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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: jfkelley on December 08, 2020, 06:52:12 PM
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I have a 1976 Chevrolet Scottsdale 4x4 … I was under it for about for an hour scraping the axle tubes looking for factory #’ s and I wanted to see if someone could confirm my axle ratio for me… Thanks in advance
KAH W159 2
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In 1976, KAH was originally configured as a 3.07:1 ratio. Based strictly on the axle code, no one can assure you that the ratio, or for that matter, the entire axle assembly has not been changed. Your safest bet is to remove the cover and count the teeth. Divide the ring gear tooth count by the pinion gear tooth count or inspect the gear stamping to determine the ratio.
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Bd ..Thanks I also got 3:07 ratio after I researched also ..I was hoping it would be a better ratio so that I could install Overdrive , but with 35” tires and 3:07 ratio might be impossible..I drive it all over the state for hunting was hoping for overdrive 🤦🏼♂️
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3.07 with 35" tires would almost be like an OD ratio. Your limiting factor is your take off. 2063 RPM @ 70 mph with your set up. My truck has roughly a 31" tire with a 5 speed with 5th gear a 0.73:1 OD and at 70 is 2270 rpm. https://purperformance.com/p-29669-rpm-calculator.html
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So if I calculated correctly with my 400 engine 3.07:1 rear gear ratio Mated to an overdrive transmission ..My final crushing Rpm on the interstate @ 70-75mph would be near 1440 RPM ..I’m a noob but that’s too low correct ?
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So if I calculated correctly with my 400 engine 3.07:1 rear gear ratio Mated to an overdrive transmission ..My final crushing Rpm on the interstate @ 70-75mph would be near 1440 RPM ..I’m a noob but that’s too low correct ?
IMO yes. Your transmission would be searching. The 700R4 came with 2.73 back in the day with tiny P195 tires. A 3.73-4.10 would be a better ratio especially with 35" tires.
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Pulled the rear cover and came up with these #’s
14:43
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Driven over drive = 3.07
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That’s too low for overdrive ..I’m gonna have to go 3.73 or 4:10 if I want overdrive
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What kind of mileage are you getting with it now? I assume that's what you're trying to get a little more out of? Maybe some work in tuning (and maybe a tune UP) will help?
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I’m getting 10-12mpg with it now on interstate ..Yes looking for better MPG a lot of the places I go hunting are 3/4 hrs away and on the highway with 3speed automatic pushing 70mph is hard on the old girl with All time 4wheel drive
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I’m getting 10-12mpg with it now on interstate ..Yes looking for better MPG a lot of the places I go hunting are 3/4 hrs away and on the highway with 3speed automatic pushing 70mph is hard on the old girl with All time 4wheel drive
10-12mpg with the NP203 full-time case is great, and quite likely near the top of the range possible with our non-aerodynamic bricks and modern gas. I have a vast collection of magazines from the era and none of the contemporary driving tests exceeded that value even with higher octane leaded fuel of the day. The 4" lift generally required to run 35" tires will degrade what little aerodynamic value these trucks have to begin with - an overdrive MIGHT get you back to your original numbers.
I have no doubt a more modern combination including an LS engine plus 4-speed overdrive could do better, but the financial calculations don't make sense. The best and only reason to go through the expense and work is simply because you can and you want to. As for myself, I drive these trucks because I enjoy it not because I think I'm saving money.
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I'd ask, what carburetor are you running? Is the HEI in good shape? What kind of timing are you running? It may be worth it to advance the timing a little to see if you can get a little more economy out of it. Is it stock cam? Is the truck running good?
If you're getting like 2100 rpm at 70 MPH, that 400 should just be loping along, well somewhat. Obviously the tires and 4wd and such are somewhat of a power sucker.
What do the plugs look like? May check at least #1 plug and see if it's time to change plugs, get new wires, etc. Rockauto usually has fairly decent prices. Also may drop by the autoparts store and see if they can test your module. If its bad, I'd consider changing the coil too. I doubt you're having this issue, but maybe, and it should be free to get it tested (other than the ride to the autoparts store).
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I have an Eldebrock 1406 carburetor.. I put a tuneup on it about a year ago, plugs , distributor cap , module , It could use some new wires.. don’t have a TH 400 transmission I have a TH350 ..
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If the edelbrock is still set from out of the box, it may be worth it to potentially do some extra tuning on it.
For hotrodding the HEI, there are some good info out there. I'd tend towards Dave Ray (ignitionman)'s info.
Why did you replace the module? Usually those don't go bad unless something else goes bad. Like high resistance forces the coil to overwork which forces the module to overwork and cook itself.
I know you have a TH350 transmission, I was referring to the 400 engine that should be loping along. That's a lot of cubes cruising at an rpm that should be in the torque curve. But you say the speed is hard on the old girl, but that's a pretty good RPM I would think? Is it struggling?
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Thanks everyone I’m gonna work on addressing the issues you guys have posted ..I truly appreciate all the help ..This one thing I enjoy about this site is everyone is helpful ..Happy Holiday’s!
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UN-couple your full time 4x4 with a miles marker part-time kit and locking hubs.
Take out the Gunther mechanical fan and install two electrics.
You'll pick up a noticeable amount of horsepower= mileage
I have the same rig, '78
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Good Looking truck ! Been looking at the following Mile Marker kit to install ...Thoughts ?
https://milemarker.com/wp-content/uploads/2018/04/503-instructions.pdf
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Good Looking truck ! Been looking at the following Mile Marker kit to install ...Thoughts ?
https://milemarker.com/wp-content/uploads/2018/04/503-instructions.pdf
Well, you asked -
Converting the NP203 to part-time has had no clear or quantifiable benefits since 1974 when the kits were first introduced. If the 1973 oil embargo had never occurred nobody would have given the NP203 a second thought, and the technology may have even benefited from a second generation of research and improvement. The magazine Pickup Van & 4WD ran repeated tests over the years until their untimely demise in 1983 and could barely measure an increase in fuel mileage using their sophisticated flow meters and meticulous methodology. By 1982 they were explicitly advising readers via the letters column that NP203 conversion kits didn't make economic sense - the rest of the drivetrain would be undriveable long before the gas savings even paid for the kit. Not a way to make advertisers like Doug Nash, MileMarker and others very happy which no doubt contributed to the magazine ceasing in 1983.
And PV4 wasn't alone in pointing out that the emperor had no clothes. Four Wheeler magazine by the late 70's explicitly stated that the maximum mileage gain after the part-time conversion that they could measure was inconsistent but always less than 10%, and even that small amount was difficult to repeat.
The kits themselves are fine and will cause no harm or be unreliable. Older kits like the Doug Nash version required 4WD to be engaged on a regular basis or lubrication issues would cause failure, but the modern kits don't seem to have that problem. Or at least they don't mention it in their instructions.
In my opinion, the NP203 was and is a technological marvel that should be celebrated for what it was designed to do and managed to achieve. The friction loss of the additional drivetrain movement is utterly inconsequential when compared to adding the mass of just two additional passengers. But keep in mind, I'm also an admitted fan of the engineering excellence of Quadrajet carburetors (also hated in the 1970's), so take what I say with a healthy dose of salt.
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I agree and converted one back in the 80's and found no measurable increase in MPG
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Main problem with running a NP203 in its natural form is that you have U-joint half shafts on the front axle and running and turning those front tires on the street all of the time adds a lot of wear and tear to the front end. U-joints run at an angle have an acceleration and deceleration profile that is typical of a U-Joint, so turning isn't smooth on the pavement even with an open front differential. It is fine at small turn angles, but you will feel it jerking around on the pavement at even moderate turn angles.
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I have an 15” electric fan I had on my 77 Trans Am ..Will that work for the truck ? Or do I need Dual fans ? ..Thanks everyone !
Good Looking truck ! Been looking at the following Mile Marker kit to install ...Thoughts ?
https://milemarker.com/wp-content/uploads/2018/04/503-instructions.pdf
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I have an 15” electric fan I had on my 77 Trans Am ..Will that work for the truck ? Or do I need Dual fans ? ..Thanks everyone !
As long as your 15" fan worked fine in the Pontiac I'd suspect it will work just as well in the truck. Both had similarly sized engines and radiators. Unless you intend to spend hour upon hour in stop and go desert traffic pulling an overweight trailer, and then I'd suggest something more robust such as the dual fans covering more of the radiator surface area.
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Main problem with running a NP203 in its natural form is that you have U-joint half shafts on the front axle and running and turning those front tires on the street all of the time adds a lot of wear and tear to the front end. U-joints run at an angle have an acceleration and deceleration profile that is typical of a U-Joint, so turning isn't smooth on the pavement even with an open front differential. It is fine at small turn angles, but you will feel it jerking around on the pavement at even moderate turn angles.
This
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I was doing some maintenance routine stuff on the truck today and pulled the thermostat housing and there was Not a thermostat in it , I just Soum the gauge was broken because it never really got hot ..So I put the correct thermostat in it and once it got up to temperature It ran a whole lot better… Adjusted the real drum brakes now stopping a whole lot better also..No hopping in the rear when I brake ..
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Okay, I'll jump in...
Did the NP203 conversion to MM last year, (don't forget the spacers/bearings PULESSSE)
Ditched the mechanical fan, huge difference.
Changed out my read diff from a 3.73:1 to a 3.41:1.
Ditched (for now) my front drive shaft
A
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At 2500 RPM "max torque", I get 15 MPG, going over a 800' hill between my office and home.
I'll post more data when I have a straight shot on level road.
Pretty good for an old box? I think so... 8)
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I was doing some maintenance routine stuff on the truck today and pulled the thermostat housing and there was Not a thermostat in it , I just Soum the gauge was broken because it never really got hot ..So I put the correct thermostat in it and once it got up to temperature It ran a whole lot better… Adjusted the real drum brakes now stopping a whole lot better also..No hopping in the rear when I brake ..
Good catch on the thermostat - proper operating temperature is a key element in how the GM engineers attempted to achieve the contradictory goals of trying to maximize peak performance for the customer and optimum emissions to keep the government happy.
Knowing that the thermostat had been eliminated suggests the previous owner probably made some other questionable changes as well. You would be well advised to double check the timing, mechanical and vacuum advance as well as the general health and operating condition of the carburetor too. To achieve maximum mileage as stated in your original post these components are critical. Setting the best overall tune of the engine cannot be understated and is generally far cheaper parts wise than messing with transfer cases or axle gearing.
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I’m going to install an electric fan ..What size do you all suggest for everyday driving ?
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Did you really edit your original post instead of making a new one? ::)
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My apologies ..I went back and fixed it
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My apologies ..I went back and fixed it
it just makes following topics easier. Any advice given is easier to find doing a search.
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You probably ought to start a new topic regarding your e-fan conversion.