XJ Horsepower

The other day I stopped at a motorcycle shop to get a new shield for my helmet and was talking to a guy that had a 98 Vulcan 1500. He had me listen to his pipes and his description of how nice his bike was. It was a nice bike. But I hurt his pride . He told me his bike 63 hp. I thought that was strange and I told him my xj650 was rated at 70+. He stammered back and said there was no way a bike half the motor size of his had more horsepower. I got on the web when I got home and that was true about his bike. In fact it seems to be a disappointment to the owners that it has low hp for that size engine. Is the difference because of the difference in gearing between the 2 bikes ?

 

Gearing doesn't affect power (except for losses). The engine generates the amount of power it does no matter the gear. Gears only exist to change the amount of torque at the rear wheel by increasing the mechanical advantage.

Basically, there are several reasons that your bike generates more power (and mine too, hee hee), but the most important and basic is that your motor runs faster! I think the Vulcan has a redline around 6500, but your 650 probably has a redline around 10k? That difference means you are hitting each cylinder more often than he is. So, even though he has a bigger engine, you are doing nearly the same amount of work simply because you are sucking nearly the same amount of air and fuel.

Of course, there's a lot more to it than that, but I wanted it to fit in a nutshell.

Sean

 

Thanks for the explanation !

 

Mines rated at 86 hp @ 9500 rpm.
Good thing too since most of my riding is 2 up

 

Part of the difference could be where the horsepower is measured.

Power ratings in cars used to be measured at the flywheel and now I believe it's measured at the output of the transmission (gross hp vs. net hp). The same may be true for bikes.

The only fair way to compare power from various bikes is to use RWHP (rear wheel horsepower) as measured on a dyno.

BTW, I bet that Vulcan owner would have been REALLY disappointed if he knew that a 600cc YZF-R6 is rated at 125 hp.

 

Don't forget the measure of torque. Sure one can go about spouting horsepower ratings but thats only half the story at best. I'd bet that vulcan makes torque enough to twist our XJ's in half.

Similar in the automotive world with the ricer who's proud to boast of his Honda civic making 200hp with only 1.7 liters. It perhaps only makes 120 lbs/ft of torque however. While the "x" muscle car owner hes boasting to perhaps makes 200hp and 350 lbs/ft torque from a 5.0 liter engine. Just using that as an example.

And for a real world automotive example (sorry, still too new to motorcycles). I researched 1/4 mile times and was delighted to find that the top of the line (now discontinued I believe) Toyota Celica which is rated at 180hp, 1.8L engine, gets the same (according to data I found) 1/4 mile time as my 2004 ecotec Pontiac Sunfire, rated at 140hp, 2.2L (and half the cost I might add). I don't want to nit pick through the data but I know my sunfire is rated for 150lbs/ft of torque and I think the Toyota was about 120lbs/ft or so. However things like axle ratio and such can make up some difference but I think it paints the picture quite well.

My XJ 650 Maxim can keep up quite well with my Uncles 1200 Harley Sportster. The difference though is that I have to make the most of my rev range while he can start to pull away in seemingly almost any gear at almost any RPM.

If you want to go really deep into the science the hp and torque ratings at various RPM make quite the difference too. Not that I have the resources to go there now. If you want to go that deep, I don't believe peak hp is even telling half the story then really. Perhaps 1/3rd. Its a combination of hp, torque, and at what RPM they all happen (not just max but all the way through) is what tells the whole tale.... Hey anyone have a Dyno we can borrow?

From what I can find that 1500 Vulcan makes 83 lbs/ft in torque and 64hp.
And our 650 Maxims make 43 lbs/ft of torque and 71hp.

That sounds about right to me.


So would that vulcan guy think its some sort of marketing propaganda to say that the top superbikes make over 175hp at under 1400cc's? By his line of thinking I guess that would be impossible. I'm not sure that Vulcan character is on the right bike. His narrow line of thinking sort of brings the letters "HD" to my mind...

 

I was looking up the 1/4 mile speed stats for both bikes-

XJ650- 12.7 122mph

Vulcan 1500-15.33 85.8mph

I am not sure what part the torque plays but with speed and time it seems the large amount of torque the Vulcan has compared to the XJ didn't make up for less HP. My limited understanding of this by things I've read is that though the Vulcan may have greater torque its' max comes at a low RPM where the XJ650 may have a lower torque its' max comes at a higher RMP. Because of this the XJ's engine speed is greater and produces more HP. Does this sound about right ?

 

Don't forget power to weight ratio. I think I offended some folks when I suggested my DRz 400 may be quicker to 60 than their 1400 v-twin. That bike was the most fun I've had on two wheels. I could get to 60 (and 5th with no clutch) in a really short space. No doubt quicker than my 80hp, 60ft/lb beemer. 550lbs vs. 280 lbs! A bike with more torque may get you there with fewer gear shifts, but the drz is a great mix of HP, grunt, and smooth shifting.

 

Very true.

Also that Vulcan probably weighs a good 200lbs more. Gearing probably set for cruising comfort rather than performance. Also apparently not enough torque to overcome the difference.

Interesting, although I would have thought that 1500 could do at least 14's.
It somewhat makes me wonder whats the point of sticking huge engines in some of those cruisers. Perhaps just so they can roll around with a "doesn't matter what gear you're in" sort of style.

 

ie: Valkyrie: Shifting=optional

I think it's a numbers game. Everyone wants the biggest engine, regardless of such "superfluous" this as efficiencey, economy, performance......

We are poor consumers, and classifying motorycycles by a simple three or four digit number is easier than understanding engine configuration, number of valves, final drive, intake, etc.....

 

I think with this situation that came up yesterday is helping me to more understand beyond the #'s. I think I have learned a little bit with regards to how efficiency works in regard to HP and torque with the 2 bikes at hand.
This is my understanding. Bikes such as the Vulcan 1500 are designed to have their max torque at a lower RPM for smoother day to day driving. After the max RPM is reached the torque lessons do to the lack of ability to "breath" at the higher RPM's. This is caused by the use of smaller valves and air intake tubes that provide the best air/fuel mixture at lower speed.

The XJ uses larger valves, air intakes and smaller cylinders that allow it to continue to breath at higher RPM's which bring about a higher HP.
I wasn't trying to "gloat" just trying to understand myself why the difference with the big difference in engine sizes. Digging into this has helped me understand this a lot better. I really like to learn.

 

Good on you Sonwatcher, knowledge is Power! It is easy to get caught up in the horsepower game.

 

In a nutshell, as said above, the Big Twins have low end power for boulevard cruising and highway legal speeds. That Vulcan only makes about 54 hp but makes about 75 lb/ft torque. Good for low 0-30 times and pretty quick 0-60 times along with good 40-60 passing times, but not 1/4 mile speedsters. Our XJ's, say my XJ650 for example, makes about 50 hp and 35 lb/ft torque, runs the 1/4 mile in 12.7-12.9 and has decent 0-60 but not so great at top gear passing. All in engine design. I prefer the top end fizz of a inline four (or twin, i enjoyed my EX500 as well), to a v twins low end pull and short breath on the top end. My Xj will handle most v twins out there, there are a hand full that are as fast or faster, (not counting the Ducati's, talking about Harlerys or Harley copies). The Honda 1800 and the Harley V rod will take me alive. They either need massive displacement or 4 valve Porsche designed motors to do so though. I have a co worker who has an S&S motored big twin that can keep up with me and is probably quicker than I am, but we agreed not to talk about it after he tried to pull on me at the light and I kept up up to 75 mph. Ineresting reading though, cruzerjd

 

I'll throw in my two cents again:

Basically, the reason people are so obsessed with power is because it tells the story of a vehicle's engine. Power is the rate at which work is done. Work, of course, is required to make a vehicle move. A motor with more power is better.

But, as people have said, there's more to it than that. Power isn't what makes a vehicle move--torque, or force, does. That means the more torque you generate, the faster you can accelerate, or the faster you can go at top speed, or whatever you're looking for.

The reason, though, that power is so important, is that you can gear a vehicle, trading speed of rotation for torque. It's not a direct relationship, though, because engines don't generate equal power at all RPMs. That's why we get beaten at low speed, but as soon as we hit the power band, we begin to dominate.

Basically, power is boss, but depending on your purposes, you might care where the maximum torque occurs. That's basically the difference between your typical rice rocket and, for instance, a Corvette, even though they might have the same horsepower.

Sean

 

I love the 900's Power going Uphill.

Leaving Concord, NH and heading North on Interstate 93-N is an adventure in Uphill motoring. Not that it's steep. Just that it is up-hill until the road finally comes to the Mt. Washing Valley.

I go up the grade in 4th ... with the Bike purring along at ... maybe 4.5K rpm's.

There are Hum-Vee's trying to pass.
Harley's wanting to get by ... and the Big Yam is yawning!
I let a guy in a Hummer get right along side and called down to the Reactor Room to lift the Control Rods out a little more as he nudged his way ahead.

I have NO idea how economically I was making power ... but, I bet the guy in the Hummer could watch his gas gauge moving from right to left.

 

ok - so.. if I'm pulling 4.5 k in my '82 XJ650, regardless of the gear.. is there a graph that can tell me the HP? If I recall.. she'll make about 71hp at about 9000 rpm...

Seems that most of my running on the local roads is at 60mph or so or less.. so I rarely make it past 5000 rpm in 5th gear...

 

I'm amazed by the level of misunderstanding when it comes to Power vs Torque, but them again I am a Mechanical Engineer so it comes naturally to me.

HP=[Torque (ft-lbs) x RPM]/5252

An engine that makes 20 HP at 1,000 RPM has the same amount of power as an engine that makes 20 HP at 10,000 RPM. Both engines will also have the same amount of torque when geared to have the same speed. In this example, the second engine would need a 10:1 reduction in order to have the same Power/Torque at the same engine speed as the first when operating at their peak power.

There is a lot of physics involved with it comes to 1/4 mile times, but that is how power and torque are related. Dynamometers measure torque and calculate mechanical horsepower base on the speed at which the torque was generated.

 

Canaweb: not really. This is just because power production is not a simple function of RPM. That's because the engine can have different loads on it. If the engine is highly loaded, you would require more force to keep it moving at the same speed.

Imagine a truck pulling a trailer at 20 mph in 2nd gear. The same truck without the trailer moving at 20 mph in 2nd gear is traveling at the same speed, with the same gear reduction. Therefore, the engine must be operating at the same speed in both cases. However, it's intuitive that in the first case, more power is required. This power comes from a higher engine loading, meaning that more air and fuel are being injected into the engine for each rotation. More heat is generated, more work is done, and a larger amount of force is generated for each piston stroke. This translates into more force per rotation, at the same number of rotations per minute.

Or, given the formula, as pvtschultz states:

HP=[Torque (ft-lbs) x RPM]/5252

Doubling the force doubles the torque, which doubles the HP, since RPM was maintained constant.

 

so.. assuming a 200 lb rider (me in full gear) on that '82 xj650j, which weighs 447 lbs dry..
2 gallons of gas (13 lbs) and a few lbs for misc... on a level road.. 5th gear..
how might I calculate the power thats being generated? Not sure how I'd calculate the torque in this scenario to plug into the formula... but it seems that the above info of rider, road and machine should be a fairly standard "load" for this machine..

 

Yeah, I'd say it's a standard load, but it's too hard to calculate. That's because the actual load the engine has to contend with is basically a bunch of friction from moving parts and wind resistance.

In other words, we would need more information, like how much power is lost in the gearbox (different for each ratio), what the frontal area of the bike is with you on it, what your coefficient of friction is with the air, etc., to have a picture of how much power is required.

This is why a dynamometer approaches the problem from the other direction--they use a machine on which they know how much torque they're generating (a "brake," or some other means) and at precisely which speed, and strap the motorcycle on it, so that everything is forced to equalize. Then they know how much power the engine is producing.