Welcome to the Corvette Forums at the Corvette Action Center!

Differential or Torque converter Change



Ok folks,

I'm trying to figure out what might be the best upgrade for my 86. Presently I have the economy gearing (2.59 or something). This car is for autocross. To get more out of it, I'm trying to figure out whether changing the rear to a 3.07 or changing the converter to a lower stall, like maybe 2000. After hard braking, it seems like forever before the car gets going again.
I'm certainly no mechanic and I proved that in my question--I just drive the Vette!


I'm no C4 expert but you need more rear end gear. Something in the 3.5 range would make all the difference in the world. Your 1st gear is lower with the overdrive auto so it has the efect of a lower rear than you install and it will still be a good road car. I would think that you would lose all around drivability with a looser converter and loose the lockup feature too. I would go with the gears. Now we need a C4 person to tell us what is available for the '86 rear end.

I'm no C-4 guy either, But......

I'm not a C-4 guy either,but I'm with Tom. Torque is accelaration, and you get torque from gears. I too, think that 3:55 or so is better then 3:07, but anything is better then 2:59's(except for gas milage and maybe the Bonaville Salt Flats). I am also no expert on autocross, but as a great movie racer once said"I've seen it on TV", you need as much control and response as possible from your car............I'd go with the gears........You'll need to work on the tranny soon enough:).........Steve
Nocones, After rereading your post I see your point about a lower stall(I'm so use to talking to guys about higher stalls,that I just read over it). I think that a stock stall should be around 1800 RPM's. You might call TCI and talk to their technical people,I've never heard of a lower stall. If, you could manually lock up your torque converter at say 1500RPM's that would be a handful and quite fun to drive, but then I'm sure you would be working on the trans sooner..........Steve

The gears are the best mod. With the L98 low rpm torque range, about 3:54's would be all the higher I would go. This will make a tremendous differnce in autocross times. Your 0 to 60 mph times will increase greatly! There not cheap, but well worth the costs.

On the torque convertor, I belive the 85's and 86's had the loosest convertors installed by G.M. from the factory of all the C-4's In the range of 2000-2400rpm stock stall speeds. That's not bad! Some "F-Body" guys with 700R4 trans would put these Corvette convertors in there cars as a cheap mod to there Camaro / Firebird in the late 80's (used from junkyard)

Next best mod in my opinoin would be" Good Sticky Tires"
My favorite street tire is the Firestone SZ50EP "Firehawks" great traction wet or dry, for a street tire.

*Also, it's possible your torque convertor is going bad and slipping too much, or not consistently. A transmission guy could probably answer that better than me.
This is great! I was really hoping to get some good responses to the question. I also prefer to go for the gear change but needed to check alternatives as well.
Tires - I use Kumho 335x11x17s all around.
Now I gotta work on convincing Li'l Miss Moneybags why I NEED those gears....

Hey NoCones.......the '85 that I recently sold originally had a 2.73 gear with an 1800 stall. I went ahead and ditched both of these:D I went with a 2500 stall and a set of Richmond 3.75 gears for the Dana 36. This was by far the most noticabel upgrade that I ever did to the car. Unfortunately I never got a chance to take it to the dragstrip after the gear swap, but it felt like a major difference in performance. How low of a gear you go depends on what you want out of your car......my '85 could not "cruise" the tollway in the triple digits anymore because my RPM's were too high (at 70mph in overdrive--2400rpm's)....but I was not looking for this type of car. I wanted a red-light terror, and this gear swap was perfect for me. Good luck on your choice:)


Tim, I won't be of much help because I run a DN 4+3, but I switched to 3.73's a while back and am very pleased with the results. The acceleration is immediate when combined with the L98, but you do need overdrive for cruising. I turn about 3200-3300 rpm at 65 mph and about 3600 at 80. Too high for cruisin' but they oughta be great for autocross, eh?
Speaking of which, does the tire size you run put you in a "modified" class? And are you satisfied with the Khumo's?

fastglass, I run the Firestone SZ50EP "Firehawks" on the street too. They not only work well, they look good too! :D
After the gears, a "Cross-bar" should be next on the list. :) ;)

Fastglass, is one of the Autocross hot rods in our area, and I think he will confirm that control of the car is priority.

Wavin at Ya' :w
I went from 2.73 to 3.07 last year. I noticed a good inprovement in the car, but mostly in 3rd and 4th gear. I am going to disagree and suggest you look into a Precision Industry torque converter of about 2400 stall. The company can advise you better then I can about the actual stall speed. These things have a history of knocking a full 0.5 seconds off a 1/4 mile time. In autocross, you are seeking to keep in the cars prime torque range, this will allow that in each gear. To do this with gears, you will need to study your speeds between turns and figure what ratio is going to keep the motor in it's sweet spot.

My suggestion is to look at a torque converter, and although expensive (not when compared to gears) Precision Industries Vigilante has no equal.
What vettenuts says makes a lot of sence too. No matter how slow you had to go around a cone as soon as you hit the gas you would be at peak torque again. Maybe a combo would be best for serious autocrossing. You might not have to go so low with the gears and this would help keep the revs lower on the interstate.

As a sidebar, back before the national 55 speed limit and overdrive automatics, a 3.36 rear would get you 3000 rpm at 70 mph and that was considered acceptable.

59Tom said:
As a sidebar, back before the national 55 speed limit and overdrive automatics, a 3.36 rear would get you 3000 rpm at 70 mph and that was considered acceptable.


Tom, ya gotta take the tire/wheel circumference into consideration as well, in order to calculate the speed/rpm relationship. ;)


Well it's obvious that not all of us will agree on the best combonation here.
With the L98 automatic, I would not go over 3:54's for gears (higher in LT1 applications).
Also I would do the gears first then if you wanted more, do the torque convertor. But doing both will create some traction problems coming out of tight corners and getting on the gas. (just my opinion)
3:07's would be a good gear also.

Good luck with lil' miss moneybags ;)
Hey, try getting her into autocrossing also :D
That's what worked for me!
Gears n tires

I run the 315x35x17 Kumhos in Group II NCCC.
I'm not sure if the tires alone would put me there.
I really like the Kumhos--for the cost they stick very well and last a while, too.

As for my wife autocrossing...she's gonna try it this season. :(

Tim :p

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors


MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Advertise with the Corvette Action Center!

Double Your Chances!

Our Partners

Top Bottom