Changing out 411's

[Jay G.]

I have a nice 66 L79 with a 411 rear end,( not great for street driving) would like to change over to a 3:55 or 3:70 is there a big difference in performance between the two?

[Mark E.]

Re: Changing out 411's

Depends on how you use the car, and what type of performance you want.

In the quarter mile, the idea is to finish the trap at red line (or peak HP) in top gear. So gearing is selected to achieve this.

For road racing, gearing is selected for the particular course, and its speed.

For the street, your L79's acceleration will be a lot of fun with a higher gear. Think through how you use the car. Cruise night? Weekend drag racing? Getting ice cream? Road trips? A higher gear is nice for the latter. Go to one of the on-line engine speed, gear ratio calculators to give you and idea. I owned a L82, M20 with 3.55 gears. 70 mph was over 3,000 rpm. I took it on a few road trips, and wished for a 3.36 or even 3.08.

[Gary R.]

Re: Changing out 411's

Jay
A lot depends on your trans, most likely you have an M21 CR. You can run a 370 no problem, probably a 355 too. Between them I would use the 373. You can use online calculators to see the RPM differences with tire sizes. You will have to change your trans speedo gears.

Now a note on gears sold today.

To my knowledge and involvement rebuilding these diffs for years there are 4 suppliers of new P&R sets for 63-79 Corvettes.
1- Richmond
2-Motive
3- Yukon
4- Tom's
Vendors buy, mark up, and rebox these gears. The first 3 are all imported now. #1 & 2 are owned by the same company, #3 is Randy's Ring & Pinion part.
#4 - Tom's is the only usa made gears sets, made for Tom's by US Gear. Tom passed away last January. Prior to that we spoke about new gears last October. At that time he told me the gears were going to made using the 2 cut process, no longer using the 5 cut process that GM and the aftermarket used. The concern Tom had was with the strength and noise level with these gears. The lash and pattern are different that with 5 cut.

My point, if you think you want to go to a 3 series gear I would call Toms asap since many of the ratios are gone now. There is no 370 USA made ratio, all imported. Tom has/had a 373 ratio that was very good. I have installed 100's of them. If you can get them now and use them later you would be better off. If they are sold out and you have to go to source them elsewhere expect to pay a lot more.

Next, if you have a GM 4 series gear then you have a 4 series posi case. You will need a series case. Many think the 4 series cases are valuable, they are not. They are good door stops now. Toms made all the vette ratios to fit the 3 series case, that includes the 411, 433, 456 ratios. This eliminated the need to swap posi cases and saved money. GM obsoleted the 3 series posi cases several years ago. These cases are still around and are better then the current cases sold today. So you will have to source the case you want to use.

If the diff numbers are not a problem you might be better served to use a mid 70's diff and build it for your car. The only difference are the casting dates and stamped housing. The posi cases & spiders were better made then the 63-71's.

[Duke W.]

Re: Changing out 411's

Unless you do a lot of freeway driving, it's probably not worth it. A 3.70 will drop revs at any speed about ten percent and another five percent with 3.55s. You don't need a web calculator. The following can be done on a pocket calculator and you can linearly interpret from there. The output is accurate assuming you have OE revs/mile tires (775).

Top gear revs @ 60 = 775 X axle ratio.

Duke

[Richard G.]

Re: Changing out 411's

After one is used to modern cars 3000 rpm at highway speeds it difficult to take. For example my C6 turns 1/2 this rpm at 70mph. My SWC has 3.36 ratio. I have achieved 18mpg on road trips. If you have stock size tires I fail to get the point of the lower ratios. Correct me if I am wrong, but isn't it all tire spin at the lower speeds anyway?
Who drives their car like they are 16 years old anyway.
I used Duke's example and posted the Rpm @ 60mph.

775	4.11	3185
775	3.7	2867
775	3.55	2751
775	3.36	2604
775	3.08	2387

[Gary S.]

Re: Changing out 411's

Jay,
I was in the same boat as you - 1972 LT1 with 4.11s and close ratio gearbox - and I really enjoyed my 4.11s until I drove from Pittsburgh to Atlanta many years ago for my first regional. Holy cannoli was that a miserable drive. When I got home I switched to 3.36s which made a world of difference for the limited time I am on the highway. But now I must slip my clutch a bit on starts from stop. I wish I had left well enough alone and kept the originals, but then I still have them and may put them back in. You have some good advice and opinions here.

Gary

[Duke W.]

Re: Changing out 411's

A few years ago I attended a local chapter meet and rode down with a member in his newly acquired, low miles, unmolested '65 FI Coupe with 4.11s. It was about a 100 mile drive down I-5 to just north of San Diego.

We kept the speed at about the 65 MPH limit and the tach was reading close to 3500. That's a lot by today's standards, but it really wasn't that noisy, but just "singing" a nice tune.

And because of the short stroke, mean piston speed is only just short of 1900 feet per minute. By comparison a HD diesel engine with a 6" stroke governed at 1800 is running at 1800 FPM, and the biggest marine diesels (3 foot bore, 7 foot stroke) are running at 1400 FPM at their typical governed speed of 100 RPM.

Back in the day the interstate highway system was just beginning to be built, and most two-lane highways of the era had speed limits of 55-60 MPH, so short gears for maximum acceleration were the rule of the day.

If you do a lot of highway driving a better idea if you want to reduce highway revs is to install a five-speed with an overdrive fifth.

Duke

[Michael J.]

Re: Changing out 411's

It is a bit of a personal choice, and of course the engine/tranny combo makes a difference. I have some C2 fuelies, and that engine, along with the '63-'64 tranny (WR) presents challenges for the lower gear rear ends. I have a 3.08, a 3.55, a 3.70, and a 4.11. The 4.11 is easily the most fun and easiest to launch, the 3.08 is the worst, and for all around driving, I prefer the 3.70.

[Mark E.]

Re: Changing out 411's

Jay
A lot depends on your trans, most likely you have an M21 CR.

Good point. Cars I've driven had wide-ratio M20s with a 2.52 first gear, and 3.08, 3.23 or 3.36 rear ends. No problem in normal driving.

An M21 or M22 with 2.20 first gear combined with a 3.08 or 3.23 rear end means launching with an effectively higher first gear. Might be fine, but never tried it.

[Duke W.]

Re: Changing out 411's

The only optional transmission for L-79 in '65 was the close ratio Muncie. The standard axle was 3.70 and 3.55 and 4.11 were optional. It wasn't until '66 that you could order L-79 with either a CR or WR four-speed.

Duke

[Joe R.]

Re: Changing out 411's

I have a nice 66 L79 with a 411 rear end,( not great for street driving) would like to change over to a 3:55 or 3:70 is there a big difference in performance between the two?

One thing to figure out first is whether your Muncie is a wide ratio (2.52 first gear) or a close ratio (2.20 first gear). Personally, I think that your L79 would be quite happy with a wide ratio Muncie and a 3.55 rear. If your existing Muncie is a close ratio, you may have more trouble pulling away smoothly from a standing start in normal street driving if you switch to a 3.55 rear.

When trying to compare how a given gear combination might feel in street driving, I like to use the overall gear ratio, which simply the product of the transmission gear ratio and the rear end ratio. Here are the numbers for the two different transmissions when combined with a 4.11 rear:

CLOSE RATIO:

1st = (2.20 x 4.11) = 9.04
2nd = (1.64 x 4.11) = 6.74
3rd = (1.28 x 4.11) = 5.26
4th = (1.00 x 4.11) = 4.11


WIDE RATIO:

1st = (2.52 x 4.11) = 10.36
2nd = (1.88 x 4.11) = 7.23
3rd = (1.46 x 4.11) = 6.00
4th = (1.00 x 4.11) = 4.11

For discussion purposes, let's assume that you presently have a close ratio Muncie. Your standing-start launch in 1st gear has an effective 9.04 ratio, and your cruising gear in 4th is effectively 4.11. This is what you are used to and you know how it feels. Simply changing the rear end ratio makes cruising more tolerable but it makes first gear more sluggish.

However, if you switch to a wide ratio Muncie at the same time you convert to a 3.55 rear, the resulting combination looks like this:

1st = (2.52 x 3.55) = 8.95
2nd = (1.88 x 3.55) = 6.67
3rd = (1.46 x 3.55) = 5.18
4th = (1.00 x 3.55) = 3.55

Note that this combination has reduced the 4th gear cruising rpm by about 14%, but 1st, 2nd, and 3rd are hardly changed at all from what you have now. I don't think the difference in the first three gears would be perceptible. If you are willing to tolerate just a slight reduction in the first three gears, you could even go to a 3.36 rear and get even lower cruising rpm.

Of course, all of this is off the table if you already have a wide ratio Muncie in the car and don't want to give up much in the first gear launch. The best fix for that is to put in a Tremec TKO-600 transmission to get that extra 5th gear. It will hurt you a bit in NCRS judging, but the standard deduction for an incorrect transmission is only 25 points. There are a few other places where you could lose points (such as not having a reverse lockout), but there are ways to fix that if you are really determined to do so.

I have a TKO-600 in my car, and its probably the best thing I ever did in terms of enjoying the car.

[Terry D.]

Re: Changing out 411's

As already mentioned it depends on how you want to drive the car. I agree with Joe on the 5-speed, put one in my 62 and we take it on 5-6 hundred mile trips and get 23 miles to the gallon. I have a 3:70 rear end and in first gear it is like having a 4:24. If you want to keep the 4-speed again it depends on your driving, the 3:55 is a better highway gear and here I have to disagree with Joe, with either 4-speed you will have no trouble starting from a stop sign.

[Gary R.]

Re: Changing out 411's

Just when everyone is pulling 411's as things of the past I am building a '66 BB 411 today in fact!

[Scott H.]

[Terry D.]

Re: Changing out 411's

Well Gary, that BB with 4:11's will be fun from light to light but you will not like it if you decide to go on any kind of trip. On the other hand you be keeping the economy going as you fill up at every other station you pass! LOL