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options for more power

Discussion in 'The Garage' started by Big GMC truck, Jun 30, 2006.

  1. Big GMC truck

    Big GMC truck Registered Member

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    Adding a 1400 lb. camper on my 1984 GMC k1500 305 4 speed auto has greatly reduced my acceleration and hill climbing power. To move in reverse on the slightest inclines, I will have to put it in 4 lo or else it will stall:eek1:. What am I to do? The engine was recently rebuilt and has a rv camshaft, edelbrock intake, and summit quadrajet. I think my diff gearing is to blame for all of this. I only spin 1400 rpms doing 65 mph. Is it expensive to re gear my diffs? Any ideas on the cost?
     
  2. y5mgisi

    y5mgisi 1 ton status

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    regearing is prety expensive (probly $350 or so per axel) of corse you could swap over to a 350 but that would cost even more then re gearing.
     
  3. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Good gearing is crucial no matter what powerplant you're running. I have noticed most truck that got the 305 or 6.2 diesel got absolutley brutal gearing from the factory. Both were pushed as 'economical' by GM and the gearing was chosen to help with that.

    1400-1600 rpm on the freeway is ridiculous and unless the truck is dead empty and the road is flat and the wind isn't blowing it's also useless.

    I'd look to gearing the truck so at regular speed on the freeway (~65-70 mph) it's turning 2000-2200 in OD. It won't hurt your fuel economy any, in fact it may improve it some. It will help greatly with acceleration and load carrying though.

    With 31's or 32's I'd personally run somne 4.56's with the 700R-4. Rpm's on the highway would be 2100 at 65 mph...but the truck would actually have some snap around town and it'd deal with loads a lot better. It'd also be a lot easier on the tranny.

    I wouldn't be surprised to find 3.08's in your diff's right now.

    Rene
     
  4. beater_k20

    beater_k20 Banned

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    add a set of 3/4 ton axle. stronger axle (14 bolt), better gears (3.73 at worst), and bigger brakes (the rig is heavier, you do need more). on top of that, add a set of air bags to help bring the truck back up to ride height when loaded. should be good to go then.
     
  5. beater_k20

    beater_k20 Banned

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    2.73s wouldnt surprise me
     
  6. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    They seemed to favor the 2.73 in the manual tranny trucks with 305's. That's what mine was...305, SM465, 10/12 bolts with 2.73's. :doah:

    Rene
     
  7. Big GMC truck

    Big GMC truck Registered Member

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    Thanks for all of the help. By the way, the truck has 31 inch tires. It sounds like everyone agrees that gearing is my problem. So, adding performance parts, such as headers, wouldn't provide much gain? Going up steep grades, I have to drop it into second gear to stay at 55 mph (3,000 rpm). Luckily, the engine pulls pretty strong at 3,000. Hopefully, lower gearing will allow me to pull 3 grand at 65 mph in third. Lower gearing will produce more gain than the addition of a 350, right? The only issue with regearing is the cost. Is $350 per axle going to be worth the money?
     
  8. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Gearing multipies torque. Going from 3.08 to 4.56 will get you about 35% more torque to the ground. You'd be very hard pressed to get a 350 to put down that much more torque than your 305. The gears you have now will tend to always have the engine lugging, so it's not in it's best rpm range for power.

    3/4 ton diff's are a cost effective way to better the gearing as someone already mentioned. Most common are 4.10's in those diff's, plus you get bigger brakes. I think it'd be a toss up cost wise because you'd also need 8 lug rims.

    If you don't plan on running anything much larger than 31's i'd just re-gear the diff's you have, and go with 4.56's. With 4.56's you'd be at 3200 rpm in drive at 65 (good for those long hills) and cruise rpm in OD would be 2248 rpm at the same speed. Ideal for a 305 IMHO.

    Rene
     
  9. Big GMC truck

    Big GMC truck Registered Member

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    Since the truck has adequate braking power, I don't want to change axles. I didn't think I would have to jump to something as numerically high as 4.56. I was thinking like 3.73. For example my dad's durango, 4 speed auto with 3.55 and 30 inch tires, does 65 mph at 3,000 in third. I would like to replicate these figures with my truck. Does the $350 per axle include labor?
     
  10. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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    You could get shorter tires for less than that (which would lower your effective gear ratio). Pop the 31s back on anytime you have the camper removed.

    By the way, that seems like a heavy load for a 1/2 ton truck.
     
  11. sweetk30

    sweetk30 professional hooker Premium Member

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    trust us.. we have answered this lots of times and learned the hard way. 700r4 need lots of gear to keep from hunting gears all over like 3-4 4-3 3-2 2-3 so on and so on.

    plus the over drive in 700r4 is kind of week in stock form. so the less a load you have on it the longer it will last. and that dodge durdrango is lighter than your truck by far and does totaly diffrent things.

    305 is not a power house by far. needs all the help it can get.
     
  12. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    why not go for right in the middle, at 4.10? Should be a good happy medium. I run 4.10s with my 35in tires and no OD so I spin about 2800rpm at 65. With 31s you'd be running into the 3000rpm range in 3rd.
     

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