Broken rear end? - FC 13B PP all motor
06-08-09, 07:35 AM
#1
Full Member
Thread Starter
Broken rear end? - FC 13B PP all motor
Are there any tricks to make the factory FC diffs last?
For some reason we keep breaking them with low power, and I'm sure people have run well into the 10's with the factory setup and a heap more HP.
With a 4.88 ratio we've managed to break a bearing cap and break the teeth on 2 crown wheels, always on the launch. Tightening the side bearing pre load to 100ft/lb didn't really help.
Car is a stock (heavy) street driven JDM TII with a 48 Weber fed PP
12.3 ET
1.6 - 1.7 60ft
ET Streets 26x10.5
Solid (plastic) engine, trans, diff mounts
TII trans
10,000rpm launch
13lb flywheel, 8.5in single clutch
I'm sure the FC diffs can take more punishment than this. Is there something I'm missing?
Do the 3rd member cases flex allowing the crown wheel to move away from the pinion? Are the solid mounts increasing the shock load?
Ideas anyone?
*edit* I just read Judge ITO's thread - 10.6 on a stock rear end. Whats going wrong with this one!?
For some reason we keep breaking them with low power, and I'm sure people have run well into the 10's with the factory setup and a heap more HP.
With a 4.88 ratio we've managed to break a bearing cap and break the teeth on 2 crown wheels, always on the launch. Tightening the side bearing pre load to 100ft/lb didn't really help.
Car is a stock (heavy) street driven JDM TII with a 48 Weber fed PP
12.3 ET
1.6 - 1.7 60ft
ET Streets 26x10.5
Solid (plastic) engine, trans, diff mounts
TII trans
10,000rpm launch
13lb flywheel, 8.5in single clutch
I'm sure the FC diffs can take more punishment than this. Is there something I'm missing?
Do the 3rd member cases flex allowing the crown wheel to move away from the pinion? Are the solid mounts increasing the shock load?
Ideas anyone?
*edit* I just read Judge ITO's thread - 10.6 on a stock rear end. Whats going wrong with this one!?
06-08-09, 12:08 PM
#3
Yeah, the only way you're going to make your stock rearend and axles last with a high rpm launch is by running higher air pressures in your slicks so that the tires spin a little or adjust the clutch so that it slips. Once the tires hook hard at that high of an rpm, the axles/rearend are toast. You can try to preload the rearend before launching too. This is harder to do without a line lock but do able. This will relieve alot of stress on the rearend. Bottom line- get a 9". It's just a matter of time before Ito's customer's rearend blows.
06-09-09, 05:17 PM
#6
Full Member
Thread Starter
Surely the rear ends are good for more than 12 seconds. If it had broken once that would be ok, but not 3 times in a row. The FC Judge Ito mentioned must be leaving the line at high rpm also, and with 400 odd hp?
Maybe it's because the 4.88 has a smaller pinion than the 4.1, resulting in less contact area on the crown wheel.
Maybe it's because the 4.88 has a smaller pinion than the 4.1, resulting in less contact area on the crown wheel.
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06-10-09, 12:30 AM
#9
Surely the rear ends are good for more than 12 seconds. If it had broken once that would be ok, but not 3 times in a row. The FC Judge Ito mentioned must be leaving the line at high rpm also, and with 400 odd hp?
Maybe it's because the 4.88 has a smaller pinion than the 4.1, resulting in less contact area on the crown wheel.
Maybe it's because the 4.88 has a smaller pinion than the 4.1, resulting in less contact area on the crown wheel.
06-10-09, 05:45 AM
#11
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06-12-09, 03:31 AM
#12
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Thread Starter
We're going to lighten it up some. If it breaks another diff then the engine/trans may get pulled and fitted to something lighter. Maybe a Starlet or something like that.
Vid of car
http://www.youtube.com/watch?v=VZGkhx29kco
Vid of car
http://www.youtube.com/watch?v=VZGkhx29kco
06-12-09, 04:09 PM
#13
We're going to lighten it up some. If it breaks another diff then the engine/trans may get pulled and fitted to something lighter. Maybe a Starlet or something like that.
Vid of car
http://www.youtube.com/watch?v=VZGkhx29kco
Vid of car
http://www.youtube.com/watch?v=VZGkhx29kco
12-08-09, 03:26 PM
#15
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Thread Starter
We changed to diff gears from 4.88 to 4.78 and they seem to be holding up just fine. The 4.78 has deeper teeth and an extra tooth on the pinion. They come out in late 90's Mazda pickup's.
12-15-09, 02:29 PM
#19
We didn't change the ring and pinion, as that could never work. We fitted the complete Mazda pickup 3rd member with a few mods. The 3rd member bolts right in, from memory the only internal mod required was the swap side gears with the stock FC ones.
Because the pickup 3rd member is shorter than the FC one, we made an angle iron brace that picked up on the factory sub-frame bolt holes. This brace was then welded to the underneath of the 3rd member.
Other than that, just a longer driveshaft is needed, and we made a urethane pinion snubber that bolted through the floor, just in case the 3rd member tried to lift on hard launches.
There's a bit of work involved but it's well worth it!!
I'll try to get pics next time the car comes in to work.
From memory, the ratios available are 3.9, 4.1, 4.3, 4.6, 4.78, 4.88, and 5.1
4.78 gears are much stronger than 4.88!!
Because the pickup 3rd member is shorter than the FC one, we made an angle iron brace that picked up on the factory sub-frame bolt holes. This brace was then welded to the underneath of the 3rd member.
Other than that, just a longer driveshaft is needed, and we made a urethane pinion snubber that bolted through the floor, just in case the 3rd member tried to lift on hard launches.
There's a bit of work involved but it's well worth it!!
I'll try to get pics next time the car comes in to work.
From memory, the ratios available are 3.9, 4.1, 4.3, 4.6, 4.78, 4.88, and 5.1
4.78 gears are much stronger than 4.88!!
12-15-09, 05:51 PM
#21
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We didn't change the ring and pinion, as that could never work. We fitted the complete Mazda pickup 3rd member with a few mods. The 3rd member bolts right in, from memory the only internal mod required was the swap side gears with the stock FC ones.
Because the pickup 3rd member is shorter than the FC one, we made an angle iron brace that picked up on the factory sub-frame bolt holes. This brace was then welded to the underneath of the 3rd member.
Other than that, just a longer driveshaft is needed, and we made a urethane pinion snubber that bolted through the floor, just in case the 3rd member tried to lift on hard launches.
There's a bit of work involved but it's well worth it!!
I'll try to get pics next time the car comes in to work.
From memory, the ratios available are 3.9, 4.1, 4.3, 4.6, 4.78, 4.88, and 5.1
4.78 gears are much stronger than 4.88!!
Because the pickup 3rd member is shorter than the FC one, we made an angle iron brace that picked up on the factory sub-frame bolt holes. This brace was then welded to the underneath of the 3rd member.
Other than that, just a longer driveshaft is needed, and we made a urethane pinion snubber that bolted through the floor, just in case the 3rd member tried to lift on hard launches.
There's a bit of work involved but it's well worth it!!
I'll try to get pics next time the car comes in to work.
From memory, the ratios available are 3.9, 4.1, 4.3, 4.6, 4.78, 4.88, and 5.1
4.78 gears are much stronger than 4.88!!
Do the halfshafts bolt in? and when you say side gears, your talking about the splines?
12-15-09, 06:48 PM
#22
Full Member
Thread Starter
3rd member = diff head for those of us in NZ/AUS. B1600, B2000, and B2200 3rd members fit.
It's probably easier to forget about changing the side gears, thats just the way we did it. It would be much easier to swap the FC carrier into the pickup 3rd member and bolt on the pickup ring gear.
If the desired ratio has a 'thick' ring gear there should be no modification required.
If the desired ratio has a 'thin' ring gear a spacer will need to be made and fitted between the carrier face and the ring gear, otherwise the carrier will end up being off set.
Half shafts will go right in.
Note: There are 2 shanked studs that hold the 3rd member in. The 2 corresponding holes in the 3rd member will need to be drilled to 8.1mm.
The 2 long bolts that bolt through the 3rd member into the housing in line with the half shafts need to be shortened so they don't push against the half shaft seals. This is because the pickup 3rd member has thinner casting where it bolts up to the housing face.
Easy
It's probably easier to forget about changing the side gears, thats just the way we did it. It would be much easier to swap the FC carrier into the pickup 3rd member and bolt on the pickup ring gear.
If the desired ratio has a 'thick' ring gear there should be no modification required.
If the desired ratio has a 'thin' ring gear a spacer will need to be made and fitted between the carrier face and the ring gear, otherwise the carrier will end up being off set.
Half shafts will go right in.
Note: There are 2 shanked studs that hold the 3rd member in. The 2 corresponding holes in the 3rd member will need to be drilled to 8.1mm.
The 2 long bolts that bolt through the 3rd member into the housing in line with the half shafts need to be shortened so they don't push against the half shaft seals. This is because the pickup 3rd member has thinner casting where it bolts up to the housing face.
Easy
