I have a 74 with an L48/TH350 that I plan to replace with an LSx/4L60E or possibly a 383 Stroker with a 2004R. I am looking to change the gear ratio to accommodate the increased performance. Driving would be mixed highway, country roads.

I was thinking 3.55 would be a good choice but would appreciate feedback /advice from others.

 

Perfect with the 2004r or a 4L60E

 

Excellent and thanks!

Sent from my XT907 using IB AutoGroup

 

The most painless swap is the 383/200R4 Because your tranny cross member mount point is the same and the drive line is the same. The speedo cable with corrected end gear can be bought from Bowtie overdrives.

As to the gear ratio. You have to think about the OD final gear ratio.

I drove around with my 700R4 for 20 years with a 4.11. I wanted a high speed Vette and 4.11 in OD would allow me to accelerate right to red line.

I always did think that a 3.90 rear end was the best compromise.

Get out the speed calculator

http://www.angelfire.com/fl/procrastination/rear.html

 

gkull,

thank you for pointing me to the calculator!

If I did everything correctly using the following parameters 3.55 works very well for what I am looking for?

RPM 2000
Axle Ratio 3.55
4L60E 4th Gear Ratio .696
Tire Diameter 27.1

Results in 65.26 mph

How do you model 0-60 acceleration and estimate quarter mile time?

 

Good Luck

I would go 3.55 or 3.73

 

This gets down to personal taste. When I spent about 4 years restoring my 68 (techno upgrade, not NCRS) I did so many great things but I forgot about the rear end diff ratio and the braking system. When I started the resto, I hadn't driven the car for about 20 years. I didn't remember any problems with the tranny or the brakes. When I drove the car for the first several times, I was annoyed that the engine was turning about 3000 rpm at 70 mph on the freeway. I didn't like the thought of engine wear..fuel burn off was another thing. And the standard brakes. At 45 years old, I had no problem with the standard brakes. When I started driving the car at 65 years of age, I became worried about braking the car when, say 85. (At 72, I can still lock up the brakes.)

Back to the tranny and rear end ratio. I have 3.73:1 diff gears (3.5:1??) and an overdrive TKO 600 5 speed now in the 68. I turn about 2000 rpm at 70 mph. This is a comfortable rpm range. For the little I drive the car, the savings on engine wear and fuel burn was certainly not worth the cost of the TKO 600 to get rpm down with the overdrive 5 th gear. I just want my tech perfect car to drive at 2000 rpm at 70 mph. I have the optional high performance diff in my 08..it's a 2.74:1!! Stock base engine was something lower. Imagine GM describing a 2.74:1 diff as high performance!...I do 1700 rpm at 70 mph. l

 

3:55 with a .68 overdrive hits the nail on the head for a great street car!!!

 

In response to your question: How do you model 0-60 acceleration and estimate quarter mile time?

There are number of formulas online that ask for horsepower, weight of the car, gear ratio, tire size and 1/4 mile trap RPM. It is a good start to determine MPH and ET. There might be some around for 0-60 calculations.

When I was younger the fella's would buy a 'dream wheel' from some custom camshaft dealer that would predict estimated ET and MPH. I spent many hours on formulas trying to change this and that. I remember it was a great time back then 'bench racing' with my buddies.

 

I just put in the 3.92 gears and they are about perfect for all driving ranges .

 

I would go 3.73 if you have an OD trans. Not a big difference in cruising RPM over a 3.55 but better acceleration.

 

It is rather ignorant to believe that just lower rpm = better gas mileage.

It is where an engine is running efficiently. Kind of determined by the cam and intake tract. The other factors are air drag, rolling resistance. and HP loss of the drive train.

Big auto transmissions take allot of HP to operate. My MPG went down when I switched from a beefed up th350 to a 700R4 because of the additional wasted HP required to run it. I gained MPG with the 700r4 going to a 4.11 rear end because my motor got into its efficiency range at my highway speed.

Same thing in my 98 vette with a 6 speed it was so hopelessly over geared in 6th that it had no get up and go. So I went to a 4.10 rear and then it really ran well and got better MPG

 

Is not a 700R4 basically a Turbo 350 with a 4th overdrive gear ?

 

No...... It was very poorly constructed attempt to satisfy increased fuel mileage in sub 300 hp cars. I don't know how many of the internal workings might be similar to a TH350.

About 1988 they figured out that to make the weak 700R4 last in heavy and or increased HP models was to increase the number of vains in the pump increasing the pressure levels and corresponding clamping force of the friction plates..................

So if you get online you can find the HP level required to just operate. Lots of the loss just goes out through the tranny fluid coolers.

The old power glide is the most efficient and the worst are the 700R4/th400/L480E I've hear numbers like 35 HP for the 700 and l460E

It also might only require 35 HP to maintain 70 mph on a level road

 

A 700r4 IS based on a TH350 with od built in and bears no resemblance to a TH400 or 4L80E. The 2004r is the weaker trans stock but it consumes less power and has better ratios, bolts in place of a TH400 and can be built to a better standard than a 700r4.

 

The '89 700R4 behind the spiced up '93 350C.L. in my '36 pickup (same weight as Vette) will get 24mpg Interstate in O.D @ 2,000rpm @ 60mph....(I have a vacuum gauge---as another member said---lower rpm does NOT equal better mpg all the time)

If I want to have some fun, I turn off the O.D./24mpg....it's similar to adding another 500rpm/90 H.P. to the rear wheels....and removing 10 mpg...LOL

(My 700R4 has 216,000 miles on it---just depends on how much you "rawhide em")

 

I think that's an opinion , not based on fact . .

http://www.webrodder.com/article.php?AID=426

Read the differences , the 700R4 is a much more stout unit. And making the 2004R stronger , well I doubt it because the internals of the 700R4 are just so much bigger and stronger then the 2004R . . . .

Of course we can differ in opinions all day but part for part the 700R4 is just a heavier built unit..

 

Ask Art Carr which one he thinks is better. I've owned both and I can say, from MY experience, that the 2004r is the better trans once it's been beefed. No doubt in my mind.

 

I am not offering an opinion , I am offering facts that are supported by an internal examination of each transmission. The facts support the claim the 700R4 is the stronger unit hands down.

And Art Carr has a dog in the race , if you want truth read that article I linked to.. I use to run a transmission business , and the 700R4 was the unit used for all heavy cars and all trucks because it has internals made stronger and larger.

Also durability can and often is reflected in the continual production of an item , such as a 350 SBC , that ran for decades ,, and the 700R4 is still in use today as a 4L60E often called the electronic 700R4..

Both transmission have there place , if your running 75 to 82 cars with stock motors go 2004R .. If you are pushing a lot of tq and hp go 700R4 . . Will Art car or others who sell these units talk them up , of course , but if you want durability and strength you have no leg to stand on suggesting the 2004R is equal to a 700R4.. The parts inside prove a different truth . . . .

 

You're flat out wrong. Ask any Turbo V6 Buick owner if he'd trade a 2004r for a 700r4...they'll laugh at you.