I am trying to understand the concept of volumetric effeiciency (VE). The way i understand it is that VE reaches its peak with the max torque output. For example, 1972 455 Buick has a torque rating of 360 ft/lb @ 2600 rpms. Since VE is controlled by the cam and heads, is the max breathing potential or VE @ 2600? If I wanted to cross the finish line @ 5500 rpm, how would I get the VE to increase to that rpm? PS What is trap speed in relation to quarter mile time?
VOLUMETRIC EFFICIENCY Imagine that you have a 100 cubic inch single cylinder engine. On the intake stroke, the piston moves to the bottom of the cylinder and creates a volume that is 100 cubic inches. The fuel/air mixture that fills this volume is what will be used to create the power. Now imagine that this engine has some type of restricted intake, such as a small carburetor or restrictor plate like they use in NASCAR. With this configuration, the intake manifold has a fairly good vacuum. In this case, even though the piston pulls a volume of 100 cubic inches into the cylinder, it is not atmospheric air. Here, you have the 100 cubic inches of the vacuum from the manifold. You can think of this as the opposite of super charging because the cylinder ended up with less fuel and air molecules rather than having more fuel/air packed into the same volume. Volumetric efficiency (VE) is used to describe the amount of fuel/air in the cylinder in relation to regular atmospheric air. If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency. On the other hand, super chargers and turbo chargers increase the pressure entering the cylinder, giving the engine a volumetric efficiency greater than 100%. However, if the cylinder is pulling in a vacuum, then the engine has less than 100% volumetric efficiency. Normally aspirated engines typically run anywhere between 80% and 100% VE. So now, when you read that a certain manifold and cam combination tested out to have a 95% VE, you will know that the higher the number, the more power the engine can produce. Basically, volumetric efficiency is effected by your carb, intake manifold, headers, and cam specs. All of these items effect how much fuel/air will flow into the cylinder. But remember, the more fuel/air that gets into the cylinder, the more power the engine will produce
(NOTE: I didn't see Alan's post above before posting this.) Volumetric efficiency is an engine's ability to completely fill the cylinder with air/fuel mix. If you calculate the volume of the cylinder at BDC (area of base x height), and the engine is able to fill that cylinder with that much air/fuel then you have 100% VE. Now, you have to realize that air has weight and obviously fuel has weight, which means they are also susceptible to inertia. When the intake valve opens, a column of air is drawn down the intake tract and past the valve. When the piston stops moving down (BDC) the draw, or vacuum, disappears, right? Not really. You can hold the intake valve open for a little longer and the inertia of the air column will continue filling the cylinder. Consequently if you hold the valve open too long, the air will start coming back out the intake valve, which is called reversion. So, when the valve closes the column of air is still moving, so it hits the back of the valve and bounces back. When it gets to the plenum the pressure wave bounces back again and heads toward the intake valve. If the valve is closed, it bounces back again, and keeps going back and forth. It may do this 2 or 3 times before the valve opens again. So, you want to tune the intake valve to open right when the pressure wave gets to the valve, and you can get a slight "ram effect". Now you see the importance of port velocity as well as raw flow numbers? A modern Pro Stock engine is running at about 127% VE. This is accomplished through VERY precise runner sizing and valve timing, which is acheived through extensive R&D work. The VE does directly effect the torque output of a given displacement engine. Why does the peak VE not occur at the peak horsepower RPM? Due to the speed of everything happening it is not physically possible to hold the valve open long enough to fill the cylinder past a certain RPM. The VE will peak at a lower RPM, which will be the torque peak of the engine. After that, the VE will drop off, but the RPMs are increasing. Now, look at the formula for horsepower. (torque x RPM)/5252. If you increase the RPM, even with less torque, you will increase HP. As the VE continues to drop off it reaches a point of diminishing returns and the HP also starts to drop off. Other factors are involved there, such as valve spring pressure, valvetrain weight, etc, that will limit the useable RPM of an engine. Hope this helps. Have fun.
There is also volumeric efficiency to be considered, along with volumetric. Also, something that hasn't been taken into consideration is a key factor in the pull efficiency of rthe cam/heads/mainfold, is the swept volume. You can have two engines one with a shorter stroke and bigger bore, the other, just the other way around, and both engines will exibit different VE/ve. The engine with a longer stroke will always show a better VE/ve, even if the bores shroud the inlet ports. It's all in the time allotted in the pull. Longer stroke, longer pull time, better VE/ve.
