kevintari

I'm wondering about the factors that go into producing acceleration...how can you have two cars with roughly equal HP & TQ yet have a good separation in 0-60 and 1/4 mi times? Like the NSX (290 hp V6) and, say a Lexus or Infiniti sedan (don't laugh, insert any comparable engine here) with a lot of hp I 'm sure gearing and weight are definitley factors, but what else?

Is the red line a factor? And what determines that, bore and stroke? What's are the advantages of a high red line?

I know, I'm asking for a degree in engineering...

Thanks!

Zed 2.0

A VTEC car may have very high peak horsepower, but it won't do as much good because it's only on for 1.5k RPMs right before redline.

Sourdough Josh

A Porche, for example, has an extremely high power/weight ratio, though the actual HP peak is often not *that* high. The car just doesn't weigh anything.

Hokie_Audi

The location of the powerband has a lot to do with how quickly a car can accelerate. Peaky engines only produce power in narrow ranges of engine speed. A wider, flatter torque spread is prefferable. All things being equal in the engine area and the differences will come out of the suspension geometry, unsprung mass, and tires; all of which help transfer the power from the drivetrain to the ground.

Concerning engine speeds... the redline is determined by the strength of the reciprocating components in the engine, along with the tolerances on piston seals amongst other things. A piston travels the length of the stroke. Stops. Changes direction and travels back. Several thousand times per minute. The forces exerted on the connecting rods are tremendous. To build a high revving engine you need very light, yet strong and tough materials for the pistons, rings, and con-rods amongst other things.

Higher RPMs will let the engine process more air in a given amount of time, generally leading to higher power. There are limits though namely the breathing efficiency of the intake and exhaust systems, valves, etc.

Greg Slater

Acceleration is just the act of getting some mass to change its velocity. In terms of accelerating a car to a certain speed, it's a function of how much force you can apply to push the mass (the car) up to some speed. The force of the tires pushing on the ground comes from the torque of the engine. Horsepower isn't really a factor (at least not directly). The engine twists (provides torque) with a certain strength, and that twist gets transmitted through the drivetrain (along with frictional losses), and then through the tires (along with their flex and slip) to the ground. So all along the chain from the engine output, there are things working to steal some of that precious torque. At the point of tire contact with the ground (also known as "where the rubber meets the road"), the engine torque has finally become the force that causes the car to accelerate. Of course, there's another acceleration that has to be accounted for, and that's the rotational acceleration of the wheel/tire combo. The energy for that also comes from the engine torque.

When you see HP/torque numbers listed for cars, those are just peak numbers, which typically occur at a certain RPM. You can't even really use "average" values for these things in acceleration calculations. Technically, you'd have to integrate the instantaneous torque values (minus all those losses mentioned above, of course) over the time period during which the acceleration is occurring. While accelerating, a car with an engine that produces a PEAK torque of 300 ft-lbs goes through various speeds where the torque is less. For example, at a certain RPM, the torque might only be 250 ft-lbs. At that instant, there isn't 300 ft-lbs accelerating the car, so there's no point in using 300 in the calculation. So two cars with similar weights and similar peak torque numbers may not accelerate the same, simply because the shape of the torque curves is so very different between the two cars.

Engine design will have an effect on the behavior of the engine and how easily it wants to spin up, and hence how much time is spent in what part of the torque curve, but ultimately the important thing is the instantaneous torque available as the primary engine output. And of course the gearing of the transmission will have an impact.

In other words, to accelerate quickly, you want very low mass, very high torque that is available at almost all engine speeds, very sticky tires for transmitting the torque to the ground as propulsive force, a lightweight drivetrain (including wheels and tires) to reduce the amount of energy necessary to get things rotating, a gearing setup where you have the shortest non-acclerative periods (like between a 1-2 shift) you can arrange, and a frictionless drivetrain to reduce frictional losses.

Or something like that.

Ray Khan

aerodynamics plays a litlte role too...with all the things everyone else has said

Jon G

a car can only go as fast as it can apply its power to the ground (traction). Quality of tires and FWD, RWD and AWD can have a huge effect. The less weight, the faster that force can accelerate it. Only in the last few years have production cars been available with autos that shift faster than manuals, but luxory cars (lexus, inifiniti) have auto trannies geared towards comfort, thus compromising shift speed. Power curve of the engine counts too, sure, an engine may have 400 horses, but it may only be for 500 RPM.

J4SON

Nice sig!!