What does power 'under the curve' mean?

[RedM]

Laymans terms please.

[michael]

"The curve" being a dyno plot - the more area there is under the curve the more power the car tends to put out over a wider rev range.

 

Car 1 may peak at 200bhp and have a curve that goes up at a steady angle, car 2 could also peak at 200bhp but have a curve that ramps up to near max power within 1500rpm and then goes almost flat to the limiter. Car 2 would have more more power "under the curve".

 

Car 1 might have a plot like this:

 

http://www.adaptronic.co.uk/CA18DET_dynoPlotLittle.GIF

 

Car 2 might be like this:

 

 

More area under the curve on car 2, car one doesn't have the low end grunt (in this example car 2 sucks at the top end but imagine it staying fairly level).

[RobSheffield]

Im glad you did that Michael.

 

I tried a few times to phrase a reply, went off looking for dyno graphs and gave up

[RedM]

Hmm... so would a car with a 6ltr V8 have loads of room under the curve and therefore be more torquey than say a turbo powered car?

[michael]

Hmm... so would a car with a 6ltr V8 have loads of room under the curve and therefore be more torquey than say a turbo powered car?

 

Depends on what the turbo car has to offer but you'd expect a 6L V8 to be pretty good on torque from low in the rev range.

[Gazboy]

Hmm... so would a car with a 6ltr V8 have loads of room under the curve and therefore be more torquey than say a turbo powered car?

 

 

Oh yes, that V8 is going to have 400lbs.ft of torque from tickover to redline, where's a Porsche 911TT won't have 400lbs.ft until 4000rpm. Ok the V8 won't rev much past 6000rpm but the Porsche is done by 7000rpm anyway. Gear ratios tend to crash the party though, give a Vette Pork ratios and the 911 wouldn't see which way the Chevy went.

[matt k]

Oh yes, that V8 is going to have 400lbs.ft of torque from tickover to redline, where's a Porsche 911TT won't have 400lbs.ft until 4000rpm. Ok the V8 won't rev much past 6000rpm but the Porsche is done by 7000rpm anyway. Gear ratios tend to crash the party though, give a Vette Pork ratios and the 911 wouldn't see which way the Chevy went.

 

 

997 turbo

Power 480 bhp / 358 KW @ 6000 rpm

Torque 620 Nm / 457 ft lbs @ 1950 rpm

 

 

Corvette C6 Z06

Power 500 bhp / 373 KW @ 6200 rpm

Torque 657 Nm / 485 ft lbs @ 4800 rpm

 

Yeah turbo is weak below 4K?

[RedM]

997 turbo

Power 480 bhp / 358 KW @ 6000 rpm

Torque 620 Nm / 457 ft lbs @ 1950 rpm

 

 

Corvette C6 Z06

Power 500 bhp / 373 KW @ 6200 rpm

Torque 657 Nm / 485 ft lbs @ 4800 rpm

 

Yeah turbo is weak below 4K?

 

See, when people post that kind of thing I don't get it. I can't relate those figures into on the road performance.

 

I'm taking it that you don't agree that the Porsche is weak below 4k?

[michael]

I'm taking it that you don't agree that the Porsche is weak below 4k?

 

With 457 ft lbs @ 1950 rpm I'd say it's far from weak

[Whitesupraboy2]

i know what you mean Red, its weird / amazing how engines can differ so drastically yet using the same 'theory / mechanics' on how they work.

[carl0s]

That's a fuckload of torque at less than 2,000rpm. Wow.

[jevansio]

Variable vane turbos for ya

[Ian C]

Yeah, but, turbocharging buggers up that figure as well because you need engine load to make boost. It's very misleading as that 1950rpm figure will be in top gear at WOT. Probably uphill. So it makes full boost pressure. In 2nd gear that Porsche won't get anywhere near 450ft/lbs probably throughout the rev range never mind at 1950rpm. Much like any other turbo engine - and if it did, then it's turbo is too small and it'll choke top end power, but that's another discussion.

 

NA engines on the other hand have the same response no matter what gear it's in.

 

As for gear ratios, that's not really relevant either, because any high rev screamer race engine is great when working the gears. Area under the curve = drivability. And for us everyday street drivers that's the biggie factor for a nice car. If you've got fat torque at 3000rpm you can row through the gears off a roundabout without having to work hard, changing up at about 4000rpm each time, lovely.

 

-Ian

[Gazboy]

997 turbo

Power 480 bhp / 358 KW @ 6000 rpm

Torque 620 Nm / 457 ft lbs @ 1950 rpm

 

 

Corvette C6 Z06

Power 500 bhp / 373 KW @ 6200 rpm

Torque 657 Nm / 485 ft lbs @ 4800 rpm

 

Yeah turbo is weak below 4K?

 

I was thinking of the 996TT & the Corvette C6 6.0, to keep it simple (as the 997 makes it too complicated and it's also the only current production car with Variable Vanes) seeing as both have around 400bhp and RedM mentioned a 6.0 V8.

 

996TT 420bhp @ 6000rpm/ 413lbs.ft @ 4600rpm

Corvette 6.0 404bhp @ 6000rpm / 403lbs.ft @ 4400rpm

 

I'm sure I could dig out some dyno sheets showing the Vette spanking the 996 until the Porsche comes on boost.

 

Edit, not that easy, the 6.0 boys only post rwhp graphs and sd of 22.34 I haven't found a single 996 graph. Here is a 5.7 flywheel graph (which is 50bhp&tq down on a 6.0, but still has oodles of torque sub 2000rpm:

 

[JohnA]

Yeah, but, turbocharging buggers up that figure as well because you need engine load to make boost. ..

 

That's a very good point actually.

Turbos have deceptively fat torque curves on the dyno but when you drive them they feel 'emptier' down low - compared to an equivalent high-capacity n/a engine.

 

So the dyno figures can be a bit misleading here.

[Syed Shah]

For a sports car, I class the relevant area under the curve to be the lowest point that revs would drop on a gear change.

 

So, if the biggest RPM drop when changing from redline drops you to say, 4.5k rpm, then it is the power between this point and redline that is important (from a performance perspective). And, of course, the power under the curve between these points.

 

 

Massive low-down torque is fun, and quicker for the unskilled driver. But personally, I love the driver involvement added by having a power band. It is why I enjoy a S2000 to a Viper on twisty roads.

[dangerous brain]

The main thing about having more power under the line is the fact that you don't have to change gear to keep shifting along. If you have a decent amount of torque (the engines actual pulling power) from lower RPM then the car will have lots of accelerative ability at much lower RPM. Makes the car alot more driveable if you don't have to keep hunting through gears to find the one where your power is optimum.

Engines that go up slowly to start then go straight up from 4k or 5k rpm have little pull under that rev figure whereas ones that go straight up from the offset and smooth out from 4.5k to 5.5k RPM have nice pull from wherever you want it. This is why diesels these days are so competitive these days.

 

In laymans terms that line is the power of the engine on a dyno graph, the more area there is underneath the line the more power you have available, wherever or however that line gets there.