Not extra, "more"

 

Thanks for the replies. To me this thread is very interesting.

Regarding the squish area and the turbulence it creates, does that mean the sparks should not fire until the piston at least gets to the squish area?
Regarding the 4 valves per cylinder, which came first the the 4 valve per cylinder design and then the 2 spark plugs or was the primary design for 2 spark plugs per cylinder and they needed 4 valves to make that work?

What is the reasoning behind 4 valves per cylinder? Does more air get sucked into the cylinder during the intake stroke because of it or is again, just to create turbulence?

 

Plug firing will vary according to a lot of things, as to what the engineers are trying to accomplish. Almost all plug firing comes just before TDC to start the burn at the piston reaches it, for max push downward. Varing the plug firing does various things, emission wise, power wise, and again depends on what application the maker is after. It also depends on compression, cam overlap, cylinder head combustion chamber, engine rpm, ect. The higher the rpm, the sooner the plug must fire.
4 valve engines have been around a long time, Ferrari even had a 5 valve engine. Its all about swirl in the cylinder. Getting a better burn on the gasses let in is where you get more power, less carbon monoxide, and a cleaner tailpipe, on a leaner mixture. Dual plugs have also been around a long time, mostly looking for more power, in the early days. Mercedes ran dual plugs from 2001 up to 2006, in all their 6 and 8 cylinder motors. While BMW doesn't run them at all. its all about that wheel now. Managing power, with a clean tailpipe, and getting mandatory fuel mileage. Notice all makers now are running turbos on almost all motors, and have decreased displacement, so they can have smaller cylinders to fill with gas, and still get the HP up. The gas most cars use today, driving at a cruise speed, wouldn't even let a 60's-70's motor even idle. Those motors would idle at anywhere between 3-6% CO, where todays motors, Idle at about .5-.75% before the cat. You can't even commit suicide by running the car in the garage anymore, you'll just get a bad headache by the time the car runs out of gas! And actually we don't even measure exhaust gases any more as a tech. We watch self adaptation, milliseconds of fuel injector opening and airflow lbs per second, going into the motor, and by that we decide on what the motor needs to be corrected. And all corrections are done in the ECU and techs have very little ability to adjust anything.

4 valves allow more air in and at more velocity, and will cause a better swirl

This is a very "readers digest" version of this subject!

 

no, spark timing and advance are more or less dictated by the engine rpm and fuel itself. the fuel-air mixture burns at pretty much a fixed speed outward from the spark plug so as the engine runs faster you have to fire the plug(s) sooner before it arrives at TDC.

# of valves and # of spark plugs have little to do with each other; it's really down to the specifics of the design of the combustion chamber. the modern Chrysler Hemi has 2 valves/cylinder, and has twin spark plugs. if I had to make a wild-*** guess, I'd say Harley had to use dual plugs for ignition reliability because of both the reduced turbulence and the fact that they have such big-*** cylinder bores. the tiny cylinders in sport bikes don't need them.

yes. all else equal you can get better airflow through an engine using multiple valves. some more exotic engines (like a few past Ferrari engines) even had 5 valves per cylinder; 3 intake/2 exhaust.