![]() Things like big valves, big cams, ported heads, tunnel rams, etc. The amount that does flow divided by the ideal amount is called the volumetric efficiency.įor your basic stock small block chevy, I think this number is around 0.85 (or 85%). With some exhaust remaining in the cylinder and the restriction offered by the intake ports and valves the actual amount of air that flows into the cylinder is somewhat less than ideal. Unfortunately, this doesn't usually happen. If we had 17 psi boost in the intake manifold, we would open the intake valve and get 17 psi in the cylinder before the intake valve closed. If life was perfect, we could fill the cylinders completely with air. (10.73 x T(deg R)) To get the volume of air: Here is the Ideal Gas Law rearranged to the two handiest forms, with the required constants: To get pounds of air: And the more pounds of air you move, the more power you will make. So we can figure out how many pounds of air the engine is moving. That is useful, since we know the pressure (boost pressure), the volume (which we calculate as shown in the first section "Engine Volumetric Flow"), and we can make a good guess on the temperature. For example, if you know the pressure, temperature, and volume of air you can calculate the pounds of air: The Ideal Gas Law can be rearranged to calculate any of the variables. If it is 80 deg F outside, the absolute temperature is 80 + 460 = 540 deg R. The absolute temperature is the temperature in degrees F plus 460. A perfect vacuum is 0 psia, or -14.7 psig. (The psi stands for Pounds per Square Inch). The "a" stands for absolute, the "g" for gauge. 17 psi is the gauge pressure, the absolute pressure at sea level is 14.7 + 19 = 33.7.Ī pressure reading is marked psia or psig. Hook it up, boost the car, and it reads 17 psi. Atmospheric pressure is about 14.7 psi at sea level.Įxample: a boost gauge reads 0 psi before it is hooked up. What are absolute temperature and pressure? Do we care? Of course we do!Ībsolute pressure is the gauge pressure (measured by a gauge that reads 0 when it is open to the outside air) plus atmospheric pressure. Where P is the absolute pressure ( not the gauge pressure), V is the volume, n is related to the number of air molecules, which is an indication of the mass (or pounds) of air, R is a constant number, and T is the absolute temperature. If you know any three of these, you can calculate the fourth. It relates the air pressure, temperature, volume, and mass (ie, pounds) of air. The Ideal Gas Law is a handy equation to have. ![]() Volume of air (cu ft/min)= engine rpm x engine cid How many pounds of air is that? That depends on the pressure and temperature of the air in the intake manifold. So, for every 2 revs the engine takes in 231 cu.in. We have a four stroke engine the intake valve on a cylinder opens once every 2 revolutions of the engine. The displacement on our cars is 231 cu.in. This equation is for finding the volume of air going into the engine. It should also offer some enlightenment of the effects of temperature, pressure, and intercooling on the engine's performance. General rule of thumb for acceptable dynamic compression ratio to run safely on pump gas is 8:1 maximum for engines with cast iron cylinder heads and 8.5:1 with aluminum cylinder heads.The purpose of this little paper is to show the reader how to calculate the volume and mass of air moving through his engine, and how to size a turbochargers' compressor to move that quantity of air. However, that same 11:1 static compression ratio engine with the radical 259/269 duration camshaft would have a dynamic compression ratio in the neighborhood of 7.5:1, totally acceptable to run on pump gas. Therefore with the mild cam it will have a high dynamic compression ratio, probably 9.5:1+ which would be way too high to run safely on 91 octane gas. If you put a very mild camshaft (194/204 duration this cam will have an "early" IVC (intake valve closing point), and will "bleed off" less compression than a radical camshaft with 259/269 duration and a considerably "later" IVC. To give an example, lets say you're considering only pump gas for your engine, and it has a static compression ratio of 11:1. Unlike Static Compression Ratio, Dynamic Compression Ratio takes into account camshaft timing by considering the intake valve closing point in relation to the piston position. Static Compression Ratio numbers are the ones you hear thrown around the most ("10:1 compression"), and it takes into consideration the full sweep volume of the cylinder in regard to its range of crankshaft stroke. This calculator will calculate both Static and Dynamic compression ratioĬompression Ratio is the ratio of an engine's cylinder volume vs.
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