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Used Differential Evaluation

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71TR6

71TR6

Jedi Hopeful
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Hello all,
Since I've had my TR6 on the road, I've struggled with trying to understand why I'm running close to 3500rpm just to keep up with highway traffic while the speedo reads 75-80 mph. After some calculations and GPS work, I've just discovered the mph difference in my 71 TR6 is caused by a 4.1:1 differential rather than the stock 3.7:1. I've located a used differential but have no idea how to determine whether it is any good or not. I'd appreciate any input on what to look for to determine whether it is a useable differential. I'd prefer not to install a differential only to have to pull it again to rebuild it. Likewise, what is a fair price for a used diffy? Any benefits to rebuilding mine with a 3.7 ring & pinion? Lastly, why would there be a 4.1:1 differential in my TR6 to begin with?

Thanks,
Ron
71TR6
 
Well, if it is a 4.1, then it is there because someone put it there. However, I doubt it is a 4.1. You should do more testing, or pull it out to make sure. OPPs, turn the wheel one turn, and the driveshaft should make a little over 4 turns. I could drive my car at 75-80 and find out, however, 3500 is probably bout right. So, you want to make sure you are not buying what you already have. Then, if you do buy a used diff, you need all new seals, and maybe a rebuild.
 
What I found out is that at 3500rpm, the speedo reads 75 or so but the GPS says I'm traveling 65. When I do the calculations, at 3500 rpm, the difference between a 3.7 & 4.1 is 73.9 mph vs 66.7 which is what leads me to believe it is a 4.1 but I will double check the input & output shaft rotations to be sure.

Ron
 
How do you know your tach is accurate ? They frequently read high.

4.1 gears are a fairly common 'performance' modification; giving more acceleration off the line in trade for higher engine rpm at speed. They were available as a factory option on the earlier TRs (in conjunction with overdrive), but maybe the option was discontinued with the introduction of IRS.

Rebuilding a diff is a royal PITA (I just did one for my Stag); but more certain IMO than finding a truly good used one. (The diff I rebuilt was sold to me as having already been rebuilt, but the seller obviously lied.)

Things to look for :
1) Excess backlash. On a healthy diff, the input flange will only turn by a few thousandths before the output flanges start to turn.
2) VERY slight stiffness in input shaft. When properly preloaded, the pinion bearings have just enough drag to feel when turning the input with your fingers.
3) Bearings should be absolutely smooth, with no trace of "grittiness", tight or loose spots.
4) Play in any of the flanges. A very slight in/out movement of the output flanges (no more than a few thousandths) is OK, but any more than that is suspect. And they should not move side-to-side at all.
5) Signs of leakage around any of the seals. The seals can be renewed of course, but you'll need a press to do the side seals, and probably have to make a lever to hold the flanges while you tighten the nuts.

Here's a shot of what I found in my "rebuilt" diff ...
 
What type of tires are on the car? If they are of a smaller diameter then the old red lines, the problem you are looking at is the tires, not the diff.

Just a thought
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Here's a shot of what I found in my "rebuilt" diff ...[/QUOTE]

Randall,

Was that the pinion bearing race?
 
Rebuilding a diff., is just like practically everything else, its a lost art. I could not find anyone who wanted to rebuild one around here, without the pinion centerline set up numbers. If I had them, it would have been easier for me to do. Some people question the quality of the steel in the new replacement gears, there is an article online about that. The good thing about your set up is it should show you are getting around 35 miles per gallon, one of those deals.
 
Maybe my math is off (it IS Saturday after all...), BUT for 4.1 gears shouldn't it be 4.1 revolutions of the driveshaft for 1 revolution of the rear wheel?? :confuse:

And for 3.7 gears it would be 3.7 revolutions of the driveshaft for 1 revolution of the rear wheel?? :confuse:

Just my 1.5 cents.... (the dollar is weaker than it used to be.....)

"Cheers!" :cheers:

-Bear-
 
Brosky said:
Was that the pinion bearing race?
Click to expand...
Yup, that's it.
Only a few hundred miles on the rebuild, but so far it looks good. No clunks or noises or drips.

Having no idea what the "checking distance" was supposed to be (numbers marked on pinion were illegible), I just used the old-fashioned method of putting it together, checking the tooth contact pattern then taking it all back apart to change a shim and try again. Fortunately only one of the shim packs had to be changed ... unfortunately it took me some 10 tries to get that one shim pack right.
 
It's a good thing that you got it out of there before it got any worse. I learned the value of a good micrometer when I used to replace ring and pinions on 442's and GTO's. The cast iron shims installed at the factory had to be replaced and matched in thickness with replacement individual shim packs to get the correct alignment in the case.

I used a lot of the bluing to check the gear engagement pattern, because not every rear end was set up perfectly from the factory. The pinion depth was pretty easy, but you could go through a bunch of the crush style collapsible spacers getting it right.
 
Well the relationship between Tach and Speedometer is fixed in any given gear.
Stock TR6's in 4th gear at 3500rpm is close to 70mph on the speedo, the tire size won't change that relationship but your ACTUAL speed (use your gps to confirm) will vary according to the circumference of the tires.
in other words a bigger tire will mean you are moving faster along the road, for the same speedo and the tach numbers.
multiply your speed in mph by roughly 50 to get the rpm in 4th gear with a regular non o/d gearbox and std diff.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Oh my!

Something new to worry about!
[/QUOTE]

Dale,

You don't need to worry about this until you get it running and begin to flog it unmercifully for a few years. You've already replaced your seals and we discussed back lash and wear last year when you did the seals. No need to panic.
 
Re: Used Differential Evaluation- follow-up

Well, today I had some time to put the TR on jackstands, crawl underneath, and check the differential turns as suggested by Searcherman. Now I'm really confused since based on the turns, it truly is a stock 3.7:1 ratio differential rather than the 4.1:1 my calculations lead me to believe.

I surmise this means that there can be three possible causes for the speed difference between actual and what the speedo reads:
1) The transmission gearing is not standard (I would guess this is highly unlikely)
2) Speedo gear in transmission is not for a standard 4 speed (maybe someone removed an overdrive?)
3) Both speedo and tach are out of wack and need to be rebuilt/recalibrated.

Either way I guess this is a positive since I don't need to worry about replacing the differential. I guess I'll just have to live with it and drive with a GPS for speed verification.

Thanks for everyone's input!

Ron
 
Re: Used Differential Evaluation- follow-up

I took the easy way out and had John at quantum mechanics supply me a rebuild Diff, again tranys, OD's diffs, are not my fortey, I am sure fortey is spelled wrong

Hondo
by the way Johns diff is performing flawlessly, no leaks, no noise
 
Re: Used Differential Evaluation- follow-up

Forte. John does nice work.
 
Re: Used Differential Evaluation- follow-up

71TR6,

If you're in PA, The Roadster Factory will also rebuild your differential for you. You might save some shipping. I know a few guys who had them do it and they do first class work.
 
Re: Used Differential Evaluation- follow-up

71TR6 said:
3) Both speedo and tach are out of wack and need to be rebuilt/recalibrated.
Click to expand...
That would be my guess.
 
Since I rebuild these diffs, and it is a lost art, Am I lost?....Oh! Dddddear (piglet)....I'd better report myself as missing! LOL!
 
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