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02-24-2011, 02:47 PM | #265 | |
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I'm confused though, why the power is not multiplied by the gearing?
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02-24-2011, 04:23 PM | #266 |
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While it is great to consider the gearing, because it does multiply torque, the spreadsheeters and simulators are still missing some of the equation. The drive of the car does not stop at the differential. My car has tires attached to the differential.
We have already learned that in reality the M3 torque is about 10% less than the 335i torque (because the N54 is underrated from the factory), which drops that 25% gearing advantage to maybe 15%. Let's factor in the tire diameters as well since they affect torque multiplication. The 255/35/18s fitted to the 335i are about 5% smaller than the 265/40/18 fitted to the M3. This has the effect of reducing the gearing -- like lowering (higher numerical) the rear axle ratio -- or multiplying torque. So now we are down to a 10% torque advantage for the M3 . . . starting from 3900 rpm. I am learning a lot from this spreadsheet simulation stuff. |
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02-24-2011, 04:42 PM | #267 | |
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Counterintuitive. |
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02-24-2011, 04:45 PM | #268 | |
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02-24-2011, 05:01 PM | #269 | |
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As a practical matter, the 4000 rpm car will be quicker in terms of acceleration, because it will have less rotational inertia to contend with. If you remember Sir Isaac's theories, you know that nothing will move, speed up or slow down without some force being applied. Car 1 has to speed up its engine and associated parts by twice as many rpm per mph gained as car 2, and since these parts don't like to do that, car 1 loses twice as much power to overcome rotational inertia as car 2, leaving less power to actually accelerate the rest of the car's weight. This of course assumes identical rotational inertia for both cars. You might consider this in terms of the M3. It is very stiffly geared, but BMW has gone to great lengths to reduce/minimize its engine and transmission rotational inertia, so it's a very free revver anyway. Last edited by bruce.augenstein@comcast.; 02-24-2011 at 05:15 PM.. |
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02-24-2011, 05:06 PM | #270 | |
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So if the engines in my example are real, can we still say high revving engines are better because they have the advantage of gearing? This is what confuses me.
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02-24-2011, 08:21 PM | #271 |
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Yes, without considering drivetrain inertia or complex loss effects, the cars will indeed accelerate identically. It is a very useful thought experiment.
As a side note on the drivetrain inertia: Using percentage drivetrain losses which CarTest does, it computes the losses to be identical for two such vehicles (when plotted vs. road speed, not vs. rpm). I'm also fairly sure that CarTest includes the effects of drive train inertia and if so it is more or less "noise" between these simulations despite the huge factor of 2 in FD ratio.
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02-24-2011, 08:36 PM | #272 | ||
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If you would just also take the simple shortcut to hp and not focus so much on either torque or torque to the wheels you would have it made.
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02-25-2011, 09:44 AM | #273 | |
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If you remember previous conversations, the smallest amount of acceleration "loss" I found in first gear compared to second gear was 12%, on a car with a three to two ratio between first and second gears, showing .50 G max acceleration in second gear, and .66 G in first. This was on an '85 Vette with an undersized trans (Borg-Warner Super T10) and an 18 pound flywheel. Every other car I tested (and there were at least a couple of dozen) showed a larger loss than that. Rotational inertia is real, and significant. I know you and lucid never bothered to check Gillespie's book, but you should have. |
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02-25-2011, 09:53 AM | #274 |
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So all in all, can we still say high revving engine is better due to gearing?
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02-25-2011, 10:25 AM | #275 | |
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High revving engines tend to make more power per liter than low revving engines, because if you're a manufacturer, why bother with forged rods and other expensive materials, etc., just to be able to safely rev higher. You rev higher so you can make more power per liter. Then, as in the M3, you gear the car aggressively so it's easy for the driver to get to automotive nirvana on the right hand side of the tach. The pinch point on naturally aspirated engines is torque related. At current levels of technology, about 90 pound feet of torque per liter is about as good as it gets, so if you want more power, you've got to make that torque higher up in the rev band. Power equals torque times rpm. Otherwise, overall gearing is not hugely important - as we've dragged some folks (kicking and screaming in this string) into the realization that it's power and weight that essentially count at any given instant. So, high revving engines are better because they make more power for a given engine size - and more power makes you go faster. On the other hand, the quote you've read in this string that says "It's better to make torque at high rpm than at low rpm, because you can take advantage of gearing" is also true. It's just another way of saying more power is better, and that you can apply that power thru aggressive gearing that let's the engine easily get up there where max power is. |
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02-25-2011, 10:51 AM | #276 | |
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Now that we have the M3 torque advantage down to 10%, let's think about what the torque is being asked to move since that should be a consideration as well. The E90M3 weighs about 100 lbs more than the E90 335i, which is about 3%. The weight affects the torque to weight ratio, obviously. Let's lop that 3% off the M3's 10% advantage. We are now down to a 7% real world torque advantage after considering ALL the factors, and that is from 3900 rpm, which is about 2000 rpm higher than the 335i torque peak. It is easy to see why the 335i makes for a great daily driver from off idle to about 4000 rpm and why it will often outsquirt the M3 around town in daily driving -- the mighty M3 needs some space to catch up. Drag racing off the line or speed contests from its torque peak certainly favor the M3, but I don't daily drive drag race or daily drive around at 3900 rpm. Again, I like my M3. Got six sets of shelves in it from Lowes last night. Had to make two trips, though. A wagon version would have been awesome. With my new ARC8s in 10 x 9.5 and 10 x 10.5, I will be running 265/35/18 and 285/35/18. The rears work out to a slight gearing improvement over the stock 265/40/18. This change is more like the 2% you were guesstimating for the 335i versus the M3 stock 18 inch tire sizes. I will add this 2% to my 7% M3 gearing advantage on my spreadsheet simulator. |
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02-25-2011, 11:01 AM | #277 | |
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Just sayin. Now, I get the impression you are math-averse by nature and so I suspect your answer to this will be "Whatever, geek!". |
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02-25-2011, 11:16 AM | #278 | |
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02-25-2011, 11:34 AM | #279 | |
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02-25-2011, 11:42 AM | #280 | |
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In other words, take your example, and make the redline RPM for the F/I engine at 6000 RPM instead of 4000 RPM. Now you can use more aggressive gearing (75% or the gear ratio of the N/A example, rather than 50%) and still get the same power down at the same speed as before. (I think I stated all of that correctly, but if not, you still get what I mean. ) |
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02-25-2011, 11:55 AM | #281 |
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I believe that we have come full circle. Is a high revving engine better due to gearing?....depends on what YOU want and how YOU like to drive.
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02-25-2011, 11:58 AM | #282 | |
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02-25-2011, 11:59 AM | #283 |
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Definetly! I can see the advantege of heaving huge amount of torque with very tall gearing.
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02-25-2011, 03:03 PM | #284 | ||
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335i: 11.2 lb/hp M3: 8.9 lb/hp That is a 26% worse value for the 335i. That is why when you choose the right gear and use the car the M3 stomps the 335i from and to any speed and in distance at a given time. If you like the flexiblity of good acceleration in a high gear at low speed and very low rpm that's fine, it is a preference. You are still missing something about tire size as well. Why don't you simply put a set of RC sized tires on the rear of your car. Torque multiplication is HUGE. For essentially free you would have the highest wheel torque car on the planet. Even if traction was not an issue there is a reason why this does not magically make a car faster by the increased percentage of torque multiplication. This is exactly why you are also confused about FD ratio modifications as well. You must consider what happens not at an instant but ACROSS multiple gears.
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02-25-2011, 03:21 PM | #285 | |
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Other than knowing gear ratios there is obviously no way of knowing how large or what materials (i.e. densities) or sized all of the relevant drivetrain components are. I checked Gillespie, he simply substitutes the so called "mass factor" for not knowing the details about each cars drivetrain. The mass factor is an approximation and certainly does not take into account actual details of the driveline. The mass factor in 1st gear between the 335i and M3 does vary by about 18%. I'll try to check CarTest to see if this "mass factor" is used as part of its acceleration calculation by specifically comparing the M3 and 335i.
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02-25-2011, 03:27 PM | #286 | |
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For actual production cars with gears and FD ratios that are fairly well matched/optimized to the particular cars engine and redline this is true. However, gearing is very important in theory and the physics of car performance. A big change in a cars gearing after already "optimized" will result in a huge performance change. We are just lucky because cars with very different engines are already matched to their drivetrains. That is the only reason gearing appears transparent in car comparisons and why we can shortcut straight to power to weight ratio.
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