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Old 04-22-2020, 11:59 AM   #1
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Crazy, impossible future project idea?

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Originally Posted by JST View Post
And you can get the exhaust done so it sounds like this.

I'm still smitten by this. Thanks JST.

I wonder if I could buy an ok-ish SL600, do a 6MT conversion, and do the Zonda sound upgrade.

That could be a cool project.

How to make a MB sound like a Zonda

A 6MT SL600

That MT conversion seems to involve sourcing a 928 GTS 6-speed.

The 6MT conversion would be the most difficult if even possible as a DIY project.
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Old 04-22-2020, 01:04 PM   #2
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Quote:
Originally Posted by FC View Post
I'm still smitten by this. Thanks JST.

I wonder if I could buy an ok-ish SL600, do a 6MT conversion, and do the Zonda sound upgrade.

That could be a cool project.

How to make a MB sound like a Zonda

A 6MT SL600

That MT conversion seems to involve sourcing a 928 GTS 6-speed.

The 6MT conversion would be the most difficult if even possible as a DIY project.
Sourcing a 928GTS 6 speed sounds like the hardest and most expensive part of the whole thing.

But, yeah, that would be cool. And you have the garage for it.

You could even do a YouTube vid and maybe pay for the project.
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Old 04-22-2020, 01:27 PM   #3
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Quote:
Originally Posted by JST View Post
Sourcing a 928GTS 6 speed sounds like the hardest and most expensive part of the whole thing.

But, yeah, that would be cool. And you have the garage for it.

You could even do a YouTube vid and maybe pay for the project.
If you freeze this video at 0:17 it mentions using a 2007 BMW transmission. That guy races his SL500.

I don't know if that transmission will be cheaper, but it might be more accessible. 2007 is not that new either.

But I do have the garage for a cool project, that car has always been special to me, and I think it would be a goo dad/boys project and maybe even something cool to put in college/job applications for them.

Looks like half-decent SL600's can be had for $15-20k, so my wife did not tell me to go pound sand right off the bat.

Had not even thought of the youtube part. My youngest wants to make a video channel (mostly to post video game exploits).
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Old 04-22-2020, 04:25 PM   #4
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So, your biggest hurdle in this project is likely to be passing an OBDII scan. I don't know for a fact that this applies to MBs, but I know that OBDII BMWs in the '90s would not pass emissions with auto to manual conversions without convincing the chassis that it was supposed to be manual. (Not that hard, but the MB is a lot more complex and has a lot less support.)

If you can put historic tags on, or use an OBDI car, you bypass that problem.

I actually don't think that the physical manual conversion is going to be *that* difficult. It's just that you need access to a machinist who can make you a custom bellhousing. That's probably a better route than trying to source an extremely rare and probably expensive transmission. R129s were sold with three pedals from the factory, so you can source the pedal box and hydraulics. (It's worth noting that in the linked thread, people point out that it's actually highly unlikely that it was a 928 transmission used and speculate that it was probably a BMW unit.)

Hell, it's likely that the V8 and the V12 share bellhousings, so there might even be an off the shelf adapter out there.
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Old 04-22-2020, 05:30 PM   #5
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Quote:
Originally Posted by Nick M3 View Post
So, your biggest hurdle in this project is likely to be passing an OBDII scan. I don't know for a fact that this applies to MBs, but I know that OBDII BMWs in the '90s would not pass emissions with auto to manual conversions without convincing the chassis that it was supposed to be manual. (Not that hard, but the MB is a lot more complex and has a lot less support.)

If you can put historic tags on, or use an OBDI car, you bypass that problem.

I actually don't think that the physical manual conversion is going to be *that* difficult. It's just that you need access to a machinist who can make you a custom bellhousing. That's probably a better route than trying to source an extremely rare and probably expensive transmission. R129s were sold with three pedals from the factory, so you can source the pedal box and hydraulics. (It's worth noting that in the linked thread, people point out that it's actually highly unlikely that it was a 928 transmission used and speculate that it was probably a BMW unit.)

Hell, it's likely that the V8 and the V12 share bellhousings, so there might even be an off the shelf adapter out there.
It looks like at least some people are doing conversions on R129 V8s. Here's a shop in England that sells a kit. It's all the money but shows it's possible with the V8, at least.

How much of that is transferable to the V12 is a good question. The max continuous torque figure for that Getrag (550Nm) is below the torque peak of the M120 V12 (580Nm), but not by much...


https://www.theslshop.com/parts-shop...l-gearbox.html
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Old 04-23-2020, 07:50 AM   #6
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Quote:
Originally Posted by Nick M3 View Post
So, your biggest hurdle in this project is likely to be passing an OBDII scan. I don't know for a fact that this applies to MBs, but I know that OBDII BMWs in the '90s would not pass emissions with auto to manual conversions without convincing the chassis that it was supposed to be manual. (Not that hard, but the MB is a lot more complex and has a lot less support.)

If you can put historic tags on, or use an OBDI car, you bypass that problem.

I actually don't think that the physical manual conversion is going to be *that* difficult. It's just that you need access to a machinist who can make you a custom bellhousing. That's probably a better route than trying to source an extremely rare and probably expensive transmission. R129s were sold with three pedals from the factory, so you can source the pedal box and hydraulics. (It's worth noting that in the linked thread, people point out that it's actually highly unlikely that it was a 928 transmission used and speculate that it was probably a BMW unit.)

Hell, it's likely that the V8 and the V12 share bellhousings, so there might even be an off the shelf adapter out there.
Thanks for the input.

Not worried about designing custom parts or having them machined. That's what I do for a living and I know dozens of places that can make it. It may get expensive, but that's another issue along with getting it all to fit in the space there is.

Is the OBDI/II thing needed for the car to work, or is it an emissions-related thing in order to get plates? I assume the latter.
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Old 04-23-2020, 07:55 AM   #7
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Quote:
Originally Posted by JST View Post
It looks like at least some people are doing conversions on R129 V8s. Here's a shop in England that sells a kit. It's all the money but shows it's possible with the V8, at least.

How much of that is transferable to the V12 is a good question. The max continuous torque figure for that Getrag (550Nm) is below the torque peak of the M120 V12 (580Nm), but not by much...


https://www.theslshop.com/parts-shop...l-gearbox.html
Good find! But yeah, that's scary expensive. I imagine that thing, shipped, woudl be closer to $15k. I suspect that a repurposed BMW 6MT would be closer to $3-5k, no?
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Old 04-23-2020, 08:49 PM   #8
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Quote:
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Good find! But yeah, that's scary expensive. I imagine that thing, shipped, woudl be closer to $15k. I suspect that a repurposed BMW 6MT would be closer to $3-5k, no?
You should be able to get a suitable BMW transmission for $1-2k, plus 2-3x that to adapt it.

but when you add in the rest of the fabrication and parts required (that appears to be a complete kit), I bet that rolling your own doesn't save much, if anything.

Of course, the question is whether or not the V12 is compatible with the same transmissions. I'm not concerned about the power rating.
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Old 04-23-2020, 08:50 PM   #9
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Quote:
Originally Posted by FC View Post
Thanks for the input.

Not worried about designing custom parts or having them machined. That's what I do for a living and I know dozens of places that can make it. It may get expensive, but that's another issue along with getting it all to fit in the space there is.

Is the OBDI/II thing needed for the car to work, or is it an emissions-related thing in order to get plates? I assume the latter.
The OBDI/II is a passing emissions issue. I don't believe that it would be a car running issue.
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Old 04-23-2020, 09:17 PM   #10
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Quote:
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but when you add in the rest of the fabrication and parts required (that appears to be a complete kit), I bet that rolling your own doesn't save much, if anything.
That may be true. If that $15k-ish get you a direct bolt-on, there is something to be said for that. But then, where's the fun?
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