robvuk

You guys are funny.

serendipity

Only true IF you had a CVT, which of course don't exist that can handle the power of any sports car. They also eat a fair amount of power themselves.

In a standard transmission, maximum accelaration in a gear is at peak torque - always. This is inherently obvious when you figure that torque at the wheels is crank torque times gearing. Nothing other than force at the wheels matters.

If you have a CVT, however, you can get maximum wheel torque by keeping the engine at peak horsepower. In a standard transmission, by the time the engine is at peak horsepower, you're not gaining any mechanical advantage by revving it higher, but you've fallen off peak torque. It makes sense to rev up to and potentially past peak horsepower, but the car will accelerate fastest in that gear at the torque peak.

ZeeOSix

That's why insane race cars have many many gears ... they only lose a few hundred RPMs between gears and if shifted optimally they can be close to a CVT in results.

Yes

I don't see how since max torque and max HP don't happen at the same RPM. The CVT still only transfers the engine torque to the wheels.

For max acceleration, you want hover +/- the peak HP point throughout each gear shift. I imagine if you had some kind of crazy transmission where it had infinite gear ratios, you could run the RPMs to the peak torque point and just keep it there until top speed is obtained due to aerodrag. Maybe that's what you meant above?

aharte

Peak power always occurs at a higher rpm than peak torque. So at a given speed, a CVT operating at peak power will have a larger ratio than one operating at peak torque. Although the engine torque there is less, the diffference in ratios more than makes up for it at the wheels. It's pretty easy to show that the best ratio is always the one that allows you to be at peak power.

serendipity

It's easy to get mixxed up. At a given vehicle speed, maximum acceleration will occur if you have a gear that puts the engine at the point of peak HP - that will maximize torque at the wheels. With a fixed gear, however, maximum torque at the wheels occurs at maximum engine torque (since torque multiplication is fixed).

A CVT is that crazy transmission. And you would keep it at peak HP, not peak Torque, for max acceleration. Here's a simple example:

Peak torque is 150lb-ft at 3000 RPM. Peak HP is 180HP, at 10,000 RPM (that's 94lb-ft of torque)

Let's say the wheels rotate at 1000 RPM at a given speed. In one gear, the engine is at the torque peak, 3,000 RPM at that speed (3:1 gear reduction). In another gear, the engine is at 10,000 RPM (10:1 gear reduction). In the taller, 3:1 gear, you're putting 3x150 = 450 lb-ft to the wheels. In the shorter gear, at peak HP, you're putting 94x10 = 940 lb-ft down - over double the torque at the wheels. You clearly want to take advantage of your engine's ability to rev high, by gearing the engine to stay at peak HP, not peak torque.

Downshifting isn't so much to put the engine in the powerband - that might make a small difference, but it's peanuts compared to the real reason. You downshift to take advantage of better torque multiplication. The ideal shift point in a simple torque curve (assuming no weird peaks and valleys) is the point at which torque at the wheels is equal in the next gear as it is in the current. With a flat torque curve, that's probably around red-line, since your torque isn't falling off enough to make losing the torque multiplication in the next gear worthwhile.

HP only matters because it gives an indication of how much torque multiplication you can get out of the engine, but torque at the wheels is the only thing that determines acceleration.

Retired GM Engr

Look folks, OK, I'm still having a really good time with all this "technical stuff",
but let's get serious. REALLY serious.

I went grocery shopping today, and forgot to buy microwave popcorn.
Oh My God (that would be, OMG), I didn't put it on my list.

This is serious ! ! !
All You'all know what happens to my posts when there is no popcorn to soak up my beer.

I may be getting senile at age 63.

Retired GM Engr

Here comes that beer without popcorn.

CVT. Snowmobiles. Big Bad *** Power snowmobiles. Fun, eh ?
The first time I ever rode a snowmobile I kicked a buddy's *** riding his wife's snowmobile.
With a lot less cubic inches, and "power" ( or "torque" ).

Of course, he wasn't a motocross racer like I was back then.

It just felt natural jumping, crossing up, and power-sliding the snowmobile.
No big deal. Easy. Just keep the throttle "full-on". It took care of "details".
Very simple compared to maximizing braking/accel on a motocross 5 or 6 speed MX bike.

But, the point I'm trying to make is snowmobiles are set up so you
"JUST keep the throttle pinned", and the pulleys do the rest.

All You'all can argue over whether that snowmobile was keeping the torque or the HP maximized.
It worked.

Velocity Steve

[QUOTE=aharte]No. In a given gear, maximum acceleration is at the torque peak.

In a given gear? How can you start that sentence with "no"? I didn't say in a given gear. I said with a continuously variable transmission.

If you had an ideal CVT, you would indeed set it to stay at the power peak for maximum performance.

Good for you. It only took you a few sentences to agree with me! You're brilliant!

aharte

You were directly contradicting the statement "Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear." Your mention of a CVT was secondary, and sounded like it was meant to support your claim about a given gear. This is clearly the way everyone else interpreted it as well.

Velocity Steve

I'M GOING TO SAY THIS ONE MORE TIME AND THEN I'M GOING TO FIND ANOTHER THREAD. MAXIMUM ACCLERATION IS, AS YOU EVENTUALLY SAY, AT MAXIMUM REAR WHEEL TORQUE, NOT MAXIMUM ENGINE TORQUE. THAT'S WHY THE CAR ACCELERATES FASTER IN FIRST GEAR THAN IN SECOND GEAR EVEN THOUGH THE ENGINE'S TORQUE CURVE DIDN'T CHANGE. THAT'S WHY A CAR WITH A 4.11 REAR AXLE ACCELERATES FASTER THAN THE SAME CAR WITH A 3.42 REAR AXLE. LET'S SAY YOU HAVE TWO IDENTICAL CARS AND BOTH HAVE THE SAME HORSEPOWER. LET'S SAY ONE HAS LOWER DISPLACEMENT AND GENERATES EXACTLY HALF THE TORQUE OF THE LARGER ENGINE. IF THE HORSEPOWER IS THE SAME, IT MUST GENERATE ITS TORQUE AT TWICE THE RPM TO GET EQUAL HORSES. IF THE CAR WITH LESS TORQUE IS GEARED TWICE AS LOW AS THE HIGHER TORQUE CAR, BOTH CARS WILL ACCELRATE AT EXACTLY THE SAME RATE. THE CAR HAS NO WAY OF KNOWING IF THE TORQUE IS FROM THE ENGINE OR FROM THE REAR AXLE.

aharte

WTF? I think you forgot to take your pills today. Besides going nuts off of something that's not very important, you said absolutely nothing that contradicts the statement you were replying to. You are both correct. At this point, I'm not sure if your problem is with English or physics.

serendipity

You don't need to shout. It doesn't make your argument any more or less incorrect.

YOU were the one confusing the issue by talking about CVTs.

Velocity Steve

Sorry for the caps. I didn't realize I hit the caps lock key. I'll be quiet and let everyone learn for themselves.

http://vettenet.org/torquehp.html

http://www.allpar.com/eek/hp-vs-torque.html

Velocity Steve

Sorry if you're so easily confused July 8, 1981. I was drag racing (at the track of course) for 18 years before you were born. If you had two cars that both weighed 3000 pounds, with identical gearing and both made 300 horsepower, you'd know that both cars would accelerate at the same rate. If you had two cars that both weighed 3000 pounds, with identical gearing and both made 300 ft lbs of torque, you'd have no idea which one would accelerate faster.

A 516 ft lb E55 Benz is not quicker than a 383 ft lb M5 BMW. They both have similar horsepower to weight ratios and the lower torque but higher revving BMW is geared 30% lower.

serendipity

We're saying the same thing, goofball. You confused the issue earlier by stating something that was irrelevant. None of the cars you've quoted, or ANY OTHER performance car has a CVT. So in those cars, maximum acceleration in any given gear happens at peak TORQUE, not peak HORSEPOWER.

Why don't you take your condescending attitude and calm down, old man?

ZeeOSix

Well, technically maximum torque at the rear wheels happens in each gear at the same time maximum torque happens in the engine.

Of course a car is gonna accelerate better in a lower gear, or with a lower geared differential.

OregonC6

IMO proper way to measure torque is to drive the car. If you like the car it has enough torque. If not you need more. Another good rule is that if the engine sounds like a rabid snowmobile at peak hp then you need a different and larger engine. Cubic inches are our friend. Always were and always will be. Cubes make torque and torque makes go.

Let's add torque and hp to the head up display then those with that option will know they have a powerful car instead of having to guess or calculate it with formulas.

The only thing better than too much torque is the wail of a big holley ( not likely to be heard on a C6 )

AU N EGL

The Feb 2006 Issure of Hot Rod put a big Holly 4 on an LS7 - then the two new GMPP Katech developed cams in the LS 7 and did a little testing.


passes and to Retired GM Engr.

black97magic

Physics doesn't lie. Read the entire article and you'll understand why I wrote what I did. Yes, gearing plays a part, but that's explained as well.
http://www.yawpower.com/tqvshp.html

Second

Maximum Acceleration will happen at the peak torque in any standard or automatic transmission. This is a fact not debatable.

I think where confusion is forming is b/c of the CVT. CVT adds the additional factor of gearing changes throughout the rpm range instead of constant gearing throughout the rpm range. This could provide a variety of different answers depending on the engine output through the rpm range. I haven't run these numbers but really it is moot to the original intention of this post.