The Stovebolt Site Forums The Shop Area Engine Shop 2:73 gears

looks like we may use 2:73 rear , not sure about the transmission yet, I was thinking more about 3:73 for my mostly in town and some hi-way,but the guy doing the work on my 52 says this will work just fine,any comments appreciated......the rear will be out of blazer so I can keep the 6 lugs for sure,,,Fred

2.73 is fine if you have a V8 in front of it with an auto tranny, but it is a little steep unless you plan on never hauling or towing or going up any steep hills. I had a 2.73 in my 49 for a few months with a strong V8 and thought it a little sluggish off the line, but it would easily do 70 or 80 in 2nd gear. If its a six you have under the hood . . . better to go with a 3.42 or taller.

Joker, whatever rear ratio you go with, your truck will need about a 9 or 10 to one overall gear ratio in first gear to have reasonable take off without having to slip the clutch a lot.

Ken and Tony, guess that I may need to talk to this guy a little more, my truck will have the same 2354 in it, he is thinking about a chev 93 to 98 transmisson manual and over drive,that doesn't make much sense then because that would even make the rear a higher gear,right,let me know so that I can talk more about this set up,Iam not worry about hauling anything and I dont have any big hills around here but,,,,Thanks,Fred

joker - can't imagine a 235 could even get rollin w/ a 273 gear set and a 3spd! sure don't need 273's and OD - less ya wanna try to go about 120mph!
note that Patricks "highway" gear set is 355. the "highway" gears for the later trucks is 338. I got hold of a couple overdrive trannies, and if I can make one work properly, that's what I'll go w/ to save the $$$ on the rear gear change.

Bill

Joker, those newer truck 5 speeds are NV 3500-4500 trannies that have intergal clutch housing using the common small block Chevy bolt pattern, won't bolt up to a Stovebolt. Stand alone trannys like the T-5 ,various Saginaws, A833 4 spd OD will work as you know.I think 235 will pull a 2.73 gear ok if you are not in hilly country or use it for towing and have the correct first gear ratio.My 94 S10 has a OD 5 spd (NV3500),160 hp 4.3 engine with 2.73 gears, weighs about 200-300 lbs less than a typical AD truck.The first gear is 3.50, and it pulls a moderate load easily in OD unless going up hill of course.I think a stock 235 has about the same torque at low speeds as the newer 4.3(262 cubes), but the old truck has way more air drag at higher speeds than a low S10.A Saginaw car 4 spd with the 3.50 first gear would be fine with 2.73's, but think about you drive the truck, the 2.73's would lack acceleration in high gear, and a down shift to the next gear will have you reving up quite a bit at 60 mph.

Has anyone tried a 2.73 with a 4 speed? It seems like it might make 1st gear useful. I am thinking of 3.08 with my 261 and a healthy cam or turbocharger.

Josiah I will soon run be running a 4 speed with a 3.73 rear end behind a mildly/moderately built 292 and I'll let you know how it goes. I know this thread is about 2.73 gears, but with my 4th gear being 0.73:1, my top gear ratio will be 2.72. I have a 3.09 first gear, which yields a 11.52 ratio overall in first. So no clutch slippage should take place and take off should be strong!

-ftyler

I have the set up you describe. 235 with a Saginaw 4 speed 1:1 ratio and the 2.73 rear. If you live in flat land country such as Texas you should be fine. You should try to find the Saginaw with the 3:50 first gear which is easiely identified by the 2 grove input shaft marking you will be ok. Another gears to consider for Texas are the 3:08 and 3:32. I would not go any deeper than that. With a properly tuned 235 and the 2.73 rear gear and the 4 speed you'll easiely get 18 mpg.

A bit of info - the NV3500 has an integral clutch housing, the NV4500 does NOT.

GMCPanel and Tony, I know we have talked about this some before, but I am kinda dumb ... the NV3500 with the integral clutch housing ... will that bolt up to ANYTHING different than what its coupled to now? (In my case, its currently the 4.3 v-6, just like Tony described). I just don't have a clear understanding of bolt patterns, bell housings and such, but I'd sure like to hook up a sturdy 5-speed to a torquey I-6, someday.

I worked out some numbers with the SM420 with 28" OD tires:

Speed at 1500 rpm (start of powerband)
SM420: rearend-> 4.11 3.73 3.55 3.08 2.73
1st 7.05:1 4.5 4.7 5 5.8 6.5
2nd 3.57:1 8.6 9.4 10 11.5 13
3rd 1.57:1 20 22 23 26 30
4th 1:1 31 34 36 41 46
Speed at 2500 rpm (optimum 235)
SM420: rearend-> 4.11 3.73 3.55 3.08 2.73
1st 7.05:1 7.4 8 8.4 9.6 11
2nd 3.57:1 14 16 17 19 22
3rd 1.57:1 33 36 38 43 49
4th 1:1 51 56 59 68 77

Safe Redline speed (3800 rpm)
SM420: rearend-> 4.11 3.73 3.55 3.08 2.73
1st 7.05:1 11 12 12.5 14.5 16.5
2nd 3.57:1 22 24 25 29 33
3rd 1.57:1 50 54 57 66 74
4th 1:1 77 85 90 103 116

My gut feel (with 28" tires) says that 3.55:1 is best for a 235 3 speed 1/2 ton or 216 4 speed. 3.08 is best for a healthy 235 or 261 with 4 speed. Taller tires will be better with 3.73 or 3.55. I fell like 68 is a good cruising speed for me, and I think 2.73 would be way too steep for hauling a load or pulling a hill but you can downshift to third below 65 if you have to. I don't think I need to go 74 in third gear. 235's can rev to 3800 regularly without damage but their life is short if run that way for sustained high speed.

Oops, I guess I need to format that better.

Is 3800 all you can safely rev a good 235. I'm pretty sure 292s can take that with no problem so certainly you can the 235 higher. No?

I've driven my current 250 for 60-80 mile trips at constant 3k and it hasn't caused any problems. But I have a huge radiator and low temp thermostat too.

-ftyler

You can rev them to 4500 but for a stock 235 no damage will happen at 3800. I consider 3800 the max to rev to under normal everyday driving. A well built and balanced 235 will stand a bit more. The redline speed is a function of the rod ratio, the stroke, the reciprocating mass, and the materials. This adds up to a number at which the rods start to stretch because the pistons are too heavy for the acceleration produced. 235's and 261's have a 3.9" stroke, 292's have a 4.12" stroke, but 292's have better rods and 7 main bearings, in addition to a longer rod ratio (length of rod to length of stroke). Harmonic vibrations tend to kill the 235/261 faster than rod stretch will, because of the main bearing configuration and general inattention to balance. A well balanced 235 or 261 with forged rods and aluminum pistons could probably hit 6000 rpm without damage. The 250 has a 3.5" stroke and can stand sustained higher rpm easily.

Usually the horsepower peaks at 3800 in a 235/251 anyway. The head can't support much more flow.

No matter what you do, remember a 50 year old truck with the stock suspension height has a high center of gravity,and was never designed to be driven at speeds over 50 mph. If stand on the brakes in a panic situation at 65 mph, most probably at least one or both rear wheels are prone to lock up and you go sideways and you may turn turtle in a NY minute.Aways drive a old truck with extreme awareness, just like riding a cycle in traffic.

4onthefloor,

If I understand your numbers above a 4.11 will go 31 mph at 1500 rpm in 4th gear, 51 mph at 2500 rpm and 77 mph at 3800 rpm. Is that right? I thought everyone complained about their 4.11's being stump pullers that top out at about 50 mph.

I have a '47 1 ton with a 5.14, 17 in. rims, and a stock 235. I am trying to decide if swapping out for a 4.11 would be worth while. If I could hit 60-65 without winding out the engine I'd be pretty happy. Is that what a 4.11 will do for me?

Thanks, Dave

A redline is something to be reached for only short periods of time. When I drive my truck at 50 to 55mph on the freeway the revs are at about maximum for sustained driving IMO. A constant 3800 and I would expect it to granade at any second.

I have a 4-speed tonner; with the 5.14 I peaked around 35-40 in top gear and could pull a house in granny low. I had 4.11's installed to get a bit more speed and keep the low end grunt for hauling. New cruise speed is around 50, and I still only use 1st on steep hills. Made the truck much less of a traffic obstruction, gave me over 10 mpg, and might just give the valvetrain a stay of execution.

The main problem is the full-floating diff in the bigger trucks limits your pinion choices. I could have spent nearly double and bought a new set of 3.90's, but I decided the used set of 4.11's would fit the weight of a panel and the hilly terrain.

GMSea, I don't know if the NV3500 has the normal GM block bolt pattern or not. I'm trying to find out if the NV4500's bellhousing has the standard pattern, I want to use one of those heavy-duty 5-speeds in my 1-ton panel. If I can do it without spending a fortune on adapters, I will be very happy. If anyone has this info, please let us know!

Boulderado,
Yes, that is correct, with 28" rolling diameter tires. I have had my truck up to 70 briefly with the 216 and it is just working the babbitt pounder too hard. They really don't like sustained high speeds.

Tony,
Good point. I recommend radial tires, new spring shackles and shocks, a good anti-sway bar, new king pins, a good alignment, and good well adjusted or upgraded brakes if you want to drive at highway speeds. A double circuit master cylinder is a good idea as well as a correctly adjusted proportioning valve.

The formula for determining engine rpm at a given speed is:

RPM=speed[mph]*rearend ratio * tranny ratio * 12 * 5280 / (tire diameter[inches] *60 )

GMC Panel,the NV3500 will bolt up to the common small block or 230-292 block, my 94 S10 has one behind a 4.3, same bolt pattern.Not 100 percent sure if the NV4500 for GM all hve intergal clutch housing ,but they have been used since around 92 so they will bolt up.You have to figure something for the clutch linkage, they use hydraulics, and the electronic speedo drive will need some work.This link shows New Venture tranny specs. These tranmissions are quite expensive even used. NV tranmissions.