The Stovebolt Site Forums The Shop Area Engine Shop '51 3800 216 gear ratios

Gear ratii? I thank God for not making me a word nerd. That don't hardly look right, ratii. Darn those old penguins for teaching me Latin. Heh. Okay, here it is: My truck, with its 5.14 diff ratio, is too slow. I want to cruise gently and economically at 50 - 55 mph. My (SM420) transmission's gear ratios are too widely spaced; I am uncomfortable with - when fully loaded starting up a long grade - using my engine's entire powerband and both verges to get up to the next gear. I live in mountainous terrain, that ain't gonna change. I intend, now, to keep the original engine - or one like it - in my truck. That ain't gonna change either. So here's the plan in its current incarnation, subject to change without notice: a 4.10 diff from a '71 3/4-tonner, and a NV 5-speed (overdrive) tranny from a Dodge Cummins. The NV is a much-maligned, but perfectly strong transmission abused by uninformed drivers who believe that the monstrous amounts of torque generated by their overpowered engines makes it okay to stay in overdrive when they should downshift to direct drive to preserve their powertrain.
This combination, with my 235/85R16 tires, will allow me to cruise at 55 mph with an engine speed of barely 2000 rpm. If I downshift to 4th the engine speed will increase to around 2800. This, I think, will give me exactly what I can live with forever in a working truck which may one day be called upon to go transcontinental.
At least, that's my theory, in its current incarnation, subject to, etc.
I want a truck which looks as close to original as possible, but works better. Opinions, anyone?

Sounds good to me. Can you adapt that trans. to that engine? I think for this transcontinental thing you will need a 235 or 261. Gears multiply torque but don,t generate power.
Being a Jewish boy it took me awhile to get the penguin thing.

Yeah, and I thought ratii was a mispell until I saw it again then it finally dawned on me what you were saying. It's going to be tough to find something that lets you cruise comfortably at 50-55 and still pull heavy loads up and down hills with a 216 but it sounds like you've got a pretty good compromise there.

Sorry, boys. I guess I might make more sense if I made more sense. But then I, myself, wouldn't know what I was talkin' about. And then where would I be? I know what you're both saying about displacement/power. But I also know that if I wanted more ponies, I could make this engine breathe and spark better with available modifications. I'm pretty sure, though, that the closer ratios will get me up any hill without winding the engine too fast, and I almost never use the low gear in my SM. The odd time I get into a real bind I could always slip the clutch to get hooked up on the NV. Since at this point power is not so much a problem as engine overspeeding, and being unable to keep up what I think of as a minimum safe speed (50 mph), I have good reason to believe that these gear mods I'm talking about will give me what I need most. Once I have this, I might want more power, but that's for later.
The NV will go, George, with a pilot shaft change.

And... here it is. Picked her up a couple hours ago, and I've driven 20 or 30 miles in a bunch of different conditions. To review, what I've done is change the differential gear ratio from a far-too-short 5.14 to 4.10. No tranny change. Nothing else substantial except for whatever bearings were exposed in the process (all of 'em), and the front wheel bearings to boot.
Here are my early impressions: First gear is now useable for something other than rock climbing. I stopped on a 15% grade. 1st gear was (of course) an easy start almost without any throttle. At about 10mph I shifted to 2nd smoothly and easily without having raced the engine, and 2nd pulled away nicely. Ditto 3rd, and I levelled off at around 30 mph in the middle of third at a moderate throttle setting. Keep in mind that at 15%, the grade I ran was almost 3 times as steep as the steepest interstate. I could easily have gone fast enough to pick up 4th, but I'd have been exceeding the speed limit. So at the top I went to 4th and idled very nicely at around 35mph at the very bottom of 4th.
Then I took to a freeway. Evelyn followed me with her certified speedometer. It's German, you know. Therefore, of course, superior. Right. First I tried for my fastest comfortably sustainable cruise: 58 or 59 mph, she said. I could do that all day. But I also tried a "fuel miser" cruise, which turned out to be exactly 50mph. At that speed I could actually downshift, dump the throttle and scream up to 55 just like that! I didn't do that - no reason to - but I could have. I have the distinct feeling that the next time Epic is asked to haul a load that would void a new truck's warranty, she'll do it without a hitch, exactly like this, but at a slightly higher rate of fuel consumption.

There is no problem here, I'm telling you.
This is exactly what I was looking for. A truck that will still lift your house, but can now eat great distances without straining. I think I'll stay with this for a while. 2nd is still a flat ground starting gear. I don't need an overdrive, and probably wouldn't unless I really started to sock on the featherweight highway miles. Even then, this would do nicely, I think.

-Mark

Hey hotshoe, exactly what did you do to change the pilot shaft?

Greg

I didn't, Greg. That is the same SM420 in there. I just did the diff.

Sorry.

Your modification sounds so darn cool.I thought there was hope for my 1/2 ton.

Greg

That NV is a heavy-duty tranny, Greg. You want the T5 from an S-10, about which there is plenty written here if you search. I thought about that one myself, but my gearhead buddy talked me out of it because he knows what I'll be doing with this truck. I don't know yet what his problem is with the Gear Vendors electric overdrive,(he has recommended against it) but I'll be picking his head apart in a couple of years if I decide I still want more. For the moment, though, I am happier than I thought I'd be with this single change.