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Why Would a Slower Car "Feel" Faster on a Torque Curve/Butt Dyno? X3 M40i vs. B9 S4.


stubaxphil123

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Greetings, individuals.

Recently, we acquired an X3 M40i (285kw variant) and on our journey back home, I was driving the S4 (the 354hp 500nm petrol model, not the TDI version) while my wife was in the X3. As we accelerated from 10mph to around 70mph, we saw that the X3 exhibited a little faster acceleration before we decelerated.

I am seeking assistance from someone with a more technical and physics-oriented mindset, with superior knowledge compared to my own, in order to comprehend the underlying reasons behind the phenomenon of seeing an object or event as quicker, despite its objective lack of increased velocity. I acknowledge the observation that diesel engines tend to provide a perception of increased speed due to their higher torque output, which is available at lower engine speeds. However, the two vehicles under discussion possess equal torque figures, with the X3 being just 150-170kg heavier.

When operating the X3, there is a discernible decrease in speed compared to the B9 S4, which exhibits a notable increase in initial acceleration upon engaging the boost. I have not had the opportunity to do a direct comparative analysis between the M340i and the S4, both of which are sedans of comparable form.

Upon analysing the Dyno charts, it is evident that the EA839 engine in the S4 attains its maximum torque output at 1370rpm, while the B58TU engine in the X3 achieves peak torque at 1900rpm. Considering the S4's ability to reach peak torque around 500-600rpm earlier than the X3, it raises the question of whether this time advantage significantly contributes to the perception of increased speed in the S4. Although the torque of both vehicles is the same, the X3 M40i exhibits an additional 25 kW of power.

For those with prior experience operating sport utility vehicles (SUVs), including ourselves who are novices in this regard, does the vehicle's weight also contribute to the perceived speed of the car?

Thank you for any contributions.
 

Edited by stubaxphil123
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The power-to-weight ratios and the form of torque curves, particularly those with a high rise, have a significant impact on the perceived acceleration of a vehicle. Additionally, the gearing system plays a crucial role in this regard.

Due to the presence of several factors, it is unlikely that a clear response can be provided.

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The torque curve, as previously shown, is a graphical representation of the relationship between torque and rotational speed. The greater torque at low rpm likely results in more horsepower. Diesel engines often provide a perception of early acceleration superiority, although their performance diminishes due to a limited capacity for high revolutions. Additionally, it is worth noting that the gearing may exhibit significant variations. Lastly, it should be noted that a brick has a lower degree of aerodynamic efficiency.

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Indeed, the diesel engine will reach its maximum power output at around 3500 RPM or thereabouts. Throughout my experience, I have mostly owned diesel vehicles with horsepower ratings of up to 400. However, I find that my M4 model subjectively seems quicker than these aforementioned diesel vehicles. Notably, the M4's engine is capable of revving above 7000 RPM. It is worth mentioning that the M4 has much less torque in comparison to my 640d, which boasts an impressive torque rating of 800 Newton-meters, for instance.

 

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The aspect that is causing me perplexity is to the fact that both vehicles are powered by petrol. Specifically, the S4 model under consideration is equipped with a 3.0TFSI EA839 engine, producing 354 horsepower and 500 Newton metres of torque, and was manufactured between the years 2017 and 2019. In my previous experience with the 435D, there was a subjective perception of more acceleration compared to both alternatives. However, upon closer examination, it was determined that the actual speed was somewhat slower.

I hypothesise that if both vehicles had a wavelength of 500nm, the automobile with a slower speed will subjectively seem to be moving at a slower pace. As previously noted, it is quite likely that the gearing mechanism is the determining factor.

 

Upon examining the juxtaposition of their horsepower and torque curves, it is plausible to infer that the Audi exhibits an approximate increase of 60 Newton-meters at 1000 revolutions per minute (rpm) compared to its counterpart. Furthermore, the Audi's torque demonstrates an earlier occurrence of spikes.

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Definitely The torque range, similar to that of a two-stroke motorcycle, imparts a heightened sensation of speed, reminiscent of the Yamaha LC 350 from earlier times, which was renowned for its exhilarating performance. In comparison, the high-torque GS 1000 Suzuki offers a different, although equally noteworthy, experience.

 

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The perception of speed is attributed to the more rapid augmentation of torque.


The incline of the Audi vehicle is more pronounced, creating a perception of increased speed, despite the absence of actual acceleration. This phenomenon is also the reason why diesel engines provide a perception of enhanced speed.

image.png.66c653d4f6c18c8145f46d9e2dc01d46.png

The incline of the Audi vehicle is more pronounced, creating a subjective perception of increased speed, despite the absence of actual acceleration. This is also the reason why diesel engines provide a perception of increased speed.

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It is quite likely that the process will be expedited. Assuming all other variables remain constant (although this assumption may not hold true in reality), an increase in torque at a given rotational speed would result in a corresponding increase in power. Consequently, in the absence of any other differentiating factors, it may be inferred that the object in question would exhibit greater speed.

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Indeed, it is not advantageous to rev a diesel engine since doing so would result in the loss of peak torque while shifting gears, owing to the non-linear torque curve.
There was an instance reported when a diesel vehicle owner engaged in revving their engine to the maximum rotational speed (5000rpm), resulting in a subsequent gear change that caused the engine speed to decrease to about 3900rpm. This action led to a total oversight of the optimal torque output, which was seen to occur at 2000rpm.

 

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One becomes used to the concept of torque. I did not know when to change gears, as I was operating the vehicle at its maximum torque output. Consequently, I resorted to using the paddle shifters, as the car appeared to rev higher when left in automatic mode. However, an individual, possibly from this platform or another social media platform, presented me with empirical evidence that demonstrated the superiority of the automatic mode. It became apparent that the car, when allowed to rev beyond the point at which I would typically shift g

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