shift points -- torque vs hp

[cojones]

interesting point made by the classroom instructor at a BMW DE event this weekend... to help determine shift points.

- HP helps sustain upper end speed
- torque causes acceleration

Most engines make their peak HP close to redline; their torque peak is offset by x (typically x ~ 1000-1200 rpm)

His advice:
- if upshifting on a straightaway, you want to maximize HP, so shift close to the HP peak (near redline)
- if downshifting into a corner, you want to maximize torque (to get acceleration into apex and track out), so shift to keep the car in the "fat" part of the torque curve

As a sidenote, sometimes "short shifting" (shifting below redline) may actually be good, regardless of location, as it puts the motor in a better location on the torque curve.

Thought it was interesting, as most folks thing HP and not torque...

 

[cobalt]

i don't think...i just do...

"...use the force, young jedi..." <grin>

Ah...the famous fifty-two fifty-two rule!

[This message has been edited by HomeDepotNSX (edited 20 May 2002).]

 

[nsxtasy]

That advice is applicable for cars other than the NSX. On the NSX, for all practical purposes, there is no "fat" part of the torque curve. The torque curve is flat enough that you optimize the car's performance by driving it in the lowest gear at all times, shifting at, or close to, redline. Remember, horsepower is a product of torque and revs in any particular gear.

For a good description of the importance of torque vs horsepower, click here.

 

[cojones]

Ken's advice is true... if the torque curve is very flat, then its less of an issue.

Other cars have less-tractable torque curve.

 

[MvM]

I don't know about other NSX'es but I can actually drive into the red area, at least 1st and 4th gear. Yesterday, I had a motorcycle on my back while doing about 170kph. On a empty piece of highway I shifted into 4th and pulled away. I had to leave the road a couple of km ahead so I just thought, why not. When a reached the exit I was doing about 8100 rpm and 240kph.
Actually, it went so smooth I did not notice I was doing 8100rpm until I looked onto the dash.

 

[naaman]

I'm kind of new to the gearhead scene so forgive me if this question sounds silly.
But what is the point of accelerating to 8000rpm if the car reaches maximum hp at 7100 (i think) rpms? And doesn't V-TECH "eliminate" the torque/power tradeoff?

 

[true]

Originally posted by naaman:
I'm kind of new to the gearhead scene so forgive me if this question sounds silly.
But what is the point of accelerating to 8000rpm if the car reaches maximum hp at 7100 (i think) rpms? And doesn't V-TECH "eliminate" the torque/power tradeoff?

You are right, yet almost always overlooked. The stock NSX makes peak power at 7100RPM. If you have a stock nsx and are racing (accelerating) and shifting at anything other then that, you're not making full potential. Just because the car 'redlines' at 8k doesnt mean shift there when racing. Shift right before you stop making power.

Also, it's VTEC...no H. Just warning before someone lamerizes you.


------------------
jack of all trades, master of some.

 

[nsxtasy]

Nope. For maximum acceleration in the NSX, you should shift at redline, not at the power peak. Actually, this is true for ANY car. For maximum acceleration in any car, you should shift where the power in the lower gear drops to the same value as the power in the higher gear, not at the power peak. In the NSX, this does not occur at any rev point lower than redline.

The following diagram shows the power curve in the NSX:

Yes, the power peaks at 7100 rpm. But at revs above that point, the power is still higher than it is where the revs will be after upshifting to the next gear. Depending on which transmission and which set of gears you are using, if you shift at redline, the rpm drop to anywhere from 4499 rpm to 6500 rpm. When the revs drop to 4499 rpm, you're getting more horsepower in the lower gear at redline (250 hp in the '91-96), than you are in the higher gear at 4499 rpm (170 hp). If you shift before redline, and the power in the higher gear will be even lower. So you will be accelerating slower in the higher gear than you were in the lower gear - NOT what you want.

The smallest drop in revs is the 4-5 shift in the six-speed, where shifting at redline will drop to 6500 rpm. Even there, with the 3.2-liter motor, the power at say 7800 rpm is higher than it is at 6300 rpm where you would wind up after upshifting, so you would not want to shift until redline there, either.

That's why you need to shift at redline for maximum acceleration in the NSX. Before that, the power never drops to the point where it's less in the lower gear than it is in the higher gear.

[This message has been edited by nsxtasy (edited 23 May 2002).]