GT6 GT6 diff choice for O/D

[Flinkly]

so...

I'm putting a d-type overdrive into the GT6 and was wondering what i should pic up for a diff. 3.89:1 is the original size, but with thinking about highway economy, a smaller diff seems better.

i've got 3.27:1, 3.63:1, 3.89:1 and 4.11:1 to choose from. 3.27 is the non overdrive original (i've got mine with a broken tooth) but 3.63 looks like a good middle ground to me.

what are the downsides to undergearing a GT6? i know a higher ratio would give me the sense of a sportier car, but can't i just downshift further before putting my pedal to the metal?

i take all kinds of opinions.

 

[SpitBang]

As you may have worked out, the 3.89:1 diff with overdrive gives the same speed per rpm when in forth overdrive as with a 3.27:1 and no overdrive, so you don't gain a lower cruising rpm with a 3.89:1 diff.

I went with a 3.63:1 when I converted my GT6, which gives a fairly relaxed 3000rpm at 70mph in top overdrive. It seems to suit me quite well.

There is a gear speed calculator you can use to see what rpm and mph you get with different diffs https://www.mintylamb.co.uk/gearspeed/

 

[pware]

Cool Site.

 

[Andrew Mace]

One problem with the 3.63:1 is that it's very uncommon in the US, so a good used differential with that ratio won't be easily found. The CW&P sets -- p/n 159802 -- are available:

US

Spitbits: $458.00 (!)
TRF: $359.95
Moss: (their p/n 676-270) $299.95

UK

Rimmer Bros.: [REMAN} $254.93 exc. VAT
Canley Classics $394.94

Of course, then ya gotta get it all set up in your differential!

All the years I drove first my 6+ and then my Mk3 GT6, both non-OD and stock 3.27:1 diffs, I never felt either that it was severely lacking in acceleration or that it was a problem at higher speeds. Roughly 3500 RPM @ 70 MPH was fine by me!

 

[TR3driver]

Where the tall gears suck is departing from stoplights and such. After that, you can just choose the gear appropriate to the speed and it doesn't matter so much. If you were happy with takeoff before, then I'd suggest looking for another 3.27 (which shouldn't be too hard to find).

And don't expect wonderful gains in fuel mileage with taller gears. It should go up some, but taller gears mean more throttle to hold the same speed, so a 10% drop in rpm does not correspond to a 10% drop in fuel consumption.

But I recall a friend of mine who installed what amounted to a 20% underdrive in his highly modified VW bug, and reported that his highway fuel consumption actually went down with the shorter gears! With the hot cam etc. the engine was actually more efficient at higher rpm.

 

[billspit]

The 3.63:1 gear set was also used in the diff of the TR7 W/4 speed tranny. I found a new one on ebay for around $100 a while back. Its now in my 80 Spitfire awaiting new carbs and reinstallation.

 

[vagt6]

It all depends on your driving habits: what percentage of your driving is done at 70-75MPH+? You need to consider this very carefully, for as we know OD ain't cheap: a rebuilt D-type unit with all the ancillaries can easily tally a couple thousand bucks unless you restore/install it yourself. A new J-type, much more.

I've installed OD in two GT6s, one D-type and one J-type, both gave excellent service. However, I'm won't do it next time because as a general rule I avoid Interstate highways while driving my LBCs. The cost/effort is not justified by my driving habits.

Plus, most LBCs feel like a go-cart Interstate at 70+MPH. When a big truck passes, I'm looking at the lug nuts on the wheels! Not a comfy feeling IMHO. Nowadays, I generally avoid Interstate like the plague in my vintage cars and motorcycle and stick to the scenic routes: it just feels natural for a car that was designed and built before Interstate highways and 70MPH speeds were around.

With a 3.89, the GT6 engine (and most other LBCs) is in it's perfect comfort zone at 55ish MPH, running cool and humming like a sewing machine. In good nick, these engines are very happy at 3,000-4,000 RPM, not stressed and rendering good fuel economy.

Again, carefully consider your driving habits before spending the time and money to install OD in your car: chances are, you may not need it!

 

[CinneaghTR]

It all depends on your driving habits: what percentage of your driving is done at 70-75MPH+? You need to consider this very carefully, for as we know OD ain't cheap: a rebuilt D-type unit with all the ancillaries can easily tally a couple thousand bucks unless you restore/install it yourself. A new J-type, much more.

I've installed OD in two GT6s, one D-type and one J-type, both gave excellent service. However, I'm won't do it next time because as a general rule I avoid Interstate highways while driving my LBCs. The cost/effort is not justified by my driving habits.

Plus, most LBCs feel like a go-cart Interstate at 70+MPH. When a big truck passes, I'm looking at the lug nuts on the wheels! Not a comfy feeling IMHO. Nowadays, I generally avoid Interstate like the plague in my vintage cars and motorcycle and stick to the scenic routes: it just feels natural for a car that was designed and built before Interstate highways and 70MPH speeds were around.

With a 3.89, the GT6 engine (and most other LBCs) is in it's perfect comfort zone at 55ish MPH, running cool and humming like a sewing machine. In good nick, these engines are very happy at 3,000-4,000 RPM, not stressed and rendering good fuel economy.

Again, carefully consider your driving habits before spending the time and money to install OD in your car: chances are, you may not need it!

Mark, those are great comments and since I am intending to convert my GT6 to OD, it provides a lot of food for thought. I find the same thing with my non-OD TR4A. I only feel I need overdrive when I find myself on a major highway. That does seem to be a lot lately since I have been venturing further away from my neighborhood roads on weekends.

 

[Andrew Mace]

The 3.63:1 gear set was also used in the diff of the TR7 W/4 speed tranny....

I knew that; honest! But I'd totally forgotten about it. Wouldn't surprise me, then, if decent used 3.63:1 diff units were readily available, considering that the TR7 still hasn't quite reached the level of collector respect. And if I'm not mistaken, one might be able to take the complete diff. unit from a TR7 axle and plug it into a GT6 case?

Regardless, I still liked my stock 3.27:1 non-OD GT6s just fine! Stoplight drag racing was never my "thing," and coming from Triumph 10s and early Heralds where 1st gear was actually not necessary when starting on a level (yeah, it actually said so in the owner's manuals, can you say "granny gear"?), it was nice to be able to run that torquey six well up in 1st gear...and then drop it into 4th!

 

[3798j]

One of the things I like about driving my non-overdrive GT6: the ideal transmission/diff ratios for most 2 lane roads. I'd be too afraid of losing/altering those with a lower (higher numerically) ratio in the differential. If the OD wouldn't be too much (tall) for the 3.27:1, I'd install one.

 

[TR3driver]

Worst case, you flip the OD off to climb hills and such.

I once drove an Audi 100LS with one side of the differential locked, giving effectively a 2:1 overdrive all the time. Very hard to drive on the street, lots of clutch slip to keep the engine running when starting from a standstill and so on. But it was pretty amazing on the freeway running 70-80 mph. Much quieter, and fuel mileage went from roughly 30 mph to around 45 mpg! Had to drop to 3rd though, to climb anything bigger than an anthill.

Note: Don't try this at home! The effort totally ruined the differential; and we abandoned the car after about 400 miles of driving that way (just enough to get home).

 

[tomshobby]

One of the things I like about driving my non-overdrive GT6: the ideal transmission/diff ratios for most 2 lane roads. I'd be too afraid of losing/altering those with a lower (higher numerically) ratio in the differential. If the OD wouldn't be too much (tall) for the 3.27:1, I'd install one.

I felt the same about the gears in my '67 GT6. But if I was to put the time and money into changing gears I would consider a stronger diff. Mine was like a shear pin in the drive line, saved the other parts. Well, except the time the tranny hatched. Other than that it was a very durable car.