Discussion Forum - Northstar Chevelle Club

SShink · 3K+ Club

12 Bolt 4.11 Gear Set and Posi

Bungy L-76 · Active Member

Didn't the OD solve that problem? You know the .67 4th gear gives you a final drive ratio of 2.75.

SShink · 3K+ Club

Bungy L-76 wrote:
Didn't the OD solve that problem? You know the .67 4th gear gives you a final drive ratio of 2.75.

The OD certainly improved it, but it's still too high IMO. I don't want to get it too low as my stall is 2400 which will slip and cause more heat, but it's running 2500 at 65 and 2700-2800 at 70 mph, and I'd like to drop the rpm 250-300 to get the engine in a lower rpm at 65-70 mph and maybe get better mileage too.

The 1-2 shift around town is within the first 15' as the 200-4R has a 1st gear of 2.74:1 which is fairly deep with the 4.11's. I think if I raise the rear end ratio, I will get more in the power band too instead of shifting so soon because of the 4.11's. At least that's my theory...

Most combo's I've seen used with OD auto trans use 3.73 gears or higher (numerically).

SteveS · Super Poster

Stan sent you a PM

Bowtieman427 · Active Member

I think you really have to change perhaps consider a 3.90 or 3.73.
You run it to low with a healthy cam it will be a real dog. Your engine will load up, not be happy and run low torque in cruise area. Perhaps the bad MPG is tunning related also.

If you are going for best cruise you need to know your peak torque you will want your cruise RPM where torque peaks or is atleast coming on for best possible effeciency of your engine.
Theory:
Here is a cool calculator.
http://www.wallaceracing.com/calcrgr.php

(MPH x axle ratio x trans ratio (.67) x 336) divide by tire diameter = RPM
SOLVE for axle ratio.
Axle ratio = (RPM x tire diameter) / ( 336 x .67 x MPH)
with a 26" tall tire I get 4.12 @ 2500RPM and 70MPH
with a 28" tall tire I get 4.44 @ 2500RPM and 70MPH

Amazing from these you can see how efficient your converter is with 4.11 you are right there with a 26" and a 28" should do you better but your tach shows 2700-2800RPM

Here is a calculator to check all these things out
http://www.angelfire.com/fl/procrastination/rear.html

You can get a 3.73 ring gear and pinion for series 4 I think someone makes 3.90 thenm for Series 4 also.

Another approach. In theory you drop 1 gear ratio you drop 1000RPM you drop .5 you drop 500. you drop .25 you drop 250. If you looking for a 250 RPM drop the 3.90 (3.86) rear gear is what you want.

If you really you feel you have to change this may be a better and more cost effective option. Then selling off your Series 4 and going 3, but if you must sell please keep me in mind. If you are selling the series 4 posi I may be interested if the price is right for the 67Impala. PM me only if Steve S is not interested.

-- Edited by Bowtieman427 on Friday 4th of November 2011 10:26:25 PM

Lost in the 60s · President

Yup, something doesn't add up here. Maybe the tach is off ? Are you using the lock up converter ? My cheap tach reads around 2300 with a 1 to 1 final trans gear, 28" tall tire and 3.31 rear gears at 65mph.

SShink · 3K+ Club

Moved this from Dashboard's project thread by Mitch:

Ah yes, I have 255's on the Camaro and they are short. You really should throw my 275's on and take it for a drive before commiting hundreds of dollars to a gear change. I can take them off the car and have them sitting here ready. It would take literally 10 minutes to have them on your car.

OK, just did some comparison sizing. A 295 is actually only 26.7" tall because it is a 50 series sidewall. Short and fat. A 255/70 tire is 29.1 tall !! It would be the same width as your current tread but even taller to help the ratio. The 29" tire may look to tall, as you said.

I have 235/60 on the front and the bias in size looks good to me but I like the rears to be wider/taller than the front for the "look". I would like to go to a 215/65 front to get the section/tread width .7" narrower and it would be 26" tall versus 26.1" for the 60 series. A 215/70 is 26.9" tall and would be a close match with a 275 rear.

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Thanks Mitch. Using the calculator for a 275 tire that is 28" tall, here's what it shows:

70 mph shows at 2417 rpm vs. 2633 rpm with a 25.7" tire (216 rpm drop)

65 mph shows 2244 rpm vs. 2445 rpm with a 25.7" tire (201 rpm drop)

Definitely better, but not sure it's enough of a drop. Probably the best thing to do is swap on your tires as you said and see how it drives and changes shift points. I'll send you a PM.

It does sound like I might be able to swap gears/posi's with Steve S, but not sure yet.

Of course, if I go with just the tire change (Estimating $300 for 2 new 275's?) that opens up thoughts of changing out all 4 BFG TA's for better tires, which means probably $600-700, which is getting close to swapping out gears/posi costs... Why isn't this ever easy!

Lost in the 60s · President

We should really swap all 4 of my Firestones on so you can see the difference in handling. If you change gears, you still need better tires anyway.. I'll be around late afternoon today and available most of tomorrow if you want to do this.

4 tires like mine are ~$450 mounted and balanced. Yeah, you can get into the ultra performance tires for that 6-800 range, but our cars aren't really built to use them to capacity.

SShink · 3K+ Club

Lost in the 60s wrote:
We should really swap all 4 of my Firestones on so you can see the difference in handling. If you change gears, you still need better tires anyway.. I'll be around late afternoon today and available most of tomorrow if you want to do this.
4 tires like mine are ~$450 mounted and balanced. Yeah, you can get into the ultra performance tires for that 6-800 range, but our cars aren't really built to use them to capacity.

Mitch, I sent you a PM about coming over. How does late morning tomorrow work for you if I come over? Or, I can do late afternoon today today too. Whichever is better for you. I'm a bachelor this weekend as the wife is in Des Moines on a 'shopping trip' with her family 'women folk'.

Bowtieman427 · Active Member

Did you factor in the converter ineffiencies also when considering all this. This is why we are asking if you are in lock up or not and when is it in lock up. You can have 2-8% loss right there in your converter. Since you want to cruise close to stall you may have more. When you ran the quarter what was your trap speed and if you dyno'd your engine you know what you have at the crank. You have to use real numbers not geusses. You can actually calculate how much loss you have in the converter and drive train I think the converter will be the big contributor. Does it in fact stall right at 2400 RPM ? Where does it lock if you lock up ?
How effecient is your converter ? There is always slipage

The converter to me is one the of the most important part of the whole car and is often over looked. A lot of them do not stall where advertised and are ineffeciant as can be espcially off the shelf. This is more of a factor in high HP and Torque solutions. If you are looking to get couple hundred RPM deltas you may consider it a more custom converter and your losses or higher rpm may actually be from the converter.

I run 275 60 15 on the rear and they fit nice but as posted in other thread Steve S car had a little rubbing with them on the street at the track it was fine.
You may have to massage a little and you may not with them.
Wish you were closer as I have the BFG set 275 60 15 (28) rear and 205 70 R15 in front (these are 26.x front) tall tires off car right now and could test. The car has a slight rake but I like it. IMO 28" best stock way to go in rear.

SShink · 3K+ Club

Mitch, thanks for letting me swap on your tires and give it a try. For anyone interested, it dropped the rpm by 150-175 at 70 mph, but not as much on the lower speeds.

Still looking for more of an rpm drop, so Steve S. and I need to talk about potentially swapping rear axles or the differentials and gear sets.

Jim, the bottom line is the gears are too deep, and I will lower the rpm by changing that component. Thanks for the feedback though.

SShink · 3K+ Club

Update to for sale ad: Only the gears are for sale, and they are 4.10's, not 4.11's. And... they are for a 3 Series carrier, not a 4 Series! Richmond ring/pinion gear set pn is 69-0304-1 (the gear is stamped 80-0428-1)

I FINALLY got lucky when I pulled the cover to check thm tonight, and the posi is a 3 series, so I don't have to buy a new posi when I change to a 3 series gear!

Here's some pics: