Octane rating question

[GMCdriver]

Rear wheel HP & Torque measure lower then crank HP & Torque but with basicly the same shape power curve.
The SAE has standardized the use of Engine(crank) Dyno testing for certifying and comparing automotive engines.
The amount of load used on a dyno is set at a constant amount that is slightly less then what would overcome the engine being tested.
The machine uses the speed of acceleration against that load and is able to calculate torque using the known resistance load and a time and acceleration equation.
Once the torque has been determined, the HP is calculated and plotted using Torque*rpm/5252.

Changeing the mass of your vehicle does not change the output of your engine it simply changes your vehicle's inertial resistance to acceleration.
You seem to think that by adding a large amount of weight to your vehicle you would somehow increase the torque output of your engine but if the torque output were suddenly increased by the extra weight then why does the vehicle loose acceleration ability.
If you accelerate at full throttle with an empty vehicle, the engine makes it's torque and you pull away briskly.
If you accelerate at full throttle with a loaded vehicle, the engine makes it's torque and you pull away more slowly.
The torque output of the engine is the same but the time to accelerate changes due to the increased load.

[gearhead]

Rear wheel HP & Torque measure lower then crank HP & Torque but with basicly the same shape power curve.
The SAE has standardized the use of Engine(crank) Dyno testing for certifying and comparing automotive engines.
The amount of load used on a dyno is set at a constant amount that is slightly less then what would overcome the engine being tested.
The machine uses the speed of acceleration against that load and is able to calculate torque using the known resistance load and a time and acceleration equation.
Once the torque has been determined, the HP is calculated and plotted using Torque*rpm/5252.

Changeing the mass of your vehicle does not change the output of your engine it simply changes your vehicle's inertial resistance to acceleration.
You seem to think that by adding a large amount of weight to your vehicle you would somehow increase the torque output of your engine but if the torque output were suddenly increased by the extra weight then why does the vehicle loose acceleration ability.
If you accelerate at full throttle with an empty vehicle, the engine makes it's torque and you pull away briskly.
If you accelerate at full throttle with a loaded vehicle, the engine makes it's torque and you pull away more slowly.
The torque output of the engine is the same but the time to accelerate changes due to the increased load.

Excellent, many thanks for taking the time to put that in layman's terms, GMCdriver!
I'm sure many Members here appreciate it.

The point I was trying to make is that although the torque curve is relatively (for argument's sake, let's call it perfectly) flat on the graph, obviously it will not exhibit the same characteristics in a real world situation.

There is no way a drivetrain can maintain a curve as plotted on a dyno and cannot react quickly enough in the real world due to moments of inertia, as we've both already noted, along with other  variables including load, wind, friction, etc.

What I'd really like to know is, if the torque curve is so flat, why do I have to wait until a certain RPM on my tachometer before I get the feeling of thrust against my back that I love so much?

Thanks again for the very interesting discussion, all of you!

[GMCdriver]

What I'd really like to know is, if the torque curve is so flat, why do I have to wait until a certain RPM on my tachometer before I get the feeling of thrust against my back that I love so much?

Thanks again for the very interesting discussion, all of you!

It's not flat, it starts low and grows steadily stronger.
They make it look flatter then it is by changeing the scale that it is plotted on but it's probably in the neighborhood of 45lb-ft difference between what is produced at 1500rpm and what is produced above 4500rpm.
The torque produced just above idle is good but it is working to overcome the most inertia with the lowest part of it's power band, as it revs up it is making more and more torque while fighting less and less inertia(although more and more wind resistanc and such) and thus that wonderfull "zoom" or "woosh" sort of acceleration that we all know and love.
They plot the torque useing 100 lb-ft graduations on the left so the line doesn't need to move up and down as far to represent the increase over the range while they plot HP in 50hp graduations on the right so they can move the line up & up at a much faster rate.
If they used the same set of graduations, hp & torque would crossover at 5252 rpm everytime.

[Steve A]

One more time on the dyno chart.

My point is that it is not a fair comparison when the numbers are taken at a different RPM.  Yes, it can be done at any RPM !!!   But - it needs to be done at the same RPM for both engines, states of tune or whatever you are trying to compare.  Taking readings at different RPMs means that the point trying to be made is meaningless or even purposely misleading.

That is my whole point about that particular chart !!!

[gearhead]

It's not flat, it starts low and grows steadily stronger.
They make it look flatter then it is by changeing the scale that it is plotted on but it's probably in the neighborhood of 45lb-ft difference between what is produced at 1500rpm and what is produced above 4500rpm.
The torque produced just above idle is good but it is working to overcome the most inertia with the lowest part of it's power band, as it revs up it is making more and more torque while fighting less and less inertia(although more and more wind resistanc and such) and thus that wonderfull "zoom" or "woosh" sort of acceleration that we all know and love.
They plot the torque useing 100 lb-ft graduations on the left so the line doesn't need to move up and down as far to represent the increase over the range while they plot HP in 50hp graduations on the right so they can move the line up & up at a much faster rate.
If they used the same set of graduations, hp & torque would crossover at 5252 rpm everytime.

Excellent, in simple terms and clearly spelled out!
Many thanks again for taking the time to share some great, real world, not "on a graph" info with us, GMCdriver!

One more time on the dyno chart.

My point is that it is not a fair comparison when the numbers are taken at a different RPM.  Yes, it can be done at any RPM !!!   But - it needs to be done at the same RPM for both engines, states of tune or whatever you are trying to compare.  Taking readings at different RPMs means that the point trying to be made is meaningless or even purposely misleading.

That is my whole point about that particular chart !!!

I'll agree with that 110%.

From memory, many supercharged or turbo charged performance factory engines had a torque curve that looked like a hill, with a definite "in your face" power band, generally peaking between 2500 to 4500 RPM then dropping off substantially after that.

They were tuned to get quick 1/4 mile, 0 to 60 or 70 to 100MPH times, for example.
Things such as top end were generally not the intended purpose in the life of such fire breathing behemoths LOL!

Comparing those types of engines to normally aspirated ones at certain RPM's really doesn't provide much useful information.

[GMCdriver]

One more time on the dyno chart.

My point is that it is not a fair comparison when the numbers are taken at a different RPM.  Yes, it can be done at any RPM !!!   But - it needs to be done at the same RPM for both engines, states of tune or whatever you are trying to compare.  Taking readings at different RPMs means that the point trying to be made is meaningless or even purposely misleading.

That is my whole point about that particular chart !!!

I get it now, you don't know how to read the graph beyond the peak point they have marked on the curve.
Pretend there are no peak marks, torque or hp, ignore them completly.
The lines are the measurments.

[Steve A]

Oh Lordy, contrary to popular belief, I can read a dyno chart.  My point is that if you quote #s taken at different RPMs, you aren't comparing apples to apples.  You are comparing an apple to an orange.

[GMCdriver]

What is the correct rpm to compare power output figures at so that the comparison is fair?

[Steve A]

There is no "correct" number.  My point is that if you are comparing two things you need to make the comparison as accurate as possible.  In this case at point, the numbers for the two things being compared need to be taken at the same RPM to make the comparison fair and accurate.  If you take readings at a different RPM - place on the trace on the dyno chart - then you are not making an accurate comparison.

That is my whole point regarding the #s derived from that particular dyno chart !!!

[GMCdriver]

There is no "correct" number.  My point is that if you are comparing two things you need to make the comparison as accurate as possible.  In this case at point, the numbers for the two things being compared need to be taken at the same RPM to make the comparison fair and accurate.  If you take readings at a different RPM - place on the trace on the dyno chart - then you are not making an accurate comparison.

That is my whole point regarding the #s derived from that particular dyno chart !!!

Why?
What info is being misrepresented by reporting that peak power was not made at the same rpm for both fuels?
I don't understand how it is "rather erroneous and misleading" to show how much power each fuel was able to make from the same engine simply because one fuel hit it's peak 100rpm higher then the other fuel.

[Steve A]

I'd agree with your theory if the purpose was merely to show the maximum power generated by fuel "A" vs fuel "B".

But for a "fair" comparison of one vs the other the data measured should be at the same RPM point on the dynomometer chart.



A rough comparison might be a horse race, you measure the results over the same distance, not one horse at 1-1/8 mile and the other @ 1-1/4 mile.

[GMCdriver]

I'd agree with your theory if the purpose was merely to show the maximum power generated by fuel "A" vs fuel "B".

But for a "fair" comparison of one vs the other the data measured should be at the same RPM point on the dynomometer chart.



A rough comparison might be a horse race, you measure the results over the same distance, not one horse at 1-1/8 mile and the other @ 1-1/4 mile.

What do you think the purpose was?

Your hores race analogy is cute but not fair or at all acurate to the subject.

[Steve A]

Ok, since it is an "overlay chart" and since the power and torque figures were not taken at the same RPM, I'd say it is a chart designed to convince the viewer that the engine produced more power and torque on E-85 fuel than on straight gasoline.  The power peak on the "gas" run is very slightly better than the "E-85" run as it doesn't drop off quite as rapidly as the other.

11 HP in 100 RPM is a little stretch, but 13 ft. lbs of torque in 400 RPM is certainly easily doable.

[GMCdriver]

Ok, since it is an "overlay chart" and since the power and torque figures were not taken at the same RPM, I'd say it is a chart designed to convince the viewer that the engine produced more power and torque on E-85 fuel than on straight gasoline.  The power peak on the "gas" run is very slightly better than the "E-85" run as it doesn't drop off quite as rapidly as the other.

11 HP in 100 RPM is a little stretch, but 13 ft. lbs of torque in 400 RPM is certainly easily doable.

So which fuel made more power at 5000rpm?

[Steve A]

At 5K the E-85 appears to be making more HP than the "gas" .