View Single Post
Old 09-26-2012, 12:51 PM   #11
let's keep going...
geometrician's Avatar
Joined: May 2008
Location: West-By-God Virginia
Oddometer: 1,034
Big Bang

Originally Posted by men8ifr View Post
If it was configured so the power strokes are closer together I would have thought this should have zero effect on power or fuel consumption.

The engine is still burning the same amount of air with the same effective spark timing and cam position relative to each cylinders TDC firing, the only difference is when the power is delivered with respect to the crank rotation.

That is quite interesting as it kind-of seems like one big power pulse (single cylinder or 2,3,4 cylinder bike with each cylinder combusting at, or close to the same time as each other) gives more rear grip, or perhaps more predictable breaking of grip (spinning up) so you can use the power more effectively. - any-one know more about this?

I guess downsides are
1) more vibration (more like a large single cyl bike)
2) more stress through crank and transmission (though not necessarily on v twin as power pulses are not happening at the identical time)
Big Bang engines aren't meant so much to put the power pulses TOGETHER as it is to spread them out- this system was used on many Formula 1 4-stroke roadracing engines as the 4-cylinders usually produced power with 90-degree crank throws- in essence, there was a power pulse every full rotation of the crankshaft- which had the marked downside in motorcycle applications that the rider must wait on partial/closed throttle for a turn to open up before they can apply power- for fear of the rear tire spinning up quickly once it has lost traction.

As roadracing bikes approached & even exceeded 200bhp the tire wear was becoming more of an issue- by spreading the power pulses out (when viewed at the rear wheel) the tire compounds have more time to recover from the massive amount of energy (ie heat/hysterisis) they must endure.

So in the end you have a more rideable bike with tires that will last longer, allowing them to use even stickier compounds that are synonymous with fast wear.

As for trickle-down, Yamaha now sells their R1 with "Cross Plane Crankshaft Technology" making their bike that much easier to ride or race


As for big-bang the difference in crank inertia pulse spacing will indeed affect fueling, esp in the overlap phase of 4-stroke valve timing when both the intake & exhausts valves are open. Overlap, not a piston drawing down & "sucking" air into the engine plunger-style, is what makes 4-strokes work at anything above a few RPMs- it's that important. Basically the exhaust gasses are super-hot as they first leave the combustion chamber via the exhaust valve- this creates a low pressure area via the inertia of the gasses expanding & cooling as they move down the header.This low pressure draws the next intake charge in as the intake valve is just beginning to open, even as the piston approaches TDC. Perhaps most importantly pressure difference is greater than would be available due to atmospheric pressure alone, which is <15psi @ sea level. The longer the duration of overlap, the spread of power will move to higher RPMs- the downside being a loss of low RPM torque. Extreme overlap is found in drag racing engines which operate almost exclusively at high RPMs. Lots of overlap is makes hot-rodded engine "lumpy" at idle, as they aren't operating at their most efficient RPM. In many case RPM must be increased to maintain idle.

Adjusting overlap with slotted cam gears will allow you to entirely change the character of your engine to torquemonster or top-speed flyer without changing other elements like exhaust or intake, etc. Simply blueprinting your valve timing will allow you to match the cylinders for a smoother engine as well.
'06 KTM 950 Adv 'S'
'03 KTM 250 EXC
'87 KLR650 - <200K miles "Like the Energizer Bunny, just keeps going & going..."

geometrician screwed with this post 09-26-2012 at 01:32 PM
geometrician is offline   Reply With Quote