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Old 04-29-2012, 07:12 PM   #1
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Default Pretty sure my trans is done, rebuild and 6-speed questions inside!

Back story: Blew my motor in January, picked up a low mileage SM shortblock and tossed on a complete '99 head minus TB, same transmission. Put a few miles on it around town over the past week but nothing substantial due to how ridiculously loud and smelly the car is.

Fast forward to yesterday (5/28) and after a ~40 minute incident free track session, followed by ~30 minutes of cool down, during my next outing the car sounded like the starter was hung up with the car in gear with the clutch out. It drove fine but it was making a terrible noise so I pulled into the pit to remove the starter with the plan to just push-start it the rest of the day... no dice. When we finally went to push-start it in reverse with the clutch in it made the same loud metallic grinding noise so I called it a day.

Then when we were pushing the car onto the trailer in neutral, obviously, it made the same noise... So it just progressively got weirder, first it was when it was engaged in gear, then it was without being engaged in gear to whenever mobile...?

I have yet to remove the trans because I'm a lazy slob but I guess I'm looking for worst-case scenario info:

1. Rebuilding a 5 speed, how much does it cost and would I only purchase OEM parts or...?

2. Swapping to a 6-speed, worth it for my current low WHP NA setup?

3. I don't really plan on ever making more than 220WHP with this chassis, should I be considering a 6-speed at all?
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Old 04-29-2012, 07:30 PM   #2
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It would be just plain silly to rebuild a 5-speed, you should be able to find them for <$200 a piece all day long. Make sure it's your tranny and not your diff. Is it a 1.6 differential?
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Old 04-29-2012, 07:50 PM   #3
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It would be just plain silly to rebuild a 5-speed, you should be able to find them for <$200 a piece all day long. Make sure it's your tranny and not your diff. Is it a 1.6 differential?
Car is a '94, original trans and rear (torsen) diff.

I don't really live in an area where Miata parts are plentiful and while I know 1.8 trannies can be had for relatively cheap, I don't mind spending the money on a rebuild, depending on how reasonable it would be, to know I have a properly functioning transmission that won't give me anymore headaches.

I'm not inclined to go to a junkyard and grab one that's in "unknown" condition just because it's cheap. I'd rather have the piece of mind that if I'm at an event I'm not relying on used parts of unknown origin to keep together and not ruin a very costly weekend of lapping.

If this didn't happen yesterday then I'd be trailering the car to a 2-day NASA weekend at Inde Motorsports Ranch next weekend and if the tranny went on me there I'd be out almost $500 in gas, lodging and entry fees... That's not alright with me. This is a dedicated track car that I can't afford to band-aid.
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Old 04-29-2012, 08:38 PM   #4
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I've been told by reputable sources, that the fastest naturally aspirated cars here in Cali, all use the 6 speed 4:10 combo.
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Old 04-29-2012, 11:52 PM   #5
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I've been told by reputable sources, that the fastest naturally aspirated cars here in Cali, all use the 6 speed 4:10 combo.
I don't know the first thing about gearing/ratios/etc... Are the 6-speeds actually helping them go fast or do they all just happen to have "upgraded"?
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Old 04-30-2012, 02:03 AM   #6
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If this didn't happen yesterday then I'd be trailering the car to a 2-day NASA weekend at Inde Motorsports Ranch next weekend and if the tranny went on me there I'd be out almost $500 in gas, lodging and entry fees... That's not alright with me. This is a dedicated track car that I can't afford to band-aid.
Then buy two $200 5-speeds, put one in your car, and put the other one in your truck/trailer. You'll spend ~$1000-1200 on the parts and labor necessary to rebuild a 5-speed, and while it will probably have better synchros, it will be no more reliable than a junkyard transmission would be.

FYI, I blew out reverse in a 6-speed a couple of years ago (accidentally tried to shift from 5th to reverse at ~120mph). It cost me $350 in parts (5/R shift fork, reverse gear, reverse synchro, reverse selector ring) and $350 in labor (reverse gear is on page ~90 of the 140 page transmission rebuild FSM). The transmission blew up 6 months later (stripped 4th under steady power). A more intelligent person would have bought another 6-speed and kept the sans-reverse box as a spare.

The 6-speed's ratios are a major improvement over the 5-speed ratios. The 6sp's 3/4 ratios are taller and more tightly spaced than the 5sp's 2/3 ratios, and the 6sp 6th is shorter than the 5sp 5th. The fact that it's stronger is just icing on the cake for N/A cars. You can expect to find 0.5-1.0sec by switching, which is why it's a +3pt mod for NASA TT/PT.

If you have 4.10s, I wouldn't bother switching them. For a naturally aspirated car with a stock (<7500rpm) rev limiter and less than 170whp, I would want 4.30:1 gears.
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Old 04-30-2012, 02:17 AM   #7
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Then buy two $200 5-speeds, put one in your car, and put the other one in your truck/trailer. You'll spend ~$1000-1200 on the parts and labor necessary to rebuild a 5-speed, and while it will probably have better synchros, it will be no more reliable than a junkyard transmission would be.

FYI, I blew out reverse in a 6-speed a couple of years ago (accidentally tried to shift from 5th to reverse at ~120mph). It cost me $350 in parts (5/R shift fork, reverse gear, reverse synchro, reverse selector ring) and $350 in labor (reverse gear is on page ~90 of the 140 page transmission rebuild FSM). The transmission blew up 6 months later (stripped 4th under steady power). A more intelligent person would have bought another 6-speed and kept the sans-reverse box as a spare.

The 6-speed's ratios are a major improvement over the 5-speed ratios. The 6sp's 3/4 ratios are taller and more tightly spaced than the 5sp's 2/3 ratios, and the 6sp 6th is shorter than the 5sp 5th. The fact that it's stronger is just icing on the cake for N/A cars. You can expect to find 0.5-1.0sec by switching, which is why it's a +3pt mod for NASA TT/PT.

If you have 4.10s, I wouldn't bother switching them. For a naturally aspirated car with a stock (<7500rpm) rev limiter and less than 170whp, I would want 4.30:1 gears.
Exactly what I was looking for, thanks so much man.

Only thing I'll add is that I wouldn't have to pay labor as my BFF fabricator would have handled it for either next to nothing or exactly nothing but given your cost run-down I doubt an entire 5-speed rebuild would be cheaper than an >$800 used 6-speed.

So it looks like I'm saving for a 6-speed...
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Old 04-30-2012, 02:58 AM   #8
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Quote:
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I don't know the first thing about gearing/ratios/etc... Are the 6-speeds actually helping them go fast or do they all just happen to have "upgraded"?
The blue Chikara monster runs a 6spd with a 4.888 final. And that car flies.
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Old 04-30-2012, 03:04 AM   #9
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OMG

I can't imagine a 6sp+4.8fd

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Old 04-30-2012, 07:14 AM   #10
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OMG

I can't imagine a 6sp+4.8fd

It's not a 6speed, it's a 5speed with a dogleg 1st (standing starts would be done in 2nd)
But the shorter 6 means that you need to rev a bit to get some speed out of it.
Our longest straight is nothing compared to yours and I reach 6500 with 5sp and 4.875 with stock power (cage an no top means about 500rpm penalty due to drag).
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Old 04-30-2012, 08:52 AM   #11
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1st is the trailer gear
2nd is for A1A Beach Front Avenue
3-6 is for the track
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Old 04-30-2012, 10:54 AM   #12
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1st is the trailer gear
2nd is for A1A Beach Front Avenue
3-6 is for the track
I'm in 2nd for the esses at ASR. No beaches here, buddy.
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Old 04-30-2012, 02:30 PM   #13
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In a 5 speed. He's talking about a 6spd. Until you drive one, you won't understand.
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Old 04-30-2012, 02:47 PM   #14
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I want to understand...

I tried Google to no avail, wich year/model Miatas came with the 6's? Should I budget ~$800 and hope I'm lucky enough to find one or do they go for less?
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Old 04-30-2012, 04:11 PM   #15
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In '99, the 10AE models had 6-speeds as standard equipment. The 6-speed was also an optional upgrade on all Miata's starting that same year. I don't believe the 5-speeds were ever phased out through the entire NB production run.
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Old 04-30-2012, 04:28 PM   #16
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Thanks man!
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Old 04-30-2012, 09:27 PM   #17
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Quote:
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2nd is for A1A Beach Front Avenue


If I ever end up building a custom gearset for the 6-speed, I will put a 7:1 1st gear in it so I can just idle it onto the trailer.
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Old 05-09-2012, 06:45 PM   #18
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Just drained a cup full of smelly fluid from my trans, I'm guessing they require more than that?

I vow to not get so drunk when I'm working on my car from here on out... God it's gonna be so boring.

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Old 05-09-2012, 10:20 PM   #19
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lololololol!!
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Old 05-14-2012, 10:26 AM   #20
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just put a 6 speed in this weekend, and hope it is stronger. did not realize it was plus 3 for nasa TT, oh well, I'm building the car I want, not going to nit pick points at this point.
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