I am determined to make this machine work the way Enzo and the Boys intended.

    Thanks in advance for the help.
     
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  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    21,159
    Texas
    Full Name:
    Steve Magnusson
    I believe the (completely linear) distributor advance curve shown in Section L corresponds with the S159A distributor and is used with the idle timing set to 6 deg BTDC on R2 (which would be 16 deg BTDC on R1 at idle). At 6000 RPM (crankshaft) the distributor adds 2 x 11 deg so the total (crankshaft) advance at 6000 RPM on R1 is (16 + 2 x 11) = 38 deg BTDC.

    The multi-slope distributor advance curve (in Section O) is for the later S159B distributor (it matches the curves shown in the 150/78 OM) and is used with the idle timing set to 3 deg ATDC on R2 (which is 7 deg BTDC on R1 at idle). At 6000 RPM (crankshaft) the distributor adds 2 x 16.5 deg so the total (crankshaft) advance at 6000 RPM on R1 is (7 + 2 x 16.5) = 40 deg BTDC.

    If you plot the overall crankshaft advance vs crankshaft RPM (i.e., the R1 idle setting + 2 times what the distributor is doing) for the two configurations, I think the high RPM range is very similar, but the low RPM range of the S159B configuration is less advanced (for emission reasons).

    Just some (hopefully educated) guesses -- but you should check your year/version OM to see if the distributor advance curves shown match Section L or Section O.
     
  4. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,891
    With the 92-93 octane we can get today, the linear advance will be best. With the crap we got in the late 1970s and early 1980s the dual slope curve would work better.

    The important part of the advance curve is between 2,000 and 3,500 RPMs. In this range, breathing has become efficient (unlike at idle) so you get a full load of mixture ready to ignite. But turbulence from piston movement is not efficient (unlike in the meat of the powerband). So timing is critically important, too early and you get detonation, too late and you can get preignition. Octane can be used to widen the window of operation by preventing detonation and preignition (or at least ameliorating smae).
     
  5. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    580
    NW Rural Nevada
    Full Name:
    Mike Florio
    I made a mistake referring to the manual rather than the distributors themselves. They are S 159 B. So that means the Section "O" in the shop manual applies. That section calls for 3 degrees ATDC static timing with both R1 and R2 enabled. It also states that R2 is disabled at 3800 engine RPM (1900 Distributor RPM) which should give an automatic 5 degrees additional advance.

    The graph in the manual shows different curves for R1 and R2, so Following the R1 graph should work (I plan on not connecting R2 at all). I also plan on using a static advance of 7 degrees. (The AF 7 mark on the flywheel). The graph in the manual shows a maximum mechanical advance of 16.5 degrees. Adding the static advance of 7 degrees BTDC plus the mechanical advance of 16.5 Degrees at 3000 Distributor RPM (6000 motor RPM) gives a total advance of 23.5 degrees at 6000 motor RPM.

    If that is the case, why is the flywheel marked for 34 degrees advance (A5 34 mark)?

    Steve: I think your math implies a 10 degree offset between R1 and R2. I'm sure the offset is only 5 degrees, and is achieved by lengthing the dwell on R2 from 34 degrees (R1) to 39 degrees.

    I really want to get this right, and I appreciate everyone's input.
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    21,159
    Texas
    Full Name:
    Steve Magnusson
    Mike -- the Distributor graphs are in "distributor degrees" (not crankshaft degrees) so just like you double the distributor RPM to get crankshaft RPM you have to double the distributor degrees to get crankshaft degrees.
    The "A5 34" mark is for 5000 crankshaft RPM so if you read the chart I think it's 14 distributor degrees at 2500 distributor RPM (don't have the charts with me) so the total crankshaft advance at 5000 RPM would be 7 + 2 x 14 = 35 crankshaft degrees (if the R1 only 1000 RPM idle timing is set to 7 deg crankshaft BTDC) -- which is close enough to the 34 mark;).
    R1 and R2 are separated by 5 distributor degrees which is equal to 10 crankshaft degrees so you need to be clear about which coordinate system you're using (note that there's 10 crankshaft degrees between R2 at 3 ATDC and R1 at 7 deg BTDC).
     
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  8. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    580
    NW Rural Nevada
    Full Name:
    Mike Florio
    Thanks, Steve - It just hit me a little while ago. Maybe I ought to work on my brain cell retard before I do any more work on the distributors!
     
  9. robertgarven

    robertgarven F1 Veteran
    Rossa Subscribed Owner

    Feb 24, 2002
    5,131
    Ventura, California
    Full Name:
    Robert Garven
    Mike, Steve, Mitch,

    Most of us have the B distributors but are using the single point set-up. I am almost positive that when I set mine up I used the section "L" specs? My car runs exceptionally well. Steve helped me with that before but it has been a few years.........

    By the way I have a simillar Allen machine as Mike and he has been instrumental in getting mine working. I have been busy with other things but I am anxious to work on my distributors soon! I would like to publicly Thank him here!

    Rob
    without you guys I would be lost!!!!
     
  10. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    21,159
    Texas
    Full Name:
    Steve Magnusson
    Rob -- If you matched your distributors to the Section L graph then I think you essentially "converted" your S159B to S159A as I believe the only difference is the advance behavior. If you're running an R1 only set-up at 7 deg BTDC idle in conjunction with the Section L graph then you'll only have ~25 (crankshaft) degress advance at 5000 RPM. Although you report good running, you might try getting the total advance at 5000 RPM up to the 34~35 deg range to see if it pulls a little better -- just a thought...
     
  11. donaldh2o

    donaldh2o Karting

    Nov 10, 2003
    143
    Irvine CA
    Full Name:
    Don
    Congratulations on getting the tester overhauled. What a nice thing to have.

    I would be interested to know what the gap setting on the points are after you've finished adjusting the dwells with your whizbang tester. Those of us without a tester can at least get close to the right settings if we know where the gap on the points end up.

    Also, If you would like to test your tester on a pair of 1976 308 single point distributors, I'd be glad to send mine to you.

    OK, they're already in the box, I just need the address.
     
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  13. 4Webers

    4Webers Formula Junior

    Nov 12, 2003
    276
    Texas
    Full Name:
    Darrell
    Mike,
    I would be very interested in what you find regarding the weights and springs, as I am planning on doing the same to my '79 distributors soon. Do you have a source of info on the S159A weights and spring rates, or are you going to do it by trial-and-error?

    Good luck and keep us posted
     
  14. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
    Full Name:
    Philip
    I had not realized the early 308s had run as much advance low down. I checked the OM for the 74 Dino gt4. Engine advance at idle was 18 degrees BTDC with the distrib adding an additional 26 degrees at 6800 rpm on up, so maximum advance was 44 degrees.

    For those of you running an Electromotive HPX, you can get close to this with 3 degrees of static, and setting the 1000, 3000 and 8000 dials to 10, 12 and 20 respectively.

    Anyone have any perspective as to how you'd modify this for a high duration cam?
     

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