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Old 01-26-2013, 02:12 PM   #1 (permalink)
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HP vs Torque vs Fuel Efficiency

Here is a post I made on the motorcycle forum. Much of the fuel efficiency material is well discussed here already but I though I would post the topic here as well as it applies to gearing. Motorcycle guys quite often change gearing since it is so easy. Multiplying the torque at the rear wheel as high as possible will make a vehicle accelerate faster even though the rear wheel power is still the same. So it would seem that maximum torque is king for acceleration. But one intrepid member charted out the rear wheel torque in every rpm and gear and showed that the highest average rear wheel torque is found when shifting on either side of the engine's power peak. Not the torque peak. And definitely not at redline with most engines that are not cammed up to a high rpm power peak.
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Quote:
Originally Posted by miprofessor View Post
Debunking Horsepower and Torque Myths
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Debunking Horsepower and Torque Myths
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Good link. This could become another good thought experiment such as some other topics here did.
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Gearing: Horsepower's Better Half - Page 3 - Honda CBR250R Forum : Honda CBR 250 Forums
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An engine torque curve and horsepower curve are really just two different ways of looking at the same thing. Power is just torque over time. People like to look at the torque curve to get an idea of what is going on below 5,000 rpm because it has better resolution on the graph due to the horsepower curve not yet having much multiplication from the low rpm. The power doesn't yet look like much and the torque looks much bigger but they are none the less showing the same information. And power is the real metric of how much work can be done. Torque can be changed any way you want with gearing but that never changes the power at the wheel because you are trading increased rear wheel torque for decreased wheel rpm. Increasing the rearwheel torque WILL make the bike accelerate quicker but when you get to the steady state top speed for any given load such as the wind on a flat top speed run or gravity on a big hill, the gear that will be fastest is the gear that puts the rpm at the power peak. Not the torque peak. And not the redline. Even a drag strip run which is all about acceleration and using the most torque multiplication, will be the quickest on a vehicle like the CBR250R when shifting the higher gears at an rpm band that is much lower than the redline and is centered on the power peak. As was generously explained in the other thread by DieselMaxPower
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The OP's link is a little too simplistic when it comes to fuel efficiency. It is possible for an engine to take in the same amount of fuel and air (disregarding the much higher losses at high rpm) at 8,000 rpm as it is at 4,000 because of the throttle plate's restriction and resulting intake vacuum. The real reason for generally better fuel economy at lower rpm is from less frictional, reciprocating, and thermal losses than the higher rpm. And from being closer to the first torque peak.
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The first torque peak usually shows where the engine is operating most efficiently as set up by the cam timing, intake runner tuning, ect versus linear piston speed losses, flame front travel and combustion gas pressure curves, ect, ect. A Brake Specific Fuel Consumption chart (Which plots 4 parameters; engine rpm along the bottom, the torque that is produced along the right, increasing engine load which can be roughly increasing throttle position up the left, and finally the topo values for how much fuel is being used versus the power produced.) is the only way to really know the engine's best efficiency range. They usually show the best efficiency at just off of full throttle on either side of the first torque peak. Which is the bad news for fuel economy. Most engines are way too powerful to operate at full throttle for more than a few seconds without going way too fast. So small engines with barely enough power, like our 250, are more often operated near their most efficient range than an engine with plenty of extra power. The only way to get the bigger engine into it's efficient range is to pulse with a high throttle opening and then glide with the clutch in, and then pulse again. But this an advanced hypermiling technique that is not for everyone. Better to stick to a CBR250R, PCX150 scooter, or new Ford Fiesta.
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Thermal losses, pumping losses, friction losses and powerband tuning, among many other factors, all come into the design equation. Using the smallest combustion chamber and the fewest of them minimizes the area which can lose heat (wasted fuel) to the head. But too small of a bore reduces the force of any given combustion pressure because it offers less area to work on. Hence the tendency for engine design to have a square (equal bore/ stroke) design. Slightly longer stroke than bore will be more efficient as it will move the torque peak to a lower rpm because of the longer crank arm. Interestingly, a square single will have less combustion chamber area to pick up heat than a square twin of the same displacement. Multiply it out for yourself. It's cool. And so is the CBR250R's engine. I'm sure you have noticed that the engine and radiator don't throw off much heat compared to your other bikes. Air cooled engines can run cylinder head temps of 350F/ 175C which, compared to liquid cooled engines which are limited to 100C to prevent boiling, waste even less heat.
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Heat out the exhaust is another waste. Which can be reduced with Atkinson cycle engines
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Atkinson cycle - Wikipedia, the free encyclopedia
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(a fancy way of saying that the cam left the intake valve open to long.) which seek to have no pressure remaining in the cylinder by the time the piston reaches the bottom. And they also reduce pumping losses with lower intake vacuum. At the expense of power versus displacement which never looks good to bench racers reading spec sheets.
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Honda motorcycles is fighting for better fuel fuel efficiency in the big bike west with the ultra long stroke CBR500 platform and ultra low rpm tuned NC700. Either one of which can almost match (maybe surpass for tall riders and two up) our already exemplary CBR250R at an average of 68 mpgUS. BMW has always been a fuel efficiency leader. The Ninja300 is another sporty but all around bike taking a step in the right direction with longer stroke and longer gearing. Some small 110cc air cooled bikes which are popular in India can sell for $1000 and get 150 mpgUS at 40 mph.
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Here is a sample BSFC map from another vehicle and a chart of the optimum shift rpm's of the CBR250R. The shift points were charted by DieselMaxPower from a dyno printout to show when the engine torque as multiplied by the drive train at the wheel would be greater in the next gear. Notice that the top three shifts are giving the most average rear wheel torque and power when shifting at about 9,300 rpm. Redline is 10,500.
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Old 01-26-2013, 03:12 PM   #2 (permalink)
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Quote:
Debunking Horsepower and Torque Myths
Good link. This could become another good thought experiment such as some other topics here did.
I actually found that a bit head ache inducing.

Quote:
The torque wrench reads how many foot-pounds of force that you are exerting on the shaft.
That sentense is enough for a death penalty, but the author decides to continue his criminal career with this snippet:

Quote:
Horsepower is a measure of work.
Like if the name of the unit itself isn't clue enough that it denominates.. well power!

After that the text just degenrates into the physics equivalent of a frat party, where someone put mescalin in the punch bowl without telling anyone.
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Old 01-26-2013, 03:23 PM   #3 (permalink)
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Old 01-26-2013, 03:29 PM   #4 (permalink)
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Isn't the NC700 series' engine the long stroke one?

But other than that, I'm curious about the FE of the CBx500 series. I can't see any info at fuelly yet, though. NC700S (I can't flat foot from 'X so I would stick with 'S, manual trans thank you) data look promising. I already like those bikes, they can be good work horses. A bit heavy, but good otherwise.
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Old 01-26-2013, 04:03 PM   #5 (permalink)
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Quote:
Originally Posted by jakobnev View Post
I actually found that a bit head ache inducing.
The original link served as a good spark to open the dialog. My own posts are quickly written and have some technical inaccuracies but are not meant to be an A+ final exam essay.
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Old 01-26-2013, 04:23 PM   #6 (permalink)
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Ok. So the 500 is a square engine. It is only long stroke compared to the super sport 600 with which it shares the bore dimension.
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500 Bore And Stroke 67.0mm x 66.8mm
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The CBR250R gets good economy by being a small single.
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250 Bore And Stroke 76mm x 55mm
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And the NC700 at
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670 Bore And Stroke 73mm x 80mm
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Which is undersquare and tuned for an ultra low rpm torque peak.
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Old 01-27-2013, 11:08 AM   #7 (permalink)
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Here is a reply from another forum which shows the classic and incorrect view of torque vs horsepower.
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"Related, but not the same. I have a Dodge Ram 3500 Cummins diesel. A BMW sportscar will smoke it in a race. It has more HP. But lets see it pull 25,000 pounds of round hay bales at 70 MPH up a steep grade! My diesel has a redline of just 3,375 RPM or so. It makes all it's power from only 1,600 RPM and up. It only has 230 or so HP, but it has gobs or torque, something like 460 ft/lbs@1600."
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Thanks for the reply. I wondered if there were any gear heads here that would want to discuss this. Yours is the common view based on intuition but it isn't quite right. This would actually make an amusing episode of Top Gear. Car vs truck: TOWING!
I found an interesting dyno test where the Ford 6.7 Powerstroke engine made 700 foot pounds and 350 hp.
The truck will be better to tow with since it has greater torque and power down low. So when you take off from a light, it is already in the power band at the torque converter stall rpm. It is easier to access it's POWER band right off the line. The car is tuned for high power closer to redline. At the expense of not having as much torque or power at lower rpm. So even in first gear it's rpms are to low to have the torque or POWER to yank the trailer to get started.
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But, if you were to race two trucks and two trailers at the drag strip, the driver who makes his shifts on either side of the engines power peak would beat the other driver who shifted on either side of the torque peak. Power is doing the work of moving the trailer. And the rear wheel torque of the winning truck will avreage out to be HIGHER than the earlier shifting truck who used his engine at the range of greater torque. The slower truck sent more torque to the trans but the faster truck stayed in a lower gear more often and used a greater POWER band to get greater rear wheel torque, and hence power, at any given speed.
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Here is the crazy part.
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Once it finally got moving so it could use it's gears at the power peak, a 400 hp car will pull the trailer up the hill at a faster speed than a 350 hp truck. Even though the car doesn't make as much torque as the truck anywhere. Power is doing the work. A lesser engine torque can be multiplied with gears but the power at the wheels is always the same as the engine is putting out.. The trans is trading higher wheel torque for lower wheel rpm. The power at the wheel is the same as the engine at any gear. And higher power will do more work than lower power. Most towing vehicles have giant engines which are tuned for low rpm so they have tons of torque, and so, power, down low to yank with, and plenty of peak power to keep moving up the hill from the large displacement. But it is always the amount of power that is doing the work.
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This is interesting. The first google search I made came up with test data on a dyno which states that the rear wheel power was the same in any gear. If anything, the power showed to be slightly higher in 5th than in 3rd. Must be the dyno is not really that linear at different speed ranges because that is the complete opposite from what most people would think where they would guess that the lower gear would give the most rear wheel power. And in reality the power would be the same discounting different losses in the trans at different gears.
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Exclusive! Dyno Testing The Most Powerful Diesels Detroit Has Ever Built - Diesel Power Magazine
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"We strapped down the 2011 ˝ Ram 3500 dualie to the rollers first. The guys at ATS made three dyno pulls with the new engine, one in Third, one in Forth, and one in Fifth gear. The results were surprising.

Regardless of which transmission gear we tested the trucks in, the power rankings were all the same"
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Old 01-28-2013, 02:45 AM   #8 (permalink)
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That's only for higher gears, and it's an artifact of how the dyno measures power and how the engine generates boost. Longer gears allows boost to build more linearly, and shows more power on some dynos.

In lower gears, that much torque will destroy a load dyno.

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Pretty much agree about towing. All that really matters is how much torque is getting to the wheels. And engine horsepower is a better indicator of how much will get there than engine torque is. Of course, the physical limitations in designing a transmission mean that you will still want low-down torque unless you can design an incredibly short first gear... and generally, running at lower rpms is more favorable for long-term reliability and to minimize efficiency losses.
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Old 01-28-2013, 05:11 AM   #9 (permalink)
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Quote:
Originally Posted by sendler View Post
Multiplying the torque at the rear wheel as high as possible will make a vehicle accelerate faster even though the rear wheel power is still the same.
This is incorrect.

If you lower the gearing, you can increase the wheel horsepower (and if you accelerate faster, you have, by definition, increased wheel horsepower). While you can't multiply horsepower like you multiply torque through gearing, you DO change the RPM that the engine is turning at. By lowering the gearing, you allow the engine to turn faster and make more horsepower, and thus get more horsepower at the wheels.

You want to maximize torque at the wheels at every wheel speed during the race. However, since wheel torque is proportional to wheel horsepower divided by wheel rpm, you are maximizing wheel horsepower as well.
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Old 01-28-2013, 06:41 AM   #10 (permalink)
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If that truck can pull 25000 pounds of hay up the hill at 70mph, then the BMW can too if it has traction, because it has more power. It just probably doesn't do it efficiently. The choice of a bigger, torquier engine is made when the anticipated operating loads are high, so it doesn't have to spin as fast to give you the same power.

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