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High RPMS

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by onlychevy6, Oct 21, 2005.

  1. onlychevy6

    onlychevy6 1/2 ton status

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    Maybe some advise will help me out. I have a 350/th350/np205 set up. i run 4.10 gears and 35" tall tires. After checking my linkage and vacuum i cannot figure this out.

    at 60 mph i am at 4500 rpms

    65 mph i am at 4700 rpms

    70 mph i am at 5000 rpms

    What is going on?

    Thanks
    Jeff
     
  2. sweetk30

    sweetk30 professional hooker Premium Member

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    yep kind of high to me as i had about the same combo at one time. are you forsure of the 4.10 gears ? and are you getting all 3 shifts forward? last i could think of is possible bad tranny slipping and ready to go. but if it was that bad it would have gone by now.

    what kind of tach are you useing? possible wrong # of cylinders selected on back of the tach for correct readout.
     
  3. onlychevy6

    onlychevy6 1/2 ton status

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    positive they are 4.10 gears had the whole suspention and axles in my old 78. As far as tranny is concerned it was completly rebuilt about a month ago from a tranny shop. As far as tach it is a factory tach.

    oh and i have a twin stick set up. and we checked my shift linkage. it shifts properly as far as we can tell.
     
  4. outlawtorn

    outlawtorn 1/2 ton status

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    Im running 4.56's and 36's and i pull about 2600 rpm at 65 :confused:
     
  5. onlychevy6

    onlychevy6 1/2 ton status

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    yeah this just baffles the hell out of me. You know i think i lost 2hi when i went to twin stick. Would that have something to do with it? I think all i have now is 4 hi and 4 lo
     
  6. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Sounds like the 205 is stuck in 2-low.

    Rene
     
  7. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Sounds like you're in low box to me. I get literally half the rpm you have with the same set up but with a 203.
     
  8. onlychevy6

    onlychevy6 1/2 ton status

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    well after this weekend turn out that my factory tach may be lying just a tad. hard to tell need to pick up an aftermarket tach to see.
     
  9. sandawgk5

    sandawgk5 3/4 ton status

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    Kinda off topic but Necromancer over on www.67-72chevytrucks.com He can repair your factory tach and he can also change the scale of it. He repaired mine and converted it to 7K and was a whole lot less than an aftermarket tach. PM if you have any questions.

    Ira
     
  10. Blazr77400

    Blazr77400 1/2 ton status

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    Using the RPM Calculator this is what comes up
    336x60(MPH)x4.11(gears)x1(trans gear) x2(Low range)
    ...........................35 = 4734rpm
    60 mph i am at 4500 rpms

    336x70(MPH)x4.11(gears)x1(trans gear) x2(Low range)
    ...........................35 =5500rpm

    70 mph i am at 5000 rpms

    You are in low range, and your tach may be a little off. Using the calculator you realy shouldn't use tire diameter. You should use the distance from the bottom of the tire to center of the wheel (when loaded) and times it by 2 for your diameter, but the diameter will get you real close.

    Here is mine at 80 mph
    336x80(MPH)x7.56(gears)x.7(trans gear)
    ...........................42 =3386rpm
     
  11. onlychevy6

    onlychevy6 1/2 ton status

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    hmmmm weird. Since we converted the 205 into a twin stick. I pull the left lever all the way back (Rear). And the right stick all the way back (front). then pop the right one up into neutral. That would put me in 2 hi correct. Could it be strangely poosible that the Hi and low got buggered up. I will put the left shifter all the way up and check today thanks for the input.
     

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