'68 Skylark Highway RPMs

Discussion in 'The "Juice Box"' started by 68 Lark, Jun 4, 2020.

  1. 68 Lark

    68 Lark All Original 68 Skylark

    I've got a '68 Skylark running the 350-4. Original trans ST-300 and rear end 2.56, tires are 245/60/R15 (26.5Inch). I'm curious what RPMs other's are seeing around 70mph cruising. I read around 3400. I have a suspicion this is over by around 1000 rpm. I do have failing torque converter symptoms, however the fluid level is good and doesn't seem burnt. Thanks for any help!
     
  2. pbr400

    pbr400 68GS400

    You’re right, those numbers don’t match. If you’re really going 70 (not speedo driven by wrong gear from a trans swap) and really turning 3400 rpm (not a tach error) and in Drive (not Low) it’s slipping bad. 3400rpm should be 104 mph (or a little less due to slippage). What are your ‘converter failing’ signs?
    Patrick
     
    Last edited: Jun 4, 2020
  3. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Maybe not shifting out of second?
     
  4. 68 Lark

    68 Lark All Original 68 Skylark

    Original trans, tire size causes the speedo to be off about 2 or 3mph but i've verified the speed in the past. Tach worked on a previous build, shifts points line up on the Tach, no reason to suspect it's off. Definitely in Drive. As far as converter, there is a decent drop between park and gear about 600 rpms (normal idle setting warmed up not high idle). Intermittently when I put the car in drive or reverse it will not engage for 5 seconds or so. I'll have to go back to park and try again or give it a couple very small throttle bumps. Pretty positive the clutch internally is worn and creating a rattle at higher rpm. Other than that fluid seem decent and the level is good. Hooked up to a trans cooler so I wouldn't believe it's a heat issue.

    Chris
     
    Last edited: Jun 4, 2020
  5. 68 Lark

    68 Lark All Original 68 Skylark

    Definitely shifts into second, hits 4500 rpm around 50mph and shifts if I lift out of the throttle.

    Chris
     
    Last edited: Jun 4, 2020
  6. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    With 2.56s 4500 at 50 is still first gear.
     
  7. pbr400

    pbr400 68GS400

    It’s an ST300; ‘out of second’ would be down to first. (Low). Not sure what the OP means when he says it shifts out of second at 50. Shifts out of first at 50/4500 maybe? OP, If you have to lift to get an upshift you may have a bad vacuum hose or modulator. Still wouldn’t explain 3400 at 75. But being in Low might. (The math isn’t exact, but close).
    Patrick
     
    Last edited: Jun 4, 2020
  8. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    My bad. Assumed it was a th350
     
  9. 68 Lark

    68 Lark All Original 68 Skylark

    Typo, shifts into second, shifts out of first. As far as the transmission it up-shifts and downshifts appropriately.

    Chris
     
  10. 68 Lark

    68 Lark All Original 68 Skylark

    Correct, I miss typed first vs second. The transmissions up shifts and downshifts when it should.
     
  11. 68 Lark

    68 Lark All Original 68 Skylark

    So with it being off that far at higher rpms, do you think the converter is slipping that much or trans and converter combined?

    Chris
     
  12. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Are you sure it has 2.56s?
     
  13. 68 Lark

    68 Lark All Original 68 Skylark

    Yes, just verified. ( actually jacked the car up and spun the wheel).
     
  14. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    That maths out to 3.55s or 3.42s depending on converter slip. 70 with 2.56s is about 2300 rpm.

    Pull the cover and check. The wheel spin method is not very accurate with an open diff.
     
  15. 65GSConv4sp

    65GSConv4sp Active Member

    Given the ST300 is a switch pitch, is it possible the stator isn't switching properly (stuck at the 51 deg torque multiplication position)?

    Per TireRack, P245/60R15 tires are 26.6" overall diameter and have 782 revs/mile. On a 2.56:1 rear axle ratio, that means the driveshaft spins 2,002 revs/mile, or 2,002 RPM @ 60 MPH, or 2,336 RPM @ 70 MPH, or 1,668 RPM @ 50 MPH.

    A 1965 ST300 has a mechanical reduction ratio of 1.765:1 in L (or 1st Gear), or an effective ratio of 1.80:1 in L accounting for torque converter slip (2%). So at 50 MPH, engine speed 3,003 RPM in L with stator in low position of 32 deg.

    If the stator is in the high position of 51 deg., it adds an additional drive reduction ratio hydraulically of 1.361:1. So in L, the overall effective ration mechanically and hydraulically, plus accounting for 2% slip, would be 2.45:1. So in this case, at 50 MPH, engine speed would be 4,087 RPM @ 50 MPH.

    Now, in drive (D) or 2nd gear, and stator in low position of 32 deg., accounting for 2% torque converter slip, engine would be at 2,383 RPM @ 70 MPH.

    But, if the stator isn't switching properly and is being held at the high position of 51 deg. in drive (D), the additional drive reduction ratio caused by this hydraulic stator would have the engine at 3,243 RPM @ 70 MPH, which is very close to what Chris is seeing at 70 MPH (he's saying his tach is reading 3,400 RPM).

    Because the math is so close to what is being seen between speedo and tach, (both what Chris sees for engine speed when in L at 50 MPH before the shift to D, and what Chris says is the engine speed in D at 70 MPH, I would be looking if the the stator isn't switching pitch correctly and is stuck at the high position of 51 deg.

    My 2 cents worth.
     
  16. 65GSConv4sp

    65GSConv4sp Active Member

    Never mind my post above - just saw this is on a 68 Skylark. The variable-pitch stator was eliminated after 1967. Unless Chris's ST300 in his 68 was replaced at some point in it's life with an ST300 from 1967 or earlier.
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    So the math is 336/tire height X rear gear ratio X MPH = RPM (no converter slip)

    336/26.6 X 2.56 X 70 = 2263 RPM. Add 150 RPM for converter slip, figure around 2400 RPM. Of course that depends on an accurate tachometer and speedometer.
     
  18. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Even if it was a switch pitch converter, with the car on a flat and at speed, there should be little rpm difference between high stall and low stall.
     
  19. 68 Lark

    68 Lark All Original 68 Skylark

    You’re correct, not a switch pitch. Original everything as far as torque converter to rear diff cover. I’m suspecting there is a combination of slippage in both. I found a smooth road today (hard to do in Michigan), I was able to produce a small shutter at low speed. Going to pull the trans pan this weekend unless someone has a better idea.
    Thanks for the help
    Chris
     
  20. 68 Lark

    68 Lark All Original 68 Skylark

    That was the Math I was using as well. Just put the Tach in it this year so unknown how long it has been doing this. Speed has been verified. Tach works in other cars, unless the dizzy is sending a bad signal.
     

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