As I understand also the shape and length of the complete intake line (including air filter) makes a difference as well
155 Kat - 143 HP, 187Nm
75 Kat - 145 HP, 186Nm
164 Kat - 145 HP, 193Nm
155 and 164 are the same engines but as you see under 164 bonnet it performs slightly better.
And yes, Chirico described that with Kat Twin Spark engine lost 5-8 HP
P.S. He also described how the choice for TS was made. In 1974, on behalf of AR, Autodelta tested 3 heads (standard, TS and 4 valve) on the standard engine block with the same carburators setup and the same exhaust. Basically it terms of the peak power the best was 16v, it had more than 160HP. But in terms of the torque and power delivery and fuel economy TS was the best.
So the decision was made, the equipment updated and in 1987 mass production started. The same time AR started to test different solutions and in 1988 presented TS with lamell valves (like in 2 strokes racing motorcycle engines) instead of VVT; 3 valves TS with VVT and 16 valves.
3 valves version outperformed 4 valves and had about 180HP. FIAT shelved this research and asked to use 16v TS head with cast iron FIAT engine block, as this solution was much cheaper.
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