rodney25

My question is what will an overdrive transmission do for a stock 75 vette? What performace gain will I see and what are the draw backs? Which make and model do you recommend and why?



Thanks

1977L48

i did a 700R4 swap back in 2006 in a 77 Monte Carlo, very easy swap, it slowed me down in the 1/8th mile, highway cruising was a blast, and 700R4 lasted about a year before the planetary gears went out in it. but i also had a mild 350 infront of it and was launching it on slicks. i love the TKO500 in my vette, but it depends on your budget, and if you want an auto or manual.

garygnu

with the right converter you will see better Ets ,the 700r4 has a 3.08 1st gear ratio.a turbo 400 has 2.4 1st gear ratio,a turbo 350 has a 2.5 1st gear.when racing with a 700r4 ,it is usually best to put it in drive .you will gain some gas mileage with the .70 over drive .

bluedawg

I have to be on it when shifting manually out of first and is easier with a shift light, but with the governor kit you can get your transmission to shift were you want under wot conditions in drive, the 3.06 to 1 first gear makes 1st feel equivalent to having the 3.70 rear ring and pinion with a 350 turbo transmission, not to mention the over drive for cruising low rpm's and some fuel savings which wouldn't pay for the transmission swap with out a lot of driving. Like gary has said, the right converter deffinately helps with the first to second gear spread.

bebezote

i have a 700r4 from Lastc3AZ's vette with 2800 rpm converter... all just built... want a T56..in trade...

bluedawg

So if i take this right your wanting to trade your 700r4 for a t56?

doorgunner

What will an Overdrive manual transmission do for stock 75 Corvette?


Overdrive: In a Manual shift transmission??????

or .......


In an automatic transmission WITH a manual-shift valve body??

If you have 2.90 to 4.11 differential gears either style transmission O.D. should help gas mileage and reduce wear on the engine (depending on the diameter of the tires---you DON'T want to lug the engine down at highway speeds causing a drop in engine vacuum which means less power/worse gas mpg's.) Try to keep the engine rpm at 2,000 or slightkly higher at 70mph cruising speed.

Garys 68

A lot is going to depend on your rear end and engine specs.
Do you know what they are?

waljr

is there a size issue that restricts the use of 6-speed manual box's in c3's?

bluedawg

I'd love to have a 6l80 in my vette.

Garys 68

Doesn't seem to be. Quite a few LS swaps run T56s.
Pretty sure I've seen an LT1 T56.
I'm running a Richmond 6 speed along with a few others here.
Not much in the way of body mods needed. Again, rear end ratio and engine is probably the biggest determination of what will work.

Mike Ward

Reduced cruising RPM.

That's it.

Shark Racer

They're also wider ratio than most of the Muncie boxes, which *generally* means worse acceleration past first.

If you're acceleration inhibited, the 5 and 6 speed boxes do come with a more aggressive first, but a larger RPM drop into second.

6T5RUSH

I know it's a different body type, but the frame's almost the same.

My .02...I looked hard at which way to go to get an overdrive tranny in my C2. I ended up with the close ratio SS700 Keisler offers and, frankly, have never looked back. My wife and I did a 5,800 + trip this past May (Route 66) with NO issues (burned a quart of oil). Averaged over 17 mpg for the whole trip (a whole lot of stop and go driving, LOL!). At 70 mph, she's taching right at 2,200.

Here's an excerpt from a thread I started over a year ago on my analysis in choosing a wide vs. close ratio 5 speed (I went with the close ratio):

"Although my present setup is using the factory wide ratio '65 Muncie that has a 2.56:1 1st gear coupled to a 3.73:1 posi. I do like the jump but cruising @ 65 or better is just a big buzz...she sure is spinnin' up there. I've read enough here about the ideal index of 10 or more (achieved by multiplying the 1st gear ratio to your rear axle ratio). Right now she's right at 9.55.

Being a wide ratio tranny my drops in gears as I go through them are:

1st (2.56) to 2nd (1.91) is 25.39%

2nd (1.91) to 3rd (1.48) is 22.51%

3rd (1.48) to 4th (1.00) is 32.43% (this is the down side of my factory wide ratio)

Now, looking at the SS700 Series, Keisler Wide Ratio reflects the following % drop between gears:

1st (2.97) to 2nd (2.07) is 30.30%

2nd (2.07) to 3rd (1.43) is 30.92%

3rd (1.43) to 4th (1.00) is 30.07%

4th (1.00) to 5th (.68) is 32.00%

Finally here is the SS700 Series with the Keisler Close Ratio reflecting the following % drop between gears:

1st (2.66) to 2nd (1.78) is 33.08%

2nd (1.78) to 3rd (1.30) is 26.97%

3rd (1.30) to 4th (1.00) is 23.08%

4th (1.00) to 5th (.68) is 32.00%

I lean towards having that 2.66 ratio in 1st gear on the Close Ratio SS700. The index I spoke of (1st gear x the posi) would be 9.92 vs the Wide Ratio SS700 (1st gear x the posi) of 11.07 (more of a stump puller...useless...as some of you commented with your TKO 600 1st gear experience.

Also my 3rd to 4th gear in my present setup (Muncie wide ratio) shows a 32.43% drop pretty much mirrors the 4th to 5th gear drop in either of the SS700 Series Close or Wide Ratio...32.00% drop.

Your thoughts as I'm leaning towards the close ratio box over the wide ratio box from Keisler.

Background on my motor:

327 bored .060 over

'70 LT1 cam, lifters and intake

1.5 ratio stamped steel rockers

'461 heads breathed on...flowing 248 I/188 E @ .050 with an exhaust pipe that exhaust # jumps to 202.

Compression is right at 11:1...I run pump gas using 93 octane with no ping.

Ignition is an MSD6AL box along with a 1st generation Pertronix Lobe Sensor (knock on wood...no issues with either and has been in place 6 years). Those that do have this 1st generation type lobe sensor know that you turn the key on to run, buckle up, then start it...she starts right away.

Here's a link to a dyno run last year where the tech had to get out of it at 5,500 r's and she still registered 296 RWH and 307 ft. lbs of torque. Oh yeh, the air conditioning was off, LOL! Enjoy!

http://s1083.photobucket.com/user/6T...tml?sort=3&o=0 "

Had the SS700 in my '5 for a year now. For a manual tranny, THIS is the ticket. Shifts are smooth as butter AND 2nd to 3rd @ 6,500 r's are as good as the Muncie, if not better.

As has been said, match the tranny gears with your final rear end ratio. My drive train combo really works well.



Jim
In God We Trust!

ps...Hope you all had a very MERRY CHRISTMAS!!

7T1vette

The answer(s) to your question are as follows:

1. It will save wear/tear on your engine at highway speeds.

2. It will increase your mileage about 25-30% while in overdrive range.

3. It will cost you about $2K to get all the necessary parts and significant installation effort....IF you DIY it...to have a new O/D tranny in your car.

P/S: You will likely never recoup the conversion expense with amortized gasoline "savings". I did it to save wear & tear on my original engine and for driver comfort [noise] when on the highway. So, in my case, the savings were a positive side-benefit.

drwet

When swapped my TH350 for a 700R4 I went from 16 mpg to 24 on the highway (imperial gallons). Much more comfortable cruise on the highway, and certainly less wear and tear. Would never go back. Currently working on installing a manual trans and spent the money to buy a new TKO 600 rather than install the Muncie I already have. That should tell you something.