3.08 to...3.55

[Bob S.]

I'm thinking of changing my rear end gears from 3.08 to 3.55. I have a 427/390 Auto. Will there be an appreciable difference if acceleration? Yes, I know...I'm giving up one to get the other...but, seems to me a 3.55 is, and always had been a good "All Around" final.

RPM @ 65MPH/3.08=2657

RPM @ 65 MPH/3.55=3062

Difference of 405 RPM

Thanks in advance,

Bob DeSimone
(9944)

[Bob R.]

Re: 3.08 to...3.55

I think you will get different answers from many different people. I think you will like the 3:55 on the streets but in my personal opinion 3:36 is a better chioce for overall use. In 63 it was the only option with a power glide transmission.

[Gary R.]

Re: 3.08 to...3.55

Yes you will feel the difference with 355's but the BB should have enough torque to drive the 308 gear. I went from 308 to 336's with my SB TH400 and didn't feel any difference, hardly any change in RPM. Most of the diff changes I do guys jump up to 373's, those run about 3400 RPM cruising and will also require a high stall TC as well.

[Duke W.]

Re: 3.08 to...3.55

It's a lot easier to install lighter springs in the distributor to beef up the low end torque than change axle gears.

Even with the lazy OE centrifugal curve it should light the tires from a dead stop if you floor the throttle. Even a properly tuned base 350/300 TH400/3.08 should do that. If your 427 won't there's probably some kind of problem.

Duke

[Gary R.]

Re: 3.08 to...3.55

Typically a jump up to 2400 stall from the stock 2000 will work better, but then people have done a lot of things over the years. I should have said higher rather then high since that leaves the door open for a lot of ranges.

[William C.]

Re: 3.08 to...3.55

I'm with Duke, most of the distributors I have done for members are so far off the spec curve that is is no wonder they had problems with "low power"

[Patrick H.]

Re: 3.08 to...3.55

I'm trying to figure out why he's asking the same question he asked months ago.
Most importantly, has the motor tune been addressed since his saga of earlier this year and if so what happened?

[Chuck G.]

Re: 3.08 to...3.55

FWIW.... I changed from 3.08's to 3.36's in my 69 BB. That car is a 427/400 THM automatic trans.

The car actually ran slower at the drag strip with the 3.36's. Ran in the 13.7--13.8 range with 3.08's. Ran 13.9+ with the 3.36. Go figure.

Chuck

[Jim T.]

Re: 3.08 to...3.55

I agree with Duke, keep the the 3:08 gears. My 1970 turbo 400 and 350/300 with 3:08 has been very enjoyable to drive, 164K original owner for over 42 years. I have often wondered with my 70 would be like with 2:73 gears. The 350/300 has good torque for a small block and still can spin the tires. It has been some time ago since I broke the speed limit with a WOT take off on a street, but the shift from first to second has broken traction WOT before. My once owned 64 convertible with 4-speed, 327/300, and 3:08 was a very good combination for my driving experience with it.

[Bob S.]

Re: 3.08 to...3.55

I'm trying to figure out why he's asking the same question he asked months ago.
Most importantly, has the motor tune been addressed since his saga of earlier this year and if so what happened?

LOL!!! I AM asking again because I'm getting closer to finishing my interior resto and just looking for more input...Hopefully from different members....(More members=More opinions) The engine problem is all fixed....Fuel Starved. New filter. And, I can't believe the mechanic made such a big, "Mysterious" deal out of it....Think he just likes "Drama" in his old age

Bob DeSimone
(9944)

[Duke W.]

Re: 3.08 to...3.55

I'm still perplexed. You used the word "required" in your original statement. You didn't address this requirement.



WHY do you feel a higher stall converter is required when fitting a lower final drive ratio?

Thanks.

It wasn't my statement, but I'll answer the question. Changing axle ratios, especially to something shorter, does not require a higher stall converter.

The change that would require a higher stall converter to keep a car from being a dog off the line is if the OE low to moderate overlap camshaft is replaced with an aftermarket high overlap camshaft that kills the low end torque.

The other effect of such a change is to turn a 10 MPG big block into a 6 MPG big block since in around town driving the converter will always be slipping significantly.

In the seventies when the L82 was available with TH400, the version used on L82 had a higher stall speed than the base engine version, and the standard axle ratio was shorter to make up for the L82s' weaker low end torque compared to the base engine.

Duke

[Gary R.]

Re: 3.08 to...3.55

Sorry guys I just got back to read this thread.

Dave, for the sake of discussion I will remove "require" from my statement. As I'm sure you know selecting a TC involves matching cam spec and rear gearing. My point was/is that many times when I am building a vette diff and changing the ratio from say a 308 to 370/373, the engine is also being worked on ( new cam) or just being replaced as with the ton of 383 stroker motors built the past 5-10 years. Now rereading the post it didn't mention a cam change so I just went beyond his original question I suppose. However I will say many, many times guys have stepped up to the 2400-2500 stall range with a 355-373 diff and really liked the setup. One setup that was popular for a while in a SB was the 262XL cam,headwork, TH400, 2400 Stall, 355's was real nice setup. Be that as it may people will do what they want and all I can do is wish them the best.

My personal opinion with this thread would be to leave the 308's in but if more acceleration is wanted then I would go to 373's over 355's.

[Bob S.]

Re: 3.08 to...3.55

Sorry guys I just got back to read this thread.

Dave, for the sake of discussion I will remove "require" from my statement. As I'm sure you know selecting a TC involves matching cam spec and rear gearing. My point was/is that many times when I am building a vette diff and changing the ratio from say a 308 to 370/373, the engine is also being worked on ( new cam) or just being replaced as with the ton of 383 stroker motors built the past 5-10 years. Now rereading the post it didn't mention a cam change so I just went beyond his original question I suppose. However I will say many, many times guys have stepped up to the 2400-2500 stall range with a 355-373 diff and really liked the setup. One setup that was popular for a while in a SB was the 262XL cam,headwork, TH400, 2400 Stall, 355's was real nice setup. Be that as it may people will do what they want and all I can do is wish them the best.

My personal opinion with this thread would be to leave the 308's in but if more acceleration is wanted then I would go to 373's over 355's.

Well, I've been following you guys for a while now, and must say I'm always learning SOMETHING!!! I'm told I should be happy with a BB and 3.08's..but, I'm just not happy with the low end acceleration....I guess I'll finish my interior resto and see how I feel...

Thanks for the input

Bob DeSimone
(9944)

[Bob S.]

Re: 3.08 to...3.55

FWIW.... I changed from 3.08's to 3.36's in my 69 BB. That car is a 427/400 THM automatic trans.

The car actually ran slower at the drag strip with the 3.36's. Ran in the 13.7--13.8 range with 3.08's. Ran 13.9+ with the 3.36. Go figure.

Chuck

Hmmmm? Could it just have been how the car was beind driven with the slightly lower gears? I'm guessing there isn't too much of a differnece between the 3.08 and 3.36...I'm hoping for a bit more low end acceleration with a 3.55 setup....

Bob DeSimone
(9944)

[Joe C.]

Re: 3.08 to...3.55

I wouldn't change the final drive ratio unless I installed a modern automatic trans with locking converter and overdrive.

You should investigate wise engine modifications that will increase torque and extend it in both the low and high range. The best way to do that is to install a roller cam. In your case, a mild hydraulic will give you what you're looking for. Cylinder head modifications, including judicious porting will give you "free" torque and horsepower with no downside. Quickening the spark advance to incipient detonation with cooler intake manifold temps and WOT mixtures in the 12.5 - 13.0 range will optimize power and torque.

I have a wonderful driver '65 which is very streetable and a joy to drive both on the "streets" and on the highway. It's a short stroke 331 (.030 over 327) that does the 1/4 mile in 12.7/113 with skinny street tires. With its 3.73 final drive and Keisler RS600 5 speed, it cruises at 80 turning 2500 RPM or 60 @ 2000, and with the short 2.79:1 first gear, it will smoke the tires forever, with the brakes released, off the line until I release the "go" pedal.