Engine flexibility - a fair fight?

[Peter Carmichael]

Beemer 330D vs. my pre-expiry 1.8 litre k-series.

 

Somebody helpfully sent me a power plot for the Beemer. I have had to trace the curve, but my numbers should be accurate to within a few percentage points

 

330D:

Peak power: 225bhp@3900rpm

Peak torque: 325lbft@3509rpm

 

1.8 K:

Peak power: 250bhp@8900rpm

Peak torque: 169lbft@6500rpm

 

By plotting the power curves against a normalised engine speed axis (0 to 1 equating to "stationary to maximum rpm"), I have a graph where nowhere in its operating range is the 3 litre turbocharged BMW diesel producing more power than that 1.8 petrol engine. Admittedly it gets better mpg and lasts longer between rebuilds.

 

What does this mean?

 

It means that if you got a primary step down ratio on the k-series, you could plug it into the same Beemer gearbox and the k-series would be better at pulling the Beemer and a caravan up a hill, all the while operating at a smaller proportion of its maximum operating rpm.

 

It means that a 141bhp/litre race engine can be as flexible as a renowned stump puller.

 

It means that torque loses.

[Peter Carmichael]

Just realised that the figures I used were for a chipped engine, taken from here.

 

[AMMO]

After some quick back of an envelope calculations I reckon that an F1 engine that revs to around 18,000 rpm makes less than 250 ft/lbs of torque. This is making some assumptions which may not be correct but that should at least be in the ball park. Lets assume that it makes 800 bhp and that peak torque occurs at around 16,000 rpm. Even if the assumptions and calculations are wrong the relationship between power and torque are still disproportionate.

 

If torque were that important F1 engines would all be turbo-diesels. The fact they are not and that F1 are chasing ever-increasing rpm levels should tell us something. If the best paid engineers on the face of the planet are going down the rpm route you have to conclude that power is more important than torque in a competition engine.

 

 

 

AMMO

[Peter Carmichael]

Just attempting a "reductio ad absurdum" argument on this one. Similiar power levels Vast disparity of torque levels. Close to a factor of 2 difference in their rev range. Similar performance potential, excluding considerations of weight, but still finding in favour of the low torque engine.

 

Stick a 2:1 gearbox on the k-series and the output would be at up to 4650rpm peaking at

250bhp@4450rpm

338lbft@3250rpm.

[Paul Ranson]

If torque were that important F1 engines would all be turbo-diesels. The fact they are not and that F1 are chasing ever-increasing rpm levels should tell us something.

If F1 had a rev limit power would still rise every year. In the current rules environment it seems that increasing the rpm is the most productive line of development. When the 'one engine per weekend' rule comes in then I think we'll see some variation in approach, and perhaps a brief drop in power output. But power will still rise annually....

 

Peter's illustration is amusing. I expect some people will still not get the point.

 

Paul

[felix.klauser]

And the point is?

 

What about fuel economy? Emissions? Engine longevity? Refinement? It's horses for courses...

[edmandsd]

Autocar this week suggests Jag F1 peak torque is 380 ft lb's.

 

Home of BDR700

[Peter Carmichael]

That seems very unlikely using normal metrics. Are they running 5 valve per cylinder heads? Is that at maximum ram from the airbox?

 

380/3 = 127 lbft /litre

Let's see. 800bhp/18000*5252 = 233 lbft.

Maximum power at 233/380*100 = 61% of peak torque?

 

Try as many 4 valve per cylinder natasp engines as you can think of with the same calculations and see if you come up with any number which isn't between 88% and 91% and any specific torque figure higher than 100lbft/litre.

 

I wouldn't trust journalists to get that right. I wouldn't suppose Jaguar would feel much need to tell the truth when their competitors are not sharing such information.

[edmandsd]

The Gould's quote of 225 ft lb's @ 9,000 rpm and 477 bhp @ 11,300rpm seems closer to the mark, and more representative of such a highly tuned engine.

 

477/2.5 = 191 bhp/litre

225/2.5 = 90 lbft /litre

477bhp/11300*5252 = 222 lbft.

222/225 = 98.7% of peak torque!

 

It could actually be because the engine gives a fair bit more power at higher rpm with pneumatic valves i.e. The wire valve springs are the limiting factor. Makes by BD seem tame in comparison..........and then we get onto the F1 engine..........we're living in the stone age compared to these monsters ! ☹️

 

Home of BDR700

 

Edited by - edmandsd on 24 Sep 2002 20:00:25

[Smokin Joe]

Plagerised but the message is clear 😬

 

Engines don’t make horsepower; they convert fuel into torque. Torque is the twisting force imparted to the crank flange and transmitted to the transmission and the rest of the drive train. To some degree torque is the grunt that gets things moving and horsepower is the force that keeps things moving.

 

First of all, from a driver's perspective, torque, to use the vernacular, RULES :-). Any given car, in any given gear, will accelerate at a rate that *exactly* matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same.

In contrast to a torque curve (and the matching pushback into your seat), horsepower rises rapidly with rpm, especially when torque values are also climbing. Horsepower will continue to climb, however, until well past the torque peak, and will continue to rise as engine speed climbs, until the torque curve really begins to plummet, faster than engine rpm is rising. However, as I said, horsepower has nothing to do with what a driver *feels*.

 

You don't believe all this?

 

Fine. Take your non turbo car (turbo lag muddles the results) to its torque peak in first gear, and punch it. Notice the belt in the back? Now take it to the power peak, and punch it. Notice that the belt in the back is a bit weaker? Fine. Can we go on, now? :-)

OK. If torque is so all-fired important, why do we care about horsepower?

Because (to quote a friend), "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*.

 

For an extreme example of this, I'll leave car land for a moment, and describe a waterwheel I got to watch awhile ago. This was a pretty massive wheel (built a couple of hundred years ago), rotating lazily on a shaft which was connected to the works inside a flour mill. Working some things out from what the people in the mill said, I was able to determine that the wheel typically generated about 2600(!) foot pounds of torque. I had clocked its speed, and determined that it was rotating at about 12 rpm. If we hooked that wheel to, say, the drive wheels of a car, that car would go from zero to twelve rpm in a flash, and the waterwheel would hardly notice :-).

 

On the other hand, twelve rpm of the drive wheels is around one mph for the average car, and, in order to go faster, we'd need to gear it up. To get to 60 mph would require gearing the wheel up enough so that it would be effectively making a little over 43 foot pounds of torque at the output, which is not only a relatively small amount, it's less than what the average car would need in order to actually get to 60. Applying the conversion formula gives us the facts on this. Twelve times twenty six hundred, over five thousand two hundred fifty two gives us:

 

6 HP.

 

Oops. Now we see the rest of the story. While it's clearly true that the water wheel can exert a *bunch* of force, its *power* (ability to do work over time) is severely limited.

OK. Back to car land, and some examples of how horsepower makes a major difference in how fast a car can accelerate, in spite of what torque on your backside tells you :-).

A very good example would be to compare the current LT1 Corvette with the last of the L98 Vettes, built in 1991. Figures as follows:

 

Engine Peak HP @ RPM Peak Torque @ RPM

 

------ ------------- -----------------

 

L98 250 @ 4000 340 @ 3200

 

LT1 300 @ 5000 340 @ 3600

 

 

The cars are geared identically, and car weights are within a few pounds, so it's a good comparison.

First, each car will push you back in the seat (the fun factor) with the same authority - at least at or near peak torque in each gear. One will tend to *feel* about as fast as the other to the driver, but the LT1 will actually be significantly faster than the L98, even though it won't pull any harder. If we mess about with the formula, we can begin to discover exactly *why* the LT1 is faster. Here's another slice at that formula:

 

Horsepower * 5252

 

Torque = -----------------

 

RPM

 

 

If we plug some numbers in, we can see that the L98 is making 328 foot pounds of torque at its power peak (250 hp @ 4000), and we can infer that it cannot be making any more than 263 pound feet of torque at 5000 rpm, or it would be making more than 250 hp at that engine speed, and would be so rated. In actuality, the L98 is probably making no more than around 210 pound feet or so at 5000 rpm, and anybody who owns one would shift it at around 46-4700 rpm, because more torque is available at the drive wheels in the next gear at that point.

On the other hand, the LT1 is fairly happy making 315 pound feet at 5000 rpm, and is happy right up to its mid 5s redline.

 

So, in a drag race, the cars would launch more or less together. The L98 might have a slight advantage due to its peak torque occurring a little earlier in the rev range, but that is debatable, since the LT1 has a wider, flatter curve (again pretty much by definition, looking at the figures). From somewhere in the mid range and up, however, the LT1 would begin to pull away. Where the L98 has to shift to second (and throw away torque multiplication for speed), the LT1 still has around another 1000 rpm to go in first, and thus begins to widen its lead, more and more as the speeds climb. As long as the revs are high, the LT1, by definition, has an advantage.

 

Another example would be the LT1 against the ZR-1. Same deal, only in reverse. The ZR-1 actually pulls a little harder than the LT1, although its torque advantage is softened somewhat by its extra weight. The real advantage, however, is that the ZR-1 has another 1500 rpm in hand at the point where the LT1 has to shift.

 

There are numerous examples of this phenomenon. The Integra GS-R, for instance, is faster than the garden variety Integra, not because it pulls particularly harder (it doesn't), but because it pulls *longer*. It doesn't feel particularly faster, but it is.

 

A final example of this requires your imagination. Figure that we can tweak an LT1 engine so that it still makes peak torque of 340 foot pounds at 3600 rpm, but, instead of the curve dropping off to 315 pound feet at 5000, we extend the torque curve so much that it doesn't fall off to 315 pound feet until 15000 rpm. OK, so we'd need to have virtually all the moving parts made out of unobtanium :-), and some sort of turbo charging on demand that would make enough high-rpm boost to keep the curve from falling, but hey, bear with me.

 

If you raced a stock LT1 with this car, they would launch together, but, somewhere around the 60 foot point, the stocker would begin to fade, and would have to grab second gear shortly thereafter. Not long after that, you'd see in your mirror that the stocker has grabbed third, and not too long after that, it would get fourth, but you'd wouldn't be able to see that due to the distance between you as you crossed the line, *still in first gear*, and pulling like crazy.

 

I've got a computer simulation that models an LT1 Vette in a quarter mile pass, and it predicts a 13.38 second ET, at 104.5 mph. That's pretty close (actually a tiny bit conservative) to what a stock LT1 can do at 100% air density at a high traction drag strip, being power shifted. However, our modified car, while belting the driver in the back no harder than the stocker (at peak torque) does an 11.96, at 135.1 mph, all in first gear, of course. It doesn't pull any harder, but it sure as hell pulls longer :-). It's also making *900* hp, at 15,000 rpm.

 

Of course, folks who are knowledgeable about drag racing are now openly snickering, because they've read the preceding paragraph, and it occurs to them that any self respecting car that can get to 135 mph in a quarter mile will just naturally be doing this in less than ten seconds. Of course that's true, but I remind these same folks that any self-respecting engine that propels a Vette into the nines is also making a whole bunch more than 340 foot pounds of torque.

 

That does bring up another point, though. Essentially, a more "real" Corvette running 135 mph in a quarter mile (maybe a mega big block) might be making 700-800 foot pounds of torque, and thus it would pull a whole bunch harder than my paper tiger would. It would need slicks and other modifications in order to turn that torque into forward motion, but it would also get from here to way over there a bunch quicker.

 

On the other hand, as long as we're making quarter mile passes with fantasy engines, if we put a 10.35:1 final-drive gear (3.45 is stock) in our fantasy LT1, with slicks and other chassis mods, we'd be in the nines just as easily as the big block would, and thus save face :-). The mechanical advantage of such a nonsensical rear gear would allow our combination to pull just as hard as the big block, plus we'd get to do all that gear banging and such that real racers do, and finish in fourth gear, as God intends. :-)

 

 

 

 

[edmandsd]

Does your drag model account for very lightweight applications Smokin' ?

 

I always reckon I can assess a car's output (of similar type and by comparison using a reliable benchmark) via it's terminal speed over a standing quarter mile.

 

 

 

Home of BDR700

[Paul Ranson]

It could actually be because the engine gives a fair bit more power at higher rpm with pneumatic valves i.e. The wire valve springs are the limiting factor.

I was told by a man who might know that over 500BHP would be available from this engine on air springs, which are now available 'off the shelf' rather than having to use the Cosworth proprietary technology. AFAIK nobody has funded this experiment yet.

 

The numbers quoted from Autocar for the 2.5 are about what I recall the man who built (or is associated with the man who built) the engine telling me. I don't think he has any reason to obfuscate what the dyno says, I'm a 'potential customer'...

 

My Vauxhall 2 litre makes about 90lbft/litre on petrol.

 

Cancerous Joe is extremely long winded...

 

Paul

[Smokin Joe]

As my moniker would imply I am usually short winded. I did say I stole the piece!

 

Lightweight of course requires less torque to move hence the good times available with small Torque in a 7.

Yes Edmands Terminal speed is a good way to calculate what is needed.

 

I have a lot of Torque, over 200lbs from 5-7500 rpm and can best describe it as instant steam. No waiting for revs to build car just pushes hard in any gear the moment you press pedal.

Joe