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35" - 4.56 - 65mph - 2845rpm. Too much rpm?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by divorced, Sep 20, 2005.

  1. divorced

    divorced 3/4 ton status

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    rpm and mph neither one are searchable :crazy:

    So, I have 4.56 geared axles with 35" tires and a th350 transmission. The carbed small block (350) that came in my K20 didn't even last a year until a rod was knocking, but maybe the motor was just old.

    Now, I have a TBI 305 to swap in. Will running the engine at 2845 rpm for extended lengths of time be ok? At 55 mph it would be 2407 rpm, which still seems high to me. Maybe I should use my 3.73 axles, but I don't think I'll like the decrease in power, and my 4.56 axles have disks rear and the front is rebuilt.
     
  2. J.R.

    J.R. Guest

    thats what i run mine at, same set up and no problems. it depends on how healthy your motor is. if its old and worn then you may have some problems. just make sure you have good oil in it and drive it and see how it goes. good luck
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    I don't like what mine turns at 70MPH, so I don't drive that fast. :)

    Motor should have no problem running that, plenty of trucks out there that came with small tires and pretty low gears that bone stock would turn about that RPM too.
     
  4. Hossbaby50

    Hossbaby50 3/4 ton status

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    It isn't great but it isn't terrible either. I personally don't like to spin that many RPM's on my motor but it is because I am not used to spinning that high because of my 700R4. I like to cruise my truck at around 60 mph in Drive at about 2600 rpm. It is a good area of mileage vs. over reving for me.

    Harley
     
  5. Leper

    Leper 1/2 ton status

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    My 71 Duster tachs 4200 at 70 mph, but I built the motor to handle that. I shift at 6100-6200 at the track and then drive it home 50 miles. If the motor isn't tired it should be ok. If it is old or hasn't been taken care of or even if it never got its legs stretched, it won't last.
     
  6. divorced

    divorced 3/4 ton status

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    Thanks guys. The motor was rebuilt, but I have no idea how long ago. It sat for about a year and would make just a puff of smoke when it starts, but otherwise seemed to run good.
     
  7. bigjbear

    bigjbear 1 ton status Staff Member Moderator

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    My experience w/ 4.56:1, 35s and a TH350 was a lot better than before I got rid of the 2.73s. The 20 yr old 350 worked less spinning the RPM than it did getting the GMC moving with the high gears. My 85 Monte Carlo spun about the same (3k), stock.
     
  8. utchev

    utchev Registered Member

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    I get pretty decent mileage with 4.11 and 35" tires (14.5 mpg, normally--mostly highway driving) in my TBI Sub. I tach about 2500 RPM about 70 (according to my GPS). I think your motor will be fine, but you might suffer a little in mileage.
     
  9. divorced

    divorced 3/4 ton status

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    YIKES!

    I put a tach in my '72 C10 today so I could check the rpm's, and they were about 2,900 at 70 MPH :yikes:
    This is the truck that's going to get my D44/14 bolt, so I guess it's ok. I checked the mpg toady, and it gets 18 mpg, so that's cool. I don't know what gear ratio the '72 C10 has, but the calculator suggests a 3.50:1 ratio. Was that available in '72?
     
  10. SubYuman

    SubYuman 1/2 ton status

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    No better excuse...

    TO go with newer, taller tires and a few mores inches of suspension!!!!!

    Or, find a th700, BUILD IT UP, then you'll be in a better place... more range for you, less dollahs for allah!
    Or, if youy can find one, a 4L80E and a closed loop setup for the shift computer.... MUCH stronger, like a 400 with an extra gear. I see them on eBay for $1k or so.... I love mine. 55 is too low right now for me, around 1700rpm.
     
  11. Jishory

    Jishory 1/2 ton status

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    i second the 4l80 notion
     

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