[Tom Hoffmann]

Hello I´m from Germany and need some help.I have a standard GM 8.5 with 2.41: 1 from a 1976 Pontiac Firebird, 350cui with Th350 automatic. I would like to accelerate better and switch to a 3.41 or 3.73. I know that this is not ideal for German autobahns, but I mostly drive in the city or country roads. I've read there are different carrier und a bigger gear won t fit. The next problem is that there is no mechanic near me who repairs US differentials. If I need a new carrier anyway, that means that the installation is easier. Could I, as a hobby mechanic, do the installation without any experience, or is it actually necessary to have an expert? Is it also necessary to change other things on the vehicle (Converter) when changing to a ratio of 3.73 or does it all go well together?
I would be happy to receive tips or your experiences.
Greetings from Germany
Tom

 

[lmsport]

Diff work is not hard, just tedious.
Use the pinion bearing spacer instead of a crush sleeve.
Acquire a setup bearing for the rear pinion (M802048) to make changing pinion shims easy.
Use red locktite on the pinion nut as well as a new pinion nut, use the old one for setup.

 

[ericnova72]

Yes, only 2.41 and 2.56 gear fits on the 2 series carrier, for anything 2.73 and higher you will need a 3 series carrier.
It can also be done with the 2 series carrier and a ring gear spacer, but it is not recommended as the ring gear will no longer fit down on the OD register of the carrier very well, putting a heavier load on the ring gear bolts.

No need to change the converter or anything else, except for the speedometer drive and driven gears in the tail of the trans if you want the speedo to read correctly after the gear change.

There is a possibility you will find out the driveshaft will have a vibration as it will be spinning faster at any given speed than the 2.41 gear was, may need a rebalance or a better shaft if still using the stock shaft.

 

[Tom Hoffmann]

Thank you for your answers. Would you describe 3.73 as too short or is it good /ok to drive. A friend told me he guess its way to short.

 

[AutoGear]

Greetings Tom!
Here is an RPM calculator, that can use mm and kph versus inches and mph.
373s at modern car speed will net you somewhere in the 2800RPM+. Only you can answer if that is 'too short/ok to drive'. Perhaps a 3.42/3.55 would be better.

 

[lmsport]

3.73 is a 50% change in final drive.
3.08 is a 25 % change.

 

[ericnova72]

Calculate GEAR, RPM, MPH, TIRE DIAMETER (wallaceracing.com)

Plug in your numbers and see what RPM you feel you can live with at your typical cruise speed.

I'm thinking you might be happier with 3.42, or even 3.23 since both are still a pretty big jump from 2.41

 

[RWENUTS]

Test drive in second gear.
that’ll be what it feels like with a close to 3.50 rear gear.

 

[75gmck25]

I had a Ford Torino back in the 70’s, and at that time Ford recommended 3.50 gearing for all around street performance, and 3.91 for a little better acceleration. US speed limits were mostly 65 or 70 mph. We were using F7014 (about 215/70R14 or maybe 225/70R14) tires.

If you are consistently cruising faster than about 100 kmph, the 3.42’s are probably a better choice than the 3.73.

 

[55_327]

Would you describe 3.73 as too short or is it good /ok to drive. A friend told me he guess its way to short.

Yes, 3.73 will be very short compared to what you currently have. Revs will be 55% higher. 😲 If you do any highway driving at all, you will be wishing for an OD trans.

Several years ago I went from 2.73 to 3.42 in a 55 Chevy car with a 327. (Diff was a 10-bolt.) Even that 25% increase in revs made a huge difference. I'd go with 3.23 if I had a do-over.

Also, a buddy of mine went from 2.56 to 3.42 in a 77 Nova with a mild 350. Acceleration was great, but with 26" tires it was terrible on the highway! He wished he would had gone with 3.08.

By the way, on my 2.73 to 3.42 change, I used a Yukon R+P set with a "thick" ring gear, so I didn't have to change carriers. That was a 7.5" 10-bolt, and I'm not sure those thick gears are available for an 8.5" 10-bolt.

 

[BrianB74SD]

Hi Tom. Ultimately you have to make yourself happy. Since you mentioned that you'd be happy to receive feedback, I will provide mine. My first car and daily driver in the '80s was a '78 Trans Am with a 400/Turbo 350 and 2.56 gears. I did all the typical mods and even went to 4.10 gears. It was fun, buzzy, and not practical at all. I finally compromised and settled on 3.42's, which I loved and never looked back. Very good acceleration and driveability for an everyday car.

 

[ericnova72]

By the way, on my 2.73 to 3.42 change, I used a Yukon R+P set with a "thick" ring gear, so I didn't have to change carriers. That was a 7.5" 10-bolt, and I'm not sure those thick gears are available for an 8.5" 10-bolt.

Unfortunately, the thick style conversion gearsets aren't available for the 8.5/8.6" GM 10-bolt.

 

[SoulSurfSD]

You'd likely be happiest with 3.73 gears and 700R4 trans... as long as the trans works properly.
You can cruise 80-85MPH all day long with that...

 

[RAYFIN]

Quick Performance | Ames, IA can tell you. You can install yourself if you have the right tools. Measurements/clearances is everything.

 

[ekimneirbo]

Let me offer a different way to go.........

First go to Amazon and buy a book on Chevy rear ends. About $25 US and it will tell you all about rebuilding as well as a lot of other stuff.

The different way..........

Gas is expensive in Germany and highway speeds are high. One of the main things that happens when people use lower (higher numerically) gears is their cars are not fun to drive on the expressway. If you can get an overdrive transmission for it, it will give you a lower first gear and a higher high gear. Acceleration will improve some and cruising will be more efficient and enjoyable.

 

[V8Square]

Hello I´m from Germany and need some help.I have a standard GM 8.5 with 2.41: 1 from a 1976 Pontiac Firebird, 350cui with Th350 automatic. I would like to accelerate better and switch to a 3.41 or 3.73. I know that this is not ideal for German autobahns, but I mostly drive in the city or country roads. I've read there are different carrier und a bigger gear won t fit. The next problem is that there is no mechanic near me who repairs US differentials. If I need a new carrier anyway, that means that the installation is easier. Could I, as a hobby mechanic, do the installation without any experience, or is it actually necessary to have an expert? Is it also necessary to change other things on the vehicle (Converter) when changing to a ratio of 3.73 or does it all go well together?
I would be happy to receive tips or your experiences.
Greetings from Germany
Tom

Where at in Germany? I lived in Wiesbaden for 2 years

 

[curtis73]

The thing to remember with diff work is that the gears are made to very tight tolerances, but the housings are not. When replacing the pinion, use the same shims that came on the original pinion and you'll be really close with pinion depth. When pinions and gears are manufactured, they are incredibly precise. The cast iron housings are imperfect to start, and then machined to be close to the same, but not always. When gears are installed at the factory, they use shims to make the gears work in the housing. Therefore, when swapping gears, chances are the same shims will be very close to correct if not perfect. Then the rest is backlash.

There will be a stack of shims on both sides of the differential. On the thrust side (the side with the ring gear) you'll notice very few thicker shims. The non-thrust side might have a bunch of thinner shims. The goal is to keep it that way. More shims means more space and more possibility of compression.

I start by installing the pinion and ring and installing with backlash sorta close. Run a pattern with yellow grease making sure to put resistance on it. I often will lightly set the parking brake and then use a ratchet on the pinion to turn it. If you just let it freewheel, it sometimes doesn't put enough pressure on the teeth to show a good pattern.

Once you've confirmed a good pinion depth, move shims around until you get the right backlash and you're done.

 

[Bonneville462]

I would keep the 2.41 and work on getting more low end torque.. Has the cam ever been changed?
For some reason people like to put cams in cars with too much duration, this gives you a loping sound at idle. They would rather have a car that sounds fast but is slow.. I would rather have a car that's fun to drive.

Put a vacuum gauge on the engine and see what the reading is warmed up and idling in Park or neutral.. It should be 18 or higher.. Even if the cam has never been replaced, you could replace the stock cam with one that will give you a stronger bottom end and mid range..
You would want something along the lines of the Pontiac 066 or 067 cams.. The compression is too low for the 068..

 

[BogiesAnnex1]

The problem your stuck in is to huge amount torque is a function of engine size, so the first consideration is something bigger than a 350. For the amount of added bigness to affect enough more torque production you're in need of at least 20 to 30 cubic inches to obtain anything useful. This eliminates anything you can get with a reborn as the cylinder wall thickness just doesn't allow it so stroking or entirely replacing the engine with a 389, 400, 421, 455 are the only practical solutions here.

More cam can be used to bolster torque output but more cam weakens lower RPM tourque and power and while improving the maximums it moves them up the RPM band which requires stiffer final obd overall gearing to chase where the improvements went which outs you right back to where 2.41’s are too lazy and while these will work on the autobahn they’re a pain in the butt everywhere else.

While stiffer gearing both adds to the torque multiplication through the leverage of the gears and those gears moving the engine RPMs up into the maximum available forces it can generate band, listening to the screaming motor does not not an enjoyment to the ride and is detrimental to life expectancy of the engine.

The North American answer has been overdrive transmissions where the manual T5 offers a fifth speed overdrive or the T56 offers a fifth and sixth gears as overdrives. For the ever popular automatics the 4th speed overdrive gained widespread use.

Electronic port or direct cylinder fuel injection adds a lot of torque and power that cannot be obtained fron a carburetor or throttle body injection because these systems eliminate the problems of getting fuel and air to flow together from one place to another. This mixture correction by cylinder adds a lot of bottom end torque without needing a bigger engine.

Short of converting to sequential EFI and can’t do much with gearing for cabin comfort at high speed you're looking at a bigger engine with the gears you have or reasonably close to them. Or mating your engine to a 700R4 gear box for a deeper launch low and an easy turning overdrive when matched with stiffer rear end gearing.

Living Europe I really don’t know how much luck your going to have duplicating the 350 HO of the late 60’s. It’s hard to do here in the states as those motors were a long time ago and there weren’t many ever out on the street in the first place, But if your stuck with the P350 then the HO is worthy of duplicating from the aftermarket if you have the finances to do so.

Bogie

 

[Bonneville462]

In 1974 I drove a 1971 el Camino 454, 4 speed, 3.23 rear end.. That car was a dream come true. I never knew there were cars made that could do what that one did.. I was spoiled. Small blocks seemed like lawn mower engines after driving all the torque...
I will only swap to the biggest cubic inches the car brand made.. The Chevy truck 454 of the seventies is one nice engine!! It needs a good old fashioned power tune up though. Start by throwing that vacuum advance in the can, then screwing the points plate to the distributors frame followed by a new NOS set of Accel points; remove the spring from the points adjuster and put the exact amount of washers were the spring was and keep tighting them until the screw is very tight and the gap is right on .017. Next remove the condenser. Next pick up a Vertex Z-6 Capacitive Discharge box. Use the points to trigger the box. The points will last an easy 10 years 50,000 miles because the CD box took 98% of the work load off them..