12g202 preliminary big valve tests

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 ?


Well  here:


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.

lift         CFM@25′

0.05     16.7

0.10      32.5

0.15       42.6

0.20       50.2

0.25       55.8

0.30       60.4

0.350     63.8

0.400     65.3


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.










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One thought on “12g202 preliminary big valve tests

  1. Keith Calver says:

    In my experience, changing data from testing at one pressure drop to another by calculation is far from acurate. And despite Superflow’s insistance that bench to bench figure are directly comparable, again in my experience, they are not. And, again, despite Superflow’s insistancethat their flow benches are accurately repeatable day to day, week to week, etc., they are not. I carried out several tests on several flow benches near to me using the same head as a test base. Took readings from these benches at different pressure drops, then cross-compared the data both collected from the flow-coms and calculated to compare different pressure drop readings by claculation. None of them were 100% identical. Some were more different than day to day testing. And yes, there were differences from day to day.

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