Timing on your Race BBB's

Discussion in 'Race 400/430/455' started by GS Kubisch, Oct 21, 2007.

  1. Bens99gtp

    Bens99gtp Well-Known Member

    I noticed after all assembled( I put the piston stop in and wheel just like to degree the cam, but had the wheel mounted to the balancer and cover on ) that when I found tdc as I would to degree the cam in that my 0 mark on my balancer was not at 0 on my timing cover, they were 3 degrees off. My balancer then was a brand new romac. This few degrees could be another reason why some are getting number near 40. Mine being +3 means I would have to look for 35 to actually be at 32. I think my keyway in the crank is off some causing this issue as to degree the cam I had to be be +6 on my 308s to actually get to the intake cl of 108.5. When we wanted to get to 104 had to advance the cam then retard the gear on crank. If someone else ever tears it apart to check they will be really confused.
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Does the piston "dwell" at TDC for a number of degrees? Might that account for the discrepancy?

    https://www.hemmings.com/magazine/hcc/2009/06/The-Mechanical-Advantage/1827793.html

    From the above link,
    "The rod-to-stroke ratio also impacts the dwell time of the piston at TDC, measured in crankshaft rotational degrees. With a longer dwell period, the piston will stay at TDC for more degrees of the crankshaft's rotation before moving down toward BDC. The benefits of a longer dwell at TDC are myriad. When the piston is stationary, the combustion region stays small longer in the crankshaft's arc of rotation, containing the flame as it tries to expand. Meanwhile, the cylinder pressure is rising, because the mechanically locked piston cannot move until the angle of the crankshaft allows it to."
     
  3. Bens99gtp

    Bens99gtp Well-Known Member

    I was under the impression that when one located tdc with a piston stop, it allowed for finding perfect tdc without dealing with the dwell at tdc. Maybe I'm wrong, but if on my degree wheel, let's say after putting the piston stop in I got a reading of 22 turning the motor one direction and 18 after turning the other way, adjust the pointer degrees, recheck and bot direcyion I get 20, that would mean once the stop was removed back to 0 would be true tdc in the very middle of the dwell. When I did this my balancer lined up with the tdc mark on the cover with the 3 btc. So on mine 3 was actually 0 so I had to subtract that 3 from all my timing to be truly refernce at tdc. Maybe I'm thinking about that incorrectly?
     
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I think you would get the center of that piston dwell. Hoping someone else can comment on that.
     
  5. D.S.R.E.455

    D.S.R.E.455 Active Member

    31* made most power in mine, romac damper with proform cover was dead on at tdc
    464 .040 quench, TA413 110ilc
    10.75:1, 329cfm E-heads, Ported SP1 950 HP holley, 1-7/8 long tube TA headers, pump gas, 76 camero 3650lb street car
    3.73 gears, 3200 stall ran 10.93 at 123mph before getting kicked off track. Fwiw
     
  6. D.S.R.E.455

    D.S.R.E.455 Active Member

    That is a correct way to get tdc, you have that right, most damper/covers/pointers are off a bit, its not really necessary but an adjustable pointer is a nice add on.
     

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