Auto trans / rear gear advice please. - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 34 (permalink) Old Oct 29th, 18, 9:20 PM Thread Starter
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Tom
 
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Auto trans / rear gear advice please.

Hey folks,

Im currently running a very weak 70 402BB in my 68 El Camino. I have a 12-bolt 3.73 Posi rear, 29 dia. tires, and a 3-spd. auto transmission. I run just over 3k RPM at 70 MPH.

My first order of business will be to replace my engine with a more potent BB. Probably nothing over 550 HP/TQ.

My plan WAS to stick with my 3.73, but to swap in an overdrive transmission to bring my cruise RPM down toward the 2,100-2,200 range. But then my hamster wheels started turning and now Im wondering if the 3.73 gears might be a mismatch for a higher-torque engine. I dont want my first gear to be only a smoke/freckle generator.

This car wont be a race car, nor will it be a long-distance daily driver. Just a fun weekend cruiser that will make me smile when I apply the throttle. So whats the advice of the masses here? Stick with my original plan of an overdrive or just knock the rear gears down to the 3.42/3.23/3.07 range?

Thanks for any advice.

Tom.
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post #2 of 34 (permalink) Old Oct 29th, 18, 9:30 PM
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Claude
 
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by any4xx View Post
Hey folks,

Im currently running a very weak 70 402BB in my 68 El Camino. I have a 12-bolt 3.73 Posi rear, 29 dia. tires, and a 3-spd. auto transmission. I run just over 3k RPM at 70 MPH.

My first order of business will be to replace my engine with a more potent BB. Probably nothing over 550 HP/TQ.

My plan WAS to stick with my 3.73, but to swap in an overdrive transmission to bring my cruise RPM down toward the 2,100-2,200 range. But then my hamster wheels started turning and now Im wondering if the 3.73 gears might be a mismatch for a higher-torque engine. I dont want my first gear to be only a smoke/freckle generator.

This car wont be a race car, nor will it be a long-distance daily driver. Just a fun weekend cruiser that will make me smile when I apply the throttle. So whats the advice of the masses here? Stick with my original plan of an overdrive or just knock the rear gears down to the 3.42/3.23/3.07 range?

Thanks for any advice.

Tom.
Tom, if I were you, I'd leave the 3:73 gears alone and swap the transmission for a 700R4 with a lockup converter. It'll bring your rpm's at about 1,950 ~ 2,000 at approximately 68 ~ 70 mph. In fact, 3:73's are the most popular gear choice when going to a 700R4 and since you already have them in your car, the next logical step would be to upgrade to a 700R4 transmission.
Good luck,

Claude.

Claude.
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post #3 of 34 (permalink) Old Oct 29th, 18, 9:36 PM Thread Starter
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by toofastforyou View Post
Tom, if I were you, I'd leave the 3:73 gears alone and swap the transmission for a 700R4 with a lockup converter. It'll bring your rpm's at about 1,950 ~ 2,000 at approximately 68 ~ 70 mph. In fact, 3:73's are the most popular gear choice when going to a 700R4 and since you already have them in your car, the next logical step would be to upgrade to a 700R4 transmission.
Good luck,

Claude.

Claude.
Thanks. I was first thinking of a 2004R, then started thinking 4L80E. But I think youre right. The 2004R just has too low of a first gear and Im kind of wanting to keep this car in the dinosaur era with as few electronics as possible.
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post #4 of 34 (permalink) Old Oct 29th, 18, 9:49 PM
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Steve
 
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Re: Auto trans / rear gear advice please.

The 2004r has better gearing than the 700r4 that matches up better with your 3.73 gears. Heck, the Monte Carlo SS came with the 2004r and 3.73 from the factory.

Ratios of 1st through 4th for each.

2004r 2.74 1.57 1.00 0.67
700r4 3.06 1.62 1.00 0.70

As you can tell, I lean towards the 2004r over the 700r4. Both have their place, I just prefer one over the other. I just don't like the drop from 12 for the 700r4.

Quote:
Originally Posted by any4xx View Post
Thanks. I was first thinking of a 2004R, then started thinking 4L80E. But I think youre right. The 2004R just has too low of a first gear and Im kind of wanting to keep this car in the dinosaur era with as few electronics as possible.
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post #5 of 34 (permalink) Old Oct 29th, 18, 9:56 PM
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by any4xx View Post
Thanks. I was first thinking of a 2004R, then started thinking 4L80E. But I think youre right. The 2004R just has too low of a first gear and Im kind of wanting to keep this car in the dinosaur era with as few electronics as possible.
A 700R4 has a 3.06 first gear. 200R4 has a 2.74 first. If you currently have a TH400 trans and plan to keep those 29" rear tires, you could swap out those 3.73's, for some 3.08's or 3.31's. A new big block making those power/torque numbers doesn't need a lot of gear.

Clint Hooper

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post #6 of 34 (permalink) Old Oct 29th, 18, 9:58 PM Thread Starter
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by Clint44 View Post
A 700R4 has a 3.06 first gear. 200R4 has a 2.74 first. If you currently have a TH400 trans and plan to keep those 29" rear tires, you could swap out those 3.73's, for some 3.08's or 3.31's. A new big block making those power/torque numbers doesn't need a lot of gear.
Thats kind of what I was thinking. I was all over the OD idea until I started thinking about how useless ANY first gear might be with the 3.73 at one end and a high torque plant at the other.
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post #7 of 34 (permalink) Old Oct 29th, 18, 10:04 PM
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by lucifershammer View Post
I just don't like the drop from 12 for the 700r4.
Yes it's true that there a drop from 1 -2 with a 700R4 but if the car had a small block engine I'd agree and say go with the 200-4R but with all the torque a big block puts out, it easily overcomes that drop in rpm's when shifting from 1st to 2nd gear. I have one in my car right now and if I'm a little heavy-footed from a standing start, not only the drop is almost non-existent but the car will light up the tires effortlessly if I don't back out of it.

Claude.
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post #8 of 34 (permalink) Old Oct 30th, 18, 7:37 AM
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Re: Auto trans / rear gear advice please.

fwiw... feel like I run the perfect gear in my 67 chevelle. 496/700r4/3.42... the low gear is a hoss. the 1-2 shift is a bit different than what I was accostomed to with the th-400 but it doesn't even matter with 600hp on tap....
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post #9 of 34 (permalink) Old Oct 30th, 18, 7:44 AM
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Re: Auto trans / rear gear advice please.

Just be careful about lowering the cruise RPMs too much. If you have a large cam, big headers, big carb and other high RPM stuff your engine will not be happy cruising below around 2500 RPMs. Everything must work together. My 427 doesn't like running below 2500 RPMs so overdrive doesn't work in my car. I run a 4 speed M23Z and it works well for me.


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post #10 of 34 (permalink) Old Oct 30th, 18, 8:35 AM Thread Starter
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by big gear head View Post
Just be careful about lowering the cruise RPMs too much. If you have a large cam, big headers, big carb and other high RPM stuff your engine will not be happy cruising below around 2500 RPMs. Everything must work together. My 427 doesn't like running below 2500 RPMs so overdrive doesn't work in my car. I run a 4 speed M23Z and it works well for me.
Good point. Ive considered this and am hoping I can get an engine built with the lower-happy-end of the torque curve in the 2,000 to 2,200 RPM range. I know that 3K isnt exactly screaming, but Id like my car to feel a bit more relaxed at freeway speeds. My biggest problem is that Im always looking for something to fix. Sometimes I need someone to tell me, its not broken.
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post #11 of 34 (permalink) Old Oct 30th, 18, 8:39 AM Thread Starter
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by the heckler View Post
fwiw... feel like I run the perfect gear in my 67 chevelle. 496/700r4/3.42... the low gear is a hoss. the 1-2 shift is a bit different than what I was accostomed to with the th-400 but it doesn't even matter with 600hp on tap....
I was leaning toward a 3.42/3.23 with a TH400 or sticking with the 3.73 with an overdrive, and it sounds like Im looking at similar power. How does your car feel at 55-60 MPH when locked up?
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post #12 of 34 (permalink) Old Oct 30th, 18, 8:50 AM
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Re: Auto trans / rear gear advice please.

Quote:
Originally Posted by any4xx View Post
This car wont be a race car, nor will it be a long-distance daily driver. Just a fun weekend cruiser that will make me smile when I apply the throttle. So whats the advice of the masses here


I drive my 11 second,all stock appearing 70 SS454 LS6 w/3.31 gear/TH400 everywhere. Including 180 miles round trip to the track. Been doing it for 18 years.
Don't need no stinkin' overdrive

I would go with a 3.31 or 3.42 gear for what you want to do..and keep your 3 speed auto.
JMHO and experience. And FWIW,a 3.73 gear would certainly make my car perform a little bit quicker/faster at the track but I don't want to have that gear on the highway.And I don't want overdrive in my car either. Keep it simple.
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post #13 of 34 (permalink) Old Oct 30th, 18, 9:24 AM
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Re: Auto trans / rear gear advice please.

any4xx,

I have a 69 Camaro with a 383 stroker with an Art Carr built 200R4 Transmission (Check out his website) and 4:11's in the rear. The 200R4 allows the car to stay in first gear a lot longer than other transmissions and it runs great on the highway with plenty of top end.

camro-guy
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post #14 of 34 (permalink) Old Oct 30th, 18, 10:19 AM
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Re: Auto trans / rear gear advice please.

I will throw this in. I wanted a driver car for cruising and long trips. So I built a 4380 Lb. 1970 Nomad Sta. Wgn. Here is my driveline and results.
The engine is a very mild 468 with a Q-Jet, ( like a LS-5 or less) Tranny is a 700R4, Rear is a 69-12 bolt with a 3:07 rear gear, tires are 225 X 70R X 14" red lines.
The two ton wagon will burn the tires down to the 70-SS wheels if you stay in it from a dead stop. The wagon will cruise on the interstate at 73 MPH. in the electronic servo cruise along with the 468 spinning 1950 RPMs @ 73 MPH.
My two ton cruiser gets 20 MPG all day and night on the big road without the A/C operating. ( it drops 1-1/12 mpg) " Who cares its a fun toy".
So far with this combination I have driven my 70 Nomad over 38.000 miles.
I built this buggy for cruisin & long trips. I do not beat the heck out of it. I just have fun and drive it a lot.
NO I DO NOT RACE IT. Its fun to hop in it and drive from Pittsburgh, Pa. to Florida. As soon as the bridge is finished, I am picking up M. Merlo and we are going to drive to Cuba...............
Fuel milage in town, It sucks! Its like flushing a toilet.
I built my engine like a class 8 Cummins Diesel. A lot of cubic inches mild " Motor home cam "= low RPMs. All of this equals a TON of TORQUE. Torque is what moves things. As I get older, Its either " Race it or Drive it ". A very nice car, Gets very messy after a few years of racing. I does cost a lot to get that extra " TENTH " of a second.
Bob


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post #15 of 34 (permalink) Old Oct 30th, 18, 11:04 AM
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Greg.
 
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Re: Auto trans / rear gear advice please.

If you put a 700-R4 in your car you will find that 1st Gear is very short and then comes the drop to 2nd in RPMs

If it is only a Cruzer then go with a 200-4R as others have said it has better Gear splits differences

So figure out how much $$$ you want to spend / what kind of performance you want and chose accordingly

My order of preferences ;
FOR CRUZIN
1. Change Rearend Gearing to ?.?? based on Cruzin RPM you want at 70 MPH = likely cheapest
2. a well built 200-4R AT and leave the Rearend Gears of 3.73 = provides great performance and Cruzin RPMs
3. Gear Vendors Unit attached to the current AT = provides great performance and Cruzin RPMs
4. 4L80FM AT and leave the Rearend Gears of 3.73 = provides great performance/dependability and Cruzin RPMs
BUT is not cheap, unless you can find a good used unit with the computer Electronics

FOR STREET/STRIP
1. 4L80FM AT and leave the Rearend Gears of 3.73 = provides great performance/dependability and Cruzin RPMs, but is not cheap
2. Gear Vendors Unit attached to a TH400 AT = provides great performance/dependability and Cruzin RPMs
3. a well built 200-4R AT and leave the Rearend Gears of 3.73 = provides great performance and Cruzin RPMs

BIG NOTE
make sure the Stall Converter you chose is Tight for Street use since a loose SC will slip to much and burn up a good trans
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67 Chevelle Malibu SS Cdn
Born 283 / PG / A51
468/TH400 2,500 stall/ S60 3.70 Gs
Prev Best on M/T ET S/S #3454 street Tires/Dress 2018/05/20
60' = 1.664
1/8 = 7.532 et / 91 mph
1/4 = 11.814 et / 115 mph
NEW Best with 3,500+ Stall TC 2019/09/08
60' = 1.547
1/8 = 7.311 et / 91.76 mph
1/4 = 11.595 et / 114.28 mph


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