These are the preliminary result of the ”big” valve test.
I ground down a 33mm intake valve form a 12g940 1275cc head and reduced it’s size to 31mm. The seat is just roughed in and I cut a minimal amount of metal from the ports, mainly to clean them up a bit from rust and dirt. The valve has no reduced stem and does not have a proper back cut but only a slight blend. Seat with is super wide at about 3mm just from enlarging the seat. Chamber is cut to the shape you see below.
So where are we now ?
Yes pretty decent gains in the mid lift department, but the port still stalls like a mother about 7mm lift (0.275 inch).
Right hand side gives % gains, so 5-12% from low to mid lift. For the amount of effort ( very little) the gains are rather good, however the high lift flow needs to come up quite a bit.
These are the numbers as published in minimagazine done on a supposedly standard head using 25 inch depression on a SF600 with flowcom.
this makes these values STD a ported STAGE 3 head recalculated at 28 inch depression.
Well.. There is definitely a lot if work to be done, although I think my bench is a tad unhappy compared to a lot of benches the numbers for the 202 head seem to be quite similar.
Although the chamber is de-shrouded to where common wisdom tells you it should be, the flow numbers in the high lift area are not much better even though the valve is significantly larger . If you want to get it to 90 CFM it needs a lot of work. The port gets quite noisy after about 5mm lift indicating that there needs to be a good deal of effort put into speed and turbulence management. Just putting in a larger valve and making the chamber look like a 295 head does not make it flow like a 295 head. That said these gains are mostly achieved by using a bigger hole.
Next test would be something a bit odd.. try a small valve on the big seat.