Rear gears with T56

[BigBlockOlds]

Just curious what rear gears everyone is running with a T-56?

I'm building a pro-touring/g-machine '87 Cutlass with a 425 big block Olds, LT1 version T-56 and an 8.5" rear. Just trying to figure what rear gears would work well.

I'll be running it in a few autocross events and would also like to try my hand at a road course or two but it will primarily be a back road fun toy.

Right now I have 3.73's with the TH-2004R trans. I'm looking to go with something in the the 4.10-4.88 range. Question is, how much is too much?

Thanks,

[chicane67]

Me.... 3.90's

But 3.73 to 4.10 would be a choice range.

[BigBlockOlds]

I'll be running a cam in the 242/254 duration @ .050 range. With the 4.10's, .50 6th gear and 25.7-26" tall tires and 65mph, that puts me around 1742rpm in 6th gear.

I'm a little concerened about lugging the engine at that rpm with that cam.

Anyone have a similar cam setup running at this rpm? Or is this not really a problem?

Thanks,

[73z-6sp]

I had the ZZ4 camshaft in my 350 with the 6 speed and 3.73 gears. I didnt like the way it would cruise at 70mph. It seemed to be just under the start of the power band and it felt sluggish. Even had a bit of a vibration at times. I would vote for 4.10 or higher.

[chicane67]

It seemed to be just under the start of the power band and it felt sluggish.

Which is right where you would get your best gas mileage. Of course its going to be sluggish.... your in 6th gear at 70 !!!

[jpdeuce]

I'm running a 259/261 @ .050 camshaft, 26" tall tire, .50 6th gear and 4.10 gears. My big block is turning around 1800-1900 rpms at 70 mph. If I can stay above 65 or so it does ok. I am seriously considering a set of 4.30s or 4.56s for my next one though. Unless you just had a set of 4.10s already I would go with 4.30s or more.

[Charley Lillard]

I have several with 6 speeds and 389 gears. 425 Olds should make plenty of torque so 389, 410 or anywhere near there is where I would be.

[73z-6sp]

Which is right where you would get your best gas mileage. Of course its going to be sluggish.... your in 6th gear at 70 !!!

When you're driving a car like these is gas mileage really that important? Maybe Im too used to my R1 where it has power to spare even if it is in 6th gear and "sluggish". What the hell do I know?

[Derek69SS]

When you're driving a car like these is gas mileage really that important?

I have found that I drive mine less, every time the prices go up... when I get my mileage up to 20+ with the LT1 swap, I will drive it more often again.

[andrewb70]

I have 4.10 gears in my RX7 with 25 inch tall tires. It is just a hair too much gear. With a 26 inch tall tire should be better and a 3.73 gear would be even better.

Another thing to keep in mind is the driveshaft. With a .5 6th gear, the drive shaft speed becomes a concern. With the engine turning 2000 RPM, the driveshaft will be spinning 4000 RPM. Spicer has a nice little critical shaft speed calculator that you can refer to. The longer the driveshaft, the lover the critical speed (failure speed). The larger the diameter of the driveshaft the higher the critical speed. Keep all that in mind when you are getting you driveshaft made. A competent driveshaft company will quiz you about your combination and make a proper driveshaft.

Andrew

[BigBlockOlds]

Yes, the driveshaft speed is definitely something I'm keeping mind. I plan on probably using a 4" aluminum shaft or even a carbon fiber shaft if need be.

From what I've read the Winston cup cars run a 48" long, 3.5-4" dia., aluminum drive shaft that is good to 8500 driveshaft rpm.

I believe the length I will need for my car is around 47-48" which is 5 or so inches shorter than stock.

Thanks,

[andrewb70]

Nextel CUp cars must run steel shafts per the rules. They are 4" diameter .065" wall. With the large diameter and a relatively short length (generally about 44 inches) critical speed is not really an issue.

Andrew

[USAZR1]

I have found that I drive mine less, every time the prices go up... when I get my mileage up to 20+ with the LT1 swap, I will drive it more often again.

Actually,I tend to drive my El Camino more because it gets such good mileage with the 6spd. Saw as high as 24mpg on last years Power Tour with the 3.50 cogs. Since I've switched to the 4.11's,mileage has dropped down to around 20-22mpg.

[Damn True]

How critical is the overdrive ratio here? .5, .64, .8?

[PRO TC]

4.10s are perfect
barry

[Norwoodx55]

What is your usable RPM range. I would think that with that big block Olds it would be a bit limited. If you go with too deep of a tail gear, your first gear will be like a granny gear in a truck. With a 6 speed, I would be a little more concerned with how the first 4 gears worked with my motor and tire diameter than the second overdrive. Unless I was planning on cruising the interstates most of the time. Just my opinion!
Brett

[BigBlockOlds]

What is your usable RPM range. I would think that with that big block Olds it would be a bit limited.

My rpm range will be from around 3000-6500/7000rpm.

[blackbeast]

i would think anything over 4.10's would really be overkill, especially down low for anything but a track car, especially with that big block.

When i originally installed my 6 speed i was running the original rear with something like 3.08's. still more noticeable power down low, but lugged in 5th and 6th. What do you expect, i was cruising at 1100-1200-rpm.

I recently installed a 3.73 rear, and the truck drives much better. cruises great, and will easily break loose the rear on a 2 or 3rd gear downshift.

I guess your decision is how radical of a driver do you want, and how much will you be cruising this thing? you don't want to much power down low you have to baby every downshift or not trust anyone else to drive it if need be.

[blackbeast]

also, in case it wasn't mentioned before 93 t-56's have different ratio's and are weaker than others. This might impact your decision if you happened to get the oddball

[CAMAROBOY69]

My T-56 specs
2.66 1st gear ratio
.50 6th gear ratio
Rear end specs
4.11 rear gear with spool 8.2 rear housing
26" diameter tire

You would probably be really happy with 4.10 gears especially if you have a .50 final gear on the tranny. I wouldnt change anything with my set up.