Historically, cam intake close had a strong correlation to compression ratio on the Harley Big Twin engines. Later intake closes allowed a higher CR. Going with too early of an intake close could move the powerband down the rev range but would increase chances of detonation.

Why does the M8, with its 11:1 CR big bore kits, suggest cams that on average would make an old Big Twin a ping monster?

Take the S&S kit, for instance. The 124" bolt on Power Pack kit comes with an 11.1:1 CR and the 550 cam with the intake close of about 30 degrees ABDC. You would never get away with building an Evo or TC engine to such parameters. Why does it seem the extra pair of valves in the combustion chamber allow this?

 

Chamber design and dual plugs is allowing that, for one.
Scott

 

I'm not really understanding how that would help, although clearly it does. Increased CR on a TC without proper protocol leads to detonation. Basically a hot spot in the CC will ignite the fuel before the spark plug does and you get ping. Visually comparing the CC of the M8 head vs. the TC head: the M8 is more of a shallow, larger dish whereas the TC is a deeper, narrower bathtub. The valves are closer to the piston on the M8, clearly due to the reduced valve lift required now that there's two valves for each side of the head. The M8 CC looks like it's about the same diameter as the bore (mostly) yet the TC is smaller, narrower, but deeper (with increased squish at the sides).

If I read into the dual plugs thing a bit, having the combustion process originate in two locations throughout the entire combustion chamber will complete the combustion process quicker than a single point of origin. Still not sure why this allows such a drastic difference in cam timing vs. CR on the M8 though, or how it alleviates the potential issues from running such a high CR with such an early intake close.

Does this also imply dual plugs on a TC engine would allow getting away with a higher CR for the same reasons? Or perhaps designing a more shallow but larger combustion chamber (with accompanying domed piston to maintain CR)?

Clearly the design works, I'm now wondering why. Look at the CC of any of the 600cc metric sport bikes. Damn near 13:1 running pump gas! And you know them squids aren't running 93 octane either.

 

Yeah, You are missing much of the theory. For 1 detonation is not ignition from a hot spot. It's a spontaneous explosion based on pressure and heat after ignition. Ignition from a hot spot is typically preignition which can cause detonation but is not necessarily the source of detonation.

The big thing about dual plugs is that the less timing can be run and still build maximum peak pressure.

Also since stoichiometric mixtures are necessary for clean burning at cruise, the mixture burns slower and the extra plug helps. Again less timing controls the detonation.

A 600cc bike can get away with 13 to 1 cr with on plug because it is liquid cooled and has a much smaller combustion chamber.

A TC can achieve some advantage from running 2 plugs as far as running a higher compression on the street but for the most part a good tune can get you real close.

Another thing to think about has to do with the 4 valve heads. While the cam timing moved from 0.053 tapet to 0.050, the 2 extra valves increase low lift flow dramatically which give some of the characteristics of a later closing and earlier opening valve. Cam timing at tappet like is simply a reference point.

 

Even just sticking to the tc world. Same compression with same cam can require different tuning strategies depending on how you got to that point. S&S with a 79 chamber and flat top compared to 91 chamber and a pop up.

 

Absolutely.. Even the shape of the chamber will require different timing / AFR.. Trying to use Singh grooves in a TC head is another.