Topic: Riddle me this

[cosmicvision]

Having operated an increasing variety of machines in my day, I can't help but notice odd patterns in their characteristics.

For example, driving up a hill in an 8-cylinder truck, one barely has to apply the throttle to maintain the climbing torque needed to keep your speed. On the same hill, at the same speed, in a 4-cylinder car, one will likely experience a loss of speed if the throttle is applied in the same manner. To maintain your speed, you must dip into the gas pedal a little heavier and maybe even bump down a gear.

Having ridden two-cylinder motorcycles, the opposite is true of our 4-cylinder bikes. The twin will pull the hill smoother than the four, which requires at least one downshift to climb most any hill. And dirt bikes seem to have the most torque response, with a single piston.

What causes this phenomenon?

If the same logic held true across the board, it seems all pickup trucks would be enormous-displacement thumpers and v6 & v8 duties would be left for the smoothness of passenger cars.

These are the things I lie awake wondering at 3am

[kghost]

stroke, intake and exhaust design.

More cylinders tend to be more efficient but higher in the rpm range.

There is however no replacement for displacement.

[333]

I wouldn't think it as the opposite, as you state, rather it's probably more equal. But my answer would be horse power and torque.  For example, take my bike, a CB350F, and compare it to it's 2 cylinder brother, the CB350G(one I've ridden). They actually perform quite similarly. And the specs are very close, horse power the same, top speed within a mile or two, 1/4 mile(from the magazine tests) very close.

I would disagree with your statement regarding smoothness. A 4 is going to be smoother than a twin.  A V-twin may seem smoother than most inline twins. This is because these inline twins are mostly single crankpin design, and are 180 degrees apart, meaning that both cylinders rise and fall at the same time, but as one comes to the top of the stroke to compress, the other comes to the top to exhaust.

[Xnavylfr]

I have to disagree about the V-TWIN being smoother than an in-line 2 or 4 cylinder..
The V-twin  has MUCH larger piston and stroke to get the torque from the bottom of the RPM range and the in-lines use mid to higher RPMS.. I went from riding a KZ1000 bored to 1075cc to a 1600cc V-TWIN. At first I thought I was riding on a cobblestone street because of the way the BIG TWIN felt. Nearly 4" pistons on a single pin crank stroking at the same time, you felt it when they both came UP!!

The in-lines have more moving parts but the parts are better counterbalanced to transmit less vibration. I know my BIG TWIN has plenty of low end grunt to pull some of the hills here in NC but when you're only turning 2500 rpms at 60 MPH and start up a LONG grade it doesn't take long and you start loping and need to drop a gear. The IN-LINES are somewhere between 4K and 5K RPMs and right in the peak of their power curve.

Xnavylfr(CHUCK)

[kghost]

There's limitations.

Piston bore is capped at about 6 inches due to flame propagation. Too big and it causes pre ignition/detonation of the unburnt fuel as the flame front moves across the cylinder from the spark plug.

Stroke is limited by rpm. Piston speed gets higher the longer the stroke.

[cosmicvision]

The twin will pull the hill smoother than the four...

There's no question here that a four is overall much smoother to ride on a flat surface. My first bike was a KZ twin and my hands would be half numb after riding the thing sometimes. What I was referring to was that the torque seems to come on smoother on a twin, whereas with the 4cyl bike there's that big dead spot from 0-4000rpm, that becomes even more apparent when climbing a hill. BUT the truck with more cylinders than a passenger car has the better low-end response.

The point about displacement is a good one. That could be the variable I hadn't considered...I suppose if there was a 5-liter 4cyl truck it would probably pull a hill about the same (would sound like a garbage disposal though)...and a 2-liter v8 would probably behave like a 4-cylinder of a similar build. Now that would be an engine to hear though!

Interesting to read that an inline engine compared to a V layout affects how much an engine vibrates. I guess the physics of it make sense now that I think of it.

[Bailgang]

As far as automotive V8's and 4cyls it's all about displacement and the same can be said at least to a point about bike engines because many V twins are nearly twice the size or more as our 750's. Many things are a factor but bore/stroke ratio is a key player as well as cam timing and what crankshaft the V twin has, I think all HD's and many Jap V twins as well as some parallel twins use a 90 or a 270 (however you want to look at it) degree crank which helps promote a stronger torque curve on the low end but suffers at high rpm. Each design layout has it's assets as well as compromises. An older brother of mine has an old H2 Kaw as well as modern Honda V twin, don't know what model but it's a big cruiser. The H2 will smoke his V twin in a drag race but he says the V twin of his is much easier to ride in slow traffic, it has a much broader TQ curve on the low end which means he doesn't have to downshift nearly as much as he would with his H2 when dealing with slow speeds.

[kghost]

One hell of a flywheel will make a difference too lol

[Markcb750]

There are many variables in the "why" one vehicle goes up a hill with minimum throttle and gear manipulation while another requires one or both to climb.


Torque at the drive wheel(s) is the root of all these reasons.    If the wheels do not have sufficient torque, somethings got to change, throttle position or gear ratio & throttle.

[Don R]

When any engine is loaded past it's torque output (the 4 cyl) more throttle opening will only lose vacuum and cause a loss of effeciency. Assuming it has a carb. Easing up on the throttle will sometimes allow the engine to build power as the rpms climb.

[cosmicvision]

When any engine is loaded past it's torque output (the 4 cyl) more throttle opening will only lose vacuum and cause a loss of effeciency. Assuming it has a carb.

The same could be said for a fuel-injected engine correct? They seem to suffer from a similar phenomenon.

Many things are a factor but bore/stroke ratio is a key player as well as cam timing...

Probably the reason I keep hearing how a 650 cam really opens up the low end in the 550 motor.

The H2 will smoke his V twin in a drag race but he says the V twin of his is much easier to ride in slow traffic...

Yeah but that H2 is a two-stroke ain't it??? Sure would love to ride one of those beasts, what a unique bike.

Interesting topic here guys, thanks.

[Bailgang]

Yeah but that H2 is a two-stroke ain't it??? Sure would love to ride one of those beasts, what a unique bike.

Yep it is and using it as a comparison was extreme because the H2 has a crazy power band, if you were to give one full throttle at low speed without downshifting the sensation would be ho-hum until its rpm got in it's power band and at that point you better have a good grip on the handlebars because its going to turn on as if you flipped a nitrous oxide switch.

The reason I mentioned bore/stroke ratio is because I played with Pontiacs for a long time. The 326, 350, 389 and 400 Poncho engines all shared the same stroke crank shaft as well as the same connecting rod length, the diff in displacement was in the bore size which means if you were to drop a 326 crank in a 400 block you'd still have a 400 or a 400 crank in a 326 you'd still have a 326. When comparing a Poncho 326 to say a Chevy 327, even though the 2 are very close displacement wise the 2 have very different power characteristics and it starts with the bore/stroke of the 2 engines. The bore of a 327 is much larger than the stoke of it's crank (4"/3.25") when compared to the bore/stroke of a 326 (3.72/3.75) in itself makes the 327 more willing to rev higher and produce more HP at those higher rpm's at the cost of low end TQ. On the other hand a 326 has gobs of low end TQ but at the cost of top end HP. Granted the cyl heads of the 2 engines play a factor but even with fully ported heads and aggressive cam a 326 Poncho has never ever been known to be a high rpm engine (at least not reliably ) while the 327 Chevy is. Being the above mentioned Pocho engines all shared the same crankshaft, as the bore size increased for each engine so was each engine's ability to produce HP higher up in rpm's. I wouldn't call a Pontiac 400 a high rpm engine even when fully modified but it is much more willing to rev than it's smaller siblings and that is largely due to it's bore/stroke ratio. To make a long story short the bore/stroke ratio of a given engine is what intially dictates at what rpm's that engine will be most efficient at whether it be low end, mid range or high rpm. It's not the one and only factor but it is a key factor.

[LesterPiglet]

Generally the longer the stroke the more torque produced, and lower RPM ceiling.

[dave500]

always the longer the stroke,rider feel when to shift and not lug too late plays a part in an up hill climb,many 360 degree twins got a cam swap to have both fire the same time in racing applications,its still done these days to fours,so you have a pair firing at a time,the idea is torque based.