Differential Performance going from 2.65 to 2.82 - PeachParts Mercedes-Benz Forum

Roncallo · #1 11-28-2018, 12:51 AM

Does anyone have any real number from switching only the differential from a 2.65 to a 2.82. 60' time improvement and 1/8 or 1/4 mile improvement.

Stretch · #2 11-28-2018, 04:37 AM

There are plenty online calculators that will help you estimate things like this (alternatively you could figure out a rough expectation by calculating it yourself)

Mighty190 · #3 11-28-2018, 02:06 PM

I've had a 124 and 210 both with the M104. The w210 with a 722.6 trans and 3.07 ratio diff feels much faster off the line than the 124 did with a 2.65 ratio diff. The 722.6 also has a 3.951 first rather than 3.87. So the total reductions are 12.13 for the 210 vs 10.26 for the 124 and the 210 reliably utilizes first. So a 3.07 or 3.27 could match that and I happen to have an open 3.27 ratio 185mm diff I might be trying to get rid of or you can find a LSD 3.27 in a 4 matic 124. The 210mm housing in 3.07 can be found in a 6 cylinder 210 and I also have one of those to move. The downside to going with a shorter gear is higher rpm highway cruising.

What is interesting is a domestic V8 car with a 3.73 rear end and 3.06 usually feels pretty peppy and that's a lot less total reduction.

Frank Reiner · #4 11-28-2018, 02:49 PM

Quote:

Originally Posted by Mighty190

What is interesting is a domestic V8 car with a 3.73 rear end and 3.06 usually feels pretty peppy and that's a lot less total reduction.

A lot less than what? 3.73 x 3.06 = 11.4

Mighty190 · #5 11-28-2018, 03:39 PM

Ok halfway between the two but a sizable amount less than a 722.6 with 3.07 gearing at over 12.

Roncallo · #6 11-29-2018, 08:59 PM

Quote:

Originally Posted by Stretch

There are plenty online calculators that will help you estimate things like this (alternatively you could figure out a rough expectation by calculating it yourself)

I haven't seen any online calculators that will accept different gear ratios. These are all just based on statistical data and if your running a 2.65 rear end you will certainly fall out of the range of the statistical data base that most drag racers fall into.

I have seen simulator programs for sale that will accept differential and transmission data, but they all require an engine dyno data as well.

Calculating the effect of a gear ratio change is a formidable task especially in the 60' range as acceleration is not constant and therefore requires dyno data.

Roncallo · #7 11-29-2018, 09:28 PM

So the reason I asked this question is because I changed two parameters in my V12 107 at the same time. Not a good thing to do but the diff was ready to go. I had a 2.65 open diff and it just caused massive single wheel spin with the M120 V12. I replaced that diff with the stock 107 2.47 LSD diff. Now the car is slightly slower with no wheel spin at all. Best 60' time went from 2.04 to 2.14.

So apparently the significant increase in traction was more than offset by the 7% change in gearing. I think there is more to this than just the 7% reduction in torque. Especially noting that the same stock 107 560SL did 60' in 2.24 seconds.

I believe what is happening is the M120 and M117 have nearly the same torque at low stall speed 1500RPM launches. At least nearly so to make the the M120 have only a slight advantage over the M117 in 60'. Also the 722.4 trans has a slightly lower 1st gear ratio than the 722.6. If that is the case than the 7% change in gear ratio will have a significant effect on the acceleration as in not just a 7% reduction in torque but a further reduction from engine being out of the power band. In other words not only did I loose 7% of my torque at the starting line but I also effectively decreased my stall speed in a way.

This will all be answered when I install the 2.82 LSD. 2.82 will be kind of an optimal gear ratio because it will allow me to run out 3rd gear in the 1/4 mile.

So I guess I will be the one to answer this question but it will be a while.

Stretch · #8 11-30-2018, 02:33 AM

Oh dear (obviously I didn't realise) you're trying to calculate things with too many variables for any online solution - I doubt anyone with that kind of information would give it up for free.

You're getting to the stage where you'd have to estimate the particular traction of each tyre on a particular surface - ummm - good luck!

Mighty190 · #9 11-30-2018, 01:42 PM

I didn't know you were trying to optimize this for a 1/4 mile in the v12 107.

What I would do is look at your projected trap speed and gear it around that. Generally you want to be a ready to shift (but hold the gear) right at the line. You can theoretically gear a car so short it hurts 60 foot time but that would be a challenge with Mercedes ratios.

Your trap speed doesn't change nearly as much as your 60 foot time or et from gearing. Do you can do some passes and record a trap speed and leave some headroom. What's your trap speed currently?

Also have you looked at a looser torque converter? In domestic cars a cheap way to get a loose converter is to use one from a smaller engine like a 4.3 converter on a 5.7. I'm guessing the v12 has a pretty tight converter.

Roncallo · #10 11-30-2018, 07:25 PM

Quote:

Originally Posted by Stretch

Oh dear (obviously I didn't realise) you're trying to calculate things with too many variables for any online solution - I doubt anyone with that kind of information would give it up for free.

You're getting to the stage where you'd have to estimate the particular traction of each tyre on a particular surface - ummm - good luck!

No my original post did not ask for any of this. It simply asked if anyone had any real numbers going from a 2.65 to 2.82 gear?

Roncallo · #11 11-30-2018, 07:35 PM

Quote:

Originally Posted by Mighty190

I didn't know you were trying to optimize this for a 1/4 mile in the v12 107.

What I would do is look at your projected trap speed and gear it around that. Generally you want to be a ready to shift (but hold the gear) right at the line. You can theoretically gear a car so short it hurts 60 foot time but that would be a challenge with Mercedes ratios.

Your trap speed doesn't change nearly as much as your 60 foot time or et from gearing. Do you can do some passes and record a trap speed and leave some headroom. What's your trap speed currently?

Also have you looked at a looser torque converter? In domestic cars a cheap way to get a loose converter is to use one from a smaller engine like a 4.3 converter on a 5.7. I'm guessing the v12 has a pretty tight converter.

Its kind of a balancing job. I'm in no way trying to make this car into a hot rod otherwise I would be talking 4+ gear ratios. 2.82 with stock tires will run the car at about 110 MPH at 6000 RPM in third gear. My thinking is that will be the balance point between reasonable performance and and a car I can enjoy taking cross country.

Current trap speed is about 104, which calculates out to around 4600 RPM in third with the 2.47 gears and large MT 255/50 drag radials.

Roncallo · #12 11-23-2019, 08:13 PM

Well its been a long time since I started this thread. Since no one could answer the question. Tomorrow I will answer it myself. Weathers looking good and I'm heading to Rockingham NC. Hope nothing breaks.

Started out with a and open 2.65. Last year tried the 2.47 LSD and now the 2.82 LSD.

greazzer · #13 11-24-2019, 08:36 AM

2:82 LSD 210 or 185mm ?

t walgamuth · #14 11-24-2019, 09:06 AM

It sounds like the gear you now will provide excellent performance all around. The LSD is a must for high power engines.

The only problem i found with a really big engine and the limited slip is occasionally when I enter a fast corner and let off the throttle abruptly the rear will lose traction with both wheels if I let off abruptly. When that happens the rear end can come around without any possibility of catching it.

I had this happen in my Lotus 7 replica and my Cobra replica when autocrossing. I don't corner that hard on the street.

Roncallo · #15 11-24-2019, 06:11 PM

Quote:

Originally Posted by greazzer

2:82 LSD 210 or 185mm ?

210. See this thread for how it was built.

Differential Build Tool Ideas