09-26-2013, 08:46 PM | #111 |
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Just to point out a few things for comparison sake.
Most GTR guys (namely the guys setting the 1/4 mile records in stock form) are short shifting the cars in first gear only (6600RPM) then letting it wind out to the 6800 redline for 2nd gear and beyond. The built motor guys are revving beyond 9000 RPM... A stock GTR peaks torque in the low 4000RPM range (roughly 420awtq) holds about 400awtq shortly after to about 5500 RPM then falls off to about 350 wheel torque at 6850 RPM (Avg. late model car on a dynojet). To say "the torque falls off you will have to short shift way early" is not looking into the quantitative factors of this equation carefully. As torque falls off (as it does in all cars), enough revolutions CAN keep the horsepower constant, or increase it and this applies to FI cars.... For our purposes, horsepower is the rate work that is done by a factor of time . More RPM = more work "can be done," assuming your torque doesn't plummet (Mazda Speed 3, 335i, E36 M3 U.S spec beyond 6800). If you calculate horsepower using torque (with a rough estimate on the graph using increments of 2000 RPM) @5700 RPM you are making 370ft/lbs which equates to 401 HP @7300 RPM you are ONLY making (rough est. as it looks to drop 110ft/lbs in 1000 RPM) about 290ft/lbs but that equates to 403 HP As your M3/M4 needle swings past 7100, the car is still doing more work in the same amount of time despite the drop in torque Vs. a lower RPM and the rate of increased torque on the graph. As you go past 7300 RPM the H.P numbers have only dipped a slight percentage from 7100. At 7600 RPM though, it has gone down to roughly 391hp.. RPM and the fact that it looks as if the BMW will only loose about 110ft/lbs of torque from 5700-7600 RPM is to thank for that. Other, less important cars (haha) drop off torque much more harshly, hence the need for an early shift. However, the trend with technologically advanced F.I cars (MP4, GTR, or any "built" Turbo car) is as long as you are spinning it fast enough, a slow drop off in torque could still mean more work being done. Once we have real dyno graph's we can calculate this with more accurate numbers and really see how much work is being done is at given RPM's
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09-26-2013, 09:08 PM | #112 | |
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09-26-2013, 09:45 PM | #113 | |
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I know very well the Clarkson episode like almost everybody else, thanks anyway. N54T is a great engine imo, but of course, the S55 will be even better. That is the part of the F8X that I have no doubts about. I don't like to compare apples to oranges that's why I don't want to say that it will be better than the S65 also, because I don't feel like it is a meaningful thing to say, they are different, like N54 and S65 are different too. There are things that one makes better than the other and I have zero doubts that S65 will remain as the better engine in terms of revving, linearity, sound (engine not exhaust) and especially throttle response. The new engine will do other things better, way much better and there are no winners or loosers here, it is a shift or a new way of going fast. In fact, I need to hear more about the steering, manual transmission, clutch, standard brakes, ride (with passive suspension), exhaust, production car's final design inside/outside and the price tag which may kill it or make shine. Engine itself seems fine really.
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BMW 1M-SOLD-: TECH: Evolve Race+N55mids, Evolve IC, Michelin PSS, ER cp, aFe filter, CDVx, Vorshlag camber plates, BMS OCC EXTERIOR: trunk spoiler, blacklines, black grills, IND goodies INTERIOR: Alcantara steering wheel, steel pedals, custom mats, MPower e-brake. Last edited by ozinaldo; 09-26-2013 at 10:46 PM.. |
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09-26-2013, 11:36 PM | #114 | |
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09-27-2013, 02:20 AM | #115 |
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Forget about dynos and gear ratios!
The best way to determine the optimal shift points is to look at real data, that is, longitudinal G. You would simply shift up if the longitudinal G in the next gear at the respective RPM is greater than the longitudinal G in the current gear at current RPM. See below. (from lapfaster.com, or what used to be there) |
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09-27-2013, 06:46 AM | #117 |
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09-27-2013, 08:09 AM | #118 | |
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It looks like a noline 3 series dash. |
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09-27-2013, 08:18 AM | #119 | |
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09-27-2013, 10:14 AM | #120 | |
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09-27-2013, 05:58 PM | #122 |
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If you're ever in the ATL area, PM me beforehand and I'd be happy to give you some seat time.
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09-27-2013, 06:36 PM | #123 | |||||
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Thanks for your blessing, unfortunately yours is not...
Positively 100% incorrect. Quote:
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Either way, due to both the math (the actual way you calculate from crank torque to wheel torque) and the fact that rpm does not "bias" torque the same way it does hp (at high rpms). I maintain that Quote:
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09-27-2013, 07:19 PM | #124 |
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too many "smart people" here lol.. go to actual track w some corners.. and do your shit talking with your lap times.. nobody gives a crap on if you shifted 500 rpm too early..
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09-27-2013, 10:37 PM | #125 | |
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P.S. Just to satisfy your potential curiosity I do take my car to the strip and to the track as well.
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09-27-2013, 10:44 PM | #126 | |
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Oh well to each their own I guess.
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09-28-2013, 01:52 AM | #127 | |
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Unless someone fully understands all the intricacies of your analysis then they may not be confident in the results. It may be more costly, but the results are easy to understand. You need to keep acceleration as high as possible and you can test to see what shift paradigm works best. Me personally, I'm struggling to reconcile the fact that max power is held pretty much to redline but your analysis shows short shifting (sub 7k RPM) to be beneficial. I guess, as you mentioned, that losses increase as RPM increases so even though the engine power is constant the effective delivered power may be going down with RPM as well. Since the M5/M6 engines have similar power delivery, I would be interested to get an owner's (or someone who's driven it) impressions about what kind of shifting worked best for them. |
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09-28-2013, 03:04 AM | #128 | ||
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1. Peak acceleration in any gear occurs at the point(s) where the engine crank torque is peak*. As a result the acceleration curve basically mirrors the torque curve NOT the power curve. 2. Comparing two vehicles identical in all aspects except engines, at a given speed the car that can produce more power will out accelerate the other (WOT of course). Engine torque is truly 100% irrelevant in this point. No not 95% or 99% irrelevant, 100% irrelevant. (* not counting corrections for drag and tire losses) Unless you can convince yourself that both of these are correct, you'll probably continue to believe that constant power delivery means don't ever upshift until redline. Even without losses (no drag, aero nor drive train losses) short shifting may still be required to obtain maximum performance. One does not obtain constant acceleration even with constant power. With constant power acceleration decreases with increasing velocity. This is more or less the definition of power. Constant torque in fact delivers constant acceleration. So again the reason to shift is not the loss of power but the loss of torque. Also, it is not just "my analysis". Yes, I have done the entire calculation by spreadsheet manually and it agrees with the software I use for this calculation, CarTest, which has been very extensively validated by me and by others, on this forum and in other places. Quote:
Hope all of that helps.
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09-28-2013, 09:44 PM | #129 | |
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I guess I'm still having a little trouble reconciling those two points above because they don't seem to be mutually inclusive. For example, at a given speed, if car 1 had more HP than car 2 but less torque then according to your second point car 1 should out accelerate car 2. But your first point says the car with the higher torque should have a higher acceleration. They seem to be contradicting each other. What am I missing? |
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09-29-2013, 02:43 AM | #131 | |
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Or more specifically just a couple people. |
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09-29-2013, 03:56 AM | #132 | |
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Would this BMW 523i have a "optimum shiftpoint" at around 4250rpm for max acceleration? http://www.ultimatespecs.com/car-spe...ries-523i.html And this one at 4800? http://www.ultimatespecs.com/car-spe...-528i-Aut.html Or am I missing something here? Because the auto trans certainly doesn't shift that low at wot acceleration... |
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