Transmission and gear ratios

[Dtret]

1 hour ago, Leghome said:

 

That looks like the numbers on my 72 with a 799r4 and 3:73 gears

Cool transmission. Who makes that? Is that from the Isuzu decision of gm, like Allison,  r4 so I assume it’s overdrive. Let us know. Curiosity you know.  Sorry had to I have the fat finger problem too. Lol. 

[Leghome]

Good ole fat fingers. I find myself doing that more every day. Yep 700r4

[Scott S.]

Hey Aaron,

If you're considering switching the rear gears, it'd probably be easier to transfer the whole unit.... 6 bolts, one brake line, and E brake cable. Plus the U joint. 

Beats taking apart the carrier and pinion.... Unless you're in need of a rebuild.

Personally, I'd recommend the 200r4. It's pretty much a bolt in for the TH350's. the only issue I ran across was the driveshaft needed to be shortened about 1".

The T.V. cable was a touch finicky to set, but the guy who did mine has been rebuilding transmissions for 45 years, and he build mine to 750 horses. 

Just remember, on the 200r4's, they need to have a new, hardened "barrel" installed. Or else any decent horsepower will just spin the input shaft right off.

With the 200r4 and 3.31 gears I went from about 14 mpg to almost 21 mpg. (imp.)

Originally had Th350 and 2.73 gears. 

Hope this helps, Scott.

[EVC]

256 gears… 700r4…  lots of low end torgue.. 15 to 17" of vacumn at 65… The engine isn't bogging down … She has the low end torgue… Sorry no pics after 80… 

When I push the pedal,,  she just keeps pulling… After she reaches the 100mph I back off … b

Ed C...

[Canuck]

I appreciate all of your input. For the 71 SS I’ll stick with the TH400 as I’m trying to keep it stock. I still have a decision to make with the rear gears but your input has helped. Thank you!

[jft69z]

I was hesitant to reply, because what I have isn't exactly what you were asking, but here goes. 

My Camaro has (had) a Doug Nash 5 speed trans. I put a 3.08 rear gear in it because the DN trans used a 1:1 ratio for 5 gear. Where they made it up was with a pretty stiff first gear of 3.27. It drove great, especially with the 454, plenty of torque. Most of the time, first gear wasn't even needed, to the point I could start off in second gear easily.

Where this becomes important, is the DN second gear is 2.13:1. That, with the 3.08 rear gears was a very drivable combination, especially if you're steering more towards highway driving vs. absolute off the line acceleration. A turbo 400 has a first gear of 2.43, so that's a bit more in your favor than the DNE second gear as far as off the line. 

3.08 or 3.27, either one would work for you here with the 454, but it basically comes down to what works best for you. I realize it's not an exact comparison that you were looking for, but still data that may help you decide.

[Canuck]

5 hours ago, jft69z said:

I was hesitant to reply, because what I have isn't exactly what you were asking, but here goes. 

My Camaro has (had) a Doug Nash 5 speed trans. I put a 3.08 rear gear in it because the DN trans used a 1:1 ratio for 5 gear. Where they made it up was with a pretty stiff first gear of 3.27. It drove great, especially with the 454, plenty of torque. Most of the time, first gear wasn't even needed, to the point I could start off in second gear easily.

Where this becomes important, is the DN second gear is 2.13:1. That, with the 3.08 rear gears was a very drivable combination, especially if you're steering more towards highway driving vs. absolute off the line acceleration. A turbo 400 has a first gear of 2.43, so that's a bit more in your favor than the DNE second gear as far as off the line. 

3.08 or 3.27, either one would work for you here with the 454, but it basically comes down to what works best for you. I realize it's not an exact comparison that you were looking for, but still data that may help you decide.

That’s helpful info. Thanks!

The area where I live requires a lot of highway driving so that factors in to my decision. It is likely I’m going to put in the 3.07 gears as that be my best compromise for the way I use the car.

[EVC]

I was reading that the gear ratio is stamped on the passenger side of the rear housing of some differentials  …

After doing the math with an online calculator, it looks like I am running 308 gearing rather than the 256…

 

 

[jft69z]

2 hours ago, EVC said:

I was reading that the gear ratio is stamped on the passenger side of the rear housing of some differentials  …

After doing the math with an online calculator, it looks like I am running 308 gearing rather than the 256…

That's just a casting number for the diff housing. The number you're looking for is on the passenger side axle tube, facing the front of the car. You'll still need to decode it.

http://chevellestuff.net/1970/rear_axle.htm   (click on the year of your vehicle near the top to get the proper codes)

http://chevellestuff.net/qd/rear_axle_info.htm

[Canuck]

I just drove it home from the shop with the 3.07 gears. I actually get to use 1st gear now, LOL. It seems to drive nice and it doesn't sit at such a high RPM on the highway. Now it is ready for the drive to the Western Meet! Thanks again for all your input.

[sbarnette]

3 minutes ago, Canuck said:

I just drove it home from the shop with the 3.07 gears. I actually get to use 1st gear now, LOL. It seems to drive nice and it doesn't sit at such a high RPM on the highway. Now it is ready for the drive to the Western Meet! Thanks again for all your input.

I’m sure that makes a huge difference. You’re going to like that on the highway. I would like to do the same with mine. That’s the main reason I’m trailering mine to the Eastern Meet. I hate running high RPMs for such a long trip.

[Canuck]

After a few cruises at low speed by the beach, driving around town and some drives on the highway I’m quite happy with the 3.07 gears. I’m actually surprised at the acceleration I still have after the swap and it is much better overall driving now. I didn’t do this to save fuel costs but that will be an added bonus.

[EVC]

Finally got the axle codes… CKD  0206B2…. According to the tech info here , It looks like the old girl has a 273 gear ratio. 

[Canuck]

On 10/4/2022 at 8:30 PM, EVC said:

Finally got the axle codes… CKD  0206B2…. According to the tech info here , It looks like the old girl has a 273 gear ratio. 

2.73 was very common in these cars

[BobW]

My '72 has the base 350 with 273 gears. Recently upgraded the 2 bbl to a Jeg's dual plane manifold with a 1405 Edelbrock. Also running long tube headers with Delta50 Flowmasters. 

Will a 200r4 OD lug too much with the 273's?

Raise the lock-up to 50 mph?

Maybe better without lock-up?

[cbolt]

I would think you could research what stock gears backed up the 200r4 on the mid '80s cars and figure that out. I know the IROC Z's and SS's from that time period had 3:73's, but not sure what an ordinary Camaro or Monte Carlo ran for gears. 

 

I would reason that whatever a stock vehicle with the 200r4 had for gears would be about as low as you could get away with. 

[BobW]

4 hours ago, cbolt said:

I would think you could research what stock gears backed up the 200r4 on the mid '80s cars and figure that out. I know the IROC Z's and SS's from that time period had 3:73's, but not sure what an ordinary Camaro or Monte Carlo ran for gears. 

 

I would reason that whatever a stock vehicle with the 200r4 had for gears would be about as low as you could get away with. 

I think you hit the nail on the head!

Found a post that listed final drive gear ratios with RPM's at 70mph. 273's at 70 would be running at 1800 RPM's.

This post related to the swap with a base 350 like mine. OD kicking in at 45mph would severly lug the engine. A high torque stroker motor 'might' pull the higher ratio better.

General consensus from that discussion felt 308's would be the highest acceptable ratio, with 331's most likely the best compromise for performance and economy.

Now my mind wonders if a non locking converter would function OK, or just shift in and out if OD?🤔

[Scott S.]

20 hours ago, BobW said:

My '72 has the base 350 with 273 gears. Recently upgraded the 2 bbl to a Jeg's dual plane manifold with a 1405 Edelbrock. Also running long tube headers with Delta50 Flowmasters. 

Will a 200r4 OD lug too much with the 273's?

Raise the lock-up to 50 mph?

Maybe better without lock-up?

It's not that bad if you're just looking for a "cruiser" in the city and stay in third gear.

But it gets very interesting on the highway when you're running at 70 mph and only pulling 1500 rpm. 

Stepping into the throttle at those speeds and having the tires chirp is something only my T-Type ever did.

[Scott S.]

5 hours ago, cbolt said:

I would think you could research what stock gears backed up the 200r4 on the mid '80s cars and figure that out. I know the IROC Z's and SS's from that time period had 3:73's, but not sure what an ordinary Camaro or Monte Carlo ran for gears. 

 

I would reason that whatever a stock vehicle with the 200r4 had for gears would be about as low as you could get away with. 

Monte SS's and Buick GN's/T-Types came with 3.42 to 1 rear gears.

[Scott S.]

33 minutes ago, BobW said:

I think you hit the nail on the head!

Found a post that listed final drive gear ratios with RPM's at 70mph. 273's at 70 would be running at 1800 RPM's.

This post related to the swap with a base 350 like mine. OD kicking in at 45mph would severly lug the engine. A high torque stroker motor 'might' pull the higher ratio better.

General consensus from that discussion felt 308's would be the highest acceptable ratio, with 331's most likely the best compromise for performance and economy.

Now my mind wonders if a non locking converter would function OK, or just shift in and out if OD?🤔

The "non locking" torque would be better, so I've read, for longevity and street racing..... But you won't be using the O.D. much in those conditions anyways.

The "locking" is meant for highway usage and fuel economy. Once the torque locks, it puts less strain on the transmission pump. Runs cooler. 

Setting the kick in higher should lessen the lugging problem, and depending on the torque converter lock up rpm, make it a smoother runner. 

But just as a "rule of thumb", I tend to stay in third gear (1 to 1) anywhere but on the highway. Reduces the wear and tear. 

[MC1of80]

1 hour ago, Scott S. said:

Monte SS's and Buick GN's/T-Types came with 3.42 to 1 rear gears.

Not true. Monte Carlo SS in 1983 came with a th350 and 3.42 gears. 1984 and up has the 200r4 and 3.73 gears in a 7.5 housing.  

The Grand National had the 8.5 rear with 3.42 gears and 200r4 trans. The olds 442/hurst olds also had the 8.5 rear with 3.73 gears and the 200r4. 

[cbolt]

3 hours ago, Scott S. said:

Monte SS's and Buick GN's/T-Types came with 3.42 to 1 rear gears.

Mine came with factory 3:73's. I purchased it new in May of 1986

[BobW]

19 hours ago, Scott S. said:

But just as a "rule of thumb", I tend to stay in third gear (1 to 1) anywhere but on the highway. Reduces the wear and tear. 

No such thing as a dumb question, right?

How do you stay in 3rd gear?

I have a B&M Megashifter that's scheduled to go in after paint & the bucket seats install. It's a rachet shifter with provision for 3 or 4 speed trans.

If I put the trans in 3rd, will it shift 1, 2, 3 or try to start in 3rd? 

[BobW]

5 hours ago, BobW said:

No such thing as a dumb question, right?

How do you stay in 3rd gear?

I have a B&M Megashifter that's scheduled to go in after paint & the bucket seats install. It's a rachet shifter with provision for 3 or 4 speed trans.

If I put the trans in 3rd, will it shift 1, 2, 3 or try to start in 3rd? 

So I guess I answered my own 'dumb' question! Put the MC in 2nd and it did shift from 1st to 2nd, presume the 200r4 will do the same.

Stopped to fill up after this morning's cars & coffee and calculated mileage at 13.3! This tank had mostly 1 to 2 hour runs at 50 to 70 mph. I think I could have safely been in OD for 80% of those miles.

Pretty sure I'll eventually do the trans swap and shift as needed.

[Scott S.]

22 hours ago, cbolt said:

Mine came with factory 3:73's. I purchased it new in May of 1986

The '84 I owner came with 3.42 to 1. As did the '86 T-Type. They must have dropped the ratio on the Montes in '85-'86.