Overdrive vs Rear Gear change - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 20 (permalink) Old Jan 14th, 20, 11:19 AM Thread Starter
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Bob
 
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Overdrive vs Rear Gear change

A new wrinkle to my research:
1969 Chevelle, 454, 450 hp, Comp Xtreme XR282HR cam, 2500 stall converter, TH400, 3:73 gear, 15 wheels (26.5in tire dia.) Mostly a cruiser with occasional street race.
To make the car more highway reasonable for longer distances, I've considered various Transmission swaps with OD 4th gear at 0.70 or a Gear Vendor Unit.
Both routes bring the RPM's below 2500 (2146 and 2365 respectively) at 65mph. Presently the 3:73 = 3100 rpm.

Read recently this low a rpm is not recommended with a 2500 stall converter. True??
If so, looks like a rear gear change is best.

Thanks!
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post #2 of 20 (permalink) Old Jan 14th, 20, 11:33 AM
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Re: Overdrive vs Rear Gear change

An overdrive trans with a lockup converter gets my vote as you could still use a 2500 stall and it would still have zero slip on lockup. A 700r4 would be my choice as long as it is built by a reputable builder that can build it to handle the torque of a rat motor. It won't be cheap but it can be done. I have built 2 units that have been used in the same type of setups as yours, but , the price tag will exceed the 3k figure if you have to source a core and a converter.A 3.08 gear would bring your rpm's to 2550 but will most likely require a center section change.
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post #3 of 20 (permalink) Old Jan 14th, 20, 3:02 PM
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Lew
 
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Re: Overdrive vs Rear Gear change

Your torque converter should be fine go with the OD that way you have the best of both worlds. Just make sure you have a good trans cooler. On the highway as long as you are pooping along you should be fine the converter is not seeing any real power and is not slipping much at all thus not generating a lot of heat.
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post #4 of 20 (permalink) Old Jan 14th, 20, 3:15 PM
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Mark
 
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Re: Overdrive vs Rear Gear change

The torque converter from the TH400 wont work with the 700r4 trans. You might get by with a non lockup converter, but the trans has to be setup for this. Prices for both converters are almost the same so using a lockup style would drop your cruise rpm's another 200 and take any question out of TC slip at cruise speeds.
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post #5 of 20 (permalink) Old Jan 14th, 20, 4:08 PM
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Re: Overdrive vs Rear Gear change

Quote:
Originally Posted by CTHawk View Post
Read recently this low a rpm is not recommended with a 2500 stall converter. True??
If so, looks like a rear gear change is best.

FWIW,I have 3.31 gears and a "tight" 4000 rpm converter/TH400 and I drive my car everywhere,including 180 miles round trip to the drag strip,and no issues. Runs high 11's/12.0 in the 1/4 then drive home.


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post #6 of 20 (permalink) Old Jan 14th, 20, 5:10 PM
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Claude
 
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Re: Overdrive vs Rear Gear change

Quote:
Originally Posted by modern malibu View Post
An overdrive trans with a lockup converter gets my vote as you could still use a 2500 stall and it would still have zero slip on lockup. A 700r4 would be my choice as long as it is built by a reputable builder that can build it to handle the torque of a rat motor. It won't be cheap but it can be done. I have built 2 units that have been used in the same type of setups as yours, but , the price tag will exceed the 3k figure if you have to source a core and a converter.A 3.08 gear would bring your rpm's to 2550 but will most likely require a center section change.
Yes, the 700R4 also gets my vote. I have a built 700R4 on my car but I found out that with 3:08 gears, the engine was not revving high enough (with 30" tall tires) so I changed them for 3:73's and now it runs great, about 1750~1800 rpm's around 60~65 mph. The price to build the 700R4 with all the bullet-proof stuff was approximately $4K (canadian $$), including the torque converter.

Claude.

1971 Chevelle SS
1987 Buick Grand National 9.48 @ 142 mph
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post #7 of 20 (permalink) Old Jan 14th, 20, 9:11 PM
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Re: Overdrive vs Rear Gear change

Our 70 has a 2.73 12bolt posi.
M22W with 2.56 1st gear.
11in bent finger diaphragm LUK clutch from Rock Auto.
Healthy SB400.
Essentially no clutch slippage from a dead stop (the 400 has plenty of torque to turn the 2.73).
No tach, but hiway rpm @ 70mph is about 2400.

Oh ya, auto trannys of ANY kind are against my religion!
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post #8 of 20 (permalink) Old Jan 15th, 20, 4:22 PM
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Overdrive is the best of both worlds and worth the added cost. It really changes the car.
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post #9 of 20 (permalink) Old Jan 15th, 20, 4:32 PM
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Re: Overdrive vs Rear Gear change

3.08 gear change or if you are fortunate like mr. 4 speed to find some 3.31 would be even better. I would not go over (numerically) a 3.42 rear gear for a big block with 1:1 final trans output. That being said and fwiw... I have a stout 496 in my '67 with a 700r4 (tci version this time) and 3.42 rear gears. it is ideal for the application.. the 700 isn't nearly as heavy duty as that th400 but the down low gearing is really peppy out of the hole and on the highway - 65 mph = 2200 rpm with 28" od. tires...
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post #10 of 20 (permalink) Old Jan 16th, 20, 9:02 AM
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Re: Overdrive vs Rear Gear change

Overdrive without a lock up converter, especially a 2500 stall unit., is really not that efficient.

You will find this out for yourself if you go that route.

'67 Chevelle Malibu, Catalina Blue, 540 inch Rat, Air Flow Reasearch 290 CNC Heads heads, Custom Hyd Roller, .675 lift, 234, 244 at .050, Lemons Headers, Holley Multiport Fuel Injection, TCI 6X 6Speed Automatic, 12 bolt Posi Eaton TruTrac, 3:31 Richmond Gears, front and rear power disc brakes, Global West Suspension, etc.

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post #11 of 20 (permalink) Old Jan 16th, 20, 9:39 AM
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Re: Overdrive vs Rear Gear change

Quote:
Originally Posted by jackie schmidt View Post
Overdrive without a lock up converter, especially a 2500 stall unit., is really not that efficient.

You will find this out for yourself if you go that route.
Yeah you got to have a lockup converter…

Claude.

1971 Chevelle SS
1987 Buick Grand National 9.48 @ 142 mph
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post #12 of 20 (permalink) Old Jan 16th, 20, 10:05 AM
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Very few carbureted, big cam hi performance engines produce much horsepower ‘till around 3000. There’s a reason Chevrolet offered their finest w/low axle ratios.

700 OD w/extreme low first is almost uncontrollable (with any serious HP) in first, then falls on it’s face with big RPM drop to second. Again, there’s a reason for close ratio Muncies.

Turbo 400 is a fine auto. Mostly every car built ran around 3000/60 till fed mandated mpg requirements.
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post #13 of 20 (permalink) Old Jan 16th, 20, 10:15 AM
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Re: Overdrive vs Rear Gear change

I'm on the lockup converter bandwagon too. Here's a vid of my LS powered S10 blazer just daily driving. The video is a slight uphill acceleration, you can see the tach needle on the left barely moving as I accelerate to 55mph until the lockup clutch kicks in around 17 seconds into the video.

This was a Precision Industries 3000 stall in a LSx based 4L60e behind a small cam 5.3L V8. Great converter! If you have a loose stall it will be creating heat until you get to speed, then continue to "slip" causing more heat to generate. The lockup converter will keep your trans cooler and allow some MPG. Don't cheap out on a converter, make sure the mfg of the converter knows what STR (Stall Torque Ratio) is and can provide you with numbers for it. The higher the STR the more the torque multiplication. This also generates heat so have a GOOD trans cooler installed. Good luck!!

https://youtu.be/ZnRvkUe9uJ8

*Jeff*

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post #14 of 20 (permalink) Old Jan 16th, 20, 11:16 AM
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Re: Overdrive vs Rear Gear change

I dunno about that - I'm quite pleased with my non-lockup 200-4r with an approximately 2800 stall converter and 3.73 rear gear.

It all depends on the converter. My converter is not super-loose, so it only flashes up to the stall range when my right foot is really in it. It is very reasonable and fun to drive.

And with a .67 fourth gear, my final ratio is 2.50 when in 4th cruising down the highway and RPMs are very reasonable.

Temperature is not a problem - the transmission never gets hot either (I have a trans temp gauge on it at all times.)

Quote:
Originally Posted by jackie schmidt View Post
Overdrive without a lock up converter, especially a 2500 stall unit., is really not that efficient.

You will find this out for yourself if you go that route.

1972 Chevelle, 454, 200-4r non-lockup, 12 bolt 3.73
1981 Camaro Z28, 355, TH350, 3.73
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post #15 of 20 (permalink) Old Jan 16th, 20, 11:36 AM
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Re: Overdrive vs Rear Gear change

Is it this cam?
https://www.compcams.com/xtreme-ener...k-396-454.html

Ovalport heads? Dual plane intake? Carb?

I wouldnt change both gears and to od just one of them. No problems running 2500 stall converter with 2100 rpm if its good quality and you have a good transmission cooler.

I was in same boat some years ago. Wanted to use car on longer trips vaccation with wife and kids.
Thought of Gear vendor or 4l80e but felt both was to much money.
Went with 3.31 from 3.73 and the drop in rpm was enough for me. Considered 3.08 but decided 3.31 was better for my combination. No noticeable difference in performance with 3.31.
I have a 9.5” converter with 3000 stall and noticed zero difference in behavior. Has used 2500 stall with 3.08 in the past worked just as good as with 3.73 with a sb.

489 10.5 in comp, brodix race rites, edelbrock RPM air gap, pro system 1000hp, 242,[email protected] .621 .616 112ls.
Th 400 with 9.5” 3000 stall converter.
275/60-15 so 28” tall tires.
Even dropped fuel consumption.
Highway 12-14 mpg with 3.73. Highway 16-17 mpg after change to 3.31.

With your smaller cam and shorter tire I would consider a change to 3.07(3.08 aftermarket) think it will work very good even though you have less cui than me if you dont mind loosing little performance. I recomend US gears they are the best brand. With all import gears some shops refuses to use anything else since they can give all kind of problems.

You dont know someone you can borrow a set of wheels with 275/60-15 or another dimension with 28” tall or taller tires from? It will put you between 3.73 and 3.08 in rpm and give you a hint.

If you can afford and dont want to loose any performane go with gear vendor over th700 with your combination imho.

Chevelle Malibu ss 64
489 BB with parts from Lewis racing
Alumitech radiator with dual spals
Th 400 Edge 9.5" converter
57 3/4” wide 12 bolt: Eaton posi, Us gear 3.31, Toms KA 33 splines axles
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