The Stovebolt Site Forums The Shop Area Driveline 235 Engine - 4 Speed Sag - 3.07 Gear -15 R

New to forum. Topic as been discussed in varying depth but perhaps there is a refresh on the topic...

Would love to hear if I should use the 4 speed sag with 3.07 gear with my 235 engine. 1952 chevy truck.

Daily Driver/Cruiser - Willing to spend $$ if necessary to complete upgrade once and be HAPPY with performance. Prefer minimal mods and I do not like to go fast but hate road/RMP noise....Open to manual or automatic transmission.....

Thanks,

It sort of depends on what the ratios in the Saginaw are. If you know how many lines were cut into the input shaft, we can tell you what the ratios are. As for 4th speed, it will work. You'll run about 25% fewer rpm at highway speed. Hint: 2 lines in the input shaft is a good situation...

The best of all worlds in my opinion is a T5 with a .86 fifth and the rest of the gears beginning with 4.03 (first). A 4.03 first will give you a starting line ratio of 12.37 and this will indubitably let you operate it like a pickup truck. Your original starting line ratio with the 4.11 rear end was 12.08. 12.37 is awfully close, so it will feel like you have the old 3 speed until you get to 4th and then rpms will drop 25% and when you get to 5th on a nice flat stretch with minimal wind, you'll drop them another 14%.

The original 3-speed is 1:1 in 3rd gear, the Saginaw 4-speed is also 1:1 in 4th gear and the T-5 is 1:1 in 4th gear, so where is the "25% fewer rpm" coming from?

If the OP installs the 3.07 rear he won't need an OD trans (T-5) as he'll never get out of 4th gear. The T-5 works well with the stock 3.90/4.10 rear or something in the high 3's.

Mike B

4.11 (original torque tube rear end) to 3.07 (what the original poster mentioned he had) = 25% reduction. If he uses a 3 speed or a 4 speed with that 3.07 rear, he will have a 25% reduction the moment he shifts into 3rd or 4th.

I've posted this at least 10 times, but here are some more wasted electrons on the matter of the T5:
Original transmission ratios (and more importantly overall ratios) GM designed into your 1952 3100:
1st was 2.94 x 4.11 = 12.08
2nd was 1.68 x 4.11 = 6.90
3rd was 1.00 x 4.11 = 4.11

T5 ratios (assuming you get the proper ones)
1st is 4.03 x 3.07 = 12.37 this means you'll be shifting out of first around 17 mph
2nd is 2.37 x 3.07 = 7.27 you'll be shifting out of second around 30 mph
3rd us 1.49 x 3.07 = 4.57 you'll be shifting out of third around 45 mph
4th is 1.00 x 3.07 = 3.07 and you'll be shifting out of fourth around 60mph
5th is .86 x 3.07 = 2.64 and if you have 27 inch tires and you're on a flat stretch you'll be cruising at 65 at around 2200 rpm and with a 235 in a pickup, that's as good as it is going to get. If you're in hilly terrain or battling a headwind, you'll stay in 4th, but in 4th at 65 you're going to be running around 2500 rpm. 2500 rpm is just fine, by the way but so is 2200. Your torque has peaked around 1600 rpm you know.

The overall ratios of the T5 (as I've said many times and proved above) can be very close to the original ratios that were used with the 3 speed and torque tube rear end. Now if on the other hand you just run out and buy any T5 without doing any homework at all, you might wind up with ratios like these:

1st ratio of 2.95
2nd ratio of 1.94
3rd ratio of 1.34
4th ratio of 1.00
5th ratio of .63

Now let's see what your overall ratios will be with a 3.07 rear end:
1st...2.95 x 3.07 = 9.05
2nd...1.94 x 3.07 = 5.96
3rd...1.34 x 3.07 = 4.11
4th... 1.00 x 3.07 = 3.07
5th... .63 x 3.07 = 1.93

With this transmission coupled to a 235 and a 3.07 rear end, you won't be able to carry anything heavier than an empty beer can, you'll burn clutches out frequently, and 5th gear will be something you never get to use, because at 65mph, your rpm will be around 1500 and that simply will not work.

But with the proper T5, some thought/planning, you absolutely can use it with the 235 and a 3.07 rear end. You can see it in the figures above.

Thanks for the explanation Jon!

Mike B

The 2 groove input sag four speed has a 3:11 first gear. So with a 3.08 rear gear you are at an effective first gear ratio of 9.57. For a cruiser, this will work but I think you need a bit more gear. With a low power 235, you need gear multiplication to get going from a stop. So for example in the 57 cars with three speed stick transmission, they had a first gear of 2:94 or thereabouts and a rear gear of 3:55 for an effective first gear ratio of 10.43. Trucks had steeper gears because they needed the gear multiplication to get going with a heavy load. Once you are in top gear in a four speed or three speed, you are at 1:1. I would want more rear gear (numerically higher) if it were me. For a couple of reasons, I think 3:08 are too low and lug these motors. Lugging the motor is really hard on crankshafts. So if 3.20s are available, you would be at 9.95 which is a little better in the get going from a stop department and is not a giant increase in rpm at cruising speed. On the other hand, I think 3:55s are a terrific all around rear gear ratio and would put you at 11.04 effective first gear. That's is pretty good and don't let anyone tell you that is a granny gear because I can assure you from personal experience, its not.

As for the sag itself, reasonable in cost to purchase, and they are generally not beat to death like a Muncie is. I bought one that I ran for a few years for like 250. Decent rebuild kits are in the 120 range (you can find them cheaper but the synchro's are total junk). The biggest issue is the shift forks. They wear out. One of the forks is common to the three and four speed Saginaw, the other is strictly four speed. So one is easy to get new, the other a little harder. I think I was another hundred in new forks. Rebuilt it in the basement "laa Boor Atory" and ran perfect first time out. So I had $500 in parts, transmission core, gear oil, sealers, paint and degreaser. Pretty good $$ for a four speed. In fact my son is now going to use it behind a 6 in a 57 car he is rebuilding.

3.07 rear end Is too high (IMHO) if you have hills. I had 3.54 gears (spicer 44) in my '60 for a few years, changed to 3.73 & much improve pulling grades, seldom need to shift out of direct (4th). Now if you live where you can see the curvature of the earth, thing are different.

Thanks so much for all of the expertise that has been shared. Obviously there is a lot of information concerning this topic and I am in the midst of a restoration and have read many journals, articles,etc...

Now the challenging part comes into play. Choosing an option and spending the money!

I am enjoying this project immensely. The truck was my grandfathers and I actually put the Sag SM420 into the truck when I was sixteen and I did not know what I was doing then, fast forward 41 years and I am still do not know what I am doing.

But the challenges, the sounds and the smell of that truck motivate me to continue going forward,

Enjoyed the many responses...

If Sag is short for Saginaw it shouldn't be used when talking about the SM-420...the SM-420 is a Muncie (SM = Synchro-Mesh and 420 is it's input torque rating).

Mike B

Thanks. Some other "experts" have looked at this transmission as I was looking to initially sell this transmission but they said it was a Saginaw. This is a family truck and my brother said it was a MUNCIE.

So will look up the numbers and see if I can document its origin. Always fun with this truck as I have to fight the family stories as well as the undocumented changes to the truck.

Thanks for the input.

Don't forget to consider tire size in your calculations.

BTW, that air cleaner is too small. Maybe it's just there to keep the dust out while you are working on the truck.