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| TECH CENTER - Tech Discussion - About Displacement, Volumetric (VE) Efficiency, and CFM
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TECH DISCUSSION
ABOUT DISPLACEMENT, VOLUMETRIC (VE) EFFICIENCY, AND CFM
FORMULA FOR VE
| VE = (CFM X 3456) ÷ (CID X RPM) |
If VE (volumetric efficiency) is less than 1 (or 100%) the amount and quality of
charge in the cylinder is reduced so less torque is produced. VE above 100% is a
supercharging effect and more torque is produced.
| Power Level | Stock | Performer | Torker II | Perf. RPM | Victor Jr. | Super Victor+
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| Peak VE% | 60-80 | 75-90 | 90-100 | 95-105 | 105-115 | 110-122 |
FORMULA FOR CID (cubic inch displacement)
| CID = NUMBER OF CYLINDERS x SWEPT VOLUME |
Note: CID = N x 0.7854 x bore x bore x stroke (all in inches)
Example: What is CID of a V8 with a “30 over”, 4 inch bore and 3.48 inch stroke?
CID = 8 x 0.7854 x 4.030 x 4.030 x 3.48
CID = 355 cu. inches
FORMULA FOR CFM
| CFM = CUBIC FEET PER MINUTE |
A measure of air flow into and out of an engine (CFM = CID x RPM x VE ÷ 3456).
Example: What CFM is consumed by a 355 CID engine at 4500 RPM if VE =
105% (1.05)?
CFM = 355 x 4500 x 1.05 ÷ 3456
CFM = 485
Example: What CFM by the same engine at 6400 RPM if VE has fallen to 95% (0.9)?
CFM = 355 x 6400 x 0.95 ÷ 3456
CFM = 625
TO CONVERT C.I.D. TO CC'S
Example: What is the cubic-inch displacement of a 5000 cc engine?
C.I.D. = 5000 ÷ 16.39
C.I.D. = 305
TO CONVERT CC'S TO C.I.D.
Note: cc = cubic centimeter, 1,000 cc = 1 liter
Example: What are the cc’s of a 350 C.I.D. engine?
cc’s = 350 x 16.39
cc’s = 5736.5
Please Note: The above equations and rules apply only to four-cycle engines. The equations have been simplified for ease of understanding. Answers will be approximate but generally will be close enough for use as a guideline.
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