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Hot Rods Flathead V8 Detonation

Discussion in 'The Hokey Ass Message Board' started by gotit, Jun 22, 2016.

  1. gotit
    Joined: Aug 27, 2009
    Posts: 360


    Hello all, I have a 59 A-B that has a weird detonation problem. I'll start with the specs. It is a 4.25 stroke and 3.3125 bore with a 400 Jr. Cam. Edelbrock block letter heads and edelbrock regular manifold with 2 97's built and jetted by uncle max and I have a new dizzy on built by bubba. The car pulls very hard but it has detonation at mid throttle. Wide open it mellows out. I pulled the choke and the car studders.

    Any ideas?
  2. alchemy
    Joined: Sep 27, 2002
    Posts: 16,082


    What type of distributor? If it's a 42, just run the car steady at 40 MPH and then stab it. If you get pinging, stop and adjust the vacuum brake screw on the top inward a half turn. Try it out again and see if the pinging goes away. If not, screw it down another half turn and drive again. This is all assuming you have the vacuum line hooked up properly to the intake.
  3. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,682

    Bruce Lancaster
    Member Emeritus

    Flathead is much like a small block Chevy on has high turbulence generated from quench area that encourages fast combustion without detonation. Open up the quench area and they start pinging.
    Chevy learned this in the '70's and '80's when they used dish shape in piston that extended under quench area of head...on a flathead, look at clearance between piston top and the low quench area of head. It needs to be tight, like 040-050, and with stroker and aftermarket heads there would be lots of opportunity for incorrect clearance.

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