Engine theory... Tuning, modernizations, and CCvsHorsePow

Riding with my uncle (late 90's Harley with a 103 in. factory upgrade engine... 1400cc or something?) my grandfather (a 2001 yamaha road star, 1200 cc I think), and my cousin (2004 ninja, 600cc), I started wondering...

My cousin's bike probably goes a lot faster than mine, even though it's a smaller engine. My uncle's bike might be able to race mine, though I'm honestly not sure who would win that, even though his engine is over twice the size of mine. My grandfather, supposing the weird would occur, would likely lose such a theoretical race to me, even though he, too, has an engine nearly twice the size of mine.

I know there's more to an engine than displacement; but I've always thought, intuitively, that it was a more crucial factor than it seems to be. What do Harleys do with all that combustion chamber there?

And what are the factors that play into my cousin's ninja being able to probably have a top speed at least 20mph faster than mine, if not 40 or more? (Haven't looked it up.)

Then, continuing... What are the relics present in an XJ that show its age? If one was made today, almost identical, with the only difference being a 'modernization' (in the sense that electric-ignition was a modernization over points), what would be changed? How much of a performance difference would it make?

I get the impression that an XJ Maxim is a reasonably fast 650... But how does it compare to modern 650's? How would its 'tuning' be described, and what are the actual areas where that tuning occurs?

 

Re: Engine theory... Tuning, modernizations, and CCvsHorsePo

An engine is an air pump. The faster it can pump air the more power output.

That Ninja probably has a red line of 15,000 RPM. That's more than double the red line on a Harley. So, that 600 Ninja can pump as much air as a Harley, but in a much lighter package.

The reason the Ninja can spin so fast is it has a very short stroke, so the piston speeds are slow even at high rpm. If you ran a big stroke engine like the Harley that fast the inertial forces would destroy it.

The down side to the short stroke is very low torque. That Ninja requires a lot of clutch slipping to get it rolling. Then lots of gear changes because first gear has a very low ratio and the ratios in the gear box are very close together.

The long stroke Harley produces a lot of torque. You can get it moving pretty easily and can pretty much cruise around town just leaving it in 3rd gear. Because of all that torque the Harley is likely faster off the line than the Ninja, but the advantage won't last long.

 

The ninja has about 20 more brake horsepower, and it's lighter than the XJ by about 100lbs, which increases the power-to-weight ratio.

But don't think that Kaw will blow the doors off your XJ in a straight line, it would simply be slightly faster.

And if the ninja didn't go above 9,000rpm (our redline) it probably wouldn't be as fast as the XJ.

 

Seriously? Why is that?

 

good read and great responses, ths is something i have always pondered as well.

 

This is a very good thread. The same thing applies to car engines. Part of the reason a 1979 ferrari 4.4 liter made over 300 horsepower yet a GM 6 liter made like 170. The Ferrari had a v12 vs the GM v8. I'm sure there's also other tech advancements but the main increase was from the smaller cylinder bore and stroke allowing for more rpms.

Does anyone know if they ever put a rotary engine in a bike? I bet that would fly.

 

The XJ650 makes it's peak 71 horsepower between 7,000 and 9,500 RPM.

EDIT I incorrectly listed the BHP for the 650 turbo (89bhp) instead of the Maxim. Corrections were made. The principal is still sound though. Sorry - Tony

The ninja puts out about 75 horsepower in that range. It doesn't make it's peak 114 horsepower until it gets above 12 grand.

So in that RPM range:
Ninja-75 bhp-lighter weight
XJ-71 bhp-heavier

Would be a pretty close race, with the XJ having a pretty good shot at it.

 

Does anyone know if they ever put a rotary engine in a bike? I bet that would fly.[/quote]

Did you say rotary on a bike?

I know of at least one....it was (if I'm not mistaken) a short lived project by Suzuki called the RE-5. I seem to remember it was plagued by cooling problems (very important for a rotary)....throttle response was also lacking in comparison to similar sized "conventional" bikes.... add to the list, none of the current mechanics knew how to work on this one bike. Gas mileage was also terrible (ever drive an RX7?).
In the end...nobody bought them...maybe it was new technology keeping them away...maybe it was the other problems.

 

Norton did also; after its original folding up, the Marque returned a couple of different times in different incarnations. During the late '80s~early '90s, they offered a line of rotary-engined bikes in the European market; the primary customers being Interpol and various police agencies, although a "civilian" touring model was also offered. It wasn't sold in North America.

 

They have an RE-5 at a really cool classic bike shop nearby called "horsepower farm" (the guys really know their shit). They even have a '40s BSA barn find there that they haven't restored for $3000 (if anyone is interested--it's fully intact, I think they even got it to turn over). The place is just filled with bikes from all parts of history.

But yeah, I freaked when I saw the rotary engine. At that time I didn't realize anyone had ever put a rotary engine on a bike outside of customs (did see a picture of a cruiser at one point where someone had installed a rotary... Great for the shows, gotta wonder how much of a PITA that is to maintain and repair, though. No forums for him... Or maybe all rotary owners have a common forum and just talk theory?).

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So to summarize the above: the redline of a machine is limited by the 'intertial force' of the moving piston itself; thus, shorter length means the ability to spin faster, and as a result, the ability to blow up more air per second, generating more horsepower.

This comes at a cost of torque because it generally means you have to have smaller displacement cylinders? I guess? Or, the smaller the displacement, the more efficient you can be with the engine, which is why it's preferred to have more cylinders instead of more cc's?

Now, what is the advantage to a longer length piston? Why does that produce a benefit? How is that related to torque?

Just for understanding's sake, if one were to boost the performance of our XJ, make it a drag racer or something, but you had to start with the original engine, what modifications would you do? Bar none, you have unlimited money, tools, skill, whatever--maybe limited by the laws of physics, I guess. You get the idea.

 

An engine's redline is determined by inertial forces of its moving parts, strength/weight/friction of materials used, as well as some other factors such as how fast the valve springs (if used) are able to lift the valves back up. If they aren't fast enough you get valve float which can cause engine failure if the valves cannot get out of the way of the piston or other valves.

Longer stroke increases torque because to get a longer stroke the other end of the connecting rod has to spin around a larger circle around the crank shaft, which provides greater mechanical advantage for the piston to rotate the crank. On the other hand, short stroke engines must rotate in a smaller circle, losing some mech. adv. and thus torque, but making up for that by having a smaller distance to travel.

Generally, oversquare (short stroke large bore) motors can make more power/displacement because you can move air more easily through them (they breathe better). Because the tops of the cylinder heads have a larger surface area (due to larger bore), you have more room for larger intake and exhaust ports, thus air isn't as bottle-necked as in undersquare (long stroke small bore) motors, which have to pull/push more air through small ports in a smaller area cylinder head.

Other modern performance improvements probably come from things like better regulation/atomization/placement of fuel via injectors, slightly higher compression while moving via ram-air (sportbikes), tighter machining tolerances, a myriad of things not having to do with the motor, and perhaps even some reliability/performance tradeoffs.

Disclaimer: I'm not an engineer, but it makes sense to me!

 

Re: Engine theory... Tuning, modernizations, and CCvsHorsePo

Boost performance/Drag bike?
NO2 - Nitrous Oxide

Not sure how long the engine would last...but bang for your buck...it's kind of like a turbo charger in a bottle.

Keep in mind...Cheap, Safe, or Fast...you can only pick two.

 

Valve float is the reason the XJ and other high revving engines tend to have cam over valve with a shim to set clearance. Any kind of rocker arm or adjusting screw adds mass to the valve train so you need stronger springs to close the valves fast enough.

A big twin has a long stroke so forces on the piston and rod limit RPM before valve float becomes much of an issue. They can get away with push rods, rocker arms and hydraulic tappets because the valves don't have to move very fast.

 

Re: Engine theory... Tuning, modernizations, and CCvsHorsePo

Just what I was thinking. XJ's don't like the lean condition with pods, bet NOS really bugs the heck out of it. I can picture a holed piston in a hurry...

 

And at the other end of the spectrum, Formula One automobile engines that have to turn 18K RPM reliably don't use springs to close their valves. Springs simply can't react at that many cycles per second; modern F1 cars have compressed-air operated valves.

 

this is quite interesting, keep going lads

 

better 'atomization'/misting was listed--that's what the yics supposedly accomplished, no? Or at least, the same effect...

Why did they stop using YICS? Does fuel injection produce a similar effect or something?

NOS... What makes NOS so great? How does it make your wheels spin faster? Does it damage the engine even in small doses?

 

Yes, the YICS helped to balance the vacuum signal and "swirl" the fuel mixture around in the cylinder to improve 'atomization'. There is an illustration of this on the forum somewhere.

I can't comment on the vacuum signal part, but I'm of the _opinion_ that computer-controlled fuel injectors can do a better job at "spraying" the fuel at just the right locations in the cylinder.

As for NOS, the basic idea is that it can combine with more fuel particles than regular air. According to Wikipedia, plain old air contains about 21% oxygen, whereas NOS (N2O, 2 parts nitrogen, 1 part oxygen) contains 33% oxygen (one part oxygen of the three atoms that make up N2O).

Once you apply the "golden ratio" of air:fuel (14.7:1) to the NOS going into the engine, you can burn more fuel in one go and thus gain more power.

The problem is that all that extra power puts a *LOT* of extra stress on the engine (stress the engine was not designed to take), in terms of explosion force and the associated heat. Thus you can only use it in very short bursts on engines not modified to take the extra stress (and often times you have to modify the engine from the get-go to even use the stuff), unless you want to risk catastrophic engine failure.

 

So, as to anyone who ever wanted more on this topic... A great educational read that really englightened me on this topic was this. A guy takes the FJ11 (the first 'superbike' that Yamaha made for the era, and a direct generation continuation of the engine design of the XJ650 and before that of the XS11) and does everything one can do to bring it up to modern standards, so that he can compete with '01 R1's and the like. Nothing is neglected. Mindblowing. A lot of it is weight.

Maybe the only remaining questions would be head/valve design, and fuel injection is of course off the table for his purposes (though some people do install fuel injectors onto FJ1200's). But he pretty clearly illustrates that the head/valve design was plenty sufficient at the time and definitely not the limiting factor of the bike. He takes the bike down to the same weight range as modern bikes, and brings the horsepower up from 108 stock at rear wheel to the 165 at rear wheel range.

I definitely have a better grasp of what it takes to make a bike go faster now.

 

Don't forget about compression ratios. The Ninja is much higher than the XJ and gets plenty of HP out of that. It's CR is around 12.8 I think and the XJ is less than 10.

Determined primarily by the bore, stroke, and most importantly the cylinder head volume (or the minimum volume of the space into which the fuel and air is compressed, prior to ignition).

As an example of importance my 240SX was an early model off the line and had a smaller CHV than the later cars listed at 135HP, so my early model has 152HP. Of course higher compression means you have to run high octane too.

No doubt each model has it's high points, but I would never sell one of these XJ's short for performance. In 1981 Yamaha's Seca750 did the 1/4 mile in 11.99 with a SHAFT DRIVE! 8O The Ninja which is much lighter, 23 years newer, and chain driven hits at 11 flat. Depending on the rider the Seca could win.

Oh yea and a 47 degree banking angle for the Seca is mind blowing for 1981.

 

I love stuff like this. Understanding how things work is awesome and you guys make it very easy to understand. If anyone knows of any good articles that breakdown how motorcycle engines work send me a link!