Altima SE V-6 vs. Accord LX V-6

Yes, the 6 speed is in the mid 14's on the Accord COUPE. If the sedan offered a 6 speed, it would be no question what car I'd go with.

Tony

Found this helpful -

Here's my take.

Power rules for acceleration, and a flat power output curve means faster acceleration. A flat power output curve is suggested by a relatively high-RPM torque peak.

It takes energy (or power, power being the time rate of energy application) to accelerate a car. An engine generating more power (in each given instant of time) will accelerate the car faster (in that instant). Period. Torque is not the same as work, energy or power. You can indeed stand on a stationary breaker bar and exceed the torque output of a typical car engine. But standing on a breaker bar won't move a car 0-60 in 8 seconds or less. Torque x rotational speed does equal power. (Must throw in conversion factors when using English units: Power [as hp] = torque [as ft-lbs] x rotational speed [as RPM] / 5252.)

In practice, if an engine produces peak torque at a relatively lower RPM, the power curve for that engine is more strongly sloped down from peak horsepower. It doesn't produce anywhere near the highest horsepower except at its peak horsepower speed. At practically all speeds in all gears (except near the peak speed in each gear) the power output is far below the engine's maximum output. Less power most of the time = Slower acceleration. But the engine will seem to be more flexible in normal driving, i.e., not requiring gear shifts all the time.

On the other hand, an engine that produces peak torque at a higher RPM has a flatter power output curve at the upper RPM's -- the power is more usable -- doesn't require a large number of exact gears to extract max. hp, and it accelerates faster. But to many drivers, it'll be a pain when strong accceleration is called for because it more often requires downshifting to keep the revs up.

--Pete

Ah, that post has made the rounds for years (not saying it is right, wrong, or lucid).

The physics is fine, but either I'm not following, or disagree with the following...

What do you mean by "flat power curve"? By their nature, IC engines tend to have flatter torque curves. If you had an "ideal" flat torque curve (and it really is debatable as to whether this is ideal), the hp curve would linearly increase with rpm. So the power curve is decidedly _not_ flat.

As an aside, note some electric motor types approach the flat hp curve. Their torque curve starts out with a high plateau at low rpm, then follows a decreasing hyperbola at higher rpm (ie flat, constant, peak power). But ICE's look nothing like this.

[skipping down ...]

Not sure this generality is accurate.

That goes for any ICE. Including high-revvers.

Again, that goes for any ICE. Again assuming the flat torque curve, then power = Constant*rpm. So fraction of peak power present rpm / peak hp rpm. Since a high-revver has the peak hp at a higher rpm, I think it would be more likely than not that you'd be further away. Which is the same as what you said about having to downshift to get at the hi-revver's power.

Agreed. Basically, if you want to drive at low rpm, get the big displacement engine.

Again, I do not see this generality as being obviously true. Sorry.

Give me two engines with the same hp, but widely different torque ratings in otherwise identical vehicles (choose gearing to best suit each) and they'll have just about the same best accel numbers. But in "normal" driving, most will perceive (scratch that, actually get) better accel in the big displacement, high torque engine since most will not normally be at the hi revs where the hi-revver makes up ground.

Your physics still holds. In *most* "normal" situations, the big-torque engine will be making more hp than the hi-revver. Unless you consider normal driving to be above 6000 rpm.

No, I meant a flat power curve. That implies a torque curve that decreases linearly with RPM.

OK.... See reply below.

My main purpose was to state some laws of physics, to clear up some false, garbled, or (at best) incomplete statements in earlier posts (Accord vs. Altima thread).

OK, I went out on a limb writing about car engines. You caught me. Went looking for evidence... took engine hp & torque ratings from Car & Driver auto reviews, and made the following table. (If the table is unreadable, set your newsreader font to Courier and maximize the window...)

Table: Horsepower at Max. Torque for Several Auto Engines

HP, Torque (mfr rating)@ Eng Spd HP @ Max Torque %HP at %RPM (@Max Torque) altima v-6 240 bhp @ 5800 rpm 86% of peak hp at 246 ft-lb @ 4400 rpm 206 hp 76% of max.HP rpm

pontiac g6 gt 200 bhp @ 5400 rpm 80% of peak hp at 220 ft-lb @ 3800 rpm 159 hp 70% of max.HP rpm

accord v-6 240 bhp @ 6250 rpm 84% of peak hp at 212 ft-lb @ 5000 rpm 202 hp 80% of max.HP rpm

cherokee v-8 235 bhp @ 4800 rpm 76% of peak hp at 295 ft-lb @ 3200 rpm 180 hp 67% of max.HP rpm

scion tc 160 bhp @ 5700 rpm 78% of peak hp at 163 ft-lb @ 4000 rpm 124 hp 70% of max.HP rpm

It looks like the engines that make max. torque at lower %RPM make a relatively higher %HP at that RPM, so this supports your view (I guess... it's a small sample and there are lots of details ignored...). ***You're saying that if two engines have identical peak HP @ x-RPM, the one that has peak torque at lower RPM will (generally) have better usable power output -- give better acceleraction?*** In other words, does this lower peak torque mean the engine was engineered 'better' for acceleration, given a typical drivetrain (limited # of gears, no hybrid help) .

Thanks for your input.

Trying again to make the table readable........

Table: Horsepower at Max. Torque for Several Auto Engines

HP, Torque (mfr rating)@ Eng Spd HP @ Max Torque %HP at %RPM (@Max Torque) altima v-6 240 bhp @ 5800 rpm 86% of peak hp at 246 ft-lb @ 4400 rpm 206 hp 76% of max.HP rpm

pontiac g6 gt 200 bhp @ 5400 rpm 80% of peak hp at 220 ft-lb @ 3800 rpm 159 hp 70% of max.HP rpm

accord v-6 240 bhp @ 6250 rpm 84% of peak hp at 212 ft-lb @ 5000 rpm 202 hp 80% of max.HP rpm

cherokee v-8 235 bhp @ 4800 rpm 76% of peak hp at 295 ft-lb @ 3200 rpm 180 hp 67% of max.HP rpm

scion tc 160 bhp @ 5700 rpm 78% of peak hp at 163 ft-lb @ 4000 rpm 124 hp 70% of max.HP rpm

Gotcha. And again, I agree with the physics. Torque v HP is one of the oldest, and even though simple, most misunderstood, auto forum debates. Part of the problem is in just getting the message across. Some people know the "truth", but either can't convey it, or comprehend someone elses discussion of it. But alas, I'd estimate that more than 50% of the folk screaming about it are clueless. Physics is the best way, but again alas, >50% are incapable or unwilling to follow.

Agreed, probably too small to make any conclusions. And I think you really have to look at the shapes of the curves. Variable valve timing, tunable intakes and systems like VTEC (added cam lobes) are doing a good job of broadening out torque curves. But there are still peaky hi-revvers (the 2.0L S2000 makes 153 ft-lb peak torque at ~7600, but off VTEC, ie ***You're

I was more referring to same hp but at different rpm. Clearly the hi-revver will be lower displacement with less torque. So if both are cruising at 3000 and you punch it, the hi-torque one is making more power (power=torque*3000 rpm), and accelerates better.

But if they have the same peak hp at same peak rpm, but one has peak torque at lower rpm, then what happens? Hmmmm. Well, if at lower rpm, then yes, the one with the lower rpm torque peak will clearly be quicker there. But overall the difference would be smaller than my case above, probably marginal. All depends on the shape of that curve. At any given rpm (and given road speed), the one with more torque is making more power and accelerates faster.

Tuning an engine for low rpm "drivability" vs high rpm power is a common tradeoff. And again, I think that's what a lot of this hi-tech variable valve technology is about. Getting best of both worlds.

Welcome, and nice to have a reasonable discussion on this topic :-)

Dave

I think you are as-backly-ackwards on this: If HP = Torque x RPM (which it does, with the fudge-factors you mentioned), then an engine requires MORE torque at lower RPM to get a "flatter" HP curve. That is, if you want HP to be more or less constant, you need MORE torque when RPMs are lower.

So, an engine with lots of torque at low rpm will give more hp at the lower rpm. This will compensate for the fact that, if torgue was constant, HP goes up with RPM. If torque is achived at a high RPM, then both RPM and torque are "peaking" at the same time, giving a very "spikey" hp curve.

Now, as you said, the flatter power curve (given by torque at LOW rpm) is better for smooth driving, fewer gear changes, etc. The "spikey" power curve gives more hp, since both RPM and torque are peaking at the same time, but results in lots of gear changes to find that spike.

Lloyd

Lloyd is right. Dig up a dyno chart for a rotary engine, which produces almost constant torque at any RPM. The HP is a straight diagonal line peaking right at redline. It's not the easiest car to drive fast, and requires careful shifting, but the Formula Mazda drivers don't complain one bit. If you want a flat horsepower curve, you need to peak your torque early and steadily decrease the torque in upper RPMs.

Dave

[snip]

No... you DO NOT want a FLAT POWER curve. Easy to show that

Acceleration = Power / (Mass x Speed)

So if power were constant (Flat) your acceleration would decrease as engine and car speed increased. The whole idea of modern engine design with all the variable valve timing etc. is to get a flat TORQUE curve. Because of the physics of combustion, there is max value of torque you can get from an engine. It's simply mean cylinder pressure x displacement. So the goal is to get the torque up as fast as possible at low rpm (by achieving high volumetric efficiency) and then *holding* it there at high rpm. Power dies when you reach the rpm where you can no longer fill the cylinders fast enough to keep up (torque falls)

Some of the previous replies are close - but I hope this makes it a bit more clear. Again, ideal IC engine would have a hp curve that is a straight line vs rpm, as stated by someone previously.

Frank

The main reason for my original post was to clarify the physical meaning of torque vs. power. Torque is a force. Force alone doesn't provide energy or power. It must be combined with movement (i.e. rotation) to make power. Power, not torque, is the 'stuff' that accelerates the car. If I could have an engine that produced maximum power over a wide range of speeds (without other tradeoffs, like much greater maximum power over a narrower range of speeds) then this is what I'd want.

Now, as I conceded in reply to Dave, I know nothing about the practical aspects of engineering a car engine for best acceleration. I am learning here as I read your replies. Sounds like physical laws governing heat engines make a flat torque curve the best thing achievable.

My thinking (w/o much knowledge as I've confessed) was that, in real-world engines of a given displacement and max h.p., if torque peaked at, say, 4000 r.p.m. instead of 3000 r.p.m., you'd get a rise in h.p. that exceeded the

3000 rpm peak-torque engine to that 4000 r.p.m. point, then a flatter h.p. curve (that's higher than the hp curve for the 3000-peak torque engine) out to the (say) 5800 peak-hp r.p.m. (Wish I could post a .jpg graph here...). I figured this was the meaning of cars getting 6-speed xmissions these days instead of 4-speed. Sounds like this is wrong -- that a real-world engine that produces max. torque at 4000 r.p.m. would be out-powered by a real-world engine (larger displacement, or same displacement but better breathing?) that produces max torque at 3000 r.p.m. but the same max h.p. at say 5800 r.p.m.

I would rather have a flat power curve -- equal to the MAXIMUM power output achievable under ANY sort of valve timing... etc. However, you've made clear that isn't achievable in the real world due to physical laws governing combustion engines... so I'll accept that a flat torque curve is the best that can be done.

Torque is a term describing a force applying to an object perpendicularly. In engine's term, torque is a twisting force at a particular rpm. Connecting these points together give a torque curve. When referring to torque being applied over time...during acceleration... the proper term is horsepower. A lot of people don't realize the proper term.

1. Most gasoline burning street engines make the same ft*lb per liter at

1000-4000 rpm range. This means that they have equivalent horsepower per liter in that range.

1. According to 1, doubling the engine means doubling the power, in that rpm range. A 4.0 put out 2x the power of a 2.0 in that rpm range...

As for your questi> It looks like the engines that make max. torque at lower %RPM make a

Well, if someone said they'd give me a 200 lb-ft peak torque engine, I'd ask that it be constant at 200 out to infinity (okay, as high as possible).

If someone said they'd give me a 200 hp engine, then I'd want it to be a flat 200 hp at all rpm. So, yes, I'd want max power at all revs.

So, it all depends on what you are offered :-)

But seriously, as everyone is saying, ICE's by their nature are much closer to the former (flat torque curve) than the latter (flat hp curve). Electric motors are closer to the latter.

So which is better? For a given power, take the motor. In theory, a CVT, or a whole lotta gears, will make the output of the ICE approach that (if it is designed and controlled to sit near the peak power rpm). But you'll need a near infinite gear ratio to match the motor at launch..