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Sportster Motorcycle General Discussion and Problems For discussing problems about your Sportster motorcycle that don't fit anywhere else.

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  #21  
Old 29th June 2009
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Here is an article I had written a while back.
I've also included some responses.

TORQUE & HORSEPOWER… NO CRAP

There are 2 basic thoughts…
1) An engine is an air pump.
2) Torque is force times distance.

For engines, this can be referred to as rotational torque.
It is simply the force of the explosion above the piston,
expressed in pounds, times the distance from the center
of rotation of the crankshaft to the center of the crank pin,
expressed in feet.

The way to increase torque is to either increase
the force of the explosion, such as…

a) Bringing in more air/fuel mixture.
b) Increasing the oxygen concentration by adding nitrous.
c) Increasing the effective opening of the intake valve by
adding yes, a second intake valve.
d) Lengthening the time the intake valve stays open.
e) Polishing the air intake path to reduce drag effects.
f) Improved scavenging techniques where the exiting exhaust
may actually help draw in more air.

or…
further offset the crank pin from the center of rotation of the crank.
That last item is usually referred to as making a ‘stroker’.

OK…
I’m sure I left out some techniques, so bear with me.
In this discussion, I’m not getting into the merits or issues
with these various techniques.

A lot of people opt for increasing the air flow through the
air filter by changing to a less restrictive filter.
The wrong way… install a filter with less filtration.
The right way… install a filter with more surface area.

Once the air filter is done, there is a slight adjustment of the
carburetor to account for the increased airflow.
A less restrictive exhaust rounds out the modification,
and, presto… we have a stage one completed.

The particular timing of the valves opening and closing
reaches a maximum efficiency for airflow at a certain RPM.
That’s the RPM where the peak torque smiles its happy face.
BTW, air is only about 20% oxygen. The rest is mostly
nitrogen that unfortunately contributes to the unwanted
production of nitrogen based emissions in the exhaust.

When the intake valve opens, all you have working for you
to help suck in air, is a vacuum that is created
by the downwards movement of the piston.
Vacuum maxes out at minus 15 PSI,
assuming we are starting with 0 PSI atmospheric pressure.
Pressure on the other hand can go into the thousands
of PSI. So… by comparison, vacuum is relatively weak.

A way to get around this, is with a turbo, which can
artificially force more air into the combustion chamber.
Notice I’m not dealing with fuel issues,
as with both an EFI and a carbureted engine,
adding more gas is not really a very big problem.

HD seems to reach their maximum torque early in the RPM band.
It is not uncommon to see an HD cruiser running comfortably
in top gear at 3,000 or less RPM.
That ability to be in top gear at relatively low RPM
gives the rider a perception of horsepower.
However, it is just a perception.
Yes… there is some horsepower there,
but what you are really feeling is a big amount of
low-end torque running through the gear box producing
a decent speed on the highway.

OK… I’ve beaten torque to death, so let’s move on.

Horsepower is essentially torque times RPM.
Horsepower is the ability to perform a certain
amount of work in a certain time.
The formula tells us that if we could maintain our great
HD low-end torque figure at higher RPMs,
we would have loads of horsepower.

Unfortunately, it doesn’t happen.
With the design of the usual HD engine,
torque drops off after the early peak is achieved.
In spite of falling torque however,
horsepower still climbs somewhat, but obviously,
the increase in horsepower is mostly due to the increase in RPM.

Imagine if we could have our huge HD torque at 10,000 RPM.
Now, that would multiply out quite nicely, thank you,
and scare the crap out of any crotch rocket egging us
for an unsafe ‘red light to red light’ showdown.

On crotch rockets, especially water cooled crotch rockets,
where the engine clearances are more tightly controlled,
the peak torque occurs later in the RPM band.
As a result, the torque value, is multiplied by the higher
RPM resulting in a very nice horsepower figure.

Horsepower is the more complete measure of power,
as it accounts for both torque and the ability to produce
that torque value high in the RPM band. You need both.

But… the HD torque that peaks low in the RPM range,
however, does give the rider a certain ‘feel’.
We feel the potato, potato rumble. Crotch rockets feel like
you are riding an electric motor.
That’s great if that’s what you’re after.
Some say there is no soul in that crotch rocket engine.
Crotch rocket owners say,
‘screw the soul… I’ve got 150 ponies at 10,000 RPM’.

It just depends what turns your crank ( no pun intended ).
One is not better than the other. They are different designs
for different people. There is room on the road for all of us.

Yes… on the road… that’s where I’d be right now
instead of writing this huge article.
Unfortunately, it’s about 32 F. degrees outside.
Tomorrow is supposed to be warmer here so,
I’ll get some more miles on the odometer before the snow flies.

If you got this far, thanks for staying with me.
As Kev likes to say… ride safe !


REPLY TO QUESTION
I used the words ‘air pump’ on purpose.
That’s a term I’ve heard used before
to describe the internal combustion engine.
Yes... obviously an oversimplification,
but it gets the idea across that you need
to suck gases in and then after combustion,
you need to pump the used gasses out.
The big problem is getting a lot of air in at higher RPM.
Once you are past the torque peak,
it’s a sign that the amount of air being drawn in
is getting less and less.
Thanks


REPLY TO QUESTION
The term 5252 is interesting.
I left it out as I didn’t want to get too mathematical.
BTW… when a bike is dyno’ed, the torque
and hp curves MUST cross at 5252 RPM.
If your dyno chart has them NOT crossing there,
then get your money back…
the dyno has, or is, screwed up.


REPLY TO QUESTION
To answer another note…
If your Sporty has a higher torque value
than a sport bike at say, 3000 RPM,
then yes… the Sporty is producing
more hp at 3000 RPM.


REPLY TO QUESTION
You also have to appreciate that HD engines
do not rev up as fast as sport bike engines.
The HD engine has a much heavier flywheel
than it’s crotch rocket counterpart.
That heavy flywheel comes into play
when there is acceleration. The faster you
want to accelerate the more hp you need.
That’s why drag racing cars have
several thousand hp. Accelerating a mass
from one velocity to another quickly eats
up hp.


REPLY TO QUESTION
Let’s assume that you are currently running a perfect air/gas ratio.
I believe that going to a bigger injector, which can inject more
fuel in a given time period will ONLY increase combustion force
if there is enough extra air to burn the extra injected fuel.
If you are not going to change the amount of air that can be
drawn in under the normal vacuum conditions, during the piston
downstroke, then the air/gas mixture will wind up being too rich
which depending upon how far off you are, from the ideal air/gas
ratio, may result in actually less performance.
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  #22  
Old 29th June 2009
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  #23  
Old 29th June 2009
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I don't want to get too picky here, because most of what you said is right, but let me just clarify a few things ...

Quote:
Originally Posted by MadMax25 View Post
2) Torque is force times distance.

For engines, this can be referred to as rotational torque.
To be more accurate, torque is literally defined as rotational force. It's the force exerted by a rotating body times the radius it's measured at.

Quote:
The way to increase torque is to either increase
the force of the explosion, such as…

a) Bringing in more air/fuel mixture.
b) Increasing the oxygen concentration by adding nitrous.
c) Increasing the effective opening of the intake valve by
adding yes, a second intake valve.
d) Lengthening the time the intake valve stays open.
e) Polishing the air intake path to reduce drag effects.
f) Improved scavenging techniques where the exiting exhaust
may actually help draw in more air.
B will not make more power without also bringing in more fuel. But then you have A.

C and D may or may not increase the cylinder fill.

E almost will never increase the cylinder fill.

F is a very effective technique for increasing cylinder fill.

Collectively, though, what you're talking about is various techniques for increasing cylinder fill. There are a whole bunch more, you didn't hardly even scratch the surface, but I think the point is that increasing cylinder fill, however it's done, increases cylinder pressure and that raises torque


Quote:
When the intake valve opens, all you have working for you
to help suck in air, is a vacuum that is created
by the downwards movement of the piston.
You forgot something that you already mentioned. If you've done your exhaust system properly and have overlap in the cams, the exhaust will actually start pulling on the intake charge before the piston even starts going down. This is a huge, huge deal.

Quote:
Unfortunately, it doesn’t happen.
With the design of the usual HD engine,
torque drops off after the early peak is achieved.
In spite of falling torque however,
horsepower still climbs somewhat, but obviously,
the increase in horsepower is mostly due to the increase in RPM.
That's because as things happen faster and faster, there's less and less time to fill the cylinder. You have to optimize the system to fill the cylinder most completely at the rpm of choice. HD engines are designed to fill it at low rpm.

Quote:
BTW… when a bike is dyno’ed, the torque
and hp curves MUST cross at 5252 RPM.
If your dyno chart has them NOT crossing there,
then get your money back…
the dyno has, or is, screwed up.
The way a dyno works, it measures two things and calculates the third. An inertia measures horsepower and rpm and calculates torque. A water brake dyno measures torque and rpm and calculates horsepower. But anyway, the calculation is based on the formula and therefore it has to cross at 5252. The only times you see the curves not crossing at 5252 is when hp and torque are scaled differently (the software lets you define the scales for each independently). We do this sometimes when the two numbers are a long way apart.

Anyway, Like I said, I don't want to get too nitpicky here, the vast majority of what you said is correct, these are just minor clarifications.
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  #24  
Old 29th June 2009
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Quote:
Originally Posted by aswracing View Post
I don't want to get too picky here, because most of what you said is right, but let me just clarify a few things ...



To be more accurate, torque is literally defined as rotational force. It's the force exerted by a rotating body times the radius it's measured at.



B will not make more power without also bringing in more fuel. But then you have A.

C and D may or may not increase the cylinder fill.

E almost will never increase the cylinder fill.

F is a very effective technique for increasing cylinder fill.

Collectively, though, what you're talking about is various techniques for increasing cylinder fill. There are a whole bunch more, you didn't hardly even scratch the surface, but I think the point is that increasing cylinder fill, however it's done, increases cylinder pressure and that raises torque




You forgot something that you already mentioned. If you've done your exhaust system properly and have overlap in the cams, the exhaust will actually start pulling on the intake charge before the piston even starts going down. This is a huge, huge deal.



That's because as things happen faster and faster, there's less and less time to fill the cylinder. You have to optimize the system to fill the cylinder most completely at the rpm of choice. HD engines are designed to fill it at low rpm.



The way a dyno works, it measures two things and calculates the third. An inertia measures horsepower and rpm and calculates torque. A water brake dyno measures torque and rpm and calculates horsepower. But anyway, the calculation is based on the formula and therefore it has to cross at 5252. The only times you see the curves not crossing at 5252 is when hp and torque are scaled differently (the software lets you define the scales for each independently). We do this sometimes when the two numbers are a long way apart.

Anyway, Like I said, I don't want to get too nitpicky here, the vast majority of what you said is correct, these are just minor clarifications.
MadMax25:
A certain level of knowledge was assumed when making my comments.
Obviously, anything that provides more air or increases the concentration
of the oxygen will also require more fuel, to be effective.

' Once the air filter is done, there is a slight adjustment of the
carburetor to account for the increased airflow. '


Anyone who knows anything about nitrous oxide systems
also knows that the systems come with provision for extra fuel enrichment... that goes without saying.
Having said all that, adding more fuel to the mix is extremely trivial
compared to getting a greater volume of mixture into the cylinder.
Any modification that results in a greater volume of mix into the cylinder will yield more power.

I repeat... every one of the following will increase torque to some extent.
a) Bringing in more air/fuel mixture.
b) Increasing the oxygen concentration by adding nitrous.
c) Increasing the effective opening of the intake valve by
adding yes, a second intake valve.
d) Lengthening the time the intake valve stays open.
e) Polishing the air intake path to reduce drag effects.
f) Improved scavenging techniques where the exiting exhaust
may actually help draw in more air.

And yes... I did probably leave out a few. I wrote...

' OK…
I’m sure I left out some techniques, so bear with me. '

You took many of my comments out of context
and it seemed that you only read parts of the post.
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  #25  
Old 26th July 2009
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The long and the short of it... Horsepower sells parts, TORQUE wins races.
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  #26  
Old 27th July 2009
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Quote:
Originally Posted by KillerWrench View Post
The long and the short of it... Horsepower sells parts, TORQUE wins races.
Wrong... horsepower wins races. There is no arguing with horsepower.
Torque is a component of horsepower. Torque x RPM = Horsepower.
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  #27  
Old 27th July 2009
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Quote:
Originally Posted by MadMax25 View Post
I repeat... every one of the following will increase torque to some extent.
a) Bringing in more air/fuel mixture.
b) Increasing the oxygen concentration by adding nitrous.
Not unless more fuel is introduced at the same time

Quote:
c) Increasing the effective opening of the intake valve by
adding yes, a second intake valve.
This isn't necessarily true. Ports have to be sized appropriately and cam timing adjusted as well. If you ever look at the cam specs of a grind that's made for a 4 valve engine, you'll see they're quite a bit different than a 2 valve grind. In short, the motor has to be designed around the extra intake valve, just adding a valve is not what makes it work.

Quote:
d) Lengthening the time the intake valve stays open.
This may or may not make more torque. On the open side, it only helps to a point, open too early and it can actually hurt cylinder fill, and it only helps at all if the exhaust is pulling right then. On the closing side, for a given engine and rpm, there's an ideal time to close the intake valve. Close it sooner or later and you make less torque, not more. So a statement like this is true sometimes and not others.

Quote:
e) Polishing the air intake path to reduce drag effects.
This is almost never true. The ideal finish in an intake port is not a polish.

Quote:
f) Improved scavenging techniques where the exiting exhaust
may actually help draw in more air.
This only helps to the extent the cams have overlap. Very low overlap cams will realize virtually no benefit from an exhaust that scavenges better.

In general, a list like this really oversimplifies what's happening in a motor, and as such, most of the things on it have exceptions.

Last edited by aswracing; 27th July 2009 at 02:53..
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  #28  
Old 27th July 2009
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Quote:
Originally Posted by KillerWrench View Post
The long and the short of it... Horsepower sells parts, TORQUE wins races.
If torque was what mattered, you'd never take your bike out of first gear. You have far more torque at the rear wheel in first gear than any other gear.

Every time you upshift, you give up some rear wheel torque and trade it for some rear wheel rpm. By the time you get to top gear, you have very low rear wheel torque but very high rear wheel rpm.

The trick to making the bike win the race, then, is not to make more torque. Making torque is easy, just leave it in first gear. The trick to making the bike win the race is to make more torque and more rpm at the same time.

More rpm and more torque is called more horsepower.

Try this experiment sometime. Get two identical bikes, running side by side, in the same gear, running at the engine's torque peak. Now, at the same time, have the rider on one bike whack the throttle wide open, while the other rider downshifts and opens the throttle to put the motor at the horsepower peak instead, and watch what happens. It won't even be close. The downshifted bike will walk away. Horsepower wins every time.

Last edited by aswracing; 27th July 2009 at 02:55..
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  #29  
Old 27th July 2009
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Quote:
Originally Posted by ReddTigger View Post
So what size shovel do you recommend for maximum scoop rate ?

J/K

Great analogy and should help some people understand what their talking about.

1340cc

I agree, great analogy.
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Old 27th July 2009
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Torque gets your moving, Horsepower keeps you moving faster and faster..

consider torque as a long wrench attached to the wheel. the more torque the engine has the longer the wrench..

(these are layman's terms and are only to describe a visual representation of the force)
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