HP on a WR426?

25 replies to this topic
  • berudd

Posted April 26, 2001 - 10:48 AM


My two cents worth on the Ti valve thing just to see if we are all talking about the same stuff. Any benefits that come from Ti valves are not directly related to their reciprocating mass. The gains come from being able to use the lighter valve springs. The springs exert considerable force on the camshaft. In extreme cases it is possible for them to actually rotate the crankshaft. So, each power stroke of the engine must over come this force. Stronger springs take my force to compress so that will mean less horse power to the rear wheel. This effect would be noticable at any rpm. This would rob you of HP and you can compare it to dragging the rear brake. How much differenct does it make. Well, I am sure that if you are going for you PhD in automotive science that would make a great reasearch project. Since this is an engine of good quality we can safely assume that some work has been done to eliminate friction in other areas which may allow our Ti valve to have a more noticable effect. If it gives you a half to one HP that may be more power than the guy next to you so every little bit helps.

  • techman

Posted May 02, 2001 - 07:35 PM


Here's a bit of mechanics. The hpxtime=work needed for angularly accelerating or delerating the crank and reciprocating masses is stored and then released again when the whole mess gets slowed down. I.e. rev your engine, cut the ignition and drop the clutch - voila, where did the power come from to launch your bike into the weeds if no gas is being burned? That's inertial energy storage, and it would affect the hp readings in an inertial-loading type dynamometer. However, under steady load, it means nothing as rpm is constant and there is no rotational mass x angular acceleration sucking hp (from a crankshaft or wheel point of view). Adding or removing primarily flywheel weight affects rate of rev building, but not engine hp output in the engine tuning sense of the meaning. I.e flywheel weight or no weight, you'll hit the same top road speed or trail speed. The only difference will be, in a practical view, how much wheel spin occurs when you break traction, wild or medium wheel spin. Guys add flywheel weight to get medium, and not wild. SX guys want wild spin because they simply operate that way in riding style: rev to the moon, let the ground catch up with the wheel.

Something to keep in mind is that besides the crank, piston, counterbalancer and valve train, there's the clutch, geartrain, sprockets, chain and rear wheel getting spun up and down. As a matter of fact, the whole bike and rider is an inertial load for the system. So it's kind of artificial to draw a line somewhere and include inertial effects in engine hp evaluations. But I do agree that where the inertia is distributed affects the nature of the wheel spin upon breaking traction. I.e no crank mass, all super light motor but heavy loaded bike (equal inertial load) on pavement with no wheel slip would accelerate the same as a super light bike with massive flywheel (of course adding up to the same inertial loading total), but, put those two in the dirt and the no-flywheel bike would buzz slots in the ground and then bog etc, while the heavy flywheel would damp it's rate of rpm changes, providing "better hookup". That all gets into the business of power delivery to the ground, which is separate from but tied to the style of engine power delivery.

The valve springs, neglecting imperfect spring material behaviour (force vs deflection hysteresis) are a simple restoring force and they will 1) load torque onto the engine as the valve is opened and 2) add torque to the engine a the valve closes - because the spring rides the down slope of the cam then! The energy put in comes back again. So, the only extra energy used by stiff springs is from additional friction they might add.

So, lighter engine parts allow higher revs because they self-create lesser inertial loads when reciprocating or rotating, and hopefully they also can endure the addditional stresses from the actual gas-burning power-creation at the same time. Higher revs usually means higher hp at the top end, but the inertial loading stuff affects the nature of "rev-a-bility" and it's effect on power delivery at the tire to ground interface.

Now don't anyone say "coefficient of friction of the mx track" :)

  • Blue_Boner

Posted May 02, 2001 - 08:09 PM


I don't want to get involved in the ti angle of the dangle, cam grinds, bore sizes or anything like that but Hick mentioned that a poorly jetted 426 will smoke a well tuned 400. I'm sorry but I beg to differ on that. I would consider my 98 WR400 tuned to perfection and it is faster than my friends stock 01 YZ426. My 400 has YZ timing, White Bros header and pipe, trimmed throttle stop, airbox lid removed and carb jetted by Stroker.

98 WR400 "Strokerized"
97 EXC400
94 RMX250

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  • yzf

Posted September 05, 2001 - 03:14 PM


titanium is not harder than hardened steel as far as wear and tear is concerened but it is stronger as far as breaking strength

  • big_G

Posted September 05, 2001 - 05:49 PM


"Mom...the engineers are fighting again..." OK fellow gearheads...lets play in reality.

IC engines are air pumps, nothing more. This is why the phrase has and will be, "There's no replacement for displacement". You cant move any more air than physically possible. Since this engine can rev out until you float the valves, you have a finite pump. Finite (with end), not infinite (without). All the valves, cams and porting in the world cannot change the maximum possible displacement. More RPM can displace more, but is this more Horsepower...

Floating the lighter TI valves at a later RPM may extend the endpoint of a dyno'ed power curve, but the area under this does not represent power. A larger area under this curve is tempting bait, but a red herring in this context. While you can measure by this, you cannot predict. The real power curve, really a power cycle, is called the "Carnot cycle" (read car-no). This is the area, multiplied by time, that leads to power. A tricky beast indeed.

If your clever, you will try to now relate cycles to revs, revs to power and BAM!!! RPM=HP right???... Dude, this is thermo-freaking-dynamics, ya right!! In today's engines, even the WR/YZ, you will undoubtedly notice that the curve at the high RPM end of dyno chart's seem to drop off a bit. So, does a TI valved pump move more air than a steel valved pump? Maybe. Does it do any good...

If faster spring/valve action leads to increased duration, and increased duration removes restriction to the pump, maybe...JUST MAYBE..., you have a shot at growing area within the Carnot plot, and therefore power. Does that put more at the back wheel?? Ooops...back to reality.

If you wanna boggle your mind tonite, check this out. It is a very nice illistration of the Carnot cycle, showing Pressure vs. Volume in a 2-stroke engine. Although we're ranting about a 4-stroke, the process is basically the same, since no work is added in the exhaust or intake strokes. The 2-stroke is power/output and repeat. the 4-stroke is power/output/waste/waste and repeat.

Ahhhh...that was fun...late, Gary

'99 WR400 with CA plates...ooooh yeah!!!

  • tbronco

Posted September 05, 2001 - 06:09 PM


techman wrote:

The valve springs, neglecting imperfect spring material behaviour (force vs deflection hysteresis) are a simple restoring force and they will 1) load torque onto the engine as the valve is opened and 2) add torque to the engine a the valve closes - because the spring rides the down slope of the cam then! The energy put in comes back again. So, the only extra energy used by stiff springs is from additional friction they might add.

THANK YOU Techman. We needed that.

This is where Ti valves _can_ give an advantage. Let me try to describe it...

As RPMs approach the point that the valves float off of the closing portion of the cam, that restoring force (that adds energy back into rotation) is diminished. The momentum of the opening valve carries it right off of the profile.
Lighter valves will therefore allow a combination of longer duration/more agressive opening & closing with the same spring tension.

Since the cam grind is reportedly the same, the potential power increase just isn't realized.

98 WR400 Dualsport!


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