Differential Gear Changes

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Hi Bob:
One of the things I am considering is the change of the rear differnetial gears in my ERA. The rear end is from a Jag XKE Series 2. Vintage is some time between 1964 and 1973 but that is all that is known.

Here is the first question:

Is the changing of the differential gears and posi unit a bench job or is it a job that can be done to the rear end in place? ie: is it a "quick change" or is it a detailed and delicate operation?

Second:

You list Michigan Driveline and SICP as possible sources for new gears. However, Michigan's and SICP phone numbers as listed in the Assembly Manual are defunct. Got new numbers on them?

Third:

Other than the gears, what else would need to be changed, ordered, etc. I assume the Toploader's speedo gear?

4th:

Do you guys have access to all the parts I would need? And can you do the gear change if it is not a quick change operation?

Much thanks,

[Bob Putnam]

Art,

We don't replace the gears here. It's a combination of lack of special tools, lack of experience and lack of time.

We send all our differentials to Reider Racing/Michigan Driveline, where they put in new gears and rebuild the Powerlock. We ship about 10 at a clip so the shipping isn't too bad. You might ask Peter whether your differential (or one of our cores) can get in the Reider queue.

Reider Racing: 734-946-4058

[REDSC400]

Thanks Bob:
Just talked to Reider Racing. www.reiderracing.com

The tech support guy there at Reider was super. He has hooked me up with a local differential specialist in my town who can perform the entire regearing job, if I decide to go that direction.

The car could be layed up about three days.
Sounds like a good job to do during the January - March snow / ice / muck season hiatus. I just have to ponder if I should leave my 4.11 rear in or go to something in the 3.x range.

Thanks for that tip on Reider Bob.

BTW, I did the trannie and differential oil and brake fluid changes this weekend during the Sunday rains. Put the car up on jackstands. A little scary doing this "elevation" for the first time. Learned that the Front goes up first then the Rear. Likewise, the rear comes down first then the front. Hydraulic jack placement is key. Especially on the descent!

Hard to find the Amalie LSD additive in my local auto supply haunts, (franky, Amalie is not too common here abouts) but a call to Amalie got me hooked up with a supplier in Ansonia, CT.

The Moroso Climbing Gear lube is also difficult to find in stock anywhere locally, but Summitt had it at the door in short order.

My old (fatigued) manual fluid evacuation pump proved inadequate for the task of replenishing the heavy trans and differential oils. So I replaced it with a new manual suction gun. What a difference having the right tool makes, eh? To be honest with you, any excuse to buy new tools is great, especailly if they are not too expensive.

So #375 was out on the roads Monday taking in the autumn splendor. Got pulled over by a Policeman in Prospect. I was a bit concerned at first. I was going slow and missed no stopsigns or lights. The officer walked up from his car carrying a cup of coffee and a donunt. He just wanted to know what the car was. He was surprised it was made in New Britain! Boy he said, Those are some Serious Seat Belts!

I hope this does not happen too often! Sheesh.

[Bob Putnam]

Art,

From what I hear from customers, getting stopped by curious cops is pretty common. The good part of their curiousity is they will frequently tolerate some "bad" behavior by a driver in one of these cars.

Just don't push your luck.

[decooney]

REDSC400,
not that you are asking, but are you sure you have 4.11 gears in the rear? With a 428 and a (wide or close ratio) Toploader, I can't imagine using anything lower than a 3.31 or 3.08. Even 3.54s are getting low in a big block Cobra for the street running with a Toploader. Maybe I'm missing something.

I've run 3.08s, 3.31s, 3.55s, 3.73s in various cars, this car, other Cobras, and really like the taller 3.31/3.08 setup in a 2500lb car with a torquey 428 big block and 4spd. Like a sling shot. The motor torques right through it pulling like a sun of a gun. Heck, if I need to I can start out in 4th gear. 3.31 or 3.08 with a wide ratio Toploader is really a nice all-around setup if you must keep the 4speed; 3.31 being more common.

In 4th gear, how many rpms are you taching at 60mph?

[REDSC400]

Originally posted by decooney


REDSC400,

In 4th gear, how many rpms are you taching at 60mph?

Hi Decooney:

At 60, if memory serves, I am doing about 3500 RPM.

I counted the # of driveshaft turns or revolutions whist my son moved the rear wheel one revolution. That came out to be about 4 and 1/10th driveshaft turns for one rear wheel turn or about 4.1 ratio. Did this while the car was up on blocks on Sunday..

[decooney]

Hey Art,
if you can, try moving the driveshaft, not the wheel, and count it again just to make sure there is not any slip going on. I'm about about 3200rpms with 26.5 inch tall tires at 60mph with a 3:31 rear. If the rear came from ERA, I was thinking maybe its a 3.54. My friends Cobra with 3.54 revs 3500 at 60; with 25.7" tall rear tires. If you have shorter rear tires, that would definitely make it rev a bit more too...but would not skew your previous count figure of 4.1 rotations; if there was no slipping going on. Sorry, I was just stuck wondering why someone would put 4.11s in there for a 2550lb car, with a 428. IMHO, you would be much happier with 3.31 ratio if you truly have something lower than 3.54 and especially if you have 4.11s. Good luck.

[Bob Putnam]

While the 4.11 gears were rare, they existed in some Jags with overdrive transmissions. We've found a few.

By the way - make sure that your rebuilder knows what gear you have now. There are a couple of different case sizes - which determine what series of gears fit.

[REDSC400]

That is a good tip Bob. Will do if I go the change route.

The previous owner/father of my car supposedly chose the 4.11 rear to keep the car in the prime torque spot for his driving style, which one could assume was going 0 to FAST as quick as possible (thus the choice of the drag racing 4.11 rear ratio). And as such it does make alot of sense on the hilly, curvey and low speed back country roads in CT where speeds in excess of 60 MPH on tree lined, stone wall lined, blind curved roads are an invite to disaster, ahem.. He supplied the rear, trannie and motor, this was not ERA's doing or council.

With the 4.11 gears, the car is a true slingshot and torque monster, I must admit. The revs immediatly get you into prime HP and torque areas of the particular engine. The dyno figures on my car are primo in the 2900 - 5300 RPM zone. A fairly flat torque curve (or should I say torque flat line) at 450 Lb-Ft +/- 15 in that range with HP starting at 241 at 2900 and then maxing at 426 at 5400 and then tapering down from there. So the 4.11 gears get you in the engines sweet spot nicely and keep you there.

The detriment is a busy 3500 RPM+ on turnpike speeds in 4th and slow overall highway top end.

In all the spares he gave me with the car, there are no other differential gear sets. Other items I have duplicates and even triplicates of.

[Bob In Ct]

Hey Art:
Just for the record, my SPF has a 3.73 rear and a Tremec tranny. At 60 MPH I turn about 1900 RPM. The 0.68 overdrive and 275/60-15 rear tires help.

Bob

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Originally posted by Bob In Ct


Hey Art:
Just for the record, my SPF has a 3.73 rear and a Tremec tranny. At 60 MPH I turn about 1900 RPM. The 0.68 overdrive and 275/60-15 rear tires help.

Bob

Hi Bob!
I do not have the overdrive with the toploader. 1:1 is the final ratio for me. I am running the BF Goodrich Euro's (now no longer available) at 305/50-15 on the rear end, real steamrollers.

[Ant]

To my knowledge you could get a 2.92 ratio in the automatic E Type Jaguar, I ran a 3.07 ratio E type rear in my FE406 and with the Toploader, but I found it even low geared on the highway, you could buzz round to about 140mph and a rotten little Nissan 200ZX Turbo would pass me, and then valve springs broke (dont run single valve springs with damper)

I have a 3.08 and 3.27 T/Bird rear end with my smallblock and that should be just fine with 1.1 top gear ratio!

My 0.02c worth

[decooney]

REDSC400,
if you think your 428 slingshots with 4.11s, and the power range (sweet spot) you specify, wait til you get below mid 3.x range. IMHO, unless you were revving the motor between 6500-8000rpms on every shift, I would bet you are shiifting your brains out almost all the time. I would also think your 1st gear is almost unusable with 4.11 rear gears and the power range you gave.

My friends stroked 427 Drag Mustang was only 1/10th faster going from 3.31 gears to 3.89s. The big-block Cobras really do well with the taller gears; with all that torque. Less shifting too. You might want to ride in someone else's big-block car with 3.31 or 3.08s to compare.... I only say this because I know of more than three guys now with 4speeds and 3.54s that changed them out to 3.31s or 3.08s instead in Big block 427/428 Cobras.

Good Luck.

[Lew Ledyard]

I have requested a 3.31 ratio in my KMP. I will be using a WR top loader and an aluminum Shelby built by Southern (probably in the 500-550 hp and torque range). Is this the right gearing or should I drop down to a 3.07? This car will be a street driven car, not a race car.

Thanks

[John 550]

Lew , I had 331 then went to 354 . I would say 331 are a good choice ! I may go back.
2nd gear with wide top loader is great with 331!

[REDSC400]

Originally posted by decooney


REDSC400,
IMHO, unless you were revving the motor between 6500-8000rpms on every shift, I would bet you are shiifting your brains out almost all the time. I would also think your 1st gear is almost unusable with 4.11 rear gears and the power range you gave.

My friends stroked 427 Drag Mustang was only 1/10th faster going from 3.31 gears to 3.89s. The big-block Cobras really do well with the taller gears; with all that torque. Less shifting too. You might want to ride in someone else's big-block car with 3.31 or 3.08s to compare.... I only say this because I know of more than three guys now with 4speeds and 3.54s that changed them out to 3.31s or 3.08s instead in Big block 427/428 Cobras.

Good Luck.

Hi Decooney:
Good point, I will have to try some of the other ERAs here in CT with toploaders and BB's and tall gears to compare. But no, 1st gear is not unsable at at. In fact in town, it is hard to get beyond second. 1st is quite usable in fact, good to 35 MPH or thereabouts. To be honest, most of my driving is done between 2nd and 3rd. I rarely find myself in 4th and if so not for long. The turnpikes and parkways here in CT are NOT the place to drive a Cobra IMHO. Too densely congested... = boring. We do not have the roads here that you enjoy in CA I would suppose. But then you have been here to CT (visit ERA?) so you know this.

Odd that you mention tall gears and drag cars.... hummm... hereabouts, most drag enthusiasts consider 4.11 the correct gearing and some go well beyond that.

From a force perspective, rear gears or the ring and pinion, multiply torque input from the driveshaft. It’s a straight mathematical proposition. If you have 100 lb/ft of torque going to the pinion and you have a 3.31:1 gear ratio, you have 331 lb/ft of torque being applied to the axle, a 4.11:1 gear ration then provides 411 lb/ft of torque or about 24% more work force. It is no wonder then that the drag crowd opts for the 4+ range of gears.


From a motion perspective, if you have a 3.31:1 ratio, the pinion turns three and a third times for every rotation of the ring gear. So altering the ratio impacts the axle torque (the potential to accelerate any given mass) and the axle rotation speed. In combination, these two events are how much and how fast work can be done.

I do not want to choose a gearing that could keep me relogated tor confined in either 1st or 2nd gears based upon the limits of our New England roads, speeds, traffic, etc. I also do greatly enjoy the loud "WHANNN-DHAAAA" that emits between gear changes. If you know what I mean.

Additionally, I do find the ability to chirp my rear tires in gear changes from 1st to 2nd, 2nd to 3rd and 3rd to 4th just a darned blast. (without ANY high RPMs or power shifting BTW!) In some instances, the tail waggle in 1st to 2nd and 2nd to 3rd when getting on it is a bit much....



This is indeed a fun topic. And in retrospect, come to think of it, perhaps what I need is a 5 speed with the 5th being overdrive for those times I am on the highway at speed. Bob, Bob, lend me you Tremec!

[Bob In Ct]

Art:
Let's do a ride this weekend if the weather is good. I know of a couple of good roads.

Bob

[REDSC400]

Hey Bob!
Drat, no can do... going the Belgium, Amsterdam and Paris to do a shoot. When I get back lets hook up.

[decooney]

Art, not to debate... but I've tried all the ratios mentioned in my car and friends cars. A Cobra is a different beast, light weight with a big block. I understand the physics and mechanical advantage of lower gear ratios. All great in theory, but its not always accurate on outcome. To me, there is a point of deminishing return when it comes to the torque multiplication factor and rear tire spin. Super low rear gears are not always better in my book. For exampel, there are many people even in drag boating that have produced better ETs by going with "taller" gearing, when using big torquey motors... In a light weight car with a torque monster motor it makes a difference. More importantly, I am saying that I've driven and compared them too side by side; not just talking from theory and physics of mechanical advantage from ratios.

In fact, I can go further to say that if two Cobras were side by side with 4-speeds wide ratio or close ratio setups, with equal 428s at 500HP (0-6000rpms only), one having a 4.11 rear and the other having a 3.31 rear, the one with the 3.31 gear wins stoplight to stoplight heat every time. I've compared this myself on more than a few tests with friends who had lower rear gears and shorter 1st gears. They are shifting gears from 1st to second, blasting the rear tires off while I'm pulling a tad longer first gear that gives me a car length every time. In a heavy Camaro, Mustang, Chevelle, etc... sure, 4.11s are great. In a light weight Cobra, with a 500HP 428, the longer 1st gear will get the jobe done almost every time.

For my car, with a custom wide-ratio large spline toploader, I believe that 3.31 is still just a tad low (with wide ratio 4speed) and a 3.08 is a tad high for an ERA at 2550lbs. 3.08 maybe good for a lighter weight Kirkham or lighter replica. A 3.23/3.27 would be perfect for my ERA and the 428 setup. If you have a friend to compare to side by side with the same motor, him having the taller rear ratio in his Cobra, race him from a stop light and see what happens... just passing along my first hand experience and comparisons. If your motor revs 7500-8000, then I could see where 4.11s might be usable.

Hence, your original question.

[REDSC400]

Hi Duane:
Yes, I can see what you are saying for sure. The weight and the force to overcome that weight (light in the Cobra instance) is nullified by the greater distance that the taller gears provide. Kind of like a linear drive situation. Probable reason some of the dragsters are going to the paddle shifted automatics as well as the new paddle shift clutchless cars. Makes perfect sense.

As it stands now, being rather new to my Cobra I will say put with the current setup for about a year and during that time, as you suggest, do some due dilligence and test some variants out.

I appreciate your input and our mutual give and take. In all instances, I hope you were taking my points, not as argumentitive or pendantic but rather as yearning to gain knowledge and a point of view. I take it that is true?

This is a fun topic and in a mathematic and logic sense one should think the outcome could and should be easy and consistant. However, there are quite a few variables to deal with.

BTW, what kind of gas do you guys get out there in CA? We are blessed with two nice powerful high tests here in CT. We have the 94 octane SUNOCO and the 94 AMOCO. There are a handfull of SUNOCO stations that sell SUNOCO 101 octane racing fuel but none are within 60 miles of my home. What does you baby get to drink out there?