[iqraceworks]

For those of you guys running 600+ hp through your ZF 6HP21 6-speed auto transmissions.....who did you have build or modify your transmission so that it holds up to all the added torque?

I'm starting to have some issues with my ZF6AT...and I'm only running bolt ons, probably only making 400hp/500tq. I've been wanting to add more power ..but at this point, the transmission is the weak link in the chain.

I've looked around online....and it seems like people who can rebuild or modify these ZF transmissions to hold more power are pretty few and far between. I've seen some guys on the Facebook groups that look like they build transmissions in there garage....not sure I would trust them to stand behind their work assuming it's $2,000+ to have them rebuild the transmission with stronger components (if they even do that).

For those of us who have the ZF 6HP21 transmissions.....what are our options as far as having one that will hold more power reliably? What transmission companies specialize bus high performance ZF transmissions?

Any info would be great!

[nyt]

Just about any trans shop can work on these, they're not difficult.

At those power levels, get the PD clutch kit, bushings, and seals. Send your valve body out for r&r or use the sonnax zip kit

[iqraceworks]

Originally Posted by nyt

Just about any trans shop can work on these, they're not difficult.

At those power levels, get the PD clutch kit, bushings, and seals. Send your valve body out for r&r.

Where is a good place to get those parts? I haven't had much luck finding parts for them.

And yes, lots of places can rebuild them.....but I'm not looking for a stock rebuild. With the PD clutch kit, what kind of power can they hold?

Thanks for the info!

[nyt]

Originally Posted by iqraceworks

Where is a good place to get those parts? I haven't had much luck finding parts for them.

And yes, lots of places can rebuild them.....but I'm not looking for a stock rebuild. With the PD clutch kit, what kind of power can they hold?

Thanks for the info!

PD is rating them at 850, but noting they've held more.

They also recommend a zip kit for a bit improved pressure, but rubber seals won't live as long as metal seals.

PR owns PD, so this is where you get the bits.

https://www.precisionraceworks.com/s...rchTerms=6hp21

I'm doing a build using their master kit now as well as billet intermediate/input shafts with two extra e clutches

[iqraceworks]

I have a nice used ZF 6HP21 I picked up a few months ago. I've rebuilt several GM transmissions in the past with good results....I'm going to give rebuilding this ZF 6 speed a try.

Thanks for the info.

[nyt]

Originally Posted by iqraceworks

I have a nice used ZF 6HP21 I picked up a few months ago. I've rebuilt several GM transmissions in the past with good results....I'm going to give rebuilding this ZF 6 speed a try.

Thanks for the info.

Not much to it. The schematics are online and there are some videos on youtube if you need to just watch it done. the 6hp19 is pretty much same thing if 6hp21 doesn't turn up anything.

[iqraceworks]

Originally Posted by nyt

PD is rating them at 850, but noting they've held more.

They also recommend a zip kit for a bit improved pressure, but rubber seals won't live as long as metal seals.

PR owns PD, so this is where you get the bits.

https://www.precisionraceworks.com/s...rchTerms=6hp21

I'm doing a build using their master kit now as well as billet intermediate/input shafts with two extra e clutches

Thanks for the link. Wow....those clutches and kits sure are crazy expensive! $1,000 just for the clutches.....I didn't think they would be that expensive. I guess it's a 6 speed, so there are a lot of them.

[924er]

Originally Posted by iqraceworks

Thanks for the link. Wow....those clutches and kits sure are crazy expensive! $1,000 just for the clutches.....I didn't think they would be that expensive. I guess it's a 6 speed, so there are a lot of them.

Originally Posted by iqraceworks

Thanks for the link. Wow....those clutches and kits sure are crazy expensive! $1,000 just for the clutches.....I didn't think they would be that expensive. I guess it's a 6 speed, so there are a lot of them.

How many miles are you at? I always thought these 6ATs can hold decent power around 600hp

[iqraceworks]

Originally Posted by 924er

How many miles are you at? I always thought these 6ATs can hold decent power around 600hp

That's what I thought too....but I'm having "E-clutch" faults on my transmission. It doesn't feel like it's slipping at all......but it has thrown that fault two times since I've had the car when I'm hard on the gas.....usually when shifting from 4-5. It did it once when it was stock....and once after all the bolt ons and stage 2+ tune. I've replaced all of the mechtronic seals, changed the fluid, and filter. Didn't seem to help. The only thing left is to change the shift solenoids. The car has around 100k miles on it.

[nyt]

Originally Posted by 924er

How many miles are you at? I always thought these 6ATs can hold decent power around 600hp

They stop shifting well around 550ftlbs. I was making about 650-700 and the shifts were not good. Pushing beyond that and you risk breaking input shafts.

I'll post details on rebuilding once I get all my parts in.

[nyt]

Originally Posted by iqraceworks

That's what I thought too....but I'm having "E-clutch" faults on my transmission. It doesn't feel like it's slipping at all......but it has thrown that fault two times since I've had the car when I'm hard on the gas.....usually when shifting from 4-5. It did it once when it was stock....and once after all the bolt ons and stage 2+ tune. I've replaced all of the mechtronic seals, changed the fluid, and filter. Didn't seem to help. The only thing left is to change the shift solenoids. The car has around 100k miles on it.

The E clutch is the smallest clutch pack in the car as it sites inside the A clutch. The A+E are used in 4th, and the E is used by itself in 5th. B+C+E for 6th. Make sure you're clearing adaptations after you make changes like you have. If you still have issues, PD clutch kit and bushings time.

[iqraceworks]

Every person who works on these things keeps telling me not to clear the adaptations......ever. The only time you should clear them out is when you pull the transmission apart and put new clutches in them. And then it sounds like it's a huge ordeal to go through the proper relearn process.....you need a lot of empty straight roads to do all the speeds, gears, starting/stopping you need to do.

As far as the E-clutch faults go.....I've never really felt it slip, and if I manually shift the transmission I can never get it to fault.....but if I let it shift on its own when I'm really hard on the throttle......it will go into limp mode with that gear sign on the dash, and then automatically lock the transmission into 3rd gear....it really scares the crap out of you when it does that. It's only done it twice....but that's two times too many.

Could it be a shift solenoid issue? ...or a sensor issue? I would think that if it really was slipping the E-clutch, I should be able to get it to do it when manually shifting it....when I changed the fluid it didn't smell burnt.

[nyt]

Originally Posted by iqraceworks

Every person who works on these things keeps telling me not to clear the adaptations......ever. The only time you should clear them out is when you pull the transmission apart and put new clutches in them. And then it sounds like it's a huge ordeal to go through the proper relearn process.....you need a lot of empty straight roads to do all the speeds, gears, starting/stopping you need to do.

As far as the E-clutch faults go.....I've never really felt it slip, and if I manually shift the transmission I can never get it to fault.....but if I let it shift on its own when I'm really hard on the throttle......it will go into limp mode with that gear sign on the dash, and then automatically lock the transmission into 3rd gear....it really scares the crap out of you when it does that. It's only done it twice....but that's two times too many.

Could it be a shift solenoid issue? ...or a sensor issue? I would think that if it really was slipping the E-clutch, I should be able to get it to do it when manually shifting it....when I changed the fluid it didn't smell burnt.

Changing solenoids changes pressure and requires a clear. Changing fluid can change the properties of it. If it's old cooked fluid you may have increased pressure and changed the friction properties of the clutches. Best to clear, imo.

As for the E-clutch code, your car knows it's slipping better than you do as it can compare the input revolutions to the output revolutions. It's not a sensor issue.

It's not a huge ordeal to adapt, you just drive your car lightly for a bit then slowly increase power and revs when shifting.

The issue you're seeing is when it goes 3->4 it's removing the b clutch and applying the e clutch, however it does not have enough friction to hold the shift with the E clutch, so it's letting go and kicking you back down a gear instead of roasting the discs completely. It's most likely a friction issue as opposed to a pressure issue. When going 4->5 it removes the A clutch and relies solely on the E-clutch. As mentioned, the E clutches are the smallest in the transmission. You can try changing the solenoids, installing a zip kit, and installing new seals in the lower section of the transmission, but you likely need to upgrade your clutches. If you just replace them, you'll probably end up right back where you are now.

[iqraceworks]

Thanks for the info ..... I always read that you had to go through that lenghty relearn process that ZF calls out in their manual. It's miles of driving at certain speeds, in certain gears...stopping for certain periods of time, then accelerating again up to certain speeds in each gear....for specific amount of time, and speeds in each gear...then repeat, repeat, repeat. Some people have said it's like an hour long process....and all but impossible to do unless you have an empty 2 mile stretch of road.

I guess that's not really "required"....just suggested? I've always been afraid of resetting the adaptations because of that process....and ZF says you can damage things if you don't do it right.

[nyt]

Originally Posted by iqraceworks

Thanks for the info ..... I always read that you had to go through that lenghty relearn process that ZF calls out in their manual. It's miles of driving at certain speeds, in certain gears...stopping for certain periods of time, then accelerating again up to certain speeds in each gear....for specific amount of time, and speeds in each gear...then repeat, repeat, repeat. Some people have said it's like an hour long process....and all but impossible to do unless you have an empty 2 mile stretch of road.

I guess that's not really "required"....just suggested? I've always been afraid of resetting the adaptations because of that process....and ZF says you can damage things if you don't do it right.

Just don't go mashing on the gas, gotta ease up to that point and drive around a bit.