大意是說, 早期採用五汽門的因素並不是因為流量, 而是由於每個汽門能做的更輕,
便可以繼續採用傳統式汽門彈簧. 等到氣壓式汽門彈簧開發出後,
從此沒有這個問題, 五汽門反而影響燃燒效應(燃燒室形狀不佳, 更複雜更難掌控)
另一個要素是進氣時擾流問題, 三個進汽門時也較難搞定.
In early ‘90s 5 valves per cylinder was present in F1 engines
(Yamaha, Ferrari and also Peugeot IIRC) but then the solution was discarded.
Currently an example of engine designed solely for race purpose and still
using 5 valves is the unit of the Yamaha M1, the MotoGp contender.
The main advantage of the solution (beside the total intake area a bit larger)
is that each valve is lighter and so this allows to increment the revs still
using metal springs. That’s no more an issue for F1 because of pneumatic
springs but it’s the first reason for the choice in the M1 revving up to
15-16k rpm. Anyway, according to Mr. Yoda (Yamaha engineer) over a certain
value of bore also this advantage is nullified and 4 valves becomes the best
choice (M1’s engine is a 4 cylinder and B is probably under 85 mm, for an F1
engine it’s 96-98 mm).
That’s because with larger bore it’s difficult to keep a good combustion
efficiency, the design of the combustion chamber is very critical, 5 valves
just make it even more complicated. Another problem is that then you have
also to control the interaction between 3 different flows at the intake
instead of two and often the bad interaction reduces the mass flow in spite
of the larger area.