Details dear Watson,

As published before by Keith Calver in MiniMagazine a small modification done to a valve can make a comparatively large difference.

In this case the tiny ridge that is on a brand new (nameless btw) vs an old single collet item with a tiny break , not even a proper back cut. The seat with is still very wide and does not match the one in the head at all (30/45/60).

Note the sharp ridge at the back of the seat area.

The old one and the new one up close

Note the very small break vs the sharp ridge on the brand new item, as well as the difference in margin ( the vertical part of the outer valve edge)

Given the very small amount of labour involved ( In this case I just chucked it up  in a hand drill and did the tiny sort of 30 degree section with a oil stone as the valve is trash anyway, took all of 3 minutes) the gains are well , pretty impressive.  The valve is not the same type and thus there is a bit of a difference between them but still they are pretty similar in general shape, even if the margin on the old valve is a lot smaller.  The effect of the change in valve is shown below. Of course I will retest with the new valve to investigate how much back cut is needed, and if the oft mentioned matching of the seats on the valve to the one in the head makes any sense airflow wise .

The reason for the  anomaly of the flow at very low (0.5mm) valve lift is still unclear. Note that the Blue vs Red flow gain is present from low lift to surprisingly high lift . As the A -series cams are lift limited anyway to about 10-11 mm max the  the right side of the graph is mainly of interest because of the fact that normally the lines should converge as the influence of the valve will diminish at very high lifts . This seems to be the case. (oh joy!)

Another interesting question is if the gains present on low and mid lift are still there/change when you add the manifold. I noticed that bare port flow is not very dependent on the shape at the port intake (see post before), however this changes when you add the valve. Adding a radius does affect readings quite a bit then. As I could not get my radius entry to stay put (got tired of it being suck into the head) I fiddled with an alternative and it turns out that just adding a 1/2 inch carb spacer does the same thing. Thus it seems that a large mismatch in diameter  is actually not all that bad, and might even be beneficial in certain circumstances. I’ll have to look into that one as well.

I remembered reading something about something similar somewhere.. I was a post from one of the people of Endyn a few years back..  yet another thing to look into


Posted by NITRO on May 11, 1998 at 23:07:12: I’ve already addressed the “R” topic.
The “air duct” should have 25% more area than the throttle body, and rather abruptly make the transition into the thrittle body with an “ideal” inlet radius which will net zero loss. The formula for designing the radius is R/D=.5, where R is the theoretical radius you’re seeking, and D is the diameter of the throttle body. Using this formula’s radius combined with the 25% increase in area in front of the throttle body allows the lower velocity air flow in the inlet “tube” to exert considerable pressure on the throttle body which is highly desirable from a power standpoint.



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One thought on “Details dear Watson,

  1. robert says:

    what a brilliant site thank you ! regards
    robert (from pts forum etc)

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