Triumphrat

Confused

My limited experience with motorcycle engines has led me to some simple (perhaps overly simple) conclusions. The cylinder configuration has very little to do with power out put. Harley's new 96 cubic inch motor puts out only 60 or 65 horsepower. Meanwhile my 500cc parallel twin (30.1 cubic inch) makes 45 HP. The reason has more to do with valve train and cam configurations. My 500 is designed to operate at a sustained rpm of 4,000 to 6,000. Harley, with the old push rod and rocker arm style valve train is designed for sustained rpm of 2,000 to 3,000 and therefore cannot approach the rpm range of other more modern designs. Since Horse power is measured as a known weight (1 pound) being moved a known distance (1 foot) in a known amount of time (1 second) yeilds a known horsepower (1 HP) the speed at which a motor turns (combined with proper torque, and valve timing) has a direct bearing on the horse power out put of the motor. (The higher revving the motor is capable of, the higher the horsepower potential) I say potential because other factors come into play.

Basically rpm is revolutions per minute (number of events over time) and horsepower is movement over time. Acceleration is also movement over time.

Kinda muddies these waters I know, but V-twin, parallel twin, Boxer, inline triple, or even radial, each configuration has benefits, and draw backs. All seem to work well, and all have their own following.

Keith

worldspeaker

Great responses here. Thank you everyone. Guess what I am looking for is all about the valve system and not the configuration. I have heard many on this forum state that they have more power with their "little" 865 than most Harleys on the road with their 96ci. Thought this was a simple " oh yeah a vertical twin is a better configuration and will have more power in a smaller engine" but I had forgotten that the Triumph engine is very modern with overhead cams. Much different than the big Harley engine with pushrods and a single pin. The 45 degree was originally done to fit the engine in a bicycle frame and the single pin design was done to save money starting with the first successful Harley twin in 1909. I find it funny that the Harley is known for it's V twin which it did not do successfully until 1909 (even though they tried in 1904) but Triumph had a successful twin in their second year (1903). This just made me think that maybe the vertical twin was a better design.

As always the experience from this forum has given me a lot to think about. Thank you so much for sharing your experience and knowledge. Motorcycles can be just machines on two wheels but it is the people like you that make this a wonderful family.

Dave

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Dave

CommandoRdster

You're quite right, of course.Realized my goof as I was falling asleep last night. I tried to come back to sneak in a fix tonight,but it was too late. Thanks. :moon:

I stand by the rest of my bullshyte!

:chug:
Cheers!
Bruce

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Jimbonnie

"I had forgotten that the Triumph engine is very modern with overhead cams. Much different than the big Harley engine with pushrods and a single pin."

And neither distinction has anything whatsoever to do with the horsepower or torque generated. Think compression and cam timing and you'll be on the right track.

"don't you have it backwards there Diego?

Long Stroke = more torque lower

Short Stroke = more hp higher"

Actually, it's neither and another common fallacy. Both long and short stroke engines of equal displacement will produce the same cylinder pressures during the power stroke for the same swept volumes and induction efficiency.

While the pistons in a longer-stroke engine are smaller in diameter and, therefore, experience less force from the same cylinder pressure (force = pressure x area) the crank arms are longer and have a greater mechanical advantage. The converse, of course, is true of the short stroke large bore engine. The net result is that equal displacement engines of unequal stroke lengths experiencing the same cylinder pressures (dependent on induction efficiency and cam timing) will produce the same torque at the crankshaft.

Stroke, however, is not an isolated variable; it affects the design of many other engine components and most importantly the available valve area. But again it comes down to cam timing, valve area, and compression ratio.

Oh yeah, one of the most important advantages to short stroke engines, and the reason they can rev so high, is that the short stroke helps keeps piston velocity below the 4,000 to 4500 fps danger zone.

A long stroke Harley has a very low rpm limit (because of piston velocity) and is designed with very mild cam timing to take advantage of its limited low-rpm power band. Our short stroke Triumphs can rev to 8,000 rpms and beyond and come with cams that can make power near the top of our rev range.

And think about this....

A triumph twin spinning at 8,000 rpms has more swept volume than a 96 inch Harley spinning at 4,000 rpms and so all else being equal (which of course it is not) would make more horsepower at those respective rpms.

[ This message was edited by: Jimbonnie on 2007-06-08 10:52 ]

rick05rocket

comparing comparably sized engines on comparably sized motrcycles, e.g., H-D 883 Sportster v. Triumph 865 Bonneville, the Bonnie's acceleration wins out because of the weight advantage; the Bonnie is only 451 lb. while the Sportster is 563 lb. (both dry wt.).

Thus, the power to weight ratio considerably favors the Bonnie.

So does the riding experience, having owned and ridden both.

ride on

rick

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Jimbonnie

Yeah. Unfortunately the Sportsters gotten really heavy with the new rubber mounted frame; I think it's gone up something like 50 lbs. But I guess they had to do something to make the vibration more tolerable; I wish they'd counterbalanced it like the softail motors and kept the frame light.

Joe-Dirt

Yep, my '03 is the last of the solid mounts, and the dry weight new was 494 pounds, but it's gone on a diet since. The vibes are just more of a heartbeat than anything else. It's nothing like the old ironhead Sportsters or Brit jackhammers. :wink:

SuperMan

don't you have it backwards there Diego?

Long Stroke = more torque lower
Short Stroke = more hp higher

Correct. More stroke = more torque (everywhere).
More bore = more WOT HP.

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Jimbonnie

"Correct. More stroke = more torque (everywhere).
More bore = more WOT HP."

Not correct. You're confusing how engine builders tune their motors in recognition of rpm limitations created by long or short strokes, and valve size limitations with large or small bores, with the physics of long or short strokes. Tuners intent and physics are neither synonymous nor interchangeable.

In fact, you only have to look as far as the Bonneville and Thruxton verses the America and Speedmaster horsepower and torque curves to see the tuners intent in action. The America and Speedmaster have curves similar to the long stroke Harleys because that's what Triumph thinks cruiser riders expect. But there's no crank magic here; it's all done with cam timing.


[ This message was edited by: Jimbonnie on 2007-06-08 16:27 ]

05Truxter

One advantage to a v-twin, as I had on my Honda HawkGT, is that it is narrow in profile. The way that Hawk was set up made it feel really "flickable" in comparison to my thruxton, but a big part of that is steering geometry and weight - the Hawk was a good 70 lb lighter than the thruxton.
Sometimes a v-twin can work against bike handling, though, because it tend to be a taller engine design and can give the bike a high center of gravity. Low center of mass is one of BMW boxer, or opposed twin engine's strengths.

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