technical difference on auto vs. manual

[zman]

Ok, this question may require some technical explanations on how a car transmission works.

I notice that I am driving my 325xi with manual transmission, the car rpm easily goes beyond 3K with a press of the acceleration pedal. Especially at the 5th gear, I notice that the rpm is beyond 3K if I drive above 55 mph. The faster my car speed is the higher the rpm of the car.

Note: I am not an aggressive driver and I always take it easy on my car.

On the other hand, I have been driving X3 with auto transmission (a loaner car). I noticed that when I put on auto (D), the car's rpm never pass 3K on every gear. In fact, it didn't go beyond 3K when I was doing 80mph. I even tried to accelerate quickly at the traffic light. The car pulls great [headbang] , but the rpm stayed below 3K.

Then, I selected "manual" mode. The rpm went a bit higher than 3K on the 1st, 2nd, and 3rd gear when I pushed it before I switched it to the next gear. But when it got to the 4th or 5th gear, once again it stayed below 3K.

It just seems that the car can produced so much power under 3K. I notice that my 325xi doesn't do that. Am I missing anything?[???1]

any ideas?

 

[frolf]

sounds like the X3 is geared a lot different, and the diff ratio is a lot lower (taller) making the final drive a lot taller

 

[William330]

BMWs produce tons of torque at all RPMs.

Most other cars can produce high torque, but at very high RPMs.

BMWs have a flat torque curve, which gives tons of power at
all RPMs.

 

[zman]

OK, I guess I am answering my own question.

I have talked to a car 'guru' and explained what I have said above.
And, he understood and gave me some explanation.
While there are many advanced terms that didn't retain in my mind, I think I get the basic idea.

Basically, in a manual transmission gears are sifted based on 'feeling' which involves experience, timing, rpm, sound of the engine, etc. In other words, you have to know what you are doing to get a good performance with manual transmission.

On the other with hand the automatic transmission, everything is taken care of by the engine itself. It has been design to know exactly when to shift which makes it 'fool' proof.

Of course with each one of them there are advantages and disadvantages.

For common people, the advantage of the manual transmission would run cooler, more efficient inside city with a lot of traffic and traffic lights. A manual transmission driver would put the gear in neutral where the engine would just idle until it is put in gear.

On the other hand, for the automatic transmission the engine would tend to run 'hot' with a lot of traffic. Breaking is basically holding back the engine to a stop. So, automatic car tends to over heat during a long traffic wait.

During takes off, the manual transmission could out perform the automatic IF the driver has had a good experience, otherwise you will see the automatic car takes off like a rocket. However, as far as engine concern the manual transmission engine would work harder because it relies on the driver skill.

And during a high speed, the automatic car would run better and more efficient. You see this where the rpm stays below 3K. It really only takes that much power to maintain a high speed.

On the other hand, the manual transmission would have rpm > 3K for high speed cruse. In other words, the engine is 'screaming' to maintain a high speed. Hence, for a long drive, the manual transmission tends to overheat.

In fact, a high performance car (i.e. Ferrari) would have automatic engine with steptronic to achieve its max efficiency level.

Conclusion:
If you drive the car for city, manual is better and will do as good as automatic IF you have the skill. Otherwise, go with automatic.

Again, this is what I understand from taking to the ‘guru’. [headbang]

 

[mjbst111]

the engine rate while driving fast depends on gear ratios. Most manuals have an extra gear than its brother automatic which would allow the engine on the manual to not have to work as hard. We'd have to pull up some charts on a new bmw's gear ratios between the auto and the manual to prove this though.

You really have to compare the same car though. You can't compare the x3 auto to the 325xi manual. They have different gearing and differentials probably....

The new 330 for example:

Transmission
Automatic gear ratios 1) 3.45/2.21/1.59 I/II/III

Automatic gear ratios 1) 1.00/0.76/3.17 IV/V/R

Final drive ratio 3.38 :1

Final drive ratio 2.93 :1

Manual gear ratios 1) 4.35/2.50/1.66 I/II/III

Manual gear ratios 1) 1.23/1.00/0.85/3.93 IV/V/VI/R

Well there you have it the sixth gear in the manual is still larger than the 5th gear in the auto which would cause the engine to "work harder" in the manual IF they both had the same differential gearing. But they have different differential gearing between the manual and auto. So...... this requires more info and math than i have at the moment....