A few points: Spark Plug heat range -- in brands other than NGK, a higher heat range number relates to "hotter" plug, lower numbers just the opposite. NGK goes backward to this orientation, with smaller numbers being hotter. "Hotter" relates to the amount of combustion heat which the spark plug dissipates or transfers to the cyl head, which means that a "hot" plug is phyisically hotter in the spark plug tip area whereas a "colder" plug will be physically cooler. Which is why an "oil burner" will get hotter plugs before a "non-fouler" attachment is needed. Oil intrusion, for example, can come "up" from the rings area or "down" from the valve guide area.
Exhaust valve size -- The larger exhaust valves have more to do with "flow" past the exhaust valve seat, NOT exhaust system back pressure. Putting more flow into an existing system might even INCREASE back pressure in the system, unless the system is similarly upgraded (i.e., single to dual, or mufflers with more flow). Exhaust system "pressure" is usually minimal at cruise conditions, but increases quickly in "kickdown" at then at WOT situations. The issue becomes how good the cylinder is ridden of the per-cycle exhaust gases and a fresh charge enters during the overlap period. The smaller exhaust valve can possibly increase residual EGR as much as a smaller intake valve might, with the end result being that either the exhaust can't get out or more clean air might not be able to get in.
In more recent times, the issues of flow are important at ALL valve lifts, not just at .500" lift. Especially at .200" lifts to get things started. Then, notice that current high compression engines might well open the intake valve a bit later, for less physical charge, then compress it more. Which means the total .050" duration might be a bit less than what it might have been years ago but power is still very high, still with a 550rpm idle in gear that is smooth (which can relate to both intake/exhaust manifold designs and EFI). Combustion and manifold flow dynamics have been well-evolved over the past 60 years.
Sorry for the diverson.
CBODY67
Exhaust valve size -- The larger exhaust valves have more to do with "flow" past the exhaust valve seat, NOT exhaust system back pressure. Putting more flow into an existing system might even INCREASE back pressure in the system, unless the system is similarly upgraded (i.e., single to dual, or mufflers with more flow). Exhaust system "pressure" is usually minimal at cruise conditions, but increases quickly in "kickdown" at then at WOT situations. The issue becomes how good the cylinder is ridden of the per-cycle exhaust gases and a fresh charge enters during the overlap period. The smaller exhaust valve can possibly increase residual EGR as much as a smaller intake valve might, with the end result being that either the exhaust can't get out or more clean air might not be able to get in.
In more recent times, the issues of flow are important at ALL valve lifts, not just at .500" lift. Especially at .200" lifts to get things started. Then, notice that current high compression engines might well open the intake valve a bit later, for less physical charge, then compress it more. Which means the total .050" duration might be a bit less than what it might have been years ago but power is still very high, still with a 550rpm idle in gear that is smooth (which can relate to both intake/exhaust manifold designs and EFI). Combustion and manifold flow dynamics have been well-evolved over the past 60 years.
Sorry for the diverson.
CBODY67
