12G202 Intake Valve Seat Antics , part 2

What happens when you put a 12g940 exhaust valve of 29 mm diameter onto a 32 mm seat.Well quite a bit is turns out


Both valves are stock so no back cut although the margin of reduced 33mm valve is a bit fatter by virtue of it being ground down and it is quite sharp. The bigger valve will , after a certain lift  ( about 5 mm) the flow is increasingly unsettled and violently turbulent on the big valve indicating a highly turbulent and separating flow.

At any practical delta P ( and you only need a tiny pressure difference) the flow is fully turbulent, as in Reynolds number Re is above 2000. This however, is ear splitting, noisy spluttering  high energy type turbulence that you can feel in the valve. In other words: A lot unwanted bouncing about of air molecules sapping energy and causing flow losses.


The gains form installing a bigger valve vs. a stock head and modifying the combustion chamber, but not doing much to the seat or valve other than making it bigger







The gain relative to the stock 202 (green) from fitting a smaller 29 mm valve onto the 32mm seat without any modifications (red). Right side axis gives difference in CFM. This is a very large gain for a head that can flow maybe about 100-110 cfm when you really max everything out . It just illustrates that the valve seat area has a huge influence on flow across the entire lift range.

Likewise Keith Calver has found flow gain, in an article on using a BB head on a small bore engine where he sank in the valves so you would not have to pocket the block. While sinking in the valve reduces the SSR, it clearly gains a whole lot more than is looses ( by making the cd of the valve better due to better pressure recovery ?).


The interesting part is what happens when you add back cuts into the mix, and I might even try 60 degree  “post cuts” near the valve margin. I really do NEED a lathe 🙂



The Rub

I fiddled and fiddled some more and found some weird positive results. Those are the sort of results that coined the phrase : never repeat a successful experiment. I of course did and now I cant for the life of me get the same results for the smaller valve, however the all of the 29 mm valve will still flow more than the 31.5mm valve, and a one which I had used to guard the valve seat while grinding so it has a sort of ground off face with a narrow margin flow a tad better that the normal ones. Then I tried flowing the whole lot without a valve (stem on guide) and it will flow 84 cfm without the stem in the way and 80 cfm no manifold with the stem protruding past the valve. With a valve in there it will go only to 75 cfm even when you lift it to 18mm. If you fill the bottom of the port with out 6cc worth of putty in the siamese section the get a halfway decent SSR you get … 84 cfm.

Ergo it needs to be a tad bigger in the push-rod area that is an estimated 6.1945 mm²  or 0.95387 in². So I will try and enlarge (and if you look at the 295 port and the work GR did to a std head, this might the the current bottle neck.


According to Pipemax the piston demand converted to 28 inch is 115 cfm for a monster +135 overbore  so i’m still quite a bit off the pace..





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3 thoughts on “12G202 Intake Valve Seat Antics , part 2

  1. Keith Calver says:

    On the sinking of the valves in to the head in my article/project test in Mini Mag, it needs to be noted that the head was not modified in any other way at all. Consequently the short side radius section that matters was as standard. Sinking the valve in to the head did not reduce the effective SSR by much, plus adding in to tghe equation the 60 degree under cut, this will have helped the SSR performance over a straight, deep cut 45 degree seat.

    And fitting a 29mm valve in to a 32mm seat seems a bit of an odd way to set about comparing valve size effects. Well – it does to me anyway, as you are not really comapring like for like in a real situation. Or is my aged, fuddled brain missing something here? Don’t get me wrong here, I am not purposely trying to ‘dis’ your efforts. Just trying to understand something of them. Again – I have massive rspect for you and your capabilities in a field I am WAY out of my depth in.

    • mowog says:

      Hi Keith,

      Well the subtitle is :join my confusion for a reason Hahaha..

      You have ground more cast iron in your life than I ever encouter in my worst nightmares (Respect! I can stand only so much grinding caaast irn before I flip out) and have oodles more practical and hand on experience with the a series

      It is not a way of trying to compare valve sizes but more of a way to compare the effect of a different aperture shape. If i had a shed full of heads and more time you could be a bit more methodical. :).

      But the smaller valve does seem to do a better job by having a more controlled outflow.

      I have a hunch that the shape after the valve can gain you more than having a higher SSR

  2. Keith Calver says:

    As I said to my very good friend Graham Russell a short time after meeting him and working with him for the first time – there are no answers, just more questions.And ity is true I have managed to turn a vast mountain of carefully cast iron back in to a mountain of dust in teh 35+ years I have been messing with the subject matter. And a whole heap of dyno testing. I blame my mate Vizard. After he produced his book, I railed at him for all the relatively incomparable data he showed, and determined to do things ina ore meaningful way. It has costs me fortunes…

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