4.10 to 4.56 or 4.88

Discussion in 'Differential & Driveline' started by fallguy, Mar 10, 2017.

  1. fallguy

    fallguy Full Access Member

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    1986 K10. 290hp 350 with mild cam and headers. 700r4 trans. 4" lift 35" tires. Currently running Dana 44 front, Corp 14 rear, both 4.10 gears.

    Gutless wonder constantly hunts between 3rd & 4th on even the slightest inclines. Prolly due to about 2k rpm. Frustratingly anemic and 6-8mpg due to the constant shifting or need to take it out of OD just to run on the highway. Towing is an absurd struggle.

    Thought 4.10 would be the solution...took people's opinions and didn't research properly. All gearing charts show me that to get back to stock ratio I need to go 4.56. However if I want a big more snap than stock and better tow ability, go with 4.88s. I'm told that mpg will increase to 12-15mpg depending and totally change he habit and feel of running down the road.

    Also told that top speed will depend upon my motor build so 80mph + will be possible with the 4.88s.....thoughts?
     
  2. crazy4offroad

    crazy4offroad Full Access Member

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    With 4.88s you would be looking at about 2300 rpm at 70 mph. Depending on your cam's powerband, if you're below the usable range it can cause bad fuel economy. I honestly doubt you'll see the high end of those mpg numbers, more likely a consistent 12 mpg. You can use this gear ratio calculator to find out what gears = what rpm at road speed...
    http://www.grimmjeeper.com/gears.html
     
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  3. fallguy

    fallguy Full Access Member

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    Comp cam xe256. 411ft lbs at 3600rpm, 330hp at 4700rpm
     
  4. crazy4offroad

    crazy4offroad Full Access Member

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    That cam's usable powerband should be 1200-5200 rpm so it's not like it needs a high stall converter or anything. 4.88s may just be the ticket.
     
  5. theblindchicken

    theblindchicken Full Access Member

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    I'm jealous you have 4 gears.... I've got a th350 with 4.11's and 35's... Sitting right about 3k at 65-70mph.

    Yeah, the higher the numerical ratio, the snappier the truck will feel. But you'll run out of top speed real quick.
     
  6. marks86

    marks86 Full Access Member

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    Sounds like your engine is tired. There is no reason why a 350 chevy would be a gutless wounder in a k10 on 35s

    The problem with going to 4.56s or 4.88 if you ever installed a real engine with real power you would have way to much gearing for everyday driving. I had 4.56 gears with a 6.2, it was fine. The second I swapped in the Cummins it was just not tolerable, so I swapped in 3.73 gears and never looked back.
     
  7. bucket

    bucket Super Moderator Staff Member

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    Ideal gearing for the powerband of a Cummins is absolutely nothing like the ideal gearing of a gas v8.
     
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  8. 77 K20

    77 K20 Full Access Member

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    A lot of that is just your engine. My dad went down that very long expensive road himself. He used to have the original 400 with a cam in it to pull his 18' 5th wheel. Did great. Low RPM on the highway as he had 3.08 gears.
    400 got old and worn out and decided to get the 290 HP 350. Gutless. Couldn't even drive the truck up the hills to his house decently running empty. He changed the gearing up to 3.73 gears (just running 30x9.50 tires). Helped "a little tiny bit". Still not acceptable.
    Next was getting rid of the stock 290 HP cam (it was for a light weight corvette). Put in some type or RV cam. Helped a little bit. Still not acceptable.
    That engine only has something like 8:1 compression and some places when measuring it got even lower than that.
    Next he bought some Edelbrock aluminum heads to to bump the compression way up. After that he gets MUCH better gas mileage and is about the same for pulling as the old 400.
     
  9. Rusty Nail

    Rusty Nail Full Access Member

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    Im not convinced the issue lies in the gearset and need convinced there is not a different underlying cause.

    Granted the opinion of "gutless" will vary. Im on 33s with 3.73s and nowhere near 290 hp - my truck is FAR from "gutless" or "anemic".
     
    Last edited: Mar 12, 2017
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  10. 77 K20

    77 K20 Full Access Member

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    Many articles on the web about the 290hp engine. Makes probably 260 ft/lbs torque at 2,800 RPM. A standard 350 will make a lot more at low RPM. Sure- this makes 290 hp... but at 5,500 RPM. My truck never sees that.

    [​IMG]

    http://www.hotrodders.com/forum/350-290hp-g-m-crate-engine-thoughts-169193.html

    Just one of the many threads and discussions about the engine when I researched it. Very low compression with dished pistons is a recipe for weakness.
     
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  11. Rusty Nail

    Rusty Nail Full Access Member

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    Dude. I decided to seek some of the many articles which you speak.

    Dude. 8.0:1 ?

    My line of initial thought was somewhere like "MAYBE he's chewed up his transmission or has a bad/loose torque convertor."
    MAYBE his timing has issues or restrictive exhaust...
    But ol boy bought a $3800 "wind em up".


    Better go 4.88s, 4.10 aint enough to do anything with... Not on 35s.
    Might look at higher speed stall convertors.
    3k? MINimum 2500.
    At least match your r.p.m. at 40 m.p.h...
    You dont light to light in 4th gear do ya? Surely you spend most of the time in Drive? Right? 4th is really only for highway use.
    That beast would spend WAY TOO MUCH time hopping in and out of 3rd, locking and unlocking...a 1500k stock convertor will not pass muster. HEAVY flywheel!
     

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    Last edited: Mar 12, 2017
  12. marks86

    marks86 Full Access Member

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    your absolutely correct. I still do believe 4.88 gears and even 4.56 gears in most cases don't belong on trucks that run on the highway.
     
  13. 75gmck25

    75gmck25 Full Access Member

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    Many of the early non-overdrive trucks used 31" tires and 4.10 rear gears for the heavy duty trucks, which worked okay during the 55 mph speed limit days. I think they ran about 2500 rpm at 55 mph, which was a decent cruise rpm for a heavy truck. On 1 ton square body trucks GM also used 4.56 gears, but most of those trucks did not see much high speed highway driving.

    Most folks want to drive 70+ mph these days, and its comfortable to cruise at no more than about 2500 rpm. If the online calculator I found is correct, to get 2500 rpm at 70 you could run 3.73 gears and 35" tires with a TH350. However, if you use a 700r4 with a .7 overdrive, you could run 35" tires and 4.88 gears, and still be well below 2500 rpm at 70 mph. That overdrive really helps a lot.

    By the way, I also bought a 350/290 hp for my truck, and was not happy with the low end power. I swapped in a cam similar to the one for the crate 350/260 hp and like it a lot better. The real fix for the basic GM crate engines is Vortec heads or good aluminum heads that are set up for better flow and about 9.5 compression. However, then you may decide you want to also swap to a roller cam, and all of a sudden it would be better to just buy a complete engine with good heads and a roller cam.

    Bruce
     
    Last edited: Mar 12, 2017
  14. bucket

    bucket Super Moderator Staff Member

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    It all depends on tire size and intended usage. My '79 runs a 454, no OD, 36's and 4.56 gears. It runs around town and goes down the highways as well. Ain't no way in hell I'd ever want to step down the gearing. It pulls like a train and that little bit of RPM won't hurt a thing.

    But remember, the OP has a truck with OD. With those 35's and 700r4, that truck needs all the gearing you can trow at it. The OP wants to do truck stuff with a truck, the current combo makes it basically useless.
     
  15. crazy4offroad

    crazy4offroad Full Access Member

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    After thinking about it, is the transmission downshifting to 3rd, back to OD, back to 3rd, etc...or is the tc just switching between locked and unlocked? When you're on the highway and it seems like it has downshifted, pull the shifter into 3rd (D). If it actually downshifts at that point it may just be the lock-up circuit, which happens a lot if you're the kind of person (like me) who drives with your right foot a lot. It's really hilly around here and I pay no mind to it, but my OD trans is a 4L60e in my 02 Sierra, it has a little more computer control than the 700R4 to prevent a bunch of lock/unlock cycles. If it is actually downshifting into 3rd gear you may just need a TV cable adjustment.
     

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