1976 rear end ratio change

[J W.]

I need some advice regarding a rear end ratio change for a 1976 L48/M40/C60 with the factory 3.08 rear end. I would like more punch off the line. The car does not do any highway driving. The drivetrain is stock except for headers and side pipes. I am trying to decide between 3.55 and 3.73. With a 3.73, would the 1-2 shift occur too quickly? And would a 3.73 require a different carrier?

[Duke W.]

Re: 1976 rear end ratio change

Keep in mind that the mid-seventies Corvettes had the lowest net horsepower ever. Part of the reason was the lazy emission control oriented spark advance map, but the single "bead bed" catalyst produced "half and atmosphere" of back pressure according to Dave McClellan's book. A neighbor has a '77 L-48 TH400 coupe with a 3.08 axle and around town IMO it performs better that what the weight and engine performance imply. Maybe bubba installed a high overlap cam along the way that just kills the low end without adding any top end

Optimizing the spark advance map is the first thing you should do, and a modern honeycomb catalyst will reduce back pressure.

Rebuilding the OE axle is fairly expensive and currently available aftermarket gears typically are noisy. IMO do the above or sell the car and buy a C4 or C5 that has better performance.

Duke

[J W.]

[David M.]

Re: 1976 rear end ratio change

Yes you will need a different carrier for 355 and 373. Or install the correct size ring gear spacer and new ARP ring gear fasteners/washers.

You could go to 4:11s with a Gear Vendor overdrive unit. Best of both worlds. Killer launches with overdrive. Step up to a higher stall converter too.

The lower the gear ratio (higher numerically) the thinner the ring gear gets which means the carrier needs to be thicker to compensate the pinion centerline thus requiring a spacer to bring the ring out towards the pinion if using the 308 carrier. But better to get the right carrier.

I'm not sure exactly where the carrier breaks are. Something along the lines of 256 to 270, 308 to 323, 355 to 373, then 411 to 430, 456 to 5+ etc.

Call around to the drivetrain suppliers they will have specific information and likely have kits to suit your needs.

Another option is to find a non-abused used rear end with the ratio you want then rebuild it with fresh fasteners, seals, bearings, races etc bolt it into the car and go.

You wont notice a huge difference from 308 to 355, especially with a stock stall converter. If you are going through all that trouble you will want a bigger bang for your buck... such as 373 with a higher stall converter. You will feel that.

Day & night with 411s GV overdrive and a higher stall converter in the 2500 stall range. With these three mods it will act like a different car. Great lunches, nice highway manners.

[Gary R.]

Re: 1976 rear end ratio change

Hi Michael,
You can certainly change out your gear set and drive the car it shouldn't be a consideration to sell the car. You need some correct information and then you can decide what you want to do.

The stock C3's with base engine and automatics came with the 308 in almost all of them. It was an ok gear for driving, there is no offline performance to speak of as you know. The headers and side pipes make noise but most of the gain will be in the higher RPM ranges and if that is what your plan is, you need to rethink some things.

355's were an option for automatics. It is not a bad gear and you will feel the difference between those and the 308's. 3.73 is an aftermarket gear ratio, GM used a 3.70 in the vette. There are more teeth in the gear set and it computes out to 3.73, no difference in feel or speedo gear between the 370 or 373.

You do not need to replace the case. The correct terminology is Carrier= housing, Case is inside the Carrier- as in posi case. Common terms to mix up so no problem.

Assuming your 76 is original, there are some things within it that need attention. There was not a perfect corvette differential between 1963-1982, including the 63 Grand Sports. They all had one or more issues within them. 76's had bad axles, posi clutches, and ring gear bolts. All of which should be replaced if you go into it.

The posi case is a 3 series, I would have to check but I don't think 4 series gears were offered by 1976 since there was only L48 slug and L82,which is better but still lacking in some areas. If 4 series gears were offered, it would have been with the L82 & 4 speed and a 411 gear. In that case the posi case would be a 4 series case.

I like 373 over 355 and have up 100's of them, they work great with OD transmissions as well.

The ratios that have been offered for years by US Gear (Toms) were 273,308,336,355, 370, 373, 390, 411, 433,456- ALL of them made the past 20-25 years by US Gear for Tom's were made to fit with a 3 series case. Spacers are not required nor recommended. That was an old trick back in the 60's-70's and didn't always work well. Some of those ratios are long gone now as well.

Next you need to decide if you are doing the work or paying someone to do it. Most guys don't know the difference in build quality or levels, thinking they are pretty much all the same. That is true for what I call vendor level builds. If that is the level you want, ok, they will work on mild cars and trailer cars and cost about $1000 give or take. You really need to educate yourself here because if you don't you may end up with a $1000 diff, which has $150 in parts and a paint job. Things like axles, gears, covers, better clutches are all extra.

I do this work every day and have repaired the work of many places - vendors, vette shops, local garage, speed shops, car clubs, and owners. I have all the pictures of what I have seen over the years, so trust me- if nothing else, learn about this work. If you rely on websites, well, I could go deeper here but won't.

You need to know what new gears are available today. Up till 2 years ago you could go to Summit, Jegs, Tom's, or most vendors since they rebox the others parts,, and buy USA made US Gear. However, they sold out and within a year closed down. The factory equipment and machinery was sold off at auction.

What is left? Look at Summit- Motive, Richmond, Yukon, and possibly Nitro Gear are what you will find. If you weed through the various online vendors they are just reboxing those gears. There is also US Standard Gear, sounds like US Gear and possibly that was/is the intention, but read about them and see what you find- very interesting.

Motive only makes 355 & 370 for the 63-79 vette. They are made in Italy and not bad gears. I have set up a few the past year and got very good results with them. They were smooth and quiet.

Richmond is owned by the same company that owns Motive, for years Richmond was known for whining. This goes back to when they were made in the USA, about 10+ years now. I set up a set of 355 USA Richmond about 12 years ago. I had a textbook pattern, perfect lash, the posi was polished and tuned and they were the worst sounding gears I ever heard. I pulled them out and installed a set of Tom's 355, again dialed in textbook perfect lash and pattern and they were quiet. I never went back to Richmond- other than for the vette 12 bolt gears. A customer just sent me a set of Richmond 390 to build, so I'll see if they are like the Motive or not.

Yukon was the brand name for Randy's Ring & Pinion, long story there. Today they offer most of the ratios US Gear did, they price the ratios differently and all are just expensive. I just looked today at Summit for a new set of Yukon 336, they are $520 now. I sourced them for about $425 shipped. Yukon set up ok and I haven't had any problems with them.

Nitro Gear never responded to my simple question a year ago, "do you make your own gears and if so where are they made" not once but twice I tried to get an answer. Nothing. What does that say?

You may find used GM gears sold on ebay. Most guys think they're gold, they're not and most are unknown. A used gear set can be fine, if you know the diff it came out of. Buying gears, hoping they are a matched set and not something that was in a garage bucket for 40 years, is risky. They're not worth more than $100 in my opinion and to go through all that work to possibly have a gear whine isn't worth it. True, real bad gears will show during installation if the builder is paying attention, but don't expect that for the common diffs for sale today. Also keep in mind many places don't even do the work, they buy and resell them. Others punch them out in 8-10 hours, and others have websites with their racing parts all over the place. A guy from CT years ago, Mr Barnum, would love it today.

As bad as new gears are so are axles. You can get new Chinese axles that are hardened but the snap ring groove seems to be located by dart board accuracy. You can get rebuilt; they are sold all over but really come from a couple of places and there are issues with those too. Some issues are no QC, others are poorly designed rebuilt axles that are doomed to fail. Seen that a few times.

So, story time is over. Yes, you can regear your diff but do your homework or as in school you will fail the test.

[Duke W.]

Re: 1976 rear end ratio change

Thanks Duke for your reply. Your thoughts are good. I am well aware of the low horsepower of a 1976 Corvette. It was my father’s since new so selling it is not an option. The distributor was recurved with new weights and springs by a reputable Corvette shop in the 1980s and I don’t remember the details but do recall that it really woke the engine up. The engine runs beautifully after a recent tune. The back pressure should be reduced with the low restriction mufflers in the side pipes. With the pipes, I am not too worried about rear end noise and the cost is not prohibitive.

So, having said all that, should I go with 3.55 or 3.73?

I recommend you do some diagnostics. Check manifold vacuum. If the cam is OE it should pull about 18" at 500 in neutral and be very smooth. A low end torque killing high overlap cam will pull less vacuum and may not even idle stably at 500. Also report idle vacuum idling in Drive or as low as possible if it won't idle in Drive at 500.

Get a dial back timing light and vacuum pump and map out and report the centrifugal and vacuum advance, also report the ID number on the VAC.

The above data may lead to an inexpensive way of improving low end performance.

The next step may be getting a chassis dyno test to determine actual engine performance. I take it it has some kind of aftermarket dual side exhaust?

I don't know if a 700R4 or 2004R will fit you car, but they have lower first gears (3.06 and 2.74 respectively) versus the TH350 2.52 with the bonus of an overdrive fourth gear.

Last, consider rebuilding you engine to "Special 300 HP" configuration with a McCagh Special camshaft that maintains the smooth low speed idle of the base engine with 90 percent of peak torque at 1200 and SHP type top end power and revs.

Duke