Voir la version complète : Qui a deja installer une poulie de crank shaft performante?
Qui a deja installer sur sa voiture ce genre de poulies ''underweight/underdrive ''? J'ai lu sur un site de tuning que chaques livres en moin de la poulie donne environ 2,7hp de plus. Si jeffectu un petit calcul je pourais aller chercher un 10-12hp sur mon moteur. 6lbs versus 2lbs. Ils mentionnent également que les gains sont 85% du au weight reduction de la poulie et 15% a leffet underdrive sur les accessoires.
Bonne journée
:evil: :twisted:
100limites
27/11/2003, 08h11
Petite précision : Tu obtiendras un gain en puissance, certes, mais une diminution du torque. Et sur les petites voitures, le torque est plutôt important... si ce n'est aussi important que le HP.
Voilà pourquoi je recommande d'y aller doucement avec les poulies ou une flywheel "ultralégère" !
Je ne suis pas du tout d'accord avec ton affirmation, pour qu'un moteur affiche plus de HP, il doit nécessairement avoir plus de torque.
Et encore là, je doutes du potentiel d'une flywheel de changer la courbe de torque.
Sur le site de unorthodox racing il affirment un gain de 5-8lbs de torque.
100limites
28/11/2003, 16h59
Je vais tenter de retrouver l'article dans le Sport Compact Car et je vous reviens la dessus.
Mais 100limites a tout à fait raison sur un point, il faut faire attention sur les poulies underdrive... c'est pas bon pour l'alternateur, ton climatiseur/chauferette va perdre de la puissance, ton power-steering va être moins assisté... ainsi que tous les autres accessoires... etc etc...
Je ne vois par contre aucun problème à la faire la plus légère possible (underweight).
Tout a fait daccord. La poulie qui m'intéresse pese 2lbs au lieu de 6lbs et offre un ratio underdrive de 10%. Anyway seulement 15% des gains de performance proviennent de leffet underdrive.
Bonne journée!
Je vais tenter de retrouver l'article dans le Sport Compact Car et je vous reviens la dessus.
J'ai trouvé ceci. Désolé pour l'anglais
Q.: Isn't there a loss of torque associated with fitting lightweight pulleys, since the flywheel effect is lost?
A.: No. In early days it was common to fit large heavy flywheels to increase torque, which to a certain extent does work, but it is important to look at the context of the application. A heavy weight acting as a flywheel on the crankshaft has a greater resistance to a change in rotational velocity, which is termed inertia. The term "resistance to change in rotational velocity" applies to both acceleration and deceleration, so a heavy flywheel or pulley does oppose deceleration in situations such as towing caravans up a hill, but it will also oppose fast acceleration.
It is easiest to think of inertia as an energy reservoir, and the laws of physics state that you can only get out the same amount of energy as you put in (actually in the real world it is often less due to irreversible losses). So an engine needs to put in a significant amount of energy to accelerate a heavy pulley before any useful torque can be extracted from it. Don't forget that when the engine is at low RPM before the turbo spools up, it is only making about 20% of its maximum power, and in this situation the amount of energy required to accelerate the pulley becomes a much larger percentage of the available power.
If you were to measure an engine's torque whilst applying a load high enough to cause the revs to drop, then a heavy pulley will show a higher figure. But if you measure the torque whilst the revs are increasing, the lighter pulley will come out on top. If you can think of a case when your 200SX or WRX is at full throttle and the revs are DROPPING, then it means your engine is probably running on 1 cylinder. In a high performance engine the emphasis is on acceleration, not momentum.
Q.: Are there any problems associated with under driving the alternator and power steering pump when using an underdrive Pulley Kit?
A.: No, the ratios are only reduced by about 15% on the alternator, and 20% on the others. This means that you get full charge at 1000 RPM instead of 850 RPM which will only cause problems if you plan to drive your car at idle for long periods of time with the fan, high beams, air conditioning, rear demister and stereo full blast.
The reason for under-driving is that from the factory the accessories (alternator, A/C, power steering) are designed to run at almost 100% at or near idle, so at high RPM they are causing excessive and unnecessary drag. Reducing the drive ratio by 15% doesn't have a large impact at idle, but at say 6000 RPM, 15% becomes a much larger proportion and the reduction in drag is much more noticeable.
Q.: What is a harmonic balancer?.....
A.: A better name for a harmonic balancer would be "torsional dampener" since its main task is to absorb the rotational pulses inflicted on the crankshaft by the pistons. Most often it is incorporated into the crank pulley by attaching the outer belt drive ring to the inner by means of vulcanized rubber. At the right RPM, it is possible for a resonant frequency to be set up torsionally on the crankshaft. Resonant frequency occurs when the pulses of the engine correspond with the natural frequency of the crankshaft and it ancillary components. However, since factory pulleys are often comparatively heavy (reasons for this are described later) it is actually the large mass (and therefore inertia) of the factory harmonic balancer and flywheel that will help to excite this natural frequency. So by dramatically reducing the weight and inertia of the crank pulley, the natural frequency of the crankshaft is shifted and its ability to self-excite is greatly reduced. So in fact it is the harmonic balancer's own weight that necessitates the dampening, and since the weight of a GFB crank pulley is typically about 20% of the factory component it cannot supply an exciting force significant enough to damage the crankshaft.
An opinion often expressed is "if the manufacturer put it there, it must be there for a reason". However, if you look at it from the car manufacturer's point of view, casting pulleys from steel is very cheap and easy, because they can be produced in large numbers and there is no waste (as opposed to machining them from billet). But because the resulting pulley weighs significantly more than one made from aluminium alloy, it requires dampening.
Manufacturers will always build cars (even high performance cars) to suit the widest possible selection of driving scenarios and drivers, which means there are always compromises. The weight of the flywheel and pulley also affect how fast the revs drop between gear shifts, and a production car is designed to only allow the revs to drop fast enough for average shifts. If you hurry the shift the revs will be too high for the next gear, resulting in a sharp jerk as the momentum of the engine transmits through the drivetrain. Reducing the engines' inertia with a lightweight pulley kit allows faster and smoother shifting.
When looking at high performance engines such as those found in Honda VTEC equipped cars and the S2000, it is obvious that manufacturers do understand the benefits of reducing engine inertia, and have utilized lightweight pulleys to help the power output and responsiveness without the use of a harmonic balancer.
However, this is not the case for all engines, many of them do require the use of the harmonic balancer to prevent failure. Skylines with the RB20, 25 and 26 are a good example of this, which is why we don't make a pulley kit for them. The pulley kits we do make are for engines that do not rely on the balancer to any significant degree
J'espère que ça éclaire des lanternes. En tout cas, ça a éclairé la mienne. Je compte mettre des poulies en Aluminium pour ma voiture.
Mais 100limites a tout à fait raison sur un point, il faut faire attention sur les poulies underdrive... c'est pas bon pour l'alternateur, ton climatiseur/chauferette va perdre de la puissance, ton power-steering va être moins assisté... ainsi que tous les autres accessoires... etc etc...
Je ne vois par contre aucun problème à la faire la plus légère possible (underweight).
L'underdrive peut-être OK si tu ne fais presque pas de ville. Les accessoires auraient la puissance voulue. Par contre, je serais un peu inquiet au sujet de la direction à assistance variable. C'est peut-être une idée que je me fais. :?
Le Baron
22/12/2003, 17h29
Il faut bien faire la différence entre la poulie d'accessoires et le "volant" (flywheel) auquel fait référence ce text. La flywheel c'est justement sa vocation de donner de l'inertie au moteur, elle a d'ailleurs un rayon beaucoup plus élevé. La poulie d'accessoires quant à elle a très peu d'impact sur le résultat final.
Le concept d'underdrive pulley agit à tout les régimes et à tout coup pour augmenter la puissance de façon modérée. Il ne diminue en rien le torque. La baisse d'inertie va plutôt donner une "impression" de diminution de torque parce que la voiture va être plus facile à staller.
Konnan: souvent ce qu'on veut dire par "moins de torque" c'est "moins de torque à bas régime". Parce qu'effectivement, pour produire plus de HP, faut soit produire plus torque, ou soit le produire plus haut.
C'est que plusieurs modifs ne font que transposer la courbe de torque vers la droite. Le même couple à plus haut régime va donner plus de puissance.
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