Does revving the engine in neutral in an automatic car make it go faster from the starting line?: originally appeared on Quora: The best answer to any question. Ask a question, get a great answer. Learn from experts and access insider knowledge. You can follow Quora on Twitter, Facebook, and Google+.
Does revving your engine make it go faster from the starting line?
In neutral? No. But I can easily see why one might think so, especially if -- as I suspect -- one has been watching live or televised drag racing. They rev it up, the green shines, and they fly down the track. So the high idle must be making them faster, and they must be in neutral since the car isn't moving, right? Well, no. If you rev your engine in neutral, then drop the gear lever into D while the engine is revved high, you aren't going to go fast -- you're going to violently and catastrophically break important stuff, and at best limp away from the line and coast to a stop, beaten and embarrassed.
Most modern drag-race cars now use an automatic transmission instead of a manual simply because the advent of computer transmission control allows the racer to shift and tune his transmission electrically precisely the way he wants it to shift. Some race cars shift the transmission by air pressure read through an air tube sensor -- some drag cars are so fast that the air can't get out of the way fast enough and impinges on the sensor connected to the car's computer with a predictable tipping point to signal the computer to shift. A modern race automatic will help a driver optimize his shift points and launch dynamics, which translate to better e.t.'s than a manual. In fact, one of the most popular drag automatics is a transmission nobody liked when it was new (1950-1974), but covet today: the GM Powerglide 2-speed automatic. It is used today as a converted electronic-shift auto. Its two forward gears made for a transmission gear ratio of either 1.82 : 1 or 1.76 : 1. Connected to the ground through the 3.31-geared rear end, a full-size Chevy with the 409 engine made famous by The Beach Boys could accelerate through first gear under full throttle and shift to second gear at around 75 mph. 75 in first gear without hurting it!
Racers like the Powerglide because it is light (not only because it has fewer parts, but beginning in 1962 models were built with aluminum cases), simple and reliable, and originally factory-built very tough ... and stronger modern parts make it very strong.
When Powerglides came out, they were nick-named the Slip'n'Slide because the engine crank is connected to the gearset via a device called a torque converter. This is an automatic transmission's clutch-pressure plate-throwout bearing-clutch fork, all rolled into a device about the size of a utility trailer tire and wheel. It weighs a little more. Torque converters don't actually have all those parts; those are mechanical coupling devices. Although once assembled to the engine and drive shaft an automatic looks like a mechanical coupling -- everything is solidly bolted together from engine block to wheel lug nuts, after all -- torque converters are fluid coupling devices, meaning, they use hydraulic fluid compressed between the shell of the converter and the converter internals to convert the rotation of the crankshaft to rotation of the driveshaft. Because there is no actual physical connection between the torque converter and the transmission itself, all automatics 'slip,' thus the nick-name from people who'd never driven anything but manuals (which slip too but by an amount determined by the driver's feet ... and you can always feel the car through your foot if you're driving a straight shift. You can't always feel road resistance through an automatic because of the slippage.)
The following is a brief (and inexhaustive) explanation of torque converters. Torque converters have 3 main parts: the impeller, the turbine, and the stator:
A torque converter is actually fairly simple. I'm keeping this simple, so I'm not covering "lockup torque converters." The impeller is the outer 'shell' of the converter (identified on the diagram); it sits inside the open front of the transmission and bolts directly onto the flexplate (bolted to the rear of the crankshaft, this is also the ring gear for the starter motor). The front of the transmission has gaskets and seals keeping fluid contained, and the input shaft sticking out the front. The torque converter slides into place over this shaft, mating its hollow output shaft to the input shaft. The impeller is actually the entire shell of the converter. This part, bolted to the engine's rotating assembly, always turns at engine speed, even whether you're going 120 or in 'Park' (this rotation also drives the fluid pump inside the transmission). The inside of the impeller has a row of fins welded to it at an angle that 'blows' the fluid inside the converter away from the impeller and toward the sides of its body by centrifugal force.
As the fluid moves away from the impeller fins, it is sucked into the turbine by its corresponding but opposite-angled fins. This causes the turbine to exert force upon the rest of the drive-train. With sufficient force, this makes the wheels turn and the car go. But torque converters are very inefficient at low RPM. At lower speeds and at idle, the turbine turns slower than the impeller (slippage), with enough RPM to turn the main pump and circulate fluid, but with insufficient force to overcome the mass and inertia of the car. In these conditions, the fluid leaves the turbine and enters the stator. The stator, which is not keyed to the output shaft but rotates on-axis around it, accelerates the fluid and sends it back under higher pressure into the impeller, where it accelerates even more, and is then sucked back into the turbine under even higher pressure. This is called vortex flow (see the arrows), and it creates the torque multiplication that gives automatic cars strong low-speed torque. One-way clutches in the stator ensure it only turns in the proper direction so as to create the vortex effect. Perhaps you may now suspect how revving the engine contributes to drag race performance.
When it comes to automatic transmissions, there are numerous internals, but really the question is actually about the torque converter.
So why do racers sit at the line at high rpm? What does that revving accomplish? That's the question.
Here's the answer, in 4 parts:
- Revving the engine causes the fluid to reach maximum pressure. Pressure is needed to circulate fluid, shift gears, and engage the clutch packs, and to hold the transmission in gear without slipping.
- Revving the engine cause the engine torque to be immediately available for acceleration as the momentum produced by heavy rotating parts is more abundant under acceleration and higher RPM.
- Revving the engine is necessary to "snug up" the drive-train before launch when a high stall speed converter is used -- and if you're drag racing, you are using a high stall converter. Stall speed is that figure in RPMs at which the fluid pressure becomes great enough to turn the turbine. Converters can be built, by changing fin angle, to "snug up" -- begin rotating the stator -- anywhere from 'off idle' (most factory original converters do this) to as much as 5,000 RPM or more. Generally, the higher stall speeds are for higher horsepower engines. One holds the brakes locked, gives it gas until you feel the car straining against the brakes, and on green, you floor the gas and lift off the brake. This produces a very fast launch, as many laws of physics are employed.
- Revving the engine is often necessary to keep the engine running, as many engines built to high performance often idle so rough they will die if not revved; it also helps prevent stalling the engine at the drop of the green.
But does revving the engine make the car faster?
Well, no, its top speed is unaffected by its idle, fast or slow.
It does make a car quicker. In racing terms, speed is how fast a car will go. Quickness is how fast a car gets to speed.
So a car sitting at the tree at idle will not start moving downrange nearly as quick as a car that is sitting at the tree revved up and snugged up when the tree lights green. To the last, unasked but implied question: why, if the car is being revved in gear, does it not leap ahead regardless of the staging light color? Because the front brakes of race cars can be locked on, so that the car can't move. The last thing the driver does at the staging line is release the front brake lock while the staging tree is counting down from red to green. He then holds the car with the foot brake until green, then lifts the brake, and away we go. Very fast.More questions on Quora: