Sportbike Racing Forum banner
1 - 15 of 15 Posts

·
Registered
Joined
·
6,188 Posts
Discussion Starter · #1 ·
Inspired by all the exhaust modders in here (buy this one and you get at least 3hp more..) and the knowledge that they loose quite some torque doing this, I finally came up with an appropriate way to express what always bothered me:

I always had the feeling it's more about torque than HP. The theory is astonishing simple:

What is the most fun while riding a high powered bike? The acceleration.

To go with Newton:
F=m*a, a is accleretion, m is mass, F is the force needed.
F=force, that means kg*m/s^2, and that's Newton not Watt, or torque when looking at your bike, not power.
So the more torque you got, the less your bike weighs, the higher is the acceleration.

Of course you need enough power as well, but only at the very high end (when you get close to the topspeed of your bike). Before that you've even to restrict the power due to the possibility of the bike flipping over.
That's why the heavy cruisers aren't as fast as sportbikes, they have lots of torque but not enough power.

But I cannot understand why someone trades in torque to power, because the torque is the limit to his acceleration, the power only in 5th and 6th gear- and then you're going at speeds around.. 250-300km/h, how often do you reach these on the road, or even on the track?

Am I wrong with my idea?
 

·
Premium Member
Joined
·
24,352 Posts
Old school gearheads (At least in the car enthusiast corner) always want more torque than horsepower.
Cruisers have so much torque because of the bore/stroke of the engine, as well as the rpm limitations, which allows them more torque than horsepower...I think that's how it goes, anyways...

I would have to think that times haven't changed that much; because torque is the power that gets you there (Speed), and horsepower is the ability to stay there. If I could increase my torque without sacrificing horsepower on the bike, I would definitely do whatever it took to get it. Besides, torque is also equated to useable power; take a look at V-twin sportbikes. You don't need to wring the living shlt out of the motor to get more power down low in the powerband.

But I think my point is moot anyways. If you get a few ponies and ft/lbs, are ya gonna feel the increase? (Unless the mod takes a flat spot out of the torque/horsepower curve of course)



/Just rambling
 

·
Registered
Joined
·
4,641 Posts
Torque and horsepower are interrelated. If you increase torque at a given RPM, you're also increasing horsepower. If you increase horsepower at that same RPM, you're increasing torque. The torque and horsepower curves always cross at 5250RPM, meaning at that RPM, you always have the same numbers for torque and horsepower. At lower RPM, the torqe number will be higher than the horsepower and at higher RPM, it will be lower. Since most cruiser engines don't spin 5250RPM, the torque numbers are large compared to the horsepower. That does NOT mean they will out-accelerate a properly geared sportbike, even if the weight is the same.
 

·
Registered
Joined
·
2,180 Posts
Horsepower = torque X rpm / 5252.

Torque is the twisting force the engine develops, and horsepower is a measurement of the amount of work that the engine produces over a period of time. So the faster you spin an engine, the more work it can do.

The drawback with spinning it faster, at least with the internal combustion engines we are dealing with, is that in order to tune an engine to breath at high rpms, it loses the ability to breath well at low rpms (and tuning is a very appropriate word - you can't play the same notes on a tuba that you can on a flute and vica versa). So your torque output falls in the toilet down low - but - it will still have good torque up high, and the higher the rpm for a given amount of torque, the more horsepower is produced.

Lets say you have an engine that makes a peak 100 foot pounds of torque at 5000 rpm. It will produce 95 horsepower at 5000rpm (and if it can still rev higher, will continue to make more power even as the torque drops off. For example, it might make only 85 foot pounds at 6000 rpm for 97 horsepower)

Another engine that makes 100 foot pounds at 10,000 rpm will produce 190 horsepower - double the power and the same amount of torque!

Problem - the 190 hp engine will likely have very little torque at low rpms, and will thus have very little horsepower at low rpms too. This is why engines that have to move heavy loads don't bother with rpm. They want large displacement engines that produce gobs of torque without having to rev to do it. They physically can't reach high revs, and don't need to.

Engines for racing will be propelling lighter vehicles and will put up with poor performance down low to reap the benefits of high rpm.

What sort of 'personality' you like from an engine is up to you :waytogo:
 

·
Registered
Joined
·
3,879 Posts
[ QUOTE ]
specialsymbol said:
I finally came up with an appropriate way to express what always bothered me:

To go with Newton:
So the more torque you got, the less your bike weighs, the higher is the acceleration.


[/ QUOTE ]


If I get enough torque, can I get my bike to be under 100lbs?? :lol:





PS. nice explanation Lime. :wink:
 

·
eStarbucks
Joined
·
17,313 Posts
I do remember somewhere on a Vette site... that and engine tuned for high HP will always be faster than high torque.

A high HP engine can always output high torque by gearing it down - a similar engine tuned for high torque cannot reach the same HP numbers that the highHP engine can.

Though... this is all moot since I"ll never find that site again.


I always see torque as the force to get you moving... once moving: it's HP that gets you flying. I have a good explanation in my head; but it's not getting to words well.
 

·
Registered
Joined
·
6,188 Posts
Discussion Starter · #8 ·
OK, I should have replaced this comma by an and...

However, It worried me all the time, and it's true the torque isn't all- but it's what gives the acceleration, so I figured it's the fun part.
Of course I see there's a need of power at high speeds... but only there.
 

·
Registered
Joined
·
1,045 Posts
Spiked and lime have it covered... but I'll add just a bit. :wink:

Torque is real, it's something you can measure. HP is just a number. You don't measure it, you calculate it.

Some friends of mine in school were big drag racers (cars). Having spent many a saturday night at the strip the following explanation became clear to me....

Torque is what gets you there quickly. HP is how fast you're moving when you get there. A car with big torque but moderate HP (big block V8 for example) will have a quick time but relatively low trap speed. If car with big HP but moderate torque (turbo 4 cyl for example) runs the same time, it will have higher trap speed.

Not sure if that helps or not...
 

·
Registered
Joined
·
2,180 Posts
Here's a little clarification based on the original question: modding an engine to produce more power doesn't necessarily make it lose torque - it moves the torque peak to a higher rpm, thus increasing power.

In general, though numbers will vary a bit, an engine will produce roughly the same amount of peak torque wether it is 'tuned' for high or low rpm operation. The torque is a product of bore, stroke, compression ratio, con-rod, and crank geometry. The horsepower depends on what rpm the torque is produced at.

Remember that a given engine has physical limitations based on this geometry - a F1 engine will never work well at low rpms, and a diesel tractor will never be able to rev to the moon.

But - within the limits for a given engine, the tuning that can be accomplished through playing with intake and exhaust events can move the torque peak up or down quite a bit. This changes the horsepower output sustantially - but the torque has stayed comparatively the same. It's just been juggled to a different rpm.
 

·
Registered
Joined
·
2,180 Posts
A little more (can you tell I'm home sick today? :tongue:) You can modify an engine to produce more torque - it's called displacement! Big bore and stroker kits are all about bumping up the torque numbers.

Intake exhaust, cams, etc all 'tune' the torque where you want it to live - more displacement adds more torque to the mix!
 

·
Registered
Joined
·
2,180 Posts
Whoops! Don't forget about the magical power adders that make an engine think it has more displacement, whether by physically or chemically cramming more mixture into the cylinders: nitrous oxide and supercharging / turbocharging. :grin:
 

·
Registered
Joined
·
6,188 Posts
Discussion Starter · #13 ·
Thanks! Those were some good explanations.

I knew torque is something physical inherent to each engine layout.
I always thought if you mess with the engine setup you automatically lose torque, e.g. flatten the torque curve, so having a lower maximum peak but moving some of that to the high rpm end (thus getting more power).

But I didn't know you can move the peak without losing torque.
 

·
Registered
Joined
·
2,180 Posts
Thanks :laugh:

Though actually, you are more apt to have a wide flat torque curve on an engine tuned for low rpm performance - and the curve will get steeper and have a higher peak if the same engine is tuned for high rpms.

This goes back to your original question again 'why put on a pipe to gain power and lose torque?' - it's not lost, per se, but it has been moved, and it might have been moved higher in the rpm ranges than you like.
 

·
Registered
Joined
·
228 Posts
Here's something I've noticed. An engine that makes more torque than horsepower (IE 100 lb-ft peak and 75 hp peak) will accelerate quickly at part throttle, but at higher revs or with more throttle, it won't accelerate significantly more quickly.

However, an engine that makes more hp than torque (IE 75 lb-ft and 100 hp) will not accelerate particularly quickly at part throttle but when you rev it up and open up the throttle, it will reward you with a noticeable increase in thrust.

My Fiero, modded with a mild cam, chip and K&N, is probably putting down about 150hp and 175lb-ft to the crank. It is reasonably peppy at part throttle, but going wide-open and running it to redline doesn't amke it into a beast of speed. My 5.0 Mustang, on the other hand, has a big cam, heads, intake, rockers, full exhaust, etc. The previous owner claimed he had built the engine to match Ford's 350hp crate motor. When I first started driving the car, I was pretty disappointed. I didn't run the revs up very high, and the car felt barely quicker than my previous bone-stock 5.0. But when I got used to the car and started running the tach to 6000+rpm at WOT, I realized that this thing feels like it's putting down close to 300rwhp, which would be in the neighborhood of 350hp to the crank. I can be at 4000rpm and at 3/4 throttle, and pushing the pedal all the way to the floor still rewards me with an extra burst of speed.

Sorry for the car talk, but these two cars have been the best examples of high horsepower vs. high torque that I have come across.
 
1 - 15 of 15 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top