Passing examination in the family car tuning school Everything has its time. Our series on modifications, measurements, and with theoretical ramble around family car tuning of a Volkswagen Passat 1.8T has come to an end. Now we wind up with among others a look at the way a traction force diagram functions. And is a sharp character always as fast as it feels? Bilsport April 2006 (In English July 2008) www.rri.se
Tuning theory, Part 5/5 April 2006 Imprint: Tuning theory, Part 5/5 April 2006 This article is prior published in the Swedish car magazine BILSPORT nr 8, 2006 Text and photo by Gunnar Ljungstedt Publisher: Förlags AB Albinsson & Sjöberg Box 529 371 23 Karlskrona www.bilsport.se Vehicle tests, technical facts and graphs by Rototest Research Institute, www.rri.se Originally published in Swedish Stainless steel, sport exhaust system and new catalyser from Ferrita Sweden AB, www.ferrita.com New intercooler from Setrab AB, www.setrab.com Engine tuning calibration, Turbo Center Stockholm AB, Claus Aichberger, www.turbocenter.se Reader Questions and comments: Please write to tuning theory@rri.se, open to December 2009. Free non-commercial downloads Except otherwise indicated elsewhere on this web server, any person is hereby authorised to view, copy, download, store, print or distribute anything on this site or obtained through it, but subject to the following conditions: The document may be used for informational purposes only. The document may only be used for non commercial, non profit purposes. The document may not be altered in any way, and must be complete. The document may not be copied to, or made available through, a web site. All other use is strictly prohibited. Copyrights Bilsport 2006 & Rototest Research Institute 2008 Rototest Research Institute 2008, www.rri, translation of Bilsport article from 2006 2/2
Power or torque? How many have not argued about torque or power being the most important factor for the acceleration? The argument never ends, and is usually topped by theories about when to change gears. Both questions can be answered by a driving power graph! How big is the power that drives the car forwards? We take a test point as an example. At 3000 rpm we measured 66.9 kw on the Passat s drive wheels. That corresponds to 810.5 Nm in drive wheel torque. Divided by the total reduction, 212.7 Nm are left of the engine s torque, after having passed through the drive line. Neat, since Volkswagen state 210 Nm at the flywheel. Here we see a show of modesty. The true torque is rather at approximately 230 Nm at the flywheel. The power driving the car forward is obtained by dividing by the effective rolling radius of the wheels. Our 205/55R16 has a radius of 308 mm. We put in all this in the formula Eq. 1. Constant speed formulas in Eq. 2 and get a total driving resistance of 457 N. From the engine s 2631 N, we deduct the 457 N that are used to keep a constant speed. We can do what we want with the rest. And what we want most is of course to accelerate! In Eq. 3 we see by how much, and if we divide this by 9.81, we get it as G power instead; in this case 0.14 G. Change gear at crossing With the same calculation at all test points tied together in one line, we have a traction force diagram for one gear. Assuming that the drive line losses are equal for all gears, we can compute a corresponding acceleration capacity for all gears. The driving power will be scaled by the relation between the gear ratios. For example, if the fourth gear has a ratio of 1.03:1 and the second gear 2.18:1, then the test point at 3000 rpm can be converted by using the relation between the gear ratios, as in Eq. 4. The driving resistance is computed to be 280 N, so now we have 5289 N to accelerate with. Divided by the weight of the car, 1560 kg, we get 3.39 m/s 2 or 0.35 g as acceleration ability. Unfortunately, we cannot use all the power to accelerate the car. A bit is used up to keep If the driving power curves the current speed, so this must cross each other it means that be deducted. more driving power can be The driving resistance consists achieved by changing gears. If primarily of rolling resistance they do not cross each other, and air resistance. At low there is too little power for the speed, the rolling resistance is gear in question. Where the of the greater importance and curve cuts the x axle, the it is influenced mostly by the acceleration ability is nonexistent and the top speed has ground but there is also a speed dependence factor. been achieved. For this calculation, it s enough to set it at a constant value. At Power accelerates the test, the car was driven at 91.6 km/h in fourth gear. We But how was it now? Which introduce this into the was of most importance power or torque? We look at the Passat again. At 4000 rpm there is 214 Nm of the engine s torque left at the wheels, or 90 kw. The corresponding value at 6000 rpm is 179 Nm and 113 kw. If we imagine that we put in the third gear at 6000 rpm, we will end up in the neighborhood of 4000 rpm after the gear change. If we look for the points in the diagram (approx. 85 km/h) we can establish that the acceleration ability is higher before the change of gear. Thus, power is the answer when you are in a hurry! The formula in Eq. 5 is another way to arrive at the answer. [] Fdriv 810.5/0.308 2631N Power is the answer when you are in a hurry! Eq. 1 F roll f m g W here f is rolling resistance of the tire 0.015 m is the weight of the car 1560 kg g weight acceleration 9.81 m/s2 F air 0.0386 _ cd A v2 W here _ air density 1.2 kg/m3 cd the cars drag coefficient 0.27 A frontal area 2.17 m2 v speed km/h Eq. 2 Facc 2631 457 2174 N Acc Facc / m 2174/1560 1.39 m/s2 Eq. 3 Fdriv vxl2 2631 2.18/1.03 5569 N v vxl2 91.6 1.03/2.18 43.3 km/h P F v Eq. 4 Power Force Speed Eq. 5 3/3
DREARY LAYOUT. Only because it performs better doesn t mean it looks better. The modifications we made in the Passat don t show much on the surface, if we disregard the temporarily fitted inter cooler. Maybe an advantage when it s time for the motor vehicle inspection. It must be emphasized that tuning is in fact prohibited, in case someone didn t know. Measurements results and Itäs great with power and torque, but most important is still the drivability. In Fig.1 we can see how the acceleration changes with the gears. Blue lines shows our Passat in original and the red one the car as it is now, with exhaust system, inter cooler and mapping. The curves are drawn to the revs stop at 6500 rpm. It was never increased since both turbo charger and injectors had already reached their limits. The first curve, at the far left, shows thus how brutally the car accelerates in first gear IF there is enough grip. The car s weight, air resistance, rolling resistance and friction against the road have been taken into account. The weight distribution is 60 percent front, and under acceleration the centre of gravity is moved to the rear, so a very good grip is required to take advantage of all the power in first gear. On ordinary asphalt spinning can be expected in second gear too, with the tuned version. Tops 245 The gear should be changed where the curves cross each other. Here we see that the standard car should always be brought to 6500 rpm before changing gear, if acceleration is to be optimal. After tuning it s enough to bring the first gear to maximum revs. The second gear is brought to 6400, the third gear to 6300 and the fourth gear to 6000 rpm. The top speed for the standard car is around 232 km/h (which is confirmed by Jonny s life experience) and in the tuned condition it is 245. The relative change in acceleration power (Fig. 2) in the three lower gears is approximately 30 40 percent, which feels jolly good. To have this extra power available if one wants to overhaul a loitering long distance lorry driving uphill is hardly a bad thing. [] The top speed for the standard car is around 232 km/h 4/4
Fig.1: The acceleration potential at the different gears. The white dots corresponds to even thousands in the revs range. 5/5
Fig.2: The acceleration improvement at the different gears. 6/6
Fit or slack? The way an engine s power curve is defined has great impact on the way the car is perceived. A pointy character will be perceived as more goahead than a flat torque curve but does not necessarily make a faster car. We compare the power curves and drivability of a Volkswagen Golf GTI and a Toyota Corolla 1.8 T Sport to make a practical example. The GTI, fitted with a 2 litre turbo engine has a high torque, 280 Nm, over a rather wide range. In fact, it resembles a Diesel engine a little bit. The maximum power is stated to 200 HP. The tested example rendered 277 Nm at the wheels, i. e. considerably more than stated. The power landed at 183 HP, showing a reasonable transmission loss. The Toyota is fitted with a high revving 1.8, 192 HP and 180 Nm engine, according to the manufacturer. The test shows 167 HP and 162 Nm at the wheels. Changes character Thus, the GTI looks on the paper like the more powerful car, with almost 10 percent more power and as much as 71 percent more torque. The advantages of the Toyota are almost 200 kg less weight and a somewhat slimmer body. Consequently, less air resistance. Fig. 9 and Fig. 10 show the acceleration ability for the two cars. The Toyota s pronounced VTEC character is clearly revealed when the engine changes character at 6000 6500 rpm. The torque increase is perceived as a horse s kick, which is emphasized by the change of thesound. The sound is clearly a part of the experience. Before the kick, everything is very gentle. A top speed test would mean maximum revs in the fifth gear, changing quickly to the sixth, where it would slowly struggle up to the stated top speed of 240 km/h. Change gears whenever The Golf s Diesel torque character means that the car feels strong all the way. The acceleration ability in first gear will guarantee wheel spin on ordinary asphalt. It would probably leave marks in second gear as well unless the traction is super. The diagram shows that changing into fifth and sixth gears should be done already at 5800 rpm, even though the difference in acceleration is only marginal. In practice you can change gears when it feels best within the range of 5800 6500 rpm, without any difference to speak of in acceleration ability. Who is the fastest then? From standstill, it is almost entirely dependent on the driver and the surface of the road. At 60 km/h, we have already changed gears in the GTI and can then keep 0.47g. The Toyota still works on in first gear and keeps 0.51g. At 70 km/h, also the Toyota pilot puts in the next gear, whereupon the acceleration is reduced to 0.28g. The GTI however, goes on with about 0.42g, dropping down to 0.33 g at 85 km/h. Optimal gear ratio The Golf s almost constant power between 5000 and 6000 rpm ties the acceleration lines together so the acceleration in the next gear is as high as before the gear change. In spite of the Golf not feeling as bright and intense as the Corolla, we would dare to bet in favour of the Golf if the two would meet on the strip. This type of diagram can be used to optimize the gear ratios in the transmission. A gear box with continuous variable ratio, so called CVT, can have a curve that ties together the right hand part of all the lines, in the same way as the GTI curve does it in the higher gears. The advantage that the engine can be kept at constant maximum power renders in total the best acceleration. On the other hand, constant engine speed does not sound so fun. Like a belt driven Daf The total experience includes the sound as well![] The torque increase is perceived as a horse s kick 7/7
Fig.3 8/8
Fig.4 9/9
Fig.5 10/10