Why more than 2 valves

My Honda VT750 Shadow has three valves per cylinder. I think the XJ700X has five per cylinder. I just saw that the Honda VFR800 has only two per cylinder, and it's a pretty hot bike. My XJ 700 N has two per cylinder. So, will one of you engineering gurus explain why multiple valves are better than just two large ones? I'm a psychology major, so give me a break and make it simple.

 

I'm gonna take a stab and say it has to do with the efficiency of filling the cylinder with the atomized air fuel charge. I'm far from an engineer but that's my guess

 

Because they charge by the valve to adjust?


Sorry....

 

I've done a little research since I posed the question. Two smaller valves allow a greater volume of air/fuel/exhaust than one larger valve. Since the valves have to fit in the cylinder, the two smaller valves actually provide a great surface area than the larger valve in the same cylinder. In order for the larger valve to give the same volume of air/fuel, exhaust it would have to open more which then brings up issues with piston contact and a higher valve weight which effects the cams, etc. So basically, multiple valves provide more volume, are lighter and apparently are easier on the cam.

 

Makes sense to me.

 

Sigh. Sad signature. Good video, though.

 

You may not be an engineer, but you understand this particular thing quite well !

 

More valves also means moe surface area for the individual valves to shed heat into the head, and they are lighter (individually). in combination that allows the valves to be able to cycle faster (higher crankshaft RMP), allowing for more work to be done in the same amount of time (more HP). As with everything, there are compromises and limits to how far that can be taken. The 7 and 8 valve engines Yamaha tried out were real screamers; very efficient, and made better power per cc than the 4 and 5 valve heads, but they were finicky and a bit fragile (as well as a ton more complex to manufacture).

 

The Yamaha Genesis engines, not just another purdy face:

http://www.motorcycleproject.com/text/cows-multi.html

Which leads to:


And in case you are interested, here are the standard compression ratios for the various Yamaha engines:

XJ550: 9.5-to-1
FJ600: 10.0-to-1
XJ600 Seca II: 10.0-to-1
XJ650 (except Turbo): 9.2-to-1
XJ650 Turbo: 8.2-to-1
XJ700 air-cooled: 9.5-t-1
XJ700-X and XJ750-X water: 11.2-to-1
XJ750 air: 9.2-to-1
XJ900: 9.6-to-1
XJ1100: 9.0-to-1
XS1100 9.2-to-1

Note the rather “sky-high” CR of the multi-valve Genesis engines in the water-cooled “X” models, which is also one of the keys to their vastly increased power output.

 

The only issue I have is this:
"Combustion chamber technology dictates very flat combustion chambers."
Why did hemispherical combustion chambers make Chrysler engines so infamous? They claim in the article that it isn't good for making power!

 

Because Chrysler hired good marketing/advertising people in the mid 60's. The "hemi" engines were available starting in the mid 1950's (392 cu. in. version, a truck motor and certain Desoto models), but didn't get any notoriety until Chrysler started spending lots of $$ in racing and dominating many classes, and then the "hemi" engine got all the credit (instead of saying, for instance, "hey, we outspent GM and Ford by a factor of 3-to-1 on racing development, therefore you should buy our entry-level Belvedere sedans, they've got really nice carpeting and a nifty push-button transmission gear selector.....").

The 426 Hemi also had cross-bolted main bearing caps, which is a very stable and durable design, able to sustain a lot of torque and bearing loading w/o spitting the crankshaft out and onto the dragstrip.