Agree, Internal combustion engines are very inefficient, good ones get around
30% of power out to the wheels.
So here is my argument. There are several HP measurements, one is break HP or
the horse power at the output shaft. But you can do other measurements of
total HP, ignoring all losses. Work of compression, friction, running the
water pump, ac, fuel pump, alternator, oil pump as well as the work it
takes to draw a fresh air/fuel charge in and the work it takes to push the
exhaust out.
So if the only thing you do in a motor is change the valves you do reduce the
amount of work(HP) it takes to draw the fresh air in and expell exhaust.
This means you get an increase in the break HP without increasing the total
becuase you have reduced the internal losses. You have improved the engines
breathing. You may or may not have changed how much total HP the engine makes.
If the volumetric intake efficency is improved you will get more air in fuel in
and increase the total HP the engine makes. In terms of heat the improvement
in efficency may outway the increase in total HP, it depends.
So large valves can hurt bottom end power because it reduces the velocity of
the air fuel charge. Same amount of air fuel flowing into the same space but
through a larger hole means it has a slower velocity. The velocity affects how
well the air and fuel mix, poor mix means poor burn and incomplete combustion.
Some large valve heads do things to increase "swirl" or improve the mixing of
the air and fuel at low RPM so there is no loss in bottom end power.
Dan
|