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Should I have different gears put in rear end?

8K views 17 replies 11 participants last post by  Torque454 
#1 ·
Hi Everyone this is my first post as I have just joined! I just bought a 55 Chevy Belair, it's just beautiful ! My question is the engine is a 454 big block, with a 411 rear end.. The engine runs perfect, but it just seems like it's running so tight at 60mph, so would changing the rear end gears solve this? I don't know a lot about that, but I am trying to learn... So let me know your suggestion and what that might cost to do... Thank You! :welcome:
 
#2 ·
Welcome. Need you to define what you mean by "tight". Is it running out of power like it has a flat spot? Or is the thing shifting very smooth. How many speeds is the transmission and any idea of what kind?

The price for a gear swap depends a lot on what you have back there to begin with. If you can post or link a picture to the diff cover that will give a good amount of info. For now count the number of bolts and report back.

You maybe better off switching your rear axle for another with a different ratio if it is a stock rear axle. If it has been heavily modified then you may be better off doing a gear change.

Pictures will answer alot of these real quick. Videos of any drivablity issues help also.
 
#6 ·
Yikes, I actually understood exactly what he meant.

He is running along the Hi Way and at the speed limit. You want to pass someone you can do it easily but at that rpm the motor seems like if it ran that way for any length of time it would go boom in short order. not scientific but just a feeling that for the speed your moving the rpm your turning it should be smooth and not winding out.

I learned a long time ago that depending on rear gear your either want snap yer head back acceleration use 4:11 gears however that my make your engine run a bit to hard on the highway.
If you want to drive down the freeway and that is your primary objective then a set of 3:50 to 308 for jammin freeway speed.

Tires play a big part in the way it runs and feels and have to be figured in the equation. :cool:
 
#4 ·
Rear end

What I was trying to say was the car is running at about 2500 rpm at 60 and trying to go any faster seems like I am pushing the engine to hard. The transmission is a 350 automatic.. I will check but I think the rear end is a 10 bolt.. I have only had the car a short time, so haven't checked that, but I will.. So is it the 411 rear end that's holding the car back? Thank you! :welcome:
 
#7 ·
Something isn't right. The RPMs at 60 MPH with a 4.11 rear end should be well over 3000. I think you have a problem with your tack or speedometer.

You can either change the gears in the rear end or install an overdrive transmission. You need to figure out which rear end you have before going any farther with the gears. There are many 10 bolt rear ends and you must know which one you have before you can get any parts for it.
 
#8 ·
What does it do at 2500/3000 rpm(your cruising rpm in 3rd) when in 2nd? If it is something similar then I would look at the transmission.

The converter could be locking up. This is to drop the transmission temperature and can cause a "bog". A torque converter change may be in order.

But, before you go running out looking for a new converter check the basics. Bit of googling should pull up proper ways to do the following.

Check your plugs. They can give you a very quick indication of many things once you read them.
Anything restricting the exhaust, fuel, or air intake?
What does the transmission fluid look like?
Pull your vacuum lines one at a time. Plug one end with your finger and blow through the other. This is the easiest way I know to find leaks. If the lines are soft, cracked, or completely plugged before you blow them out with air(you did that before getting a mouth of grit right:p)just replace the line(s) with new.

Tune the engine it or have someone properly make sure the car is tuned. You could have a fuel pump or regulator that is not supplying enough fuel at that rpm. You could have the secondaries kicking in just enough at that rpm to cause a slightly rich condition. Your timing chain could be off. The distributor could have shifted ever so slightly. The distributor may not be advancing at the right rate. etc etc.

Without actually being under the hood or in the car while it is acting "tight" the list of guesses are endless.
 
#9 ·
If you have the stock rear in it you can change the complete assy in no time at all. I use to have 3 different carriers and I was able to swap them out in under 1 hour no problem.

You simply remove 4 nuts on each rear wheel and pull the axles out a bit
Remove the Carrier Bolts and take the entire assy out
Put the new one in and tighten the bolt
Push the axles back in and tighten those bolts
BAM YOUR DONE...



You can get the complete Carriers on eBay for @ 200.00 just have to keep your eyes open

There's a NON posi 3.55 set up on eBay right now and it has one bid on it for 159.99
1955 1964 Chevrolet Belair Impala Rear End Ring Pinion Gear Housing Carrier 3 55 | eBay

Here's one that appears to be in very good shape for 225.00
1955 1964 Chevy Rear End | eBay

If you keep your eyes open you can pick up a complete carrier for a very reasonable price and as said it's an extremely easy install

Here's a How To on it in case you want to rip it apart completely but I think your much better off swaping the entire carrier.
I use to swap carriers back and forth all the time between 3 carriers with 3 different gear ratio's, it's a very quick easy swap

http://www.classicchevy.com/assets/pdf/classicchevy/1955-64 POSITRACTION CARRIER INSTALLATION.pdf
 
#10 · (Edited)
Eng RPM =
(MPH X Rear Gear Ratio X 336 X Trans High Gear Ratio) / (Tire Dia inches X Torque converter efficiency)


Torque converter efficiency in your case would be about 90%. If you had a lock up torque converter or a manual trans the efficiency would be 100%.


So plugging in your numbers and assuming your rear tire diameter is 28" (you can confirm your tire diameter and recalculate).


Eng RPM = (60 X 4.11 X 336 X 1) / (28 X .90)


Eng RPM = 3288


That's screaming at 60 MPH and feels like you're not even in high gear.


My brother in law has the same car as you with a 3.35 : 1 rear gear. So in his case at 60 MPH:


Eng RPM = (60 X 3.35 X 336 X 1) / (28 X .90)


Eng RPM = 2680


In your case, you can change your rear end ratio, or like newer cars, put in an overdrive trans with lock up torque converter.


I'm building a car right now with the following details:


3.73:1 Rear Gear, .67:1 overdrive 5 speed manual trans, 28" dia rear tire


Plugging my numbers:


Eng RPM = (60 X 3.73 X 336 X .67) / (28 X 1)


Eng RPM = 1799


The advantage of using an overdrive trans is you get great highway speeds with the engine loafing along. You also get great performance from a stop with the first 3 or 4 gears.


Back to your case, the cheapest and easiest change would be a Rear Gear change.
 
#11 ·
Tight.

A 454 has plenty of power, and a 4:11 gear will provide awesome acceleration - but past that, it just isn't practical. A street car is not a strip car, and we drive on freeways sometimes, at 75 mph. It'll take a tremendous amount of expensive gas, and wear out the engine much more quickly.
Just my opinion, but I think three speed transmissions are archaic for street use.
You could take the money you'll spend in wasted gas and buy one of those new six speed manual transmissions that have a high torque capacity. Ditch that old automatic and get a trans with two OD gears that will make your car able to drive across the country and back - and still have awesome acceleration!
JT
 
#13 ·
Overdrive transmission would give the best of both worlds; lower rear end gears (4.11) for 1-2-3, overdrive to cut it down to the 3.55-3.70 or about, level, possibly even better. Not sure how long the TH350 will last behind a BBC without significant modification anyway. Same for the stock rear end without beefing it up. junior stocker.
 
#14 ·
I agree unless that T350 was really beefed up and made bullet proof it's just not the right trans for a powerful Big Block.

I also agree a 3 Speed Auto is just not good for Parkway Driving if you have even decent gears. Small Block Big Block any Block I want a bare min of 3.5 gears but prefer 3.7 to 4.1, the problem is unless you have a final ratio better than 1.0 even 3.7's could be to much if you do a lot of parkway driving.

Rear tire height does play a HUGE part of this though, right now my current Toy has 3.5 Gears but because it only has 26-1/2" tall rear tires it runs out of gear on the parkway in 3rd gear. The good news is I have a built AOD auto so I put it in OD and I can bury the Speedo. With my T400 ProStreet Sierra I ran 4.1's BUT I had 33 x 21.5 rear tires so I could actually cruise all day long at 70 MPH and keep it under 3,000 rpm :thumbup:
 
#15 ·
The thm-350 behind a warm 454 isn't ideal, but neither is the 55-64 drop out style 8.2" rear axle assembly.

The traction device for this rearend are very expensive. The Eaton clutch type Posi is around $635. P/N EAT19686.
Even most ring and pinions are priced North of $300 brand new. Some ratios are almost $400 for the rng and pinion.

As stated by Pro StreetRob, these rear ends make it easy to chage ratios if you have 2 or more dropout center sections. One with an acceleration gear(low gear) and the other with a highway/cruising gear(higher gear ratio)

With the 454, low gearing really isn't needed for all around driveability as much as an engine that doesn't make as much torque, such as a 350.



I like this calculator as you enter in your stats and it gives you the cruise speed with a varying number of rear gear ratios.
MOTOR - A Rear Axle Calculator

3825rpm is need to cruise at 75mph with 4.10 gear, 27" tires, 1:1 3rd gear ratio.
3325 rpm is need for 75mph speed with 3.55 gears(this is a common gear for manual trans equipped cars of the era)
3200rpm is needed for 75mph cruising with 3.42's with the rest the same.
3150rpm is needed for 3.36 gears. This is a very common ratio as this is the gear that Powerglide equipped cars came with in the 1957 era. Ive got 3-4 center sections with varying gears in the garage.

The 1st gear final drive ratio with 4.10's and the thm-350's 1st gear of 2.52 gives you a 10.33:1 1sr gear final drive ratio.
With 3.36's that drops to 8.4:1.

In my 97 pickup I run the 700r4/4l60e with its 3.06:1 1st gear ratio and a set of 4.10's. That gives a 1st gear final drive ratio of 12.5:1 which is very short for street use. This setup requires 2675 to cruise at 75mph in Overdrive with the same 27" tires. I find this geared a bit to low for freeway driving at 75mph. A swap to 3.73's would bring my 75mph cruise speed down to 2550rpm.

It all depends on the intended use of the vehicle.

The good thing about the thm-350 (2.52 1st gear to 1.52 2nd gear)and even the 400(2.481st gear to 1.48:1 2nd gear) is that the rpm drop between the gears keeps the engine in the powerband better than the thm700r4/4l60e(3.061st gear to a 1.63:1 2nd gear). A higher stalling torque converter does help with this.

The newer 6 speed autos are even worse with their low 4.03 1st gear and 2.36:1 2nd gear. Larger rpm drop than even the 700r4/4l60e/65e/70e.

IMO the thm-200r4 is the best all around trans fpor gearing 2.74-1st gear 2.74 1.57 2nd gear 1:1 3rd and the high 0.67:1 OD ratio.(same OD ratio as 6th gear in the 6l80e trans).
A 4k85e with 200r4 gear ratios and weight loss would be the ideal trans IMO

peace
Hog
 
#18 ·
Wow. 4.11's with a TH400 in a car (which generally has shorter tires, which also increases RPM).

Id be looking at a 4L80e or even better a 6L80E. 6L80E has a wicked low first gear, makes the "low" 3.07 700R4 first gear look pretty high. (6L80E has a 4.02 1st, 2.36 2nd, 1.53 3rd, 1.15 4th, 0.85 5th and 0.67 6th)

TH400 has a 2.48 1st, 1.48 2nd, and 1.00 3rd. 4L80E is identical, with the addition of the overdrive gear which is .75....

The TH400 doesn't have a lockup converter either, so that raises the RPM even more. The 4L80e DOES have a lockup converter...
 
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