choi000

Not too sure if its true but with a higher final drive ratio, your top speed will be reduced. One of my buddy had an E36 M3 with a dinan diff and his car accelerated as if it had a supercharger in it, but he couldn't go faster than 120mph.

pennetta

Think of it as if you were riding a bicycle. You are producing the same power as the rider but in first gear ( larger sprocket at the rear wheel) you can accelerate faster but will have less top end speed than Sixth gear (smaller sprocket at the rear wheel). Lower gears (higher numerically) give you a better mechanical advantage.

hollywoodtrash

you are the man

znod

You are very welcome. I was in a hurry when I replied. So, if you need clarification of anything I said, then please post back.

vegastrashed

Znod,
Can you confirm with what pennetta and choi000 said about less top end speed???? Not that I would actually ever have the need to approach 152mph

UUronL

That's technically true, but it might not really hurt anything if the car is speed limited and not rev-limited to enforce the 155mph "limiter". I suspect the cars can do quite a bit more than their limited speeds, so you might still have enough leg room (gearing) to hit 155 if the car is only limiting based on speed and not a certain RPM in 5th or 6th. The 530i Automatics have a final drive even higher than what you're going to put in your car (3.64:1), and it is still capable of reaching the 155mph limited top speed. In old muscle car terms, gearing for acceleration is generally considered to be over 4:1 ratio, so we're still solidly in the range I'd consider "normal".

I found some formulas... I'll post more later.

ImolaRedM

I don't mind paying a premium for good stuff but $1999 for the Dinan Diff seems a little expensive. Anyone know what an OE BMW diff runs? I can't seem to find it on RealOEM. And is the stock E60 M5 diff an LSD?

Formulas on how the gearing will change things -- check this page out...

http://www.koalamotorsport.com/diffs.asp

UUronL

Yeah, I found some forumlas too. My results for a few gears. Looks like there would be enough gearing in 6th for the top speed, but BMW may limit RPM per gear and not simply on speed.


550i "standard" gearing (non-automatic) with 2.93:1 ratio


1st gear @ 6500rpm = 40.35mph

5th gear @ 6500rpm = 163.83mph


550i "standard' gearing (non-automatic) with 3.45:1 ratio


1st gear @ 6500rpm = 34.27mph

5th gear @ 6500rpm = 139.13mph

6th gear @ 6500rpm = 159.93mph



I used Car and Driver's 530i "MPH in each gear at 6500rpm" data to find the circumference of the 18" tires (the only missing variable). I used that figure (78 inches) in my calculations for the 550i based upon published gear ratios for the gears I have results for.

znod

A stock manual/SMG would hit about 156 mph in 5th gear at 6k RPM. With the 3.45 rear, it would hit 132.5 = (156)2.93/3.45 in 5th at 6k. If it could actually turn 6.5k in 5th, it would reach only 143.5 = 132.5(6.5/6). So, to reach 156, it would need to be in 6th. 6th is a much more viable gear for top end with a lower rear-end ratio. Calculationally, but unrealistically, the car could hit about 179 in 6th gear at 6k RPM. So to get to 156, it would have to turn only about 5.229k RMP = 6k(156/179) to get to 156--which should be totally easy to do.

The final drive ratios for the 2.93 car and the 3.45 car, respectively, are about 2.55 and 3--yielding a ratio of about 1.177 = 3/2.55. It's hard to know how exactly much the cars would be affected by their differences in rear-wheel torque moving towards their top speeds, but the 2.93 would be limited, relatively, in terms of RPM because of its lesser rear-wheel torque. Let's assume it is limited by 10% and that the 3.45 car can turn 6k RMP in 6th. Then, the theoretical maximum top speed of this car is about 179 MPH 156(6/5.229), while the corresponding value for the 2.93 car is about 189 MPH = .9(179(1.177)). The cars would be equal in top speed, under the 6k assumption for the 3.45 car if the 2.93 car's RPM loss was about 85%. That is, 179 = about .85(179(1.177)). If we continue along these lines, one can easily see that it is quite possible for a car with a numerically lower rear end to have a higher top speed than one with a numerically higher rear end--all other things equal except the relative RPM losses associated with top-speed attempts.

UUronL

The formula I was using was:



Speed = RPM x circumference x 60
________________________

gear x final drive x 63371.4 (last number is correction factor for inches to miles)


Given the 78 inch circumference I calculated based upon known BMW 18" equipped data, I feel my numbers are correct.



EDIT - I'll show my work...


6500 x 78 x 60
_____________
4.06 x 3.45 x 63371.4 (4.06 is 1st gear)



6500 x 78 x 60
_____________
1.00 x 3.45 x 63371.4 (1.00 is 5th gear)



6500 x 78 x 60
_____________
.87 x 3.45 x 63371.4 (.87 is 6th gear)