Calling SLK 32 Auto Owners - Mercedes Benz SLK Forum

SLK R170 General Discussion SLK 200, SLK 230K, SLK 320, SLK32 AMG

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#1 Old 07-13-2019
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Calling SLK 32 Auto Owners

Hi Guys,

Just after a bit of info about the 3.07 Diff ratio in SLK32 R170 with Auto Transmission ..........................

At 80mph when in top gear light throttle, how many RPM is your Engine doing ??

Please don't take a guess, I'd like to know a definite figure within say 50 RPM

I do know I can Calculate it, but I'd have to go look up things like Tyre Sizes, Trans Ratios etc to work it out.

My N/A V6 Car with stock diff does 3000 RPM @ 80 MPH, which is too low geared for the forthcoming V8 swap

I am torn between using a 2.82 C Class Diesel Diff, (same ratio as S500 V8 but dearer s/h and not so many about) or use the 3.07 SLK32 ratio, which is an easily available and cheaper to buy diff as it was fitted in a lot of other petrol MB's

TIA, Dave
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#2 Old 07-13-2019
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@Mn car guy
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I know not all have a 32, but you folks are the brains on this sort of thing.
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#3 Old 07-13-2019
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Got some things going on, but I will have the Toy out Tuesday and give it a shot. 😏
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#4 Old 07-14-2019
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Made an opportunity today. 2,750 at 80 with light throttle. Don't think it affects the numbers at light throttle, do have a small S/C pulley.
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#5 Old 07-14-2019
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I concur with Idiot101, 2,700-2,750 rpm is where mine likes to sit when cruising.
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#6 Old 07-15-2019
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Hi Guys,

Thanks for that, much appreciated, sounds like, (bearing in mind the R170 is significantly lighter than the W220 V8 S Class), the 3.07 isn't going to make a significant difference ..................

I say this considering that the V8 has a large Torque increase and using the 2.82 (stock V8 Ratio) will make it turn at 2500 RPM @ 80MPH, that I feel now will have a significant effect on Fuel Consumption, and I doubt it will have much, if any, effect upon acceleration ...................

I learned years ago that with powerful V8's you have got to use the Torque of the Motor, rather than revving the nuts off it as the HP / Torque is tailing off higher up, and thus it's efficiency

Light Throttle was asked for, because at this speed you can guarantee that the Converter is locked up, whereas squeeze that throttle a bit and the rpm will jump up as Converter unlocks

I suppose I need to do some timing over SS acceleration, Fuel Consumptions etc and at least try a 3.07 to be absolutely certain, I guess that will be the development phase once the V8 is in and sorted, I am a firm believer that there is no substitute for hard facts rather than "seat of the pants" dyno !!

Once again, thank you guys for that useful info

Cheers Dave
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#7 Old 07-15-2019
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my 2.82 will do ~2500 @80. The 6 speed has the same ratio in 6th as the auto in 5th.

I will tell you.. the car is a million times improved over the 3.27

I believe anything under 3.07 will achieve 60mph in 2nd gear.
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#8 Old 07-15-2019
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Quote:
Originally Posted by Turdo2 View Post
my 2.82 will do ~2500 @80. The 6 speed has the same ratio in 6th as the auto in 5th.

I will tell you.. the car is a million times improved over the 3.27

I believe anything under 3.07 will achieve 60mph in 2nd gear.
Hi,

Thanks Aaron,

I know you have done a lot of timed pulls to 60 mph, I'm guessing the V8's should be quicker 0-60 with the 2.82 as ............

1) It only shifts once

and

2) Higher diff gearing is taking a real advantage of the Torque

Does that sound correct to you ?

I know for a fact the 3.27 V6 Diff will be all but useless, I am also now leaning towards the statement that 3.07 will not really be ideal either, again due to the massive Torque, the S/C V6 SLK32 is a more revvy motor, BHP is developed higher up the rev range, but the V6 S/C has less torque, also higher up, so I can see why MB chose 3.07 for the 32K Cars ......................

I am definitely leaning heavily towards the 2.82 and save up the folding to fit it with a plate LSD

I just don't believe that 250 rpm will make enough of a difference, but the 500 rpm from 3.27 to 2.82 will do

Cheers Dave
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#9 Old 07-15-2019
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Quote:
Originally Posted by Dave2302 View Post
Hi,

Thanks Aaron,

I know you have done a lot of timed pulls to 60 mph, I'm guessing the V8's should be quicker 0-60 with the 2.82 as ............

1) It only shifts once

and

2) Higher diff gearing is taking a real advantage of the Torque

Does that sound correct to you ?

I know for a fact the 3.27 V6 Diff will be all but useless, I am also now leaning towards the statement that 3.07 will not really be ideal either, again due to the massive Torque, the S/C V6 SLK32 is a more revvy motor, BHP is developed higher up the rev range, but the V6 S/C has less torque, also higher up, so I can see why MB chose 3.07 for the 32K Cars ......................

I am definitely leaning heavily towards the 2.82 and save up the folding to fit it with a plate LSD

I just don't believe that 250 rpm will make enough of a difference, but the 500 rpm from 3.27 to 2.82 will do

Cheers Dave
I was getting really close to 5 seconds 0-60 with my 3.27.. by the time I had my 2.82 and my manual swapped in, it had become hot out and I also had different tires that didn't hang on as well. My last run (on youtube) was closer to 6 seconds. So I wont claim it will be faster but manuals are never quicker than automatics so I expected to lose a little just based on that.

I did the 2.82 for two reasons. 1) less work on the highway and 2) wouldn't have to use 3rd in autocross. so I'm happy.
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#10 Old 07-15-2019
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Hi,

Thanks again Aaron

An SLK 32 will do 0 - 60 in around 4.6 secs, the V6 Kompressor is very close to the same weight as an n/a V8 which has more torque .......................

I will be very disappointed if I don't get a worst case 4.4 secs with decent sticky 255's on the rear

The 5.0 should give me that perhaps a little better with that SDS Developer tweak I discovered in the ECU Codings to the ignition advance and premium pump fuel

While I've got it in bits to check and freshen it a little, a shift kit is definitely going in the V8 Tranny, and I've already got working paddles :wink

Finally, if needs be, a well ramped LSD will improve it significantly with a light sniff of NOs , trouble then might be breaking rear drive shafts but I have a solution for that as well if needs be

Cheers for now, Dave
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