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Differential & Driveline Front and Rear Axles, Lockers, Limited Slip, Yokes, etc


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Old 03-12-2017, 05:25 PM   #16
 
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Here's a chart from Grimm Jeeper with the 700R4, NP208 (at 1:1), 35" tires, and on the left is 4.10 gears and on the right is 4.88. Should help you see what to expect, looks like closer to 2300 rpm at 70 mph. If your 4.10 rpms match the chart when it downshifts, I would go ahead and pull the trigger on 4.88s. I would still only expect 10-14 mpg, 10 towing, 14 under the best of conditions. Keep in mind driving these trucks on the highway is like trying to carry a sheet of plywood in a windstorm, definitely hits the mpg hard. Hope for a tailwind.
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Old 03-20-2017, 07:38 AM   #17
 
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I have roughly skimmed through this thread and there seams to be some conflict.
I daily drive my 87 V10 short bed. It has 3" of lift on 33" tires, 5.3ls and built 4L60E, 12 bolt out back with 4:10's. At 70mph my truck is at 2k rpm exactly. 700R4 and 4l60E are same od ratio.
Use this calculator, I find it much easier and straight forward to use.
https://www.ringpinion.com/calculators/Calc_RPM.aspx
My truck NEVER hunts for gears, and as long as I keep my rpm 2k or lower, I get 16 miles to the gallon. Now I understand the argument that I've installed a newer more efficient engine/trans combo, but if the OP's engine is making an honest 290hp at the crank, he's within 10hp of my engine. As long as the trans is healthy, he should be able to achieve close to my figures.
If I ran 35" tires I would consider 4.56 gears.
My speedo reads 2-3mph slow at these speeds
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Old 03-20-2017, 04:51 PM   #18
 
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First of all, make sure your ignition timing is optimal, and I don't mean just at idle. And if the truck doesn't have low restriction dual exhausts, add them.

If that's the 290hp GM crate engine, then it's not gonna make much torque under 2500 RPM because the cam is WAY TOO BIG for 7.8:1 compression. Yes, 7.8:1 is what the base and 290 engines actually have. Don't know what GM was thinking using 76cc combustion chambers and dished pistons.

There are a ton of smaller cams out there that will make more torque at much lower RPM, but limit revs to 4500 or so. I can tell you from personal experience that the cam below is a torque monster from 1500-3500 RPM in a low compression 350. In my 69 C10's 350 Goodwrench engine, there was a night & day difference between it and even a 204/214 Edelbrock cam. It was just what the doctor ordered for driving in the Colorado mountains with 29" tires and 3.08 axle.

Crane series 2020 -- Cam & lifter kit P/N 114112

Its very early intake valve closing angle makes the engine think it has 9:1 compression or more.


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Old 03-24-2017, 08:48 PM   #19
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Interesting thread but many apple to oranges examples. 700r4 + 35" ....
absolutely 4.88 if you have power. If not, 5.13s is not too wild at all.

If a 700r4 hunts back and forth from 3rd to overdrive you will reach nuclear meltdown temps and say gbye to that transmission.

Trust the math in the gear ratio website, it's a great tool.
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Old 04-12-2017, 01:54 AM   #20
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4.88 !
My last K30 had a 350 ,4.56's, and 33's, and you could pull a steam roller in granny low & 2 High. Overdrive would've been perfect if I'd had one and cruised empty. It could carry a ton and nearly achieve 12 mpg doing it (11.7 - checked), once I ditched the pathetic Holley 600, (a 600 Edel will be no better), for an excellent 750 Q-jet.

I didn't build it's engine , but I added headers and mandrel bent 2 1/2" exhaust with an H-pipe (already had a performer intake), and a home modified qjet - better mileage AND more power !
You could hear the cam very slightly - so it likely was a little bigger than the smallest, but worked fantastic with the manual trans. You'll need less cam and to be very careful if you purchase a higher stall convertor - absolutely no cheap convertor will hold-up.


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Old 05-09-2017, 05:03 PM   #21
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Originally Posted by MikeB View Post
First of all, make sure your ignition timing is optimal, and I don't mean just at idle. And if the truck doesn't have low restriction dual exhausts, add them.

If that's the 290hp GM crate engine, then it's not gonna make much torque under 2500 RPM because the cam is WAY TOO BIG for 7.8:1 compression. Yes, 7.8:1 is what the base and 290 engines actually have. Don't know what GM was thinking using 76cc combustion chambers and dished pistons.

There are a ton of smaller cams out there that will make more torque at much lower RPM, but limit revs to 4500 or so. I can tell you from personal experience that the cam below is a torque monster from 1500-3500 RPM in a low compression 350. In my 69 C10's 350 Goodwrench engine, there was a night & day difference between it and even a 204/214 Edelbrock cam. It was just what the doctor ordered for driving in the Colorado mountains with 29" tires and 3.08 axle.

Crane series 2020 -- Cam & lifter kit P/N 114112

Its very early intake valve closing angle makes the engine think it has 9:1 compression or more.

Upon MIkeB's recommendation I went with the Crane 2020 in my new build. 1984 K20 with rebuilt 700R4, Np208, Gm crate engine, 4.88's and 315 (35") Cepek Extreme Country tires. The truck left Chico CA last Friday with my buddy driving it with a load on the flat bed deck and towing two empty boat trailers, one on top the other. He's on his way to Alaska and passed through Haines Junction this morning. Unfortunately, the speedo cable broke 100 miles into his trip so he has nothing to go by but has said that the truck has good pulling power in the hills, loves 60 -65 mph but the tires start to drift at around 65. I have the 700r4 hooked into a vac switch next to the brake booster for lock/unlock. He said for a work truck, the 4.88's are ideal with the 35" tires. He thinks however that he's getting around 10 mpg so I think with the load on the deck and towing, the Qjet is getting into the secondaries more than I anticipated. Its 3100 miles so its going to be an expensive fuel bill!


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Old 05-09-2017, 08:48 PM   #22
 
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Looking forward to hearing how it does with a "light" 2000 lb load of fish!


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Old 05-10-2017, 12:00 AM   #23
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Originally Posted by MikeB View Post
Looking forward to hearing how it does with a "light" 2000 lb load of fish!
Me too, I've got some big bills to pay after the minor 15K rebuild on this rig!


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Old 05-10-2017, 09:40 AM   #24
 
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Ouch! Of course you can always call it an investment in your business. Also, that beats $50K for a new 2500HD.


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Old 05-10-2017, 10:09 AM   #25
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Originally Posted by MikeB View Post
Ouch! Of course you can always call it an investment in your business. Also, that beats $50K for a new 2500HD.
Many have asked why I put so much $$ into such an old truck. Working on the beach in the salt water kills both the electronics and the sheet metal so I don't want anything with a computer in it if possible. So, if I bought some old 3/4 or 1 tons in AK of the vintage prior to computers, they are all rusted out. Newer trucks have better undercoating but have all the electronics. So I've found I'm better off to buy stuff outside AK, rework them and ship them up.

I have a few pics posted in the Gallery forum.


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