Ideal port dimensions?
Ports that are bigger will flow more, but, if we want any decent low-mid speed performance as well, then we must very carefully keep the minimum port area at the correct dimension and also position. As an example did Alfa themselves reduce the inlet port of the 1600 engine from 32 mm to 29 mm to improve performance.
In the figure I have included the effective cross section areas for the three 1300 heads and also my interpretation of Kartamalakis and Vizards respective recommendations (valve guide and stem areas are accounted for). It also includes my take on ideal section areas, which unfortunately cannot be achieved without port filling, and I have therefore also included a proposal of more practical section areas where I stick to the original port-runner dimensions. My proposals are based on my understanding of how minimal section areas (MSA) should be placed to maximize the pressure recovery from maximized flow speed.
All original cylinder heads and also Kartamalakis suggestion for 1300 (if I understood his book correctly) have the MSA where the valve guide protrudes into the port, around 30 mm from the valve. There are at least two problems with that. The first is that the distance for pressure recovery is very short and the second that guide and valve stem lead to flow disturbance and turbulence (exactly at MSA) which effectively limits the peak flow speed. Modern four-valve engines have their MSA 100 mm or more from the valve, and my take, with the limitations of the 1300 ports and runners, is that we can in best case have the MSA around 60 mm from the valve in a relatively turbulence free area and from there open up towards the valve seat to recover pressure.
The figure shows that the 101 heads have slightly bigger port area before the valve guide than the 105 head (29.8 mm mean diameter versus 29.2 mm) but there is quite some variation from cylinder to cylinder, especially for the 101. I don’t claim that my cylinder heads are representative - maybe there are 101 heads around with smaller ports/runners? My “ideal” proposal suggest a MSA of 28.2 mm, but we are not allowed to add material.
The 101 normale is very narrow at the valve guide and especially in the bowl where the mean diameter is only 30.0 mm versus 32.3 mm for the 105 head. The valve seat inner diameter is 31.5, 33.0 and 32.8 for the 101, 101 Veloce and the 105 respectively.
As you notice, my “practical” proposal is based on keeping the ports/runners unmodified and heavily rework the ports by opening up substantially around the valve guide and in the bowl and seat areas. The final 10 mm is even more aggressively opened up than the proposals from Kartamalakis and Vizard. This particular area is important to slow down flow enough before it turns at the valve opening. This requires a very open valve seat area blended into the bowl, probably without any extended 30 to 45 and 70 deg three-angel cuts but rather going directly from 30 to a short 70 deg cut. For the 1300 I suggest 35.3 mm seat inner diameter, 34.4 mm where the seat ring starts and 33.7 mm 10 mm from the valve. The “Ideal” proposal has 36.2, 34.8 and 33.1 mm, but that would only leave a 0.5 mm wide valve seat, only suitable for bespoke racing cars.
With that said I want to point out that both Kartamalakis and Vizard have probably forgot more engine builds than I have ever started. So, if you follow my ideas it is on your own risk. For now the “practical” proposal is only theories that I would like to try out in the flow bench.