Gear change or converter reworked?

Discussion in 'Got gears?' started by 64 Hardtop, Sep 22, 2017.

  1. 64 Hardtop

    64 Hardtop Founders Club Member

    My question is gear change or converter reworked?
    I changed tire to a 275 drag radial and now I'm on the 6400 rev limiter chip in 1/4 with 410 gears. When I ran 29" slicks and the tire growth was perfect at trap speed. Will going to a 3.73 gear be better or send my 9 1/2" converter back for rebuild?
    Last edited: Sep 22, 2017
  2. knucklebusted

    knucklebusted Well-Known Member

    Did your 60 ft time go up or down when you went to the 275 vs the 29" slick? A 275 is only about 28" by calculations so I'm guessing that's why your RPM went up. Is going back to 29" slicks not an option?
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    If you have an accurate tachometer, figure out what the RPM at your trap speed should be with no converter slip. The formula is 336/tire height X rear gear X MPH = RPM. Then use your observed RPM and figure out what the converter slip is. That will tell you whether you need to send the converter back.
  4. hugger

    hugger Well-Known Member

    Will it still pull past 6400?
  5. 64 Hardtop

    64 Hardtop Founders Club Member

    My 60 foots are about the same, maybe a little better 1.58 The problem is no tire growth on top end. Slicks are now not an option.
  6. 64 Hardtop

    64 Hardtop Founders Club Member

    Not quite sure how this works, can you help me out here. 336/28= 12x4.10=49.2x121=5953.2 So if I divide that number from my RPM that is my slippage amount? (1.075)
  7. knucklebusted

    knucklebusted Well-Known Member

    So, you are wanting a tighter converter? That will probably hurt your 60ft time. I would think a good converter would already be locked up at well past stall speed if you are spinning it at 6400 across the line.

    What is your MPH at the end? With no slip, 27.5" tires would be 128MPH. 28" tires would be 130MPH and 29" tires would be 135MPH.

    I think 3.73 gears would be too low. If you could find 3.90 gears for that rear, that would be closer.

    With 3.90 gears, your 6400 RPM would net 27.5" tires at 134, about the same as your 4.10 gears at 29" tires.

    With 3.73 gears, your 6400 RPM would be 140 with 27.5" tires, a bigger gap than your tire size.

    It all depends on what you want to tune it to cross the line RPM wise.
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    At 121 MPH, your RPM with no slip should be 5953. You are seeing 6400, so the converter is slipping 500 RPM.

    6400 -5953.2 = 446.8 (446.8/5953.2) X 100 = 7.5%

    Not sure what is acceptable, but that seems pretty good.
    Last edited: Sep 28, 2017
  9. 64 Hardtop

    64 Hardtop Founders Club Member

    Ok Guys I can tell you that with the set up I have,by changing the 4.10 to the 3.73 gear and leaving my converter alone, my 60 ft got a little better and my RPM dropped 400 rpm. So that brought me back the 6400 rpm that I'm comfortable with @ 126.51mph.
    Thanks for all the input.
  10. knucklebusted

    knucklebusted Well-Known Member

    Sounds good. Happy or still looking for more?

Share This Page