If it chokes on the bench at say 120"...wouldn't that say that it would also choke on the engine when it pulls 150"? I.e. it's not optimal?MadBill wrote:But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?
Rick360 wrote:plovett wrote:Then next thought I had was that an engine with a properly size induction system usually sees less than 1.5" of Hg in vacuum in the intake tract. So I looked up the conversion from inches of mercury to inches of water. It is 13.6185 so 1.5" of Hg = 20" of H2O.
Your thinking about the vacuum in the intake is backward. The 1.5"Hg vacuum is differential pressure across the carb, not the port.
If the intake manifold is 1.5"Hg below atmosphere it still has an absolute pressure of 28.41"Hg (sea level, std conditions) remaining to "push" air into the cylinder. The piston is creating lower pressure inside the cylinder. That is the potential differential pressure across the intake port.
How much actual differential pressure between the cylinder and the plenum, (thats what causes the flow and determines the velocity), depends on how well the port keeps up with the piston demand from the downward motion of the piston in the cylinder.
MadBill wrote:But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?
SWR wrote:If it chokes on the bench at say 120"...wouldn't that say that it would also choke on the engine when it pulls 150"? I.e. it's not optimal?MadBill wrote:But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?
So, should we use 200" water depression when flowing at low lift, and 80" depression at high lift?
Should we put the piston in the flow test bore to simulate TDC, open the ex. and in. valve to the height they are at TDC during overlap, then pull 200" water on the header tube, and measure how well the piston top/in port/ex port, all flow together during overlap? I would think how well,(poorly), that combination flows backwards would be important also.
Would a momentary negative pressure spike during overlap, significantly affect the depression the piston pulls on the port at peak piston speed, some 75* or so, of crank rotation later? And therefore the power required to simulate that depression at high lift, on a flow bench?
MadBill wrote:(good luck though with the 100 psi needed to simulate low lift opening of the exhaust valve..)
Keep in mind though that some builders say one advantage of seat angles above 45° is that they restrict low lift flow, (perhaps thereby providing higher suction and velocity/inertia later in the cycle? )
Much to ponder...
If choking occured at low lift because of extreme depressions during over-lap, wouldn't this have to be a seat angle/size/shape, problem? The air speed in the port would be slow, as compared to the speed of the air through the valve window area. The window area being the limiting factor, due to it being a smaller area at that lift, vs the csa of the rest of the port, would make me think this is where the problem would be.
Stef wrote:Some good posts. I've thought of measuring the intake valve lift at BDC and then testing at that lift at a very high depression on the bench. If you can improve the discharge coefficient around that lift point you should improve the intake ramming process. But, how would that effect the rest of the intake process? I guess with intake port and cylinder pressure plots for the engine your working on you could optimise the discharge coefficient in both directions for what you had determined as important lift ranges. However, that's probably nit-picking and you would just be better off (and have more hair) if you just tried to improve the discharge coefficients across the board.
Ron E wrote: I've never heard a reasonable argument (on hi-lift N/A) that allows for the compromising of "over-the-nose" flow.