Lock up transmission questions

Discussion in 'Transmission & Driveline Topics' started by flowjoe, Oct 18, 2017.

  1. flowjoe

    flowjoe Moderator Staff Member Lifetime Gold Member

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    I see some builders recommending that the lock up converter and affiliated parts be omitted in retrofit applications. I've read the logic behind that but the question is this: if the published final drive ratio of, say a 200-4R, is .67 is that number achieved with out lock up? Or does the tranny need to lock up in order to hit the .67 final drive number? If the lock up is omitted what happens to that final drive ratio?
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Final drive ratio is the transmission ratio. Lockup converter eliminates converter slip. There is no reason not to use a lockup converter unless this is a 100% race car.
     
  3. flowjoe

    flowjoe Moderator Staff Member Lifetime Gold Member

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    In those same "opinion" pieces (I think it was Hot Rod quoting Art Carr - and I do read magazines like that with a grain of salt;)) they stated that not using lock up was going to add 200-300 RPM...which is what got me thinking about final drive ratios. I've read statements like that elsewhere (possibly just repeated on the inter web from the same "source")...some even say the RPM drop from the tranny locking up is like having a 5th gear.

    So if I am reading you correctly then the final drive ratio is the final drive ratio and lock up does nothing to impact that ratio...just removes the inefficiency from slippage. Yes?
     
  4. Bandit723

    Bandit723 Veteran Member

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    When i lock out my 700r4 i drop about 100 rpms so your question does bring up a good point. I suspect it does not change the gearing but as corky stated eliminates the converter slip. There are full lockup trans setups for racing. I will try locking mine out off the line just to see what the results are. I do know on the downshift having this trans locked out is not good car will get jerky...
     
  5. Gary S

    Gary S Administrator Lifetime Gold Member

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    Correct. The ratio of the tranny stays the same with or without the lockup, but without it, you get slippage so that final ratio is compromised by the percentage of slip as the engine must turn that much faster. So, actual ratio stays the same, but effective ratio gets worse and you lose some of the overdrive advantage. And, since the goal of overdrive is lower engine rpms, it gets worse as the converter slips more and more as the rpms drop until you get 100% slippage at idle.
     
  6. flowjoe

    flowjoe Moderator Staff Member Lifetime Gold Member

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    Ok, that makes sense. And I agree, since the whole point of an OD is get the RPMs down you're really shooting yourself in the foot if you omit the lock up.
     
  7. SRGN

    SRGN Veteran Member

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    I got 28 MPG with a non lockup converter in my 10 second GN. If you go with an inefficient converter, it needs the lockup to be efficient. Anything below the stall speed, the converter multiplies torque. The torque multiplication and efficiency are 2 key factors when converter shopping. Its very simple to convert from lockup to non lockup, and vice versa. If you go realllly big on the converter(4400+ RPM), you may need the lockup to keep street manners. For most street/strip stuff I prefer a non lockup in a 200. My C5 flash stalled to 5000 RPM with a 4L60E, I needed lockup in that one since that converter was really loose. Locked up that car got 28+ MPG as well. Post up the rest of your combo and goals for the car. I'll give you my feedback on what I think you should go with. Opinions will vary, so I won't be offended if anyone disagrees :)
     
  8. flowjoe

    flowjoe Moderator Staff Member Lifetime Gold Member

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    Oh, it was an idle question brought on researching 200-4R build ups to retrofit. It's a 71 RS/SS with mild 350 (~400HP). I have the 8.5 rear built already with 3.73 and an Eaton LSD. Knew I was going 200-4R (hence the 3.73s) for ease of install and the better 1-2 gear spread (as compared to 709R4). Just located a 200-4R core so the tranny building phase (professional) now begins.
     
  9. Philip

    Philip Veteran Member Lifetime Gold Member

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    I have a 4L80E in my wagon and I can feel the rpm drop when the convertor locks. It does feel like another gear, tach shows about a 200 rpm drop at around 55 mph.
     
  10. SRGN

    SRGN Veteran Member

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    Central NJ
    What cam, rear tire size, and more street or strip?
     

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