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Discussion Starter #1
The diff ratio on my SL500 is very long. It shifts from 1st to 2nd gear at about 80km/h. I was wondering if I could replace it with an SL 320 diff to get better acceleration. Will this work?
Would the SL 320 diff handle the torque of the bigger engine and how would I adjust the speedometer to display the correct speed?
 

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The diff ratio on my SL500 is very long. It shifts from 1st to 2nd gear at about 80km/h. I was wondering if I could replace it with an SL 320 diff to get better acceleration. Will this work?
Would the SL 320 diff handle the torque of the bigger engine and how would I adjust the speedometer to display the correct speed?
Something not right with those figures. Surely your engine is screaming it's nuts off at 80km/h in first? Your box should have changed gear at a much lower speed. IMHO you should be looking at the autobox not the diff, but I'm not an expert in transmission matters. What happens if you select W (winter) on the gear selector and therefore move off in 2nd?
 

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Try this:

1. Turn the ignition key to the on (not start) position.

2. Press the gas pedal to the floor and hold for five seconds.

3. Turn the key to the "off" position (don't remove the key), then release the gas pedal.

4. Wait at least two minutes for ECU to reset.

Doing this resets the ecu for the automatic transmission back to it’s factory settings.
 

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I have thought about this myself but I've come the conclusion that it won't add any performance if your car is otherwise stock.

The gear ratios chosen by Mercedes are designed to accelerate the car as fast as possible without wheel spin (or minimal wheel spin). You put a lower ratio in the diff and you will actually be slower accelerating because the traction control will throttle you back until grip is regained. Keep in mind the SL is not a light car and needs a lot of traction to accelerate from a stop with a torquey V8 engine.

The ONLY way lower gearing would help is if you did extensive engine modifications that moved the powerband of the engine from the low and mid ranges (idle to 4000 rpm) to the high end ranges (4000+). The lower gearing would allow the engine to rev faster. Since the low end torque is reduced from the engine in the low speed ranges traction is not an issue even with the lower ratio.
 

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Discussion Starter #5
Sorry I ment it reaches that speed when its flat out and kick down engaged. For normal use it shifts early
 

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I have thought about this myself but I've come the conclusion that it won't add any performance if your car is otherwise stock.

The gear ratios chosen by Mercedes are designed to accelerate the car as fast as possible without wheel spin (or minimal wheel spin). You put a lower ratio in the diff and you will actually be slower accelerating because the traction control will throttle you back until grip is regained. Keep in mind the SL is not a light car and needs a lot of traction to accelerate from a stop with a torquey V8 engine.

The ONLY way lower gearing would help is if you did extensive engine modifications that moved the powerband of the engine from the low and mid ranges (idle to 4000 rpm) to the high end ranges (4000+). The lower gearing would allow the engine to rev faster. Since the low end torque is reduced from the engine in the low speed ranges traction is not an issue even with the lower ratio.
Thanks for the information, but would this apply for the AMG wheels too? I noticed the wider tires add a lot more grip and ESP light never comes on in dry weather meaning rear wheels don't slip. The differential was originally designed for narrower tires so wont changing the ratio help?
 

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I thought the differentials were the same for the 6 cylinders?
Even if they are a slightly lower gear ratio, it sounds like a lot of work for very little gain.
 

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The gear ratios chosen by Mercedes are designed to accelerate the car as fast as possible without wheel spin (or minimal wheel spin).
That's not true - there's a number of factors considered including most notably fuel economy. The SL500 is a 2.65 ratio and you could in fact improve acceleration with an SL320 rear axle. SL320 ratios bounced around from 3.45 to 3.70 which would be a big difference, in both acceleration and fuel economy. Some 140s would fit too. You need to get access to the EPC and check out part numbers and ratios for the various models.
 

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Wow, I didn't know that the 500 had such a high ratio.
I would think that changing the ratio that much would play games with the computer, though?
 

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Discussion Starter #11
I don't think it will have any effect on the TCU because it doesn't take any input from the wheel speed sensors. It will change the reading on the speedometer because the speed sensor is located on the transmission. I'm not sure how I would change that.
The acceleration increase should be significant tho. Probably 0-100 should improve by 0.5 - 1sec.

This vid is a good comparison:
 

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I would think that traction would still trump any hoped for gain.

You could put a 7.00 gearset in the diff and if the car broke traction, the little yellow triangle with the exclaimation point in the dash would light (throttle reduced and rear brakes applied AUTOMATICALLY to offending rear wheel, game over). Mileage in the SL500 and 600 is marginal at best to begin with and putting a higher ratio gearset out back would only make it worse. Unless you are talking about a drag car and then you'd probably want to disable the traction control (actually all the electronic engine/traction governors really), remove excess weight (4400 lbs, get cracking porky) and get some slicks.

If you want to accelerate faster in an SL, get better tires.

I have an SL600 with the 18 inch AMG rims with AA rated traction and temperature tires AND the rearend can EASILY be made to lose traction. Putting in a higher ratio gearset is not the answer given the traction controls and lack of LSD in these cars.
 

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I don't think it will have any effect on the TCU because it doesn't take any input from the wheel speed sensors. It will change the reading on the speedometer because the speed sensor is located on the transmission. I'm not sure how I would change that.
The acceleration increase should be significant tho. Probably 0-100 should improve by 0.5 - 1sec.

This vid is a good comparison:
M3 CSL (3,64 diff) vs M3 E46 (3,91 diff) - YouTube
The TCU does take inputs from the wheel speed sensors over the CAN bus. It will fault if you change gear ratios. I found this out the hard way.
 

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Hi John, thank you for re-confirming the above. The question is, what are the options for resolving this issue? :confused:
I was unable to find anyone who could reprogram the TCU. You say Kleemann can do it. I wish I would have known that. I ended up modifying my subframe to install the SL600 129 rear. If you find another car of about the same weight with the desired ratio you could possibly use that TCU.

Only other option is the DIY TCU by PCS

PCS - Powertrain Control Solutions - Home
 

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Oh ok. Then I guess its not worth it. Thanks for your help tho everyone
Keep in mind that it's your model that's pertinent, not someone else's. The donor car would have to have ASR, which is rare on 320s. Worth it? It was never worth it under any circumstances. It was what you wanted to do.
 
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