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Old 02-20-2015, 11:02 PM   #21
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Quote:
Originally Posted by guttedmiata View Post
Load changes with rearend gearing as well. As soon as we dropped from 4.10s to 3.63s, the 5spd broke.
I hope this is the case. I'm putting in a 4.778.
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Old 02-20-2015, 11:27 PM   #22
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I hope this is the case. I'm putting in a 4.778.
I should note, that's 2nd gear in autocross. You'll still be loaded in high gear.
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Old 02-20-2015, 11:46 PM   #23
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I should note, that's 2nd gear in autocross. You'll still be loaded in high gear.
4.778 will be used with a Quaife wide 5speed and a 8600 rpm rev limit for atocross use of second gear. 3.909 will be used on tracks.
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Old 02-21-2015, 02:01 AM   #24
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That should help you, Bob.

The one thing that hasn't been mentioned so far is the grip the car is running. A 300wtq miata on star specs is a very different beast than a 300wtq miata on 275 hoosiers on 11's. The latter case is much more likely imo to shatter a gear than the first. The engine is relentless, and the grip is relentless, and the transmission gives up in the middle.
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Old 02-21-2015, 02:41 AM   #25
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What are the thoughts on gearbox temp vs life in high torque applications? Has anyone who's broken a transmission installed a temp gauge to see what the oil temps were like?
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Old 02-21-2015, 02:58 AM   #26
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Originally Posted by Mobius View Post
That should help you, Bob.

The one thing that hasn't been mentioned so far is the grip the car is running. A 300wtq miata on star specs is a very different beast than a 300wtq miata on 275 hoosiers on 11's. The latter case is much more likely imo to shatter a gear than the first. The engine is relentless, and the grip is relentless, and the transmission gives up in the middle.
Transmission life went down big time when I switched from 225 Nittos on 9"to 245 Hoosiers on 10" for the track. I've only broken second gear once in the 6 speeds. It is usually 4th that goes. 5 speeds will break 2nd 3rd or the input gear that takes out everything but 4th at about equal probability.

I have also learned recently after taking a bunch of transmissions apart the NB 5 speed has wider gears throughout than the two versions in the NA particularly noticeable on the input gear. An NB 5 speed is most defiantly going to be a bit stronger than one from an NA. The 6 speeds have much thicker gears than the 5 speed as well but the 5 speed has 3 bearings per shaft instead of 2 which I think might be a design advantage to resist shaft flex. 6 speed was ~12 lbs heavier than the 5 speed Quaife with NB nose, bellhousing and NA tailstock I put together most of that weight I think is in the shafts and gears.
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Old 02-21-2015, 03:02 AM   #27
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@Pat

The short answer is: Higher temps break down fluid which affects its ability to maintain surface film, and regardless of TQ numbers this will accelerate wear on the tooth. Higher TQ through the gearbox will increase that wear accordingly.
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Old 02-21-2015, 03:03 AM   #28
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Bob destroys them in autocross, I don't think there's time for the gearbox to get very hot. I could be wrong.
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Old 02-21-2015, 03:04 AM   #29
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Ok so based on that, would a good transmission cooler and keeping high quality fluid in it improve reliability in high torque applications? I'm considering installing a trans cooler on my 6 speed.
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Old 02-21-2015, 03:06 AM   #30
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If you are tracking, I would think yes. There's definitely heat rejection advantages for track. For autocross, I can't recall any threads on this board with instrumented readings of trans temp, but given the relationship of other known cooling mods, autocross heat <<<< track heat.
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Old 02-21-2015, 03:09 AM   #31
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Bob is breaking trannies because his car is overcoming the physical strength of the gears. The teeth are just not strong enough to hold the power.

A trans cooler will help as long as the engine is not going to be a monster like Bob's. Keeping the fluid at an optimal operating temp is always a good thing. Not only does the cooler keep the fluid cool, but keeps the entire trans colder, helping with any wear/breakage problems related to heat induced fatigue of the gearset and case.
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Old 02-21-2015, 03:38 AM   #32
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I don't know for sure what my trans temps have been but I have never seen burnt trans fluid. always looks clean except for broken off gear teeth.

Last edited by bbundy; 02-21-2015 at 03:55 AM.
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Old 02-21-2015, 03:40 AM   #33
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IIRC Bob breaks trannies at autocrosses, where tranny temps should be a non-issue. I used to think that a fluid cooler would help improve shaft alignment by keeping the bearings cooler, but I think breaking trannies at an autocross debunks that pretty soundly.
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Old 02-21-2015, 01:19 PM   #34
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When there's enough power to overwhelm the gearset in short courses like an auto-x where no heat is generated then there's no cooler that will help. But in a lower TQ car (not enough TQ to shear teeth at will), I think a cooler would be beneficial if it is driven hard on track where heat generation will be a factor based on the points I mentioned above.

That being said, I don't personally have a track-dedicated Miata, let alone a small fleet of them, nor do we have any Miata customers at this time. If Andrew or Emilio or any of the other serious track guys are not seeing any benefit from a cooler then that's got to be considered as a hard data point.
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Old 02-21-2015, 03:02 PM   #35
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Bob has also broken them in straight line under load on the track. So not even on hard starts. Last one I read about was 4th sheared off on the back straight of PIR or pacific raceway.
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Old 02-21-2015, 04:17 PM   #36
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Yes. I think it's been established that Bob's engine will destroy gearboxes regardless of heat or shock load. He has insane grip and tons of torque.

Some have grenaded gearboxes on the track at considerably lower torque load, no? I would think that a trans cooler could really help a 6-speed live behind, say, 275 wtq instead of Bob's 300+.

Total conjecture, though, as Matthew said.
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Old 02-21-2015, 05:16 PM   #37
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Bob has also broken them in straight line under load on the track. So not even on hard starts. Last one I read about was 4th sheared off on the back straight of PIR or pacific raceway.
In current level of grip form a 6 speed has not lasted for more than two days at PIR. 4th gear goes on the back straight out of a corner taken in 4th gear so no shift is involved. I do hold the overall Miata lap record around the track by a good bit however and still holding on to the TT2 lap record. Nobody even got my Pacific raceway lap record even after repaving turn 8 last year helped most people drop about 2 seconds.
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Old 02-21-2015, 09:18 PM   #38
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I also don't run full boost on the track I limit it to right around 320-325 hp ~ 285-295 ft-lbs though.
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Old 02-21-2015, 10:55 PM   #39
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Have you broken a 6spd autocrossing?
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Old 02-21-2015, 11:39 PM   #40
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Originally Posted by guttedmiata View Post
Have you broken a 6spd autocrossing?
Just one.
Broke second gear headed for the finish shoot. I run full boost autocrossing but if I datalog a run I never seem to see full load. Can't load the engine that much in second gear its either accelerating too quickly through the RPM range or spinning the tires. I mainly crank up the manual boost controller cause it seems to build boost quicker at lower rpm when I do and it feels more responsive to throttle inputs. I might be able to do better with electronic boost control but I have never gotten one good enough to be consistant and smooth and repeatable like the manual one. on the track I will see sustained periods of full load through several gears through powerband rev range. Autocrossing is a spastic looking thing by comparison.

Last edited by bbundy; 02-22-2015 at 12:08 AM.
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