427/390 Auto with a 3.08 Final

[Bob S.]

I'm thinking of dropping my final drive from 3.08 to a 3.36 for a bit more low end acceleration. I'm told that's about as low as I can go since my car is an automatic, and it wouldn't be able to handle anything lower. Any info on this?

Also, would the switch from the 3.08's to 3.36 show an appreciable difference on acceleration?

No question about it, my 68 is a fine freeway cruiser with loads of passing power, but I've always loved that occasional "Eyeballs to the back of the head, thing" from a roll....(Just every know and then, mind you

Thanks,

Bob
(9944)

[Larry M.]

Re: 427/390 Auto with a 3.08 Final

Bob:

What year car??

I would go to 3.55.......that should be enough to feel the difference yet still be highway friendly. But if you still want more "snap", then perhaps a 4.11 would be a better choice.

Larry

EDIT/ADDENDUM: My all around favorite gear ratio is the 3.70, but it might be a little bit more than you want for highway driving. You could always consider a "street/strip" higher stall torque converter. Somewhere around 2200-2500 RPM.

[Duke W.]

Re: 427/390 Auto with a 3.08 Final

It would be a hell of a lot easier to develop a more aggressive spark advance map to improve the low end torque. As built the map is set up for emissions, so it's basically lazy and sacrifices a lot of low end torque and fuel economy.

Download the '66 and '68 information packages from the GM heritage site and compare the spark advance maps. Even the '66 is conservative and can be improved to make more low end torque and a little more top end power.

Download my national convention presentation, which has some recommended starting points.

A few years ago I drove a '69 with a base engine (350/300HP), TH400, and a 3.08. It had a set of 350 truck heads with 1.72/1.50" valves. It wasn't bad off the line, but it was wheezing at 3500. The owner obtained a set of correct 1.94/1.50" valve heads and massaged them to my recommendation. I drove it when done and the new configurations would light the tires frome a dead stop if you held the brake, floored the throttle, and released the brake.

Lighting the tires may be fun, but it ain't fast. Just flooring the throttle from a dead stop barely hazed the tires, and it took off like a scalded cat, pulled strong to 5500, and wanted to rev higher.

If a 427/390 won't do what that base 350 with massaged heads can do with the same TH400 and 3.08 there must be something terribly wrong with the tune.

Duke

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

It would be a hell of a lot easier to develop a more aggressive spark advance map to improve the low end torque. As built the map is set up for emissions, so it's basically lazy and sacrifices a lot of low end torque and fuel economy.

Download the '66 and '68 information packages from the GM heritage site and compare the spark advance maps. Even the '66 is conservative and can be improved to make more low end torque and a little more top end power.

Download my national convention presentation, which has some recommended starting points.

A few years ago I drove a '69 with a base engine (350/300HP), TH400, and a 3.08. It had a set of 350 truck heads with 1.72/1.50" valves. It wasn't bad off the line, but it was wheezing at 3500. The owner obtained a set of correct 1.94/1.50" valve heads and massaged them to my recommendation. I drove it when done and the new configurations would light the tires frome a dead stop if you held the brake, floored the throttle, and released the brake.

Lighting the tires may be fun, but it ain't fast. Just flooring the throttle from a dead stop barely hazed the tires, and it took off like a scalded cat, pulled strong to 5500, and wanted to rev higher.



If a 427/390 won't do what that base 350 with massaged heads can do with the same TH400 and 3.08 there must be something terribly wrong with the tune.

Duke

Wow...Lot more info than expected, but certainly enlightening...THANKS!!! I've heard anything lower than a 3.36 is "too much" for the transmission....Your thoughts on this? Oh, and where would I go do download your presentation?

Thanks,

Bob
(9944)

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

Bob:

What year car??

I would go to 3.55.......that should be enough to feel the difference yet still be highway friendly. But if you still want more "snap", then perhaps a 4.11 would be a better choice.

Larry

EDIT/ADDENDUM: My all around favorite gear ratio is the 3.70, but it might be a little bit more than you want for highway driving. You could always consider a "street/strip" higher stall torque converter. Somewhere around 2200-2500 RPM.

My car is a '68. I have to agree on the 3.70. My favorite as well..Now if I were 30 years younger, I'd be using it. I originally asked my mechanic about the 3.55 but was told that's too "Low" for the automatic trans....Your thoughts on this.

I'll look into the converter

Thanks Larry,

Bob
(9944)

[Chuck G.]

Re: 427/390 Auto with a 3.08 Final

FWIW....

My 69 BB is a 400HP automatic, originally with a 3.08 rear end. I took the car several times to the Mason Dixon Chapter Vintage Thunder drag races in that configuration. The drags were always held on Father's Day weekend.

I could run that car dead stock in the 13.8 range. My wife actually beat me and ran a 13.7...........

Anyhow...the rear was acting up several years later, and I pulled it out...took it to a buddy....and had it rebuilt with 3.36 gears, thinking it would aid acceleration.

Back to Vintage Thunder for several years after that.... The car would run 13.9's and low 14.0's at best. EVERYTHING else remained the same.

So, in my unscientific experiment, my 69 was actually slower with the 3.36's.

Chuck

[Larry M.]

Re: 427/390 Auto with a 3.08 Final

Bob:

Duke's presentation is at the top of the forum page. John Waggoner posted it there after the convention.

Your car/auto should be able to handle about any gear you want to run. I have had less robust auto transmissions with a 3.70 in the rear without any problems. You may find that you have more wheelspin when launching the car, which could result in slower 1/4 mile times, unless you upgrade the tires. I think this is what Chuck was saying above. However, the "fun factor" is certainly improved......or at least it was when I was 18.

Larry



EDIT: In 1967, with the auto trans (2-speed Powerglide) the standard ratio for your engine was a 3.36 posi......with 3.08, 3.55, and 3.70 as options.

[Timothy B.]

Re: 427/390 Auto with a 3.08 Final

FWIW....

My 69 BB is a 400HP automatic, originally with a 3.08 rear end. I took the car several times to the Mason Dixon Chapter Vintage Thunder drag races in that configuration. The drags were always held on Father's Day weekend.

I could run that car dead stock in the 13.8 range. My wife actually beat me and ran a 13.7...........

Anyhow...the rear was acting up several years later, and I pulled it out...took it to a buddy....and had it rebuilt with 3.36 gears, thinking it would aid acceleration.

Back to Vintage Thunder for several years after that.... The car would run 13.9's and low 14.0's at best. EVERYTHING else remained the same.

So, in my unscientific experiment, my 69 was actually slower with the 3.36's.

Chuck

Chuck's car does not run slower, his reaction time is slower...

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

FWIW....

My 69 BB is a 400HP automatic, originally with a 3.08 rear end. I took the car several times to the Mason Dixon Chapter Vintage Thunder drag races in that configuration. The drags were always held on Father's Day weekend.

I could run that car dead stock in the 13.8 range. My wife actually beat me and ran a 13.7...........

Anyhow...the rear was acting up several years later, and I pulled it out...took it to a buddy....and had it rebuilt with 3.36 gears, thinking it would aid acceleration.

Back to Vintage Thunder for several years after that.... The car would run 13.9's and low 14.0's at best. EVERYTHING else remained the same.

So, in my unscientific experiment, my 69 was actually slower with the 3.36's.

Chuck

Chuck,

Thanks for the information...However, I'm more enthralled that your wife pulled a 13.7!...Now isn't that the woman we ALL want as our mates in this madness we call "Corvette!" You be a lucky man, my friend...I have trouble getting my wife to go on a simple "Run" (although she always winds up having fun and meeting new people)

I'm off today to my mechanic and will really grill him on this...Funny, I've been asking a lot fo members if an auto trans can "Handle" a lower ration than 3.36. I'm told "No"...Your thoughts on that subject

Thanks so much,

Bob
(9944)

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

Bob:

Duke's presentation is at the top of the forum page. John Waggoner posted it there after the convention.

Your car/auto should be able to handle about any gear you want to run. I have had less robust auto transmissions with a 3.70 in the rear without any problems. You may find that you have more wheelspin when launching the car, which could result in slower 1/4 mile times, unless you upgrade the tires. I think this is what Chuck was saying above. However, the "fun factor" is certainly improved......or at least it was when I was 18.




EDIT: In 1967, with the auto trans (2-speed Powerglide) the standard ratio for your engine was a 3.36 posi......with 3.08, 3.55, and 3.70 as options.

Larry

Got it....Thank! My mechanic is someone I trust implicitly, but can be very opinionated. I'm not looking to hit the drag strip, nor light up the tires..but, when someone says, "Can you take me for a ride in that thing?" I feel obligated, somewhere in that timeframe to slow to about 10 MPG and boot her to 90+...You know the deal....Pin their Arse back into the seat and give them the thrill I got with my very first ride in a Corvette...a VERY early 1963 Fuelie with a 3.70....I was a kid..and, as they say..."That started it all"

Ain't this a wonderful addiction???

Thanks again,

Bob
(9944)

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

Chuck's car does not run slower, his reaction time is slower...

LOL!!!!!!! Well, that's what I had in mind, but kept my mouth shut......

[Duke W.]

Re: 427/390 Auto with a 3.08 Final

Transmissions are rated by maximum input torque and maximum input speed, so axle ratio has nothing to do with it. A shorter rear gear will increase the torque load on the half shafts. John Hinckley has posted photos of a pair of big block half shafts that were obviously twisted beyond the yield strength of the steel.

Also, high grip tires will load up the entire drivetrain, but the typical tires we run on vintage Corvettes don't have much more grip than the OE tires from decades ago, and that helps protect the drivetrain from excessive loads.

The TH400 has a 2.48:1 first gear and the converter can briefly multiply torque times about two on initial startup, so the total gear multiplication is 2.48 x 2.00 x 3.08 = 15.3, which is about double first gear of the WR 4-speed with a 3.08 axle.

When I was in grad school, one of my associates had a '67 427/400 with a CR 4-speed and 3.70 axle. He never tried a "launch", and I have no doubt it would have just burned the tires down to the cord, but to this day I think that '67 was one of the most fiercely accelerating cars from 30 to 80 that I ever experienced.

A hydraulic lifter big block with a TH400 and a 3.08 axle is a nice combination because it's not turning excessively high revs at freeway speed, yet it should have enough low end grunt to effortlessly light the tires if you floor the throttle from a dead stop.

Duke

[Steven B.]

Re: 427/390 Auto with a 3.08 Final

Bob, no need to worry about a TH 400 taking more than a 3.36. My TH 400 behind 400 HP and 400+ tq had no problems with 3.55's and now with 3.90's still no problems. The first thing I did was remap as suggested by Duke earlier. Steve

[William C.]

Re: 427/390 Auto with a 3.08 Final

I'm with Duke, the distributors in these cars can get a little "lazy" in the curve over the years for a number of reason (old grease for one) and before changing the rear. I'd try a Complete tuneup including checking the distributor curve. Easier, less expensive, and should get you where you need to be. How many miles?

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

I'm with Duke, the distributors in these cars can get a little "lazy" in the curve over the years for a number of reason (old grease for one) and before changing the rear. I'd try a Complete tuneup including checking the distributor curve. Easier, less expensive, and should get you where you need to be. How many miles?

Thanks, Bill

9K on a total rebuild. The engine is in fine shape with loads of compression. Just got back from my mechanic....He feels the 3.08 is fine, sighting that the factory had two rear end options back in '68. 2.73 and 3.08...he did, however suggest a stall convertor...we shall see.

Thanks for your input....

Bob
(9944)