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2006 S600
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242 Posts
Discussion Starter #1 (Edited)
I need to think out loud for a bit and bounce some ideas off other heads. I'm quite keen on tuning, and so many cars have turbos these days it's easier than ever. The kind of big cars with big engines that I'm interested in usually have automatic transmission as well. They can be a bit of a bottleneck, as turbo tuning generally means more torque, and gearboxes are usually characterised by their torque capacity rather than their power capacity.

Turbo-charged engines are often boost limited by the transmission they're mated to, but that never stops people turning the boost up, and (usually) getting away with it. Of course manufacturers don't push the limits because they want to avoid the unreliability that goes with it. Nonetheless most automatics seem to have 30-50% spare capacity in them. Is there something systematic about that?

Most autos have torque convertors, and increase the input torque by 50 to 100% at the stall (full throttle and wheels stationary), and falls away rapidly as you speed up. Therefore I presume, manufacturers must make sure the rest of the transmission can take the doubled torque (give or take). Now, the input torque at the torque convertor is the rated torque of the transmission, so the gearbox itself - the epicyclic gear sets and the clutches and the brakes etc - must be good for a lot more than the rated capacity.

Therefore as long as you're not abusing the transmission by holding on the brakes to launch a quarter mile using drag radials, I think that means you normally have some margin with an auto box. These days they use lots of gears and lock-up clutches and only use the torque convertor for setting off from rest.

I know it's a bit more complicated than that, and you have to take the vehicle weight into account, but I think it means that when you're simply accelerating through the gears with the TC clutch locked-up, there's normally a decent margin in hand for tuning.

What do you think - does that make sense?

Nick
 

W220 Moderator
Joined
6,040 Posts
Hi Nick,

Talking 722.6 here ...............

Mechanically speaking I don't think you'll have a problem except if you go really high power increases.

V12 and V8 Boxes do have significantly stronger Planetary Gear Sets and extra Frictions & Steels in the Clutches / Brake Clutches.

Torque Converter is only locked at light throttle Cruise, accelerating through the Gears it will be multiplying ;)

The real problem lies in the "Electronic Torque Limiter" which is coded not only in the Engine ECU, (can be altered with SDS Developer), but also there is a Torque Limiter in the EGS 52 (TCU), which cannot be altered via SDS unless you can Decode the SCN String :(

So, look at my project, M113 V8 going into an R170 SLK V6 .............

That Car (to use it's V6 722.6) needed the Torque Value upping in the TCU and Developer won't do it, I had to find a man who could, because others who have converted V6 to V8's found that at much over 3/4 Throttle the Trans would go into limp mode !! I will be using the Stock Trans initially, so if I get any gremlins they will only be ME related.

Once I'm happy with the Motor, I will then be fitting the V8 Trans, and a different Diff, which then needs a different TCU as Ratios are different, also I can't use a stock V8 2.82 Diff (too big casings), so that TCU had to be custom coded to a W203 2.87 Diff ...............
Whilst he was in there he also added a Manual Mode. He can also alter shift points and a whole host of other stuff, does it via Team Viewer on your SDS ;)

That said, there is a tolerance involved, in other words the programmed Torque Capacity in EGS is a bit higher than Engine Output, but not high enough in my case ;) .................

As you are just tuning the existing Motor, maybe you will not exceed it ????????????????????

HTH,
 

Registered
2006 S600
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242 Posts
Discussion Starter #3
Gosh thanks Dave. Yes, just tuning the existing motor, though I was thinking of transplanting the gearbox.

When you say a V8 can cause a V6 gearbox to go into limp home mode, do you know what the DTC is? The transmission problem I've usually seen is unfeasible gear ratio, meaning the two internal speed sensors don't agree with each other. It's a sign of a clutch slipping, or much more likely a contaminated sensor, often fixed with a new conductor plate.

The interface to the gearbox itself isn't that complicated, not like an engine, and there are only so many faults that can be detected. Could it be that the TCU is seeing a torque output value from the ECU that's too high? I think this is a common driver behind having the TCU remapped when the ECU is being remapped.

Nick
 

W220 Moderator
Joined
6,040 Posts
Hi Nick,

I don't know the actual Code number, but it is a specific "Torque Limit Exceeded" Code and I do believe it is transmitted via CAN Comm from info processed by the ME such as Throttle Opening, Engine Load and a few other PIDs ;)

HTH,
 
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