Hello everyone,

My car is a stock 81 4 speed with the 2.72:1 axle ratio. The car is a bit of a dog for a Vette in my opinion. I was thinking about upgrading the the ring and pinion to a 3.54:1 ratio for better driving performance. I would just like to know has anybody done this before on an 81 or similar car and what is their opinion on the car afterward. Does it really make all that much difference? And is 3.54 a good ratio to go with or should I consider another ratio? Any input is greatly appreciated!

Thanks

 

I use 3.55 in all my old cars, Vette included. I have a 72 so cant speak to your year but great all around performance.

 

I put a 3.54 in my 81 and I love it. I have a th350 not a 4 speed. It does get up in rpms at 60 mph plus. Nothing a small block chevy can't take. Great improvement in around town driving.

 

Another option is to swap in a trans with a different gear set.My 6spd Richmond has a 3.28 1st gear and a 6th gear .76 OD.Great 1st gear for launches and an OD for Hiway cruisin.Yes it's pricy but sure makes the Vette nice to drive...

 

You're best off hopping on a gear ratio calculator and figuring out what you want.

The ratios of the '81 4 speed, according to google, were 2.88, 1.91, 1.33, 1.00.

With that spread, personally I'd be going for something like a 3.08 diff or maybe 3.36. 3.55 will make 1st rather short... but that might be what you want.

A 3.55 in one of the earlier 4 spd close ratio vettes that had 2.2/1.64/1.28/1.0 is a different animal to one in the later vettes which had a 2.88 1st. With the 1981 box, you'll be shifting at 5000rpm and 39 mph, in the earlier close ratio, you'll be shifting at 5000rpm and 51 mph.

I actually like the spread of gears in the '81 box and with a 3.08 or 3.36 you'll be able to rev out 1st and 2nd for some decent power at typical road going speeds and then still be able to cruise at highway speeds in 4th (70mph gives 2700rpm with a 3.08 or 2900 rpm with a 3.36 vs 3100 rpm with a 3.55).

But it all depends where you want the power, what speed and rpm you want to be changing gear and what highway cruise RPM you're willing to live with.

It often surprises me what gear/diff ratios people go with, some people either have very different tastes to me or simply don't understand gear ratios, lol. So figure out what RPM and speed you yourself want to be driving and then decide from there what ratios you want.

 

Hmmm...

Well thanks for all the feedback guys! Maybe perhaps I should consider a different (lower) ratio. I still want to be able to cruise on the highway without the engine sounding like it wants to go to the moon, but I want a little better acceleration. I know that the older c3's were faster than mine but I forgot that the fast ones had the close ratio tranny which the 81's did unfortunately not have. I guess anything is better than the 2.72 thats in it now. Anyways I have all winter to think about it because the car is going into storage tomorrow for the season. Thanks again guys...

 

In some ways the ratio you have now would be great if your engine had a little more power. Your 4sp and 2:72 is probably great for hwy cruising. If you have a stock 81 engine, it wouldnt be hard to double the hp for a reasonable cost.

 

I actually prefer the wider ratio. 1st gear in your car (2.88 1st, 2.72 diff) is a 7.83 ratio. That's equivalent to an older C3 close ratio with a 3.55 diff (2.2 first).

So you have the same effective ratio in 1st with a 2.72 as an older C3 would have in 1st with a 3.55. If you go to a 3.55, it'll actually be the equivalent of a 4.65 diff ratio with the older close ratio box.

The wide ratio box lets you use a lower diff ratio, get the same acceleration in 1st (and maybe 2nd), but still get half decent RPM on the free way.

I think the close ratio box was best suited to something like an L88 which had huge power and you could use a really low diff ratio and get away with it for road racing (which is I believe how Zora Duntov did it), or for a dragster with a high diff ratio where you want to get through the gears by the end of a 1/4 mile and don't care about highway cruising. On a more modestly powered street machine, I'd much rather a wider ratio box, gives the best of both worlds without laboring the engine.

So it all comes down to what you want to do. For a 4spd, I don't think the 81 had bad ratios. But you could go for a 5spd if you have the money, or just keep the 4spd and go for the rear ratio that gives you a highway RPM you are willing to live with. The Tremec TKO600 has a 2.87 1st with a 0.64 top, so you can run a 3.73 ratio rear and still be sitting below 2100 rpm on the highway at 70mph... however it's not a cheap option in the slightest, that same money spent on the engine will make you go quite a bit faster as well!

Have a play with this and you might get an idea of what you might like...

http://www.f-body.org/gears/

But remember, just because you have a redline doesn't mean you want to be shifting there all the time on the street. According to that calculator I should still be in 1st at 41mph in my car, but realistically I'm typically out of 1st by 30mph.

 

Well as far as I plan, the 4 speed transmission is not going anywhere. I thought about it and id like a ratio that will still give me good rpm range on the highway which I have narrowed down to will be either a 3.07 or 3.31. Both are available for an 80-82 diff. I guess my main question is will these ratios increase acceleration at all? Im not looking to make the car super fast, Im just looking for a little more pep from a stock setup with the wide ratio 4 speed.

 

For me, I don't want to have to turn more than 2000 rpm at 70 mph. That's what my 68 does with a TKO 5 speed manual transmission with a 3.73 rear end. My 08 does about 1700 rpm at 70 mph. Good gas mileage, low engine wear, and low noise. (PS to get 1700 rpm at 70 mph in my 08, I had to buy an optional "performance" rear end gear ratio...repeat a 08 performance rear end ratio option gets 1700 rpm at 70 mph!!!!!!...we are not not talking late 60's 4.11:1 performance rear end gear ratios here!!) I have a 3.73 and TKO 600 in my 70 also.

You can buy modern auto trannies that allow cruise at low rpm, yet will still allow a great lower gear for acceleration. See Keisler.. a forum sponsor.

 

Yeah, 5 speeds are nice... though the cost of a 5 speed you could probably add another couple of hundred horses to the engine, I'd sooner keep his 4 spd and add a hundred horses than swap to a 5 spd

Basically, yeah, in any given gear, you'll have more torque at the rear wheels and you'll get through the gears faster. Will it be worth it? Really hard to say! Is the engine still mostly stock as well? Maybe you could do a few small things to the engine to get a bit more pep as well.

But yeah, a 3.07 will increase your RPM by 12.9% at any given speed (meaning you have to shift at 11.4% lower speed as well), in any given gear it'll increase the torque at the rear wheels by 12.9%. So in a given gear and a given RPM, you'll feel like you have around about 12.9% more torque (but you'll be going 11.4% lower speed at that RPM).

Going to 3.31, that difference is 21.7% vs a 2.72, so for a given RPM you'll feel like you have 21.7% more torque, but be going 17.8% slower at that RPM and will be shifting at a speed 17.8% lower.

Likewise, you'll be cruising at a 12.9% or 21.7% higher rpm for 3.07 or 3.31 than you are now with your 2.72.

So go take your car for a drive and decide if you can live with cruising at 12.9 or 21.7% higher RPM, and whether you'd like to have 12.9% or 21.7% more torque in a given gear at a given RPM

 

To be frank, the problem with your car has nothing to do with gear ratios. In stock form it has 100hp less than a new V6 Mustang or Camaro. The gear ratios you have should make for nice rpm at hwy speed. The motor needs some muscle!!

 

I installed 3.70 gears in my 80' with the stock 4 speed. Nice on country roads, but on the highway at 60 in 4th I was at 3100 rpms.

After looking at the prices for the 5 speed kits, I figured that the only way I was going to afford getting a trans with overdrive was to figure out a low cost way of piecing it together myself. I found a used T56 out of a 94' Trans Am on Craigslist for $750 and bought it. I held on to the trans for several months trying to decide on the best way of making the swap on a low budget. After doing a lot of research I decided to removed the original 4 speed and clutch linkages, get them sold and go for the swap. After selling the old parts, I bought a conversion kit so I could reuse the new clutch and flywheel I had from my 4 speed. The kit came with a hydraulic clutch master, new bellhousing, clutch line, and hydraulic throw out bearing. I sold the bellhousing that came with the T56. After completing the install, the total cost of the swap to the T56 6 speed trans and hydraulic clutch after selling the parts I didn't need came in at a little more than $1000. The T56 in 6th gear at 60 puts the engine at 1550 rpms.

 

I have had a 4spd `81 with stock rear end and stock box and gave it a 450 hp engine ( needed clutch modification ) .
For performance the stock ratios of box and rear were really good, also on landroad speeds no need for different rear with the 2.72.
Only for constantly higher speeds on Autobahn too high on revs.

Having a new build now which is getting 4.11-rears and a T-56 with 0.50 in 6th.
The T-56 can be found in two different OD`s.
Would i be in your place, i would again add horses first and would plan for later on a used T-56 and higher rear-gearing.
Except you now use the car for constant high speed driving i would start with another trans.
But anyway the `81 stock machine did never kick me off, so this would always be my first mod.

 

You'll be surprised what a transmission change will do to the feel of your car.

I swapped a Richmond 5 speed into my '81 with the stock engine and rear and it feels like I gained 100 horses. 1st gear with my rear is equal to a 4 speed with 4.33s in the rear, 4th gear is like 4th gear on a 4 speed with 3.55s, and my 5th has my old 1:1 with the 2.87s on the highway. The only thing I would have did different is get the Richmond 6 speed, because at 85 I could use one more gear for cruising.

 

Sure with the Richmond you come strong out of the hole, at that time i was thinking of a Richmond also, only missed an OD on it as final gear was the same like stock.
I think the Richmond-6spd was not available back then, it was 1990 and i then did the engine swap.

qwank, is this the one you have :
First Gear Ratio: 4.06:1
Second Gear Ratio: 2.22:1
Third Gear Ratio: 1.57:1
Fourth Gear Ratio: 1.24:1
Fifth Gear Ratio: 1.00:1

Like you say the six speed would be better for cruising - just for performance the 5spd sound good, especially first two gears vs. the stock box...

With a bigger engine, when you don`t really need the extreme first and second gearing, i like the T-56 with it`s two OD`s more.

 

Only in 1st would it feel like you're adding 100hp, any higher gear and it should only feel like adding a few hp at best (depending on your engine's torque curve).

Although it can make you feel like you're going faster simply because you're changing gears more often, realistically it would only give big gains in 1st gear.

You can also get big gains in really revvy engines with narrow power bands, but that's not what a 350 should give.

 

Unless you change to a torque monster BB that gearing isn't going to get you off the line.I also tried to increase HP without gearing and it doesn't help much.What happens is you install cam ,large carb,intake that needs RPM's to get the power out of that small block,these mods usually kill the low end torque.Without gearing to get those rpm's up quick your going now where fast.Rule of thumb is a 10:1 first gear ratio.After my trans swap the stock 3.08 rear gear with the new trans 3.28 1st gear made for quick take offs and transformed the car.Sure you can change rear gears but it's a compromise between quick launches and hiway cruising.That's the reason for 5&6 speed transmissions.It's gonna cost you to change your rear gears,save up more funds and do a trans swap one time call it done.I preferred to go with the 6spd due to the closer gear ratio's.Unless your going racing forget all the high RPM engine mods,you need low end torque for the street.My Vette is blast to drive with the 6spd! You'll be amazed how much a trans swap can do for your car...

 

Ive given it some thought and it looks like im favoring the 3.31. Does anybody know what type of speedometer gear I would have to get if I install the 3.31 gears, or what I should do to calculate it? I dont really know that much about speedometer gears or what to do regarding them lol...

 

There is a thread on the C3 Leman's race car. It ran the exact gear ratio you have now. Gearing cannot fix a car with a 190hp engine. A 450hp engine, which can be done with a new 383 rotating assy and a new top end, will transform your car.