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C6 Transmission and Performance

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  • C6 Transmission and Performance

    As I understand it, the stall speed of a torque converter is the engine RPM at which the transmission will cause the engine to stall if you are in Drive with your foot on the brakes. What is implied is that lower RPMs result in a lower transmission fluid pressure that allows the trans to "slip". At or above the stall speed, RPMs have increased the fluid pressure to the point where the connection between the engine and the drive train is virtually solid (as if it was a manual transmission), in effect no "slip".

    It had been recommended to me that modifying the engine such that the RPM power range is changed requires that a torque converter be installed whose stall speed matches the new engine power range. I modified my 390FE with a new long duration roller lifter cam shaft with a power range starting at 2400 RPM. So I installed a converter with a 2400 stall speed. (The stock range starts at 1800 RPM. I still have the original stock converter stored in the attic).

    I also changed the rear axle ratio from 3.0:1 to 3.7:1.

    In practice the car does not accelerate quickly off the line (you can't spin the tires). However, at say 50 MPH, depressing the gas pedal causes a significant surge in acceleration. My guess is that is because the RPM stall pressure has been exceeded, directing all power to the wheels without power loss through "slip".

    I must say that I'm a bit disappointed with the sluggish acceleration off the line. It's not what you'd expect from a hot rod.

    It turns out that my new torque converter has a small fluid leak at the transmission seal. (As it was me that installed it, I have to take the blame.) I've removed the C6 trans to take to my trans guy for repair.

    While I've got the transmission out, can anyone offer any advice to improve performance?

    Being a member of Squarebirds has been a blessing. As always, my sincere and humble thanks for all those who have advised me over the years. Happy Thanksgiving, Merry Christmas, and Best New Year's Wishes to Y'all.

    Cheers,
    Richard

    PS... At least the car looks good! Check it out: http://www.hwythunder.com/
    Richard, '66 Thunderbird Hardtop, 390FE, Edelbrock Al heads, Comp cam, Street Demon 650 carb. Visit my restoration blog at hwythunder.com.

  • #2
    Your power train is a system where all the components MATCH. A 'stall converter' comes in different RPMs depending on how much slip you want before it locks up.
    Your cam is a good place to start. If your cam doesn't start producing torque until 2,500 RPM but you have a 2,500 rpm converter, your converter will slip but lock up just as your engine STARTS producing appreciable HP. So, now it's a dance between the engine and transmission and they better not step on each other's toes. 'Slippery' torque converters also produce more heat which may require an additional transmission cooler.

    Did you install a 'shift kit'? You see, there is a lot to this. That's why it is important to get together with a speed shop that has done different drive train combinations. They should know what works and what doesn't. Rear end gears offer more torque but it has little to do with your stall converter other than the fact that a low RE gear will get to engine speed sooner. - Dave
    My latest project:
    CLICK HERE to see my custom hydraulic roller 390 FE build.

    "We've got to pause and ask ourselves: How much clean air do we need?"
    --Lee Iacocca

    From: Royal Oak, Michigan

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    • #3
      And to add to what Dave said, a 2,500 stall converter's actual "stall" will end up different than advertised depending on the engine's torque, car's weight, etc. there are a lot of factors where a converter really stalls. For an example I'm running a small loose "4,500 stall" converter in my Model A with a SBC 400. It's true stall ended up very close to where I wanted it, 2,400 rpm. If you want a converter that "work's" the best, go to a shop that builds them for the strip. For example we have a local company that builds them to your exact application. You tell them the car, the motor, the trans, rear gear, how you want to use the car. They build you a converter to fit that. For me anyway, it wasn't much more than the off the shelf converter.

      Bryan

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      • #4
        Nicely said, Bryan. I meant to bring out your facts but I got sidetracked.
        OEM converters are cheap compared with high-rpm stall converters.
        Here's another 'problem'. Most people like their engine to run at LOW RPMs even when they're on the interstate. If a 'mild' came grind starts producing torque at 2,000 RPM and that's about where you want your RPMs to be, what stall converter do you get? A 2,500 stall is too low in one sense but too high because it will always slip, turning your gas into heat. That's why most people get cams that are NOT suitable for the intended service of their engine.
        STANDARD transmissions don't have this problem because YOU control the clutch slip and the gear that gives the best torque (for the speed of the car). In other words, you will hold the trans in lower gears longer just to get RPMs up to your cam's torque curve.
        My latest project:
        CLICK HERE to see my custom hydraulic roller 390 FE build.

        "We've got to pause and ask ourselves: How much clean air do we need?"
        --Lee Iacocca

        From: Royal Oak, Michigan

        Comment

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