The traditionally accepted " rule" for the Exhaust port was 72% - this was used by Yamaha for decades in the RD based TZ range of customer race engines.
But these had very generous corner rads that , as Frits drawing shows , restricts the Blowdown area available.
I worked on a set of later model 4DP TZ cylinders ( 56 bore ) a while ago that had been " tuned " by a famous ex Roberts tech.
The Exhausts had ben widened to 75% with no change to the radi.
The owner wasnt overly thrilled when after the initial dyno runs , and two practice sessions , the new pistons timing edges were obviously belted to shit , and the rings were on the verge of trapping due to mental piston rock.
Angle area as a port metric has been also used for years , but imho , this metric when combined with rpm , that describes a ports effective open area and how long that area is available at a specific
engine speed ( STA ) , combines all of the data into a useful one for predicting power capability at a chosen rpm.
EDIT - added info. In a water cooled 125 race engine the power drop from running at 65*C exit temp Vs 45*C is 1.5 Hp in near 50.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
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