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Technical Topics B48 4-Cylinder Turbo Engine / Drivetrain / Exhaust Modifications RANT: Misconception that EVERYONE makes about Torque vs Horsepower

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      11-21-2021, 08:14 AM   #1
Knifeedge2k1
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RANT: Misconception that EVERYONE makes about Torque vs Horsepower

Torque at the engine matters very little since torque at the WHEELS is what determines your acceleration. Torque at the engine is almost a meaningless number since it changes via the gear ratios. The same torque figure at 2000RPM isn't "worth" the same as the same torque figure at 5000RPM.



In any given gear, you'll feel the greatest acceleration where ever the engine produces peak torque (neglecting air resistance) that is absolutely true.



But a much more useful and relevant question to ask would be ... "what gear should I use at a given speed in order to maximize my acceleration ?"... in that case the answer is "you want to be in whatever gear gives you the highest power output"



Power is a measure of the ability to add/remove energy into/from a system. Acceleration is the change of velocity i.e the addition of kinetic energy into a system. Clearly, when your engine is making the most power (lower gear, higher revs), it's ability to help your car accelerate will be superior to when it's making less power (higher gear, lower revs). This also determines your shift points, you want to shift up when your engine makes the same power at the new RPM as your current RPM, in most cases because you'll make less power in a higher gear than whatever power the engine makes at redline because that's how cars are geared, it means you simply shift at redline.



Side note, you can evaluate your brakes in the same way by expressing it's ability to convert kinetic energy into heat AND it's ability to dissipate this heat into the air as a "power" figure, though this is very rarely done in consumer circles.



Fat torque curves are "easy" to drive because they hide your mistakes with the shifter. Basically no car from the factory (current or historical) come equipped with engines which feature predominantly downward sloping power curve (it might dip near the end but by and large, you make more power the higher up the revs you go), they almost ALL make their peak power either at redline or more likely 10-20% of the rev range below redline. This means in almost all cars, if your goal is to accelerate the quickest, you want to be in the lowest gear possible without blowing the engine (overrev). In a car with an engine with a very fat torque curve (or one where there's an excess of torque at the bottom end like modern turbo engines) you still have a lot of power down low so when you mess up a shift, you don't suffer from as dramatic a loss in acceleration. Easy for those who are not good with the shifter but ultimately that's what makes these cars less engaging for an enthusiast (we WANT to be punished when we screw up, this makes the reward when we get it right ever more sweet)
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      11-21-2021, 08:34 AM   #2
Bc2005
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The torque curve for a well tuned turbo engine will be much different than a non-turbo. The older Honda 4 cylinder engines were much more fun to drive with a manual transmission because they needed rpm to get up into the right torque/horsepower range. My Honda spent a lot of time in the 4500-5500 rpm range. However, BMW is using turbo 4 engines like the N20, and were able to achieve a torque curve that hits its high point at only about 1500-1800 RPM, and then stays nearly flat all the way to redline. Low rpm torque makes the engine feel more powerful because most drivers spend a lot of time in that rpm range.
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      11-21-2021, 08:45 AM   #3
Knifeedge2k1
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Quote:
Originally Posted by Bc2005 View Post
The torque curve for a well tuned turbo engine will be much different than a non-turbo. The older Honda 4 cylinder engines were much more fun to drive with a manual transmission because they needed rpm to get up into the right torque/horsepower range. My Honda spent a lot of time in the 4500-5500 rpm range. However, BMW is using turbo 4 engines like the N20, and were able to achieve a torque curve that hits its high point at only about 1500-1800 RPM, and then stays nearly flat all the way to redline. Low rpm torque makes the engine feel more powerful because most drivers spend a lot of time in that rpm range.
absolutely

there are very good reasons to have meaty torque curves for driveability and "street useablity"

but for the purposes of "driving hard" or outright acceleration the bottom 2/3rds of the RPM "don't matter" if you get your shifts correct

obviously no one is perfect and pretty much EVERYONE will benefit from having that power available down low even on the track since we're not gonna get it right all the time ... but i still maintain, having all that power down low takes away from the experience somewhat, it's for precisely this reason that ferrari tones down the boost in the lower RPM range in the lower gears, to keep that increasing linear power delivery characteristic that is so often lost in today's turbo engines
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