Jim Hand
12-24-1999, 12:21 PM
Here are two formula from SuperFlow Corp concerning airflow. The first is used to estimate possible peak HP of an engine based only on used airflow. It is not related to displacement nor rpm. Multiply the airflow (as measured at 28") entering the cylinders by the factor of .257. That will be the max possible HP generated by one cylinder. Multiply by the number of cylinders for total possible power. (For same information using flow at 10" pressure, multiply by the factor of .43)
Example: The airflow through the entire intake tract of air cleaner, carb, manifold, and head ports is 220 cfm @ 28". 220 X .257 = 56.54 HP per cylinder. Multiply by eight cylinders, and the engine could develop as much as 452 peak HP. Whether it would depends on intake and exhaust tuning, cam selection, carb operation, correct selection of intake, and so on. I will post some measured values of flow through the entire intake system in a separate post.
At what rpm would the above predicted peak HP occur? It depends primarily on the size/displacement of the engine. SuperFlow provides two formula for this one: Depending on the efficiency of the engine, the rpm changes. Highly developed race engines will peak at lower rpm, whereas our stock type will not use as much air and a given amount will force the rpm higher. For an all out good race engine use this formula:
Peak HP will occur at: A factor of 1196 divided by the displacement of one cylinder: Multiply that result by the cfm airflow entering the cylinder at 28" pressure. (Again, if using 10", use a factor of 2000 rather then 1196). Example: Using the values of above for a highly developed 462 race engine, we have: One cylinder displacement equals 57.75. Dividing 57.75 into the factor of 1196 (for 28" pressure), the result is 20.71. Multiply that result of 20.71 times the airflow @ 28 of 220, and the result is 4556 rpm. This tells us that the nominal peak HP will occur close to 4556.
What if we used the same heads/flow on a 406? The displacement of one cylinder of a 406 is 50.75. Dividing 50.75 into the 1196 factor, we get 23.57, and multiplying that by 220 cfm flow, the answer is 5185 rpm. The same amount of airflow that finds its way into the cylinders will raise the peak HP rpm by over 600 rpm!
For a less developed engine more like our street engines, the formula changes to these values: Divide the displacement of one cylinder into a factor of 1316. Using the same flow of 220@28" and a 462, the result is: 1316 divided by 57.75 = 22.79. Multiply 22.79 by the 220 cfm flow equals 5015 rpm. In other words, our less developed engines (less VE) will peak at a higher rpm with the same flow!
The 405 shows the same - 1316 divided by 50.75 (one cylinder ci) equals 25.93, and that value times the 220 cfm flow equals 5704 rpm!
This formula clearly shows the differences in operation of different displacement engines.
What about using heads that flow 300 cfm of air through the ports. A highly efficient intake system can allow as much as 98 percent of the head flow capability through the entire system and into the cylinder, so we can use an airflow value of 300X.98= 294 cfm actually entering the cylinder. Using the formula for a top notch 462 race engine, we find that the engine will peak HP at this value: 1196 divided by 57.75 = 21.71, and multiplying that value by 294, we find the race engine will peak in the area of 6380 rpm. In order to use the peak HP effectively, the engine should be shifted at some higher rpm.
Again, these values will be affected by cam selection, and other aspects of engines efficiency, but by using these formula to compare the effects of flow values on identical size and developed engines, we begin to get an idea of what is needed in the area of flow values, or what kind or rpm is required to use given amounts of airflow. Jim
Example: The airflow through the entire intake tract of air cleaner, carb, manifold, and head ports is 220 cfm @ 28". 220 X .257 = 56.54 HP per cylinder. Multiply by eight cylinders, and the engine could develop as much as 452 peak HP. Whether it would depends on intake and exhaust tuning, cam selection, carb operation, correct selection of intake, and so on. I will post some measured values of flow through the entire intake system in a separate post.
At what rpm would the above predicted peak HP occur? It depends primarily on the size/displacement of the engine. SuperFlow provides two formula for this one: Depending on the efficiency of the engine, the rpm changes. Highly developed race engines will peak at lower rpm, whereas our stock type will not use as much air and a given amount will force the rpm higher. For an all out good race engine use this formula:
Peak HP will occur at: A factor of 1196 divided by the displacement of one cylinder: Multiply that result by the cfm airflow entering the cylinder at 28" pressure. (Again, if using 10", use a factor of 2000 rather then 1196). Example: Using the values of above for a highly developed 462 race engine, we have: One cylinder displacement equals 57.75. Dividing 57.75 into the factor of 1196 (for 28" pressure), the result is 20.71. Multiply that result of 20.71 times the airflow @ 28 of 220, and the result is 4556 rpm. This tells us that the nominal peak HP will occur close to 4556.
What if we used the same heads/flow on a 406? The displacement of one cylinder of a 406 is 50.75. Dividing 50.75 into the 1196 factor, we get 23.57, and multiplying that by 220 cfm flow, the answer is 5185 rpm. The same amount of airflow that finds its way into the cylinders will raise the peak HP rpm by over 600 rpm!
For a less developed engine more like our street engines, the formula changes to these values: Divide the displacement of one cylinder into a factor of 1316. Using the same flow of 220@28" and a 462, the result is: 1316 divided by 57.75 = 22.79. Multiply 22.79 by the 220 cfm flow equals 5015 rpm. In other words, our less developed engines (less VE) will peak at a higher rpm with the same flow!
The 405 shows the same - 1316 divided by 50.75 (one cylinder ci) equals 25.93, and that value times the 220 cfm flow equals 5704 rpm!
This formula clearly shows the differences in operation of different displacement engines.
What about using heads that flow 300 cfm of air through the ports. A highly efficient intake system can allow as much as 98 percent of the head flow capability through the entire system and into the cylinder, so we can use an airflow value of 300X.98= 294 cfm actually entering the cylinder. Using the formula for a top notch 462 race engine, we find that the engine will peak HP at this value: 1196 divided by 57.75 = 21.71, and multiplying that value by 294, we find the race engine will peak in the area of 6380 rpm. In order to use the peak HP effectively, the engine should be shifted at some higher rpm.
Again, these values will be affected by cam selection, and other aspects of engines efficiency, but by using these formula to compare the effects of flow values on identical size and developed engines, we begin to get an idea of what is needed in the area of flow values, or what kind or rpm is required to use given amounts of airflow. Jim