[tuco44]

Originally Posted by Lotus7

No.

The 245/45-19 and 275/40-19 PZeros have a spec of 754 revs/mile as opposed to 751 revs/mile for the 245/50-18's that I used for the initial calculation. the difference means the 19 inch tires will be turning 0.3% faster (2129 RPM vs. 2123 RPM at 75 mph) an insignificant difference that's less then the difference between a new tire and a slightly worn one.

OK, thanks.

[Lotus7]

Originally Posted by kozzi

Is this the case for both the 28i and the 35i...149 mph with SAP or MS? What if they were not limited? ...

Don't know if the SAP removes the 130 limit on the 28i. The BMW specs don't say either way.
If the artificial limiter is removed, the 28i should easily exceed 130 mph, but probably won’t quite reach 150.

The ungoverned speed is a difficult calculation to do correctly since the force needed to move the car is exponentially related to the air drag. Doubling the speed requires exactly four times the forward thrust or horsepower, and to calculate the real drag, the coefficient of drag and the frontal area must be known with a very high degree of accuracy,

The 35i’s N55 engine has been independently measured on several dynos (see the DynoJet chart below), and the results published. On 91-octane fuel, the peak hp at the wheels is typically around 280-284. Given the exceptionally efficient driveline of the X3, the power transmission loss is probably around 11 to 12 percent, which means that the engine is actually putting out around 314 hp at the flywheel. The measured hp peaks at 5800 to 6200 rpm.

Given the X3’s transmission and final drive gearing, and using the published drag coefficient and a rough estimate of the frontal area, it looks like the car cannot possibly reach full engine power in 8th gear and that it’s a tiny bit over-geared for max speed in 6th gear. The engine torque drops off too quickly above 5000 rpm. That leaves 7th as the best choice for achieving a drag limited top speed. There’s too much drag to reach the maximum hp peak in 7th (6100 rpm). My "quick and dirty” gestimate calculation given the X3 35i’s aerodynamics gives a (ungoverned) result of 162.6 mph (261.7 kph) in 7th gear at 5800 rpm.

That’s my story and I’m sticking to it!

[kozzi]

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Good story indeed and a great read...thank you very much.
Makes me wonder why BMW would even bother to limit the F25 in the first place and that the 8 speed tranny is just to help save fuel.

[Lotus7]

Originally Posted by kozzi

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Good story indeed and a great read...thank you very much.
Makes me wonder why BMW would even bother to limit the F25 in the first place and that the 8 speed tranny is just to help save fuel.

The 130 mph limit is probably mostly for political reasons, as is the "voluntary" European manufacturer's limit of 250 kph for all but the most "high performance" models.

[jbinbi]

i am doing some research in prep to get the x3. i have a 128i i want to trade and live in the NE, so probably best for me to wait until late march early april when convertibles come to peoples mind...

but bothering bmw salespeople and figures here are 80/20 28/35.

either this forum is representative of enthusiasts only, or I live in a skewed neck of bmw world...

[Lotus7]

Originally Posted by jbinbi

...either this forum is representative of enthusiasts only, ---

Most on-line car forums are exactly that, and to many "non-enthusiasts" a car is just an appliance used to get from "A" to "B" and and a 28i does that quite competently.

If BMW offered the M3 4L V8 in the X3, I suspect a few of the regulars on this Forum would have ordered one.