Saturday, August 27, 2011

Comparing bananas and lemons - Part 1 : Power




When it comes to motorbikes, there is a lot of partisan attitude, some fanboyism and cliquishness…

Right here we have the Bullet chaps (moi included), the Yezdi/Jawa fans (me included), the RD350 lovers, and the RX boys – Inevitably, there is debate about the relative merits of each and much bragging about top speeds and ¼ mile times.

But, the problem is: How would you go about comparing say a 100cc Hero Honda Splendor with a 1300cc Suzuki Hayabusa? It’s all very well to say that they’re made with different aims and can’t be compared, but actually they can – even though the only common thing about the two is the fact that they’re Japanese designs and 4 strokers.

Looking at the fundamental aspect of a motorbike – the engine – we can define certain parameters that let us place a bike as a point in some multidimensional space and see how it compares to others along different dimensions.

Power
The scientific definition of power is “Work performed per unit time” – no doubt this is the absolute measure of a bike, but then things are also relative – Which is more impressive? An elephant lifting a quarter of its weight, or a slim weightlifter hoisting four times his own weight overhead?

Thus, to measure different classes of bikes we need to normalize the power values.

An engine produces a certain amount of power at a certain RPM with a certain displacement – Ideally, (and this is true to a large extent as you will see later) we should assume that if you double an engines displacement, and maintain the same RPM, you should expect twice the power. The same applies to doubling RPM while keeping the displacement same.
The reason is simply because an engine is essentially an air pump, and the power it produces is purely dependent on how much air flows through it.
 
Thus, the correct way to measure relative power output is to divide the actual value by RPM and displacement.

For example: If a 100cc engine makes 7.5 HP @ 6000 RPM, we should expect a 200cc engine of similar design to develop 15 HP at the same RPM, since it displaces twice the volume as the former in a given interval of time.

Thus one of the first variables we can derive (in no meaningful SI units) is HP/Liter-KRevs - Namely, how many peak horsepower are developed per liter of engine per 1000 RPM – We will call this the K - factor

Let’s look at some famous (mostly from the Indian market) 4 stroke bikes past and present and see where they fare… Note that this data is gleaned from manufactures specs and some of it from reading wheel dyno charts, and adjusting upwards by around 15% to get crankshaft figures.

Japanese bikes
Model`
CC
RPM
HP
K
Hero Honda CD100
100
7500
7.5
10.00
Bajaj Pulsar DTSI 220
220
8500
21
11.23
Honda Karizma ZMR
223
7000
17.6
11.27
Yamaha R 15
150
8500
17
13.33
Honda CBR 250
250
8500
26.4
12.42
Honda CB750
750
8000
67
11.17
Suzuki Hayabusa
1300
12000
190
12.18
HD Sportster 883
883
6000
50
9.45

What can we read from this?
The ubiquitous Hero Honda, seen as the epitome of efficiency in India, is actually close to the bottom of the list – The CBR 250 does 24% better. If you metaphorically grafted 2.5 CD100 engines together and revved it up to 8500 RPM, you would get 7.5 * 2.5 * (8500/7500) = 21.5 HP – The CBR makes 5 HP more! So either more fuel is being burnt, or fuel is being burnt better.

In fact, the Hayabusa is quite astounding! If you grafted 5.2 CBR250 engines together and revved it to 12000 RPM, you would get 26.4 * 5.2 * (12000/8500) = 193.8 HP – Just about 2% more than what the Hayabusa makes on paper – Getting 1300cc to rev at 12000 RPM is no joke, it’s hard rocket science to be able to scale up an engine so well! The Hayabusa is an engineering marvel.

The R15 seems quite amazing, with only 150 cc, it makes almost as much as the 220 Pulsar – Seems like the most advanced engine amongst its peers.


The Harley is nowhere near the rest, even though it is a highly engineered design, but then again it’s a pushrod engine with an extremely under-square design, built for an almost flat torque-curve.

Let’s look at the two popular mid level bikes Pulsar and Karizma – Isn’t it startling that despite all the state of the art technology, fuel injection and what not, a 1970s CB750 of much larger capacity comes within 1% of the figures in terms of our metric?

I tell you sir! They don’t make ‘em like they used to! They just hype ‘em louder.

Of course, we aren’t looking at fuel efficiency (yet), so perhaps this conclusion is flawed; perhaps the bikes that have higher K are burning much more fuel. But nevertheless, if the question is: “How much power can you get out of a liter of engine regardless of fuel consumption?” then the above table shows the answer.

Royal Enfield’s

Model
CC
RPM
HP
K
Diesel bullet
435
3600
7.5
4.79
Std 500
500
4750
22
9.26
Std 350
350
5000
18
10.29
Lightning 535
535
4750
26
10.23
Enfield CL500
500
4750
27.2
11.45
ACE Fireball bullet
535
5750
42
13.65
Hitchcocks 612 stroker
612
5750
45
12.79
Enfield Constellation
700
6250
51
11.66
Enfield Interceptor
750
6750
56
11.06


This had to be in a separate table!
Not much to say here, except that Tom Lyons’ (aka ACE) Fireball kit can take a Bullet well beyond Jap bike territory: The dyno charts for the Fireball charts show about 36 wheel HP, so 42 engine HP is a reasonable estimate.
The Hitchcocks stroker 612 does great too, despite having an even longer stroke, and the CL500 is much better than the old Enfield’s, even considering it has to contend with Euro 4 emissions norms.
Quite surprising that the Connie and the Interceptor of yore do so well for air-cooled long stroke parallel twins.

These numbers are derived from observing several wheel dyno charts, rather than manufacturers specs (which are quite off), 6 HP added to get crankshaft horsepower estimate.

Legendary Classic Brit bikes

Model
CC
RPM
HP
K
BSA Goldstar
500
7000
42
12.00
Velocette Venom
500
6500
41
12.62
Triumph 650 twin
650
6500
46
10.89
Norton 650SS
650
6800
49
11.09
Vincent 1000
998
5500
55
10.02

Just astounding! These were mostly OHV pushrod bikes (except the Norton which was an OHC), ranging from the 40s to the 60s.
The Velocette even beats the Hayabusa in this metric! No doubt it ran on “real gas” but even so – this is a 50s bike with 8.75:1 compression! It still holds the 500 cc 24 hour endurance record at an average of 100 mph+, that too with a stock engine. All the employees of Velocette rode these bikes themselves and the build quality was so good that a stock bike was race ready. A few years after this record, they attempted it again with a souped up 350cc bike, which was going great for hours, unfortunately after 6 hours averaging > 105 mph, the piston blew ( this was not a Velocette manufactured one, but sourced from elsewhere) and the record attempt was thwarted.


The ACE fireball is on par with the Goldie on paper, and that speaks a lot about its potential performance!

I ask you!

Two strokes and four strokes
A four stroke has one power stroke for every two revolutions of the crank, whereas a two stroke has one. That means that a two stoke will (ideally) pump twice as much air as a four stroke of the same displacement at a given RPM.

Thus if we want to measure using the above metric then we need to include a factor of two – This is unfair because usually a 2 stroke of a given displacement makes only about 30 to 50% more than a 4 stroke cousin of the same volume – For e.g. take the Yamaha RX100: It makes 11.5 HP vs. 7.5 for a CD100 – that’s 53% more power, which is impressive, but still not double, even though it displaces twice as much air per unit time!

But fair or not, that’s the metric so let’s look at 2 strokes of all kinds:

Legendary two-strokes from India

Model
CC
RPM
HP
K
Jawa 250
250
4750
12
5.05
Yezdi Roadking
250
4750
16
6.74
Jawa 350
350
5000
23
6.57
Yamaha RX100
100
7500
11.5
7.67
Yamaha RX135
135
7500
14
6.91
Suzuki Shogun
108
8500
13.8
7.52
Yamaha RD350
350
6750
30.5
6.46

The RX 100 is the absolute champ here! Squeezes out every ounce of power from that 100cc, the Shogun does quite similarly. The Roadking has its reputation as a rally and motocross bike and for a 70s design, it is quite impressive.

None come anywhere close to the four stroke bikes, so it’s obvious that if you ride a two stroke, you are throwing a lot of fuel down the exhaust chute…. But then again a two stroke has an exhilarating, almost linearly increasing torque curve all the way up to the top, and who can not be enchanted by the coarse whine of the RX, the scream of the RD350 or the burbling purr of a Jawa?

Really amazing bikes
Model
CC
RPM
HP
K
Maico 760
760
4000
43
7.07
Maico 760 @ peak torque
760
1200
26
14.25
Suzuki RS67
124
16500
41
10.02
Honda Rc 165
250
10500
60
11.43
NSR500
500
19000
240
12.63

Yes, all those numbers are correct! The Maico 760 was a two stroke single, the largest ever made, and perhaps the most “gruntiest” of any bike that I have ever heard of… It makes even a thumper like a Bullet look like a revvy whiner. Stomach churning torque, I say! Look at the K factor at peak torque! That is true “bang” for the buck! Only 6 were ever made…

The other Japanese race bikes are again crazy - the RS67 had 4 cylinders, 12 gears and hit 137 mph – unbelievable!
The NSR 500 is probably the most powerful two-stroke made anytime, anywhere and needs just about 2cc to make a HP!

One thing that’s quite clear to me from real life experience is that manufacturers’ figures are way off reality; we all know how optimistic our speedos are… Dyno results are the most believable, and even they have some contradictions.

Read on to the next part where we talk about power to weight ratios…


5 comments:

  1. Finally somebody sensible :)

    Want to blog at a bigger platform ? email us at editor@iamabiker.com

    ReplyDelete
  2. Hiyee...i liked it..But why don't you Put in the R15 in the stats??? It makes every other Bike Go to Wind ryt??
    17/(8.5*0.1498)=13.35!!!!!! wowwww Thats Yamaha Baby!!!

    ReplyDelete
  3. Thanks guys, I'll add the stats for R15 too!

    ReplyDelete
  4. Great post; but the data isn't appropriate for the analysis.

    The HP/L*KRPM metric is kind of a "power efficiency", since you're normalizing by displacement and RPM. But you're only comparing scalar points, tabulated at (probably?) max HP. But that's not where your metric would peak. That happens somewhere else on the RPM function.

    The function tells the story. Take the Maico 760 @ 4000 and 1200 RPM. The power metric doubles because the RPM is 1/3. There is a lot happening on the RPM function we can't discern from max HP data. Problem is, acquiring said data.

    I'm looking forward to reading the rest of the article. It's certainly interesting.

    Tyson

    ReplyDelete
  5. @Tyson - I was looking to measure efficiency at all out RPMs...
    Probably including figures at peak torque would paint another picture - but I suspect that the relative values between the bikes wont differ too much, except for the thumpers who will come out better off because of broader torque curve.

    If my knowledge is right, peak torque is the point of maximum efficiency since I believe BMEP is maximized at that RPM...

    I'll add some more data and see

    ReplyDelete