4.77 rear... who has experience?
As you probably know you can put in the small axle LSD, big axle LSD or Miata torsen (which needs big axles). I have done all 3 and they work fine.
I didn't read the rest of thread and it may say this there already.
Carl
I didn't read the rest of thread and it may say this there already.
Carl
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Joined: Sep 2012
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From: Denmark
just my experience in my rally sprint car i have 140 whp FC NA transmision and i started out racing last year with a 4,10 gearset but swapped it out with a 5,13 and i can tell you that is crazy good it really woke up the car i love it. So much i forgot not to rev it to high and broke the egine 
but anyways it´s a blast haha i love it
but anyways it´s a blast haha i love it
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irritating nuisance
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From: Central PA
Thanks for the concise info Carl. I'd consider the 5-speed gears as well as the torsen if I could find 'em.
sa22, good to hear... makes me even more confident the 4.77 will be fine.
Thanks again guys. Can never have too much info.
I just upgraded to the Al flywheel and stage 1 clutch/pp... It's all ready to go, I just need to bolt the tranny back in b4 I can get this diff project off the ground.
sa22, good to hear... makes me even more confident the 4.77 will be fine.
Thanks again guys. Can never have too much info.
I just upgraded to the Al flywheel and stage 1 clutch/pp... It's all ready to go, I just need to bolt the tranny back in b4 I can get this diff project off the ground.
The driveshaft harmonic happens at about 5300 driveshaft RPM. That is about 80mph with original gearing/tires. With the 4.78 and stock tire diameter, this is about 65mph. You can't really cruise at 65, just higher or lower.
The other downside is that the fast driveshaft eats the transmission's tailhousing bushing. Mazda Comp used to sell a hardened yoke and a roller bearing setup which required extensive transmission modification to use. (Weld up the OD at the end so it can take the larger bore, and reboring the housing after finding center again since welding will warp everything all to heck) but this stuff is NLA.
My car eats maybe three transmissions for every tailhousing bushing that gets wiped out, so while it is a problem for me, it's not the biggest fire that I have to put out.
As far as actual engine revs are concerned. If you have an early 12A trans, then swapping to 4.78s is like driving with the 3.91s and 4th gear. Mazda sold many thousand SAs with four speed transmissions. If you have a late 12A trans or any 13B trans, the 5th gear revs will be somewhere between your current 4th and 5th. 4.78 sounds like a HUGELY SHORT GEAR OMG but in reality it is only 22% different. It's not like going from 2.73s to 4.11s in your Chevelle.
I don't really do any city driving. I rallycross and take long highway trips.
The other downside is that the fast driveshaft eats the transmission's tailhousing bushing. Mazda Comp used to sell a hardened yoke and a roller bearing setup which required extensive transmission modification to use. (Weld up the OD at the end so it can take the larger bore, and reboring the housing after finding center again since welding will warp everything all to heck) but this stuff is NLA.
My car eats maybe three transmissions for every tailhousing bushing that gets wiped out, so while it is a problem for me, it's not the biggest fire that I have to put out.
As far as actual engine revs are concerned. If you have an early 12A trans, then swapping to 4.78s is like driving with the 3.91s and 4th gear. Mazda sold many thousand SAs with four speed transmissions. If you have a late 12A trans or any 13B trans, the 5th gear revs will be somewhere between your current 4th and 5th. 4.78 sounds like a HUGELY SHORT GEAR OMG but in reality it is only 22% different. It's not like going from 2.73s to 4.11s in your Chevelle.
I don't really do any city driving. I rallycross and take long highway trips.
I just purchased a speedsource 4.875 rear end and was wondering about the driving impression of them.
With an FC, would I have the same issues that you had with the driveshaft?
Old [Sch|F]ool
Joined: May 2001
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From: Cleveland, Ohio, USA
No, the FC driveshaft is ~6 inches shorter than the FB, so its resonance speed will be higher. This is also why the Competition Book for the 1st-gen had you putting a 2 piece driveshaft in. Shorter shafts = much lower resonance. And, since the two shafts were different lengths, they never entered a harmonic with each other either.
Plus, you don't have as many issues with driveshaft angles, since it's not a moving target with the FC. I have my pinion angle set so it drives best when the hatch is loaded with gear, because most of my long distance high speed driving involves going to or coming home from an event.
You WILL still have output bushing wear issues. Keep the trans full to overfull (fill with left side jacked up) to help mitigate this, but it will still be there as an issue.
Plus, you don't have as many issues with driveshaft angles, since it's not a moving target with the FC. I have my pinion angle set so it drives best when the hatch is loaded with gear, because most of my long distance high speed driving involves going to or coming home from an event.
You WILL still have output bushing wear issues. Keep the trans full to overfull (fill with left side jacked up) to help mitigate this, but it will still be there as an issue.
Last edited by peejay; Apr 22, 2015 at 04:46 PM.
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ancient wizard...
Joined: Sep 2014
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From: Maryland
Front Kia diff.
Unless you buy a 1st gen chunk with LSD and 4.77 gearset there's no bolt in.
You're after the Kia diff for the gearset. Couple ways you can go here. You can disassemble your diff and put LSD section into Kia housing in an attempt to not have to set up gears or you can take Kia gearset and install into your housing with your LSD. Either way,you're doing bearings,or you should. Seems you've asked these questions in a prior thread,my answers there were the same. Also believe i mentioned to strive to keep your original 3rd member with 3.9 gearset intact. You will find trying to drive on street with 4.77 any distance to be a pain.
Having the option to go back to 3.9 gearset for driving car on street is nice.
You can change a 3rd member in @ an hour after you've done it several times.I'd look for a used GSL center section,or whole rear axle and pull the chunk and transfer LSD(after rebuilding) to Kia chunk and go with that.
Unless you buy a 1st gen chunk with LSD and 4.77 gearset there's no bolt in.
You're after the Kia diff for the gearset. Couple ways you can go here. You can disassemble your diff and put LSD section into Kia housing in an attempt to not have to set up gears or you can take Kia gearset and install into your housing with your LSD. Either way,you're doing bearings,or you should. Seems you've asked these questions in a prior thread,my answers there were the same. Also believe i mentioned to strive to keep your original 3rd member with 3.9 gearset intact. You will find trying to drive on street with 4.77 any distance to be a pain.
Having the option to go back to 3.9 gearset for driving car on street is nice.
You can change a 3rd member in @ an hour after you've done it several times.I'd look for a used GSL center section,or whole rear axle and pull the chunk and transfer LSD(after rebuilding) to Kia chunk and go with that.
Joined: Mar 2001
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From: https://www2.mazda.com/en/100th/
additionally the 99-2005 Miata 5 speed is a 4.3, the 99-2002 can be a Torsen, and i think after that they switched to something less premium. the 1994-1997 miatas are 4.1
the 4x4 B series trucks from the 80's are 4.44 front diffs as well.
and Kia sells the ring and pinion fairly inexpensively new (~$300, Mazda is $500)
the Rx8 is 4.4 and 4.7, but uses the bigger turbo diff, so its not compatible
the 4x4 B series trucks from the 80's are 4.44 front diffs as well.
and Kia sells the ring and pinion fairly inexpensively new (~$300, Mazda is $500)
the Rx8 is 4.4 and 4.7, but uses the bigger turbo diff, so its not compatible
Thanks guys, I know I asked the questions before but when I searched for those instructions a few threads came up suggesting the rear was a bolt in affair as well, as did pics of someone just bolting in the Kia front.
There is a pool of gear oil under my car as it sits for the winter, so it's all coming apart in the next month or two, I just need part numbers for everything I need to order or source from junk yards. Plan is to build up my spare FB trans with a miata gear set and I was hoping I could have two LSD 3rd members that I could swap between, but sounds like I need to just go all in with the gear change.
I'm aware of the highway cruising issue, I did a few drives to work in 4th gear...I survived.
There is a pool of gear oil under my car as it sits for the winter, so it's all coming apart in the next month or two, I just need part numbers for everything I need to order or source from junk yards. Plan is to build up my spare FB trans with a miata gear set and I was hoping I could have two LSD 3rd members that I could swap between, but sounds like I need to just go all in with the gear change.
I'm aware of the highway cruising issue, I did a few drives to work in 4th gear...I survived.
ancient wizard...
Joined: Sep 2014
Posts: 2,335
Likes: 262
From: Maryland
Be aware that the higher drive ratio #4.77,the faster the driveshaft turns and this accelerates the wear on out put bushing in tailshaft of transmission.
Putting a new bushing in tailshaft should be part of any higher# gearset and consider having driveshaft balanced.
You’ll want to deliberately overfill transmission to be sure enough oil is getting to output shaft bushing area to lubricate it.
Mazda used a roller bearing conversion to combat wear issues during endurance races.
Had occasion to see one of these modified boxes.
Looked like they cut off whole rear section and welded on another housing that contained bearings. At one time I think you could order this kit from Mazda,it was a long time ago.
Putting a new bushing in tailshaft should be part of any higher# gearset and consider having driveshaft balanced.
You’ll want to deliberately overfill transmission to be sure enough oil is getting to output shaft bushing area to lubricate it.
Mazda used a roller bearing conversion to combat wear issues during endurance races.
Had occasion to see one of these modified boxes.
Looked like they cut off whole rear section and welded on another housing that contained bearings. At one time I think you could order this kit from Mazda,it was a long time ago.
Thanks guys, I know I asked the questions before but when I searched for those instructions a few threads came up suggesting the rear was a bolt in affair as well, as did pics of someone just bolting in the Kia front.
There is a pool of gear oil under my car as it sits for the winter, so it's all coming apart in the next month or two, I just need part numbers for everything I need to order or source from junk yards. Plan is to build up my spare FB trans with a miata gear set and I was hoping I could have two LSD 3rd members that I could swap between, but sounds like I need to just go all in with the gear change.
I'm aware of the highway cruising issue, I did a few drives to work in 4th gear...I survived.
There is a pool of gear oil under my car as it sits for the winter, so it's all coming apart in the next month or two, I just need part numbers for everything I need to order or source from junk yards. Plan is to build up my spare FB trans with a miata gear set and I was hoping I could have two LSD 3rd members that I could swap between, but sounds like I need to just go all in with the gear change.
I'm aware of the highway cruising issue, I did a few drives to work in 4th gear...I survived.
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