Engine and Transmission Tech

Ive got a 3.00 axle ratio I’m running now which came with the car ( 74.5 MGBGT V8). It has a PowerTrax LSD installed as well. I’m assuming the builder (deceased) liked some top end and maybe better fuel economy. I’m not concerned about cruising down the interstate as much as feeling the acceleration at take off. So, I’m thinking about swapping he 3.00 gear and pinion out for a 3.73 or similar ratio. The issue might be that the gearing of the LT77 5 speed may make the lower gears spool up too quick. I know I’ve seen that this transmission when used by Rover used a 310 gear or something close. Ive looked at the spreadsheet program on here and obviously the change would raise my RPM and highway driving will be noisier; I don’t really take the car on those roads, mostly on the local mountain roads. The car has a stroked 215 with a few go fast upgrades but not too crazy. Any suggestions or advise would be appreciated.



Edited 1 time(s). Last edit at 09/15/2022 08:32PM by Gswest236.

British Leyland used this gearbox in several cars. Rover SD1 versions had 2.84:1, 3.08:1, 3.45:1 and 3.9:1 gears, depending on the engine. All V8´s had 3.08:1 ratio and Vitesse V8´s had 2.84:1.

LT77 has a 3.32:1 1st gear ratio and combined with 3.73 may be more or less useless. Good for very low speed crawling, though. Again, more revs all the time and the engine will wear faster. Which rear axle do you have, Ford 8" ? I might consider something around 3.3 - 3.5:1 but that is of course a question of individual preferenses.

I used to have LT77 behind a healthy Rover V8 in my Morris Minor. With all the mods the car is around 2100 lbs. (pretty close to a MGB GT) Rear axle is a narrowed TR7 axle with 3.08 gears and Quaife ATB. I have run the car at 1/4 mile and never really liked the ratios for that use. T5 ratios would be better but that is quite an expensive option over here. So, I looked at other options available and chose a ZF 6-speed from a BMW 1-series diesel. Not a direct swap by any means but a local company can mate BMW box to just about any engine with a matching bellhousing. Total cost including the uprated clutch was around 1000 USD.

Now, since this box is from a diesel model it has a really useless 5:1 1st gear but 2nd to 6th gear ratios are pretty spot on. Very shooth gear change and will not break behind a Rover V8. I do have a set of 2.84 gears and most likely will fit those. I am close to 60 years of age and (sadly) have to confess that I don´t like high screaming engines anymore:-)

We need to know the make of your rear axle. The ratio sounds like 8" Ford. MGB had 3.91 stock and 3.07 and 3.31 aftermarket ( same ratio's as MGC ), Automatic cars had 3.70 which is too close to 3.91. Have you thought of changing the transmission to a T5 with different ratio's?

I think I've offered my experience before, but I'll throw it out.
I have a Ford 5.0 (302) with some mild modifications. I have a Mustang T5 with
the 3.35 1st gear and a Nissan R200 rear with 3.54 ratio. The combination is 11.859,
and for me it is a little too low. My TR6 is 2510# without me, and a start from a stop
seems to require revving a little high in 1st to keep up with traffic. I would have
preferred to have a 2.95 1st gear, giving a total ratio of 10.443. Your 3.0 rear ratio
with the 3.32 1st gear in the transmission gives a 9.96 total. Maybe this isn't low enough for your
install, as my Ford is a little larger on displacement, but maybe let that be a "wait and see" deal.
Dennis

Thanks guys. It has a Ford 8 inch rear end. So, It sounds like maybe a 3.55 axle gear ratio might be as high as I should go to keep the revs down. First gear isn’t that useful as it sits, so that will get worse but I’m willing to compromise for some low end grunt. The car has good power, just not off the line. If the LT77 fails, a T5 would be the replacement. As it is now the 2nd gear synchro is starting to go, but otherwise its fine.
Appreciate the input!

Since you 5th gear is either a .79 or .83, I would recommend using no more than a 3.31 - 3.42 rear gear. Your 3.32 1st gear will still be usable & still give you the takeoff acceleration you want.

Thanks Carl,
Of course I have no idea which 5th gear I have, but your suggestion is appreciated. I hope the effort of changing out the gears proves worth it.
Sometimes its more about the journey than the destination. So, I’m ordering a ring and pinion set.

Getting ready to pull the axles and remove the 3rd member. We got the 3.40 ring and pinion kit (Motive Gear) and the installation kit.
Since I have no experience with a ford 8 inch rear end or any others to be honest, there is a learning curve going on here. First off, all the ford 8 inch pictures i see online don’t look like my axle to wheel flange. The nut at the end doesn’t look like the ford wheel flanges I see online. It looks like a MGB flange; 4 lug with a retaining nut. The fords all don’t have the nut as far as I can see online. Is the axle shaft not a ford?? Or modified maybe. I’m sure someone here knows what this is. Hoping for that guy who knows these axles to school me.

Scott, You may have a Hybrid. Ford 8" with MGB hubs. Glenn Towery used to make those. 8" is banjo type where the center section comes out the front. No separate rear cover. Do you have a picture?

First Ford 8 inch/late MGB hybrid that I ran across was Dale Spooner's.

[www.britishv8.org]

Jim Stuart also used one.

[www.britishv8.org]

The MGB vintage racers use a Banjo MGB/Salisbury MGB hybrid.

Yes, it is the type that it all comes out the front, no rear cover. Thanks for the help! So, its a hybrid. Does that mean the axle shafts are modified?




Edited 1 time(s). Last edit at 09/15/2022 11:19AM by Gswest236.

They cut off the flange and splined to fit MGB hub, The housing ends and bearings are MGB also.



Edited 3 time(s). Last edit at 09/15/2022 12:33PM by mgb260.

This video may help you:

[www.youtube.com]

OOPS, I got the install kit for the Ford 8 inch, We’ll see if any of it is compatible. The bearings and races probably are fine, but may have to be changed to do the ring and pinion swap. Like I said, learning curve! I’ll post some updates as I go. Thanks for the help guys!

You have the right kit for the center section. The outer axle ends are MGB.

Awesome, thanks.



Edited 1 time(s). Last edit at 09/15/2022 08:34PM by Gswest236.

Got the drive axles out today. Seems the Ford axles can come out without removing the brake components. Not the case with these as the backing plate is between the flange and the hub.?? Axles look good and will get the drive shaft dropped and get on to the third member. Greasy mess!

Got the diff out today. Deciding wether I want to change out the ring and pinion or let a pro do it.

I have always let the professionals do that for me. They have the tools and experience, yes they are expensive, but they have to earn a living too.

A twelve bolt Chevy or Ford 8.* is more difficult. An 8 or 9 inch Ford is easier & doable. I would give it a shot myself.

Disclaimer: I had the gears swapped in my 12 bolt Chevy in my 68 Camaro by a Chevy mechanic that did it at his house.... for $100. ;) Been a bunch of years, though.