Gladiator-LS3

I have a 72 w/ a 350 in it and after a lot of good advice on the forum I have decided to upgrade to a 383ci engine and considering a 700r4 or possibly just replacing my TH400. Not looking to drag race but would like good acceleration plus drive comfortably on the interstate. Need advice for differential gears. Any help? Thanks

Gordonm

I am doing a 383 with a 700r4 this fall for a forum member and am going with a 3.55 rear. Should provide plenty of grunt off the line with good cruising rpm.

68Thunder427

Gladiator-LS3

What are your thoughts on a strong TH400 as opposed to a 700r4?

Shark Racer

I'd probably go 3.08s or less if you were planning on long cruising w/o an OD gear (TH400, TH350).

That said, 3.55 coupled w/ a 700R4 is a nice little combo.

scottyp99

If you are going to go with the 700r4, make sure your engine has a nice, wide, flat torque curve. There's a pretty wide space between first and second gear, so rpm will drop quite a bit on that shift to second gear. I think they were designed to work with the CFI and TPI engines, which create a lot of low end torque, and can cruise at very low rpm, lower that a carbureted engine can. The dilemma here is that if you go too low on the diff gear, first gear will be like driving a tractor, because of the 3.06:1 first gear, and if you go to high, rpms on the highway will actually be too low, because of the .70:1 overdrive. Somewhere in the mid-threes (3.36:1-3.73:1) is a good compromise. If your engine is going to be a high-revving, relatively narrow powerband engine, take a look at the 2004r, it has an overdrive, and the gear spread is a little closer together than a 700r4 (not as low of a first gear ratio) Either way, I would not bother with a non-overdrive transmission in this day and age. Hope this helps you make your decision,

Scott

CWerner

I've got a modified ZZ4 tied to a 700R4 in my 64 roadster. Great combo, including the lockup converter for cruising on the open roads.

Kacyc3

406/700r4 with 355's is 2k down the interstate in lockup at 70 with 255/70/15 keep that in mind when buying a cam.

cv67

3.55 and the 700r you cant go wrong period will work perfect with a mild 383. Will pull hard accelerating and good cruise rpm best of both worlds
Youll never go back to a non OD and tall gears again promise!

scottyp99

What's it got for gears now?

Scott

johnt365

Here is a great little online calculator to play around with different transmission, tire and rear gear configurations at different speed and rpms... in each gear.

http://tri-5.chevyrides.com/rpm.php

I went with 3.55 rear and a 200-4R because it seemed like an easier conversion. If I ever need to change out the rear gears I might go with 3.70 to get right at a 10:1 final drive ratio in 1st.

68/70Vette

After my 68 for about 25 years, I did a modernization/restoration. Of the tremendous number of planning hours, I didn't think about the rear end gear ratio. When I'd driven it before, I must not have been concerned. However, when I got it on the road finally, I was shocked to see 3000 rpm at 60 mph. This is a lot of engine wear on an expensive engine, a lot of gasoline consumption, and a lot of engine noise.

I have replaced the stock transmission with a TKO 600 five speed. I now get 2000 rpm at 70 miles per hour. Much less wear and tear, less fuel burn, and less noise.

I'd recommend a tranny/diff combination that does no more than 2000 rpm at 70 mph. My 08 Corvette with stock automatic tranny, and the "performance" differential does 1700 rpm at 70 mph. BTW. A C6 Corvette with the base engine can just blow away ..what...95% ..of all the cars on the street and highway..and never exceed 2000 rpm.

qwank

I'd go with 3.70s

diehrd

I went with 3.92 .. That gear change that was mentioned earlier on the 700R4 is fine as long as when it goes to second your not dropping out of the engines power band.

As far as drive ability I never lug the motor down even with the cam I am using. I attribute a lot of that to the gear ratio I chose. That includes if I let it lock up at speeds of 40mph the engine does not chug or struggle at all.

350 CI
Fast burn heads
GM 847 cam
2 inch primary tube headers
3 inch true dual exhaust
EZ EFI ( supports up to 650hp)
Single plane GM intake
700R4
3.92 rear gears.

You hit the go peddle and it runs hard and fast. The gear change first to second I have set at 6500 and I get no loss of power.The motor eats through second like a fat chick rolling down the side of Mt Everest .

CWerner

Just realized I forgot to add the rear gear info

3.55

2200 rpm in lockup at 75

atp1234

if you go with the 400 the 308 rear would be perfect. we had a same combination of my son's 73. We converted to a 4 speed. Much more fun.

7t9l82

the 2004r is a better transmission in this case