how would a 4.10 FD affect 1/4 mile times?

[PorscheRacer]

The gears are $3K installed so $4k for everything installed is about right. I won't say "fair" because that is a lot of money but it is about what it costs. Some guys are dropping $3K for mufflers!

[J08M3]

it's $2,600 for diff. Then about 5 hours labor plus gear fluid. Shouldn't quite hit $4k but def over $3k

[e36jakeo]

The main reason I was considering the 4.10 final drive was that on MOST back roads you never exceed 100 MPH, and you rarely go slower than 45 mph. Thus I am in third gear all the time. The 4.10 would still allow me to always be in third gear, but would effectively pull 6.5% harder all the time. On top of that, there is a sweet spot in the torque curve around 7000 RPM that I rarely get to see (too fast) so with the shorter gearing I'd hit the sweet spot more often and be able to say "Wheeeee!" a lot more!!!

Overall I think the gearing in gears 2-4 is too tall (105 MPH in 3rd, 140 MPH in 4th!!) that the shorter gearing makes it a lot better.

One thing to consider: The DCT tranny goes 95 MPH in 3rd, 120 mph in 4th - and thus pulls better than our 6 MTs even with 4.10 . . .

[niqui]

Originally Posted by J08M3

it's $2,600 for diff. Then about 5 hours labor plus gear fluid. Shouldn't quite hit $4k but def over $3k

im talking good old canadian dollars.

[PorscheRacer]

I agree that it seems to suit the range where I drive as well. Do you have a speed in gears chart for the M3 or know where to find that info?

There are really only two mods I'd like for the M3. One is the shorter rear gear and the other is a slightly more vocal exhaust.

[e36jakeo]

Originally Posted by PorscheRacer

I agree that it seems to suit the range where I drive as well. Do you have a speed in gears chart for the M3 or know where to find that info?

There are really only two mods I'd like for the M3. One is the shorter rear gear and the other is a slightly more vocal exhaust.

this is a good analysis

http://www.m3post.com/forums/showthr...hlight=gearing

[mixja]

Originally Posted by PorscheRacer

I looked around for a gear chart to see what speed the M3 6sp reaches in each gear but couldn't find it. I could calculate it but I don't know the ratios either and the tire diameter. I'm sure there is a chart somewhere. I'd like to see where the shift points would be for a 4.11 to see if it would be faster at the speeds I drive or if the shifts would be intrusive.

Does the M3 reach top speed in 6th (with no limiter)? I'm assuming it does but if it doesn't, a shorter rear gear might mean a higher top speed. Doubtful given the torque curve of the M3 but possible in some applications where the engine power can't overcome wind resistance at top speed but could at a higher RPM.

Here are the gear change points in km/h for the stock 3.85 FD:

• 67.1
• 114.9
• 172.1
• 228.4
• 272.2
• 312.2 Max Speed

Here are the gear change points in km/h for the 4.10 FD:

• 62.97
• 107.79
• 161.42
• 214.23
• 255.36
• 292.84 Max Speed

Gear change calculations are based upon:

• 8300RPM Shift Point
• 265/40/18 Tires

The M3 can't reach 312km/h stock but would be able to reach the top speed with the 4.10FD...

[PorscheRacer]

Thanks for the table. I actually have 19" wheels but I'm guessing the overall diameter is the same.

Here are the same numbers converted to MPH:

Stock 3.85 diff MPH:
41.6
71.2
106.7
141.6
168.8
193.6

4.10 diff MPH:
39.0
66.8
100.1
132.8
158.3
181.6

[mixja]

Originally Posted by PorscheRacer

I stand by the analogy of the change being effectively the same as adding power to the engine. Remember HP = torque X RPM/5252 so power is just torque over time. An effective increase in rear wheel torque has the exact same effect as a horsepower increase at the engine with the caveats listed above having to do with shift points and increased RPM for given speed. I understand that it DOES NOT actually increase engine output.

It's a bad analogy - yes you are adding 6.5% extra torque but you are decreasing the vehicle speed range over which that torque can be applied by 6.5%. If the engine was making an extra 6.5% extra torque with the stock FD, then the car would be faster than the other car with 6.5% higher FD, because it can apply the extra torque for 6.5% greater speed range.

Anyway enough arguing about the finer points - I agree a higher FD greatly changes the dynamics of the car, and the car certainly should "feel" quicker as it will be more responsive. I'm seriously thinking about the DCT 3.62 Diff which is 15% shorter than stock...

[mixja]

Originally Posted by PorscheRacer

Thanks for the table. I actually have 19" wheels but I'm guessing the overall diameter is the same.

The 19in OEM wheel/tire combo is slightly smaller in diameter so your shift speeds will be slightly less...

[J08M3]

Originally Posted by mixja

It's a bad analogy - yes you are adding 6.5% extra torque but you are decreasing the vehicle speed range over which that torque can be applied by 6.5%. If the engine was making an extra 6.5% extra torque with the stock FD, then the car would be faster than the other car with 6.5% higher FD, because it can apply the extra torque for 6.5% greater speed range.

This is absolutely true, you are decreasing the vehicles speed range over which torque can be applied. But the point is the change in time it takes to cover that speed range. The point to changing the gears would be lowering the time. which essentially is quicker acceleration. Every car is different and the key is to find that sweet spot that gets the lowest amount of time while still having the biggest speed range possible.

[bbnks2]

Originally Posted by IND-Distribution

A shorter final drive like a 4.10 would make getting traction more difficult, as power at the ground would be improved. Technically a taller ratio like a 3.43 would improve traction, but only by hurting overall acceleration.

I personally don't follow the logic of getting taller gears to improve traction. Turbo guys do this in the US all the time. What good is having a high horsepower car if you're going to diminish acceleration by running a numerically lower gear ratio?

Turbo cars need LOAD to make boost. Taller gears = load = exhaust gas volume = spin turbo.

[roastbeef]

I'm sure after 12 years, they appreciate this info.