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8L90 Transmission Questions

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Old 07-02-2017, 09:37 AM
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Hi guys, I'm new to the forum, and could not find anyone with similar issues.

Ive got a 17 Yukon with a 6.2 and a 8 speed auto and a single turbo set up. It's a 67 mm turbo, running 12 pounds of boost with an alky control meth kit, mild cam, forged drop in cylinder kit, dod delete and lt4 injectors and pump.

I only have 1500 miles on the Yukon, and 1000 miles on the set up. I'm having trouble with it shifting into second properly. It starts building boost at 2800 rpm and by the time you get to the end up first, it's touches boost and it's time to shift. About half of the time, it will shift into second and feels like it is hanging up. So much so, it is either binding up or pulling so much power that it pulls noticeably harder in third gear.

Is is this a "learning issue" where the trans just needs time to adapt? Could a bigger converter help me get the truck into boost earlier so the trans isn't overwhelmed with it hitting full boost then needing to shift?

Any advice would be much appreciated.
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Old 07-02-2017, 11:08 AM
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Nice build,who tuned it? what software used? post pics
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Old 07-03-2017, 07:56 PM
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It was built and tuned by Chad and Jeremy at CBI Street Cars. They used hp tuners. I think they are the only company the 17s can be tuned with.

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Old 07-03-2017, 08:05 PM
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that is a sweet ride,and done right.I think you are right ,HP might be the only ones for 017s,My understanding is you have to send the ECM to HP and they mod it so you can tune the trans.Do you know if it was done? Have they been doing trans tuning?
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Old 07-03-2017, 08:38 PM
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I do not believe they tuned the transmission. I don't know if they were aware you could send off the ecm to do that. The truck was built in February, and that may. not have been offered yet. Do others running turbo applications on the new 8 speeds retune the transmission to get them to shift better?
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Old 07-03-2017, 08:45 PM
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You pretty much have to, tuning anything on those new trucks can throw off the torque model and cause problems on both the engine and transmission side.

Being that everything 2014+ in the GEN 5 world is torque based unlike previous controllers, it's more steps to take to make sure everything is working together. It may require various settings on both the engine and transmission side to make sure the transmission knows when to shift and how quickly so it doesn't cause problems.
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Old 07-03-2017, 10:58 PM
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seems it would be acting real screwed up if it had no trans tune.what are the shift points rpms?

with your mods and boost I would expect closer to 700hp.If the trans tune doesnt seem right I would suggest to not beat on it
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Old 07-03-2017, 11:07 PM
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found this:the tmc is sent in to be unlocked,

https://theshopmag.com/news/hp-tuner...gm-eight-speed
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Old 07-03-2017, 11:55 PM
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It made 602/675 rwhp/tq a mustang dyno. I'm guessing they did do some tuning to the trans or else it wouldn't be driveable? It has been getting better which made me think it was a trans learning type of thing. Sounds like I need to see if they did any tuning so I'm not putting another trans in it before my first oil change lol
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Old 07-06-2017, 11:28 AM
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They could have left it stock. While the Gen V is all torque based the trans module just receives the delivered torque signal and reacts accordingly. That output is well within the capabilities of the 8L90. Provided they didn't scale the torque coefficients the operation should not be affected adversely enough to create massive issues. It wouldn't be optimal but definitely would still function. I believe trifecta can also write to the new TCM. That thing is awesome by the way.
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