Cd maps : beyond simple flow bench numbers. ( still being edited)

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A flow bench is nothing more than a Hoover with a measurement device attached coupled to whatever object with a hole in it you want to measure.

Whatever value you measure is dependent on how hard you pull at the straw. There is a perpetual debate at what depression (negative pressure) one should test heads at. First you had the small Superflow SF100 desktop benches that measured at 10 inch water depression. That is a pressure ratio of 1,025 ( or 0.025 bar pressure difference). Then it moved to 25 inch H20 for the popular SF600 benches (There are a lot of other ones as well but I’m not about to write a short history of flow-benches). Then you have the 28 inch apparently propagated by Smokey Yunick ( A Nascar legend with a cowboy hat that few people at my side of the pond knew about before the internets) of which a lot of people think is THE magic number. The magic being that is was supposed to be close to the depression as produced by a running engine (I don’t know if smokey actually thought that but ”people say”) . That is total codswallop it turns out.

Do you manic depression ?

Do you need full on manic depression ?

How much depression does a person or a flow-bench need ? The answer is a little more complex than you think at first. The actual running pressure on an engine varies with rpm and valve lift.

What a standard flow bench test does normally, is test at one fixed pressure and a lot of valve positions. Then it measures the amount of flow in CFM , CMM or whatever you happen to think is convenient. There are formulas that are somewhat usable when you want to convert from one depression to another, as long as you do not try to convert from 10 inch depression to 200 inch depression.  200 inch! I hear you say . Well it turns out that actually the running pressures at lower valve lift are 200 inch H20 and then some, at full valve lift you are looking at about 60 odd inch H20 depending on what engine you have.

A very nice write up can be found in PDF here ( by Vannik developments)

Back to the conversion and why it only works when you convert from a value quite near to the one you have. The reason is that the efficiency of a poppet valve is dependent of the pressure differential. So 25 to 28 probably no problem, 10 to 150 probably not so good, but that depends of course of the head in question.

So how do the people who actually know what they are doing ( i.e. not me), make sense of this? Well they use a sort of flow bench, but not a blue one with a bunch of Hoover motors. It is basically the same concept but build using sturdy metal pipe and it does not use a bunch of vacuum cleaner motors but a cellar.. Yup a big 150.000 Liter bunker that they suck all the air out until they have a 700mm Hg vacuum. That is 375 odd inch H20. Open the tap and hey presto you have a pressure ratio of 2.

Now instead of one pressure and many lift points, you plot many pressure ratios vs valve lift. The end result is a Cd map that looks a bit like a contour height map, but instead of drawing lines between point with equal height, they are drawn between points with equal Cd.

GP Blair : SAE R186

GP Blair : SAE R186 . These are maps for an intake valve but if you swap flow directions then it is an exhaust

Ok Looks pretty but how can it be useful ?

Cd is basically the Discharge Coefficient which is a metric for the efficiency of a valve. the Cd(actual).

                              Effective throat area

            Cda=  ————————————

                          Geometrical throat area

Read as follows : How much of the available area is actually used. If you have a very efficiently shaped valve aperture you will use more of the available area at higher pressure differentials, moving more mass. If you have a less efficient aperture it will/could be more non linear .

The figure above tells you that up to a quite high lift point the Cda of the valve at inflow is pretty much independent of the pressure ratio. In this case for a 2000cc BL engine ( I suspect it is a BL-O series, not the pinnacle of British engine design I must say) with 2 valves at 90 degrees with a cam lifting L/D 0.66 ( or 9.6mm) . You could come to the conclusion that testing at super high depression is not of much benefit for most of the lift curve, just on the very last bit it might make a difference.

This is not the case for the outflow and exhausts in general  especially  for higher lifts and lower pressure ratios (the right bottom corner of the map) the Cda suffers quite badly. Slightly counter-intuitive, but by lifting the valve less is gets quite a bit more efficient.. Of course you still have to have enough mass flow capability, A 100% effective, but tiny valve aperture is still not going to flow the amount of mass you need. At a certain point the potential gain of the extra lift will be offset by its increased inefficiency.

This is corroborated by people running identical 1/4 mile drag race times with exhaust cams that have been wiped (that is lingo for ruined and missing a large part of the nose of the cam reducing lift substantial) and the common practice of running less lift on the exhausts in American V8’s, as does Porsche with some 911’s (you can’t really say ‘the 911’ can you.).

Of course it depends on the port and such, but the plots are uncannily similar across designs including pent roof 4v’s so it seems to be a general trait.

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