Sorry if I was unclear.... (archive)

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As for US driving condtions.... I don't live in the city.... have lots of hwys here in the west.... I also run on the track (where I hit 140mph).

One of my points is that most US driving conditions are unlike euro driving conditions. This means that most people here are not doing the 100+ mph autobahn cruising they would do in Germany (or France, etc.). This means you are not optimizing the car. In the northeast, it is extremely rare that you top 80 mph. (And I'm talking reasonable speeds here). So, why not optimize the car to get the better fuel economy, and be in the power band where you need it. I really appreciate being able to stick it in 5th and cruise along between 65 and 80 without having the need to shift. And yes, O/D with a 3.25 I have the need to shift at those speeds between 4th and 5th gear.

OK...here's the deal. If you're going to do engine work (in E12 case, it means cam, porting, bigger valves, maybe higher comp, pistons, headers, etc.) (in E28 case, it means chip (yes, there were chips available for high comp engines in europe), headers, cam, etc.), all of these engine mods, if done correctly, will raise the horsepower and torque peaks both in absolute terms and up the rev range. Take a look for yourself at the Alpina or Hartge engine specs. In order to take advantage of the power up the rev band you'll want to adjust your gearing (for track and street). More specifically for track you'll want to remain in the peak power band during all situations, and in street you'll want to be able to use as much of the power band as possible.

I was using the Dinan figures as an example....Since I couldn't get a hold of the power curves for the M30 B34 engine, I went ahead and took a look at the curves Dinan had for the M30 B35. The motors are similar architecture and have similar power bands to the L-jet 3453cc and B34 motors. Therefore this was using a concrete example to get the figures.

The reason I used the dinan chip equipped curves was to give an example of power curves on an M30 that has had mods. The power band moves up the rev range.

I quote you Chris "Now, according to the calculations, you'll be at around 6166 rpm with a 3.25 and C/R and at 5825 rpm with a 3.07. So interpolate a bit to get more accurate torque #s. At 6166 rpm, stock torque will be
around 140 lb-ft, and at 5825 it's around 160 lb-ft. Multiply by their respective torque multipliers (the diff), the rear wheel torque would be 455 lb-ft with the 3.25 @ 6166 rpm, vs 490 lb-ft with a 3.07 @
5825 rpm. This is with a "stock" torque curve."

2) Since when is the euro M30 engine rated at 140ft/lb of torque at 6166rpm? Also isn't the 3.70 @ 5825 rpm rated at a higher rear wheel torque per your calculations?

It is not, I was using the M30 B35 specs. I never said I was getting numbers for the B34, US or Euro, or the L-jet. I was using Dinan's data for "stock" and "modified" B35 engines as an example. Though not 100% similar, it comes very close. If I didn't make it clear, I'm sorry, but writing at 2 AM isn't condusive to perfect clarity.

And yes, keeping the motors stock, using a 3.07 will get you a higher peak top speed. However, using a modified motor will change that. And getting the data points and equivalent torques is necessary to compare stock to modified and 3.07 to 3.25.

You want to tune the car to the conditions. If it is modified, the engine that is, the power curves will be similar to the Dinan chipped example. To optimize the performance of this engine you will want to use a different diff depending on the driving environment.

Believe it or not, the Hartge H5P used a C/R with a 3.07 and the H5SP used a C/R with a 2.93 diff. Why? To optimize the power of the engines (which differed significantly) to the use of customers (europeans who used the cars to get places quickly - i.e. autobahn.)

And like I said, in the US, we don't consistently do 100+ driving. We are effectively doing what BMW should have done for the US market - adjusting the performance of the car to suit the market conditions.

Actually looking at the data for the M535i, max HP is reached at 5200rpm, not 5700 (like I thought). So I'm already over the HP powerband of the engine with peak HP power already obtained. Max Torque is reached 224ft/lbs at 4400rpm, and I believe is pretty flat up the scale hovering around 210 ft/lb or so.

Remember this is an M30 B35, used as an example.
B34 motors are similar in that according to the figures, max bhp is made around 5.2k, and torque at 4.0k.

Also, after max torque is reached, it remains close up until around 5.5k rpm, then it drops. That is how these engines perform stock. If you want to raise the power up high, you'll need to modify them. A chip on the B34 and B35 effectively raise the upper rpm performance of the engines very well. For the L-jet motor you'll need to do other mods for the same results.

If going to an even higher rpm (through a rear diff change) chances are the hp will fall off some more. as well might the torque of the engine, causing high speed to drop then as well.

Yes, that is why I introduced the "chip" to show that modifying the engine "requires" diff modification to be able to more effectively use the new power where you want it, and a new diff "requires" modifying the engine so that you're not screwing around with falling off of peak hp and torque too early. A Dinan chip with a 3.73 with O/D is a perfect set-up for USA B34 cars since it really raises performance levels and use-ability.
I always would recommend getting a chip and diff over just one or the other.

Take into account aerodynamics and the car is still limited to about 140... and won't change unless you add more power...

That was my point. And if it wasn't clear, I'm sorry. The B35 engine stats and my example calculations were to give a concrete example. You would want to get the figures for the other engines and do the calculations yourself to see exactly what happens when you change diffs, and where you'll be. If you want the car to behave like that, then perfect, it's the mod for you. And in most cases, in the US driving environment this is what happens. BMW's gearing on these cars was made for autobahn cruising, not US point and shoot driving. Again, if you live out-west, then you'll have to debate, but in higher density locals (like New Jersey where I live), the lower diff makes perfect sense - and works even better with the "required" engine mods.

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