It’s easy to see why volumetric efficiency would be less<\/em><\/strong> than 100%.\u00a0 The engine is hotter than the incoming air, which reduces it’s density, and the amount that can fit in the cylinder.\u00a0 Any air restrictions going into the engine or exhaust restrictions going out of the engine further reduce air flow.\u00a0 Then you have the typical intake closing event at seat timing (when the valve actually seats the valve and\u00a0completely seals) which can easily be 60 to 100 degrees after bottom dead center (BDC).\u00a0 That means the intake valve doesn’t close until the piston is maybe 30% back up the bore, meaning 70% volumetric efficiency should be the best possible (100% – 30% = 70%).<\/p>\n With all this working against volumetric efficiency, it’s amazing we can do much better than the 70-85% mentioned earlier.\u00a0 And, if engines ran at about 100 RPM, that would be the case.\u00a0 However, at high RPM there are all sorts of dynamics going on that really help the engine breathe.\u00a0 These dynamics involve the air in the intake port and runner, and exhaust port and runner (header primary), which produce tuning pulses<\/em><\/strong>.<\/p>\n In simple terms, once the “slug” of air in the intake runner starts moving toward the cylinder on the suction stroke, it wants to keep moving toward the cylinder even past BDC.\u00a0 The same is true on the exhaust side.\u00a0 The exhaust starts moving away from the cylinder down the primary, it wants to keep moving away from the cylinder.\u00a0 This is called inertia tuning<\/em><\/strong>.<\/p>\n I’ve included some graphs from our Engine Analyzer Pro to illustrate these points.\u00a0 These tuning pressure\u00a0graphs can be produced as the Engine Analyzer Pro <\/a>calculates the simulated engine’s volumetric efficiency at the various RPMs.\u00a0 The horizontal axis (X axis) is crank degrees.\u00a0 Important points in the 720 degree cycle are marked off, like TDC (top dead center), BDC, EO (exhaust opening event on the cam), IC (intake closing event on the cam), etc.\u00a0 To the right is a simple piston and valves diagram so you can visualize where in the 4 stroke cycle you are at when these graphs are being drawn.<\/p>\n The top graph shows intake (turquoise)\u00a0and exhaust (dark red) port pressure, and cylinder pressure (gray).\u00a0 The blue dotted line is atmospheric pressure.\u00a0 Any pressures above the line are positive, high pressures.\u00a0 Pressures below the line are actually a suction pulse.<\/p>\n The bottom graph shows flow by the valves.\u00a0 Flows in the correct direction (which improve power) are shown above the 0 pressure line.\u00a0 This intake flow (turquoise)\u00a0which flows into the cylinder and exhaust flow (dark red) which flows out of the cylinder.\u00a0 Reverse flow (which hurts power) happens when the graphs fall below the zero velocity line.<\/p>\n The graphs are shown in pairs, one with normal length runners, of about 12 inches on the intake (port and intake manifold) and 34 inches on the exhaust (port plus header primary).\u00a0 These graphs show the normal pressure pulses found in a race engine.\u00a0 The comparison graphs are for very short runners, 2 inches on the intake and 4 inches on the exhaust.\u00a0 The Engine Analyzer Pro’s software will not run with zero length runners.\u00a0 However, these runners are short enough to illustrate approximately what would happen with zero length runners, which would eliminate all the tuning pulses we’ve been talking about.<\/p>\n The graphs below show that when the exhaust valve first opens, there is a huge outflow of exhaust, called the “blowdown” pulse.\u00a0 After that, the piston acutally pushes the rest of the exhaust out.\u00a0 Both of these actions gets the exhaust flowing out of the cylinder, down the exhaust header primary.<\/p>\n (Click the images to enlarge them.)<\/p>\n The next set of\u00a0graphs (below) show that at TDC overlap (both intake and exhaust valve are open) that the Typical 34″ exhaust header keeps the flow moving out of the cylinder.\u00a0 There is also a suction pulse set up in the exhaust port.\u00a0 This is created by the inertia of the exhaust flow wanting to keep flowing in the exhaust direction.\u00a0 However the very short exhaust primary shows that there is no suction pulse, and the exhaust actually flows back into the cylinder (reverse flow).\u00a0 There was not enough exhaust flow inertia to keep the exhaust flowing in the out direction.<\/p>\n On\u00a0the intake side, the piston is at its approximate max velocity sucking in air and fuel.\u00a0 With the Typical 12″ runner, you can see a suction pulse being created in the intake port.\u00a0 With the very short 2″ runner, the suction pulse is much smaller.\u00a0 Both runners show an intake flow being high and in the correct direction.<\/p>\n The set of graphs below now show the pressures at the most critical point in the 4 stroke cycle, intake closing<\/em><\/strong>.\u00a0 With both length runners, the piston got the intake air moving toward the cylinder.\u00a0 However, with the very short runner, there was not enough inertia to keep the air flowing into the cylinder once the piston stopped and changed direction at BDC (bottom dead center).\u00a0 After BDC the piston actually pushed the intake air back out into the intake port (reverse flow).\u00a0 This condition produced about 90% volumetric efficiency.\u00a0 If we truly had zero length runners, to eliminate all inertia tuning, the volumetric efficiency would be even much lower than this.<\/p>\n With the Typical 12″ runner, there was enough inertia to keep air and fuel flowing into the cylinder.\u00a0 There is no reverse flow, and you can see a 7 psi pressure pulse occuring during this time.\u00a0 (Baro press is about 15 psi, and this green\u00a0pulse is well over the 20 psi line.)\u00a0 That is almost the same as having a supercharger installed which can create 7 psi of boost.\u00a0 This engine produced 111% volumetric efficiency, more than a\u00a020% increase in volumetric efficiency, just by having\u00a0typical length\u00a0ports and runners.<\/p>\n These tuning pulses are present in every running engine, because all engines have ports and runners to some degree.\u00a0 The size and duration\u00a0of the exhaust suction pulse during TDC overlap, and the intake pressure pulse at intake closing depend on many things, like length and diameter of the runners and header primary, cam timing, piston speed, etc.\u00a0 That is why some engines will only get to 65% volumetric efficiency at full power and others will get to 130%.\u00a0 However, it should now be easy to see how you can actually be better than perfect<\/em><\/strong>.<\/p>\n![\"Typical](\"http:\/\/performancetrends.com\/blog\/wp-content\/uploads\/2009\/01\/typical-length-runners-btdc-300x202.gif\" "\\"typical-length-runners-btdc\\"")
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