How to Calculate Max Boost
Calculating max boost depends primarily on three variables. The max CFM of the blower, rpm where you want the boost to peak, and the displacement of the motor. This based on the same reasoning used to calculate needed carb CFM. The principle is that an engine is an air pump and it is easy to calculate how much air it pumps if you know the displacement, speed (rpm) and efficiency (VE) of the “pump”. You can ROUGHLY estimate max boost as follows. The 0.9 factor is the ~VE for the typical intake tract under non-boosted conditions. The number 3,456 is just a mathematical factor to make the units come out properly.
CID*rpm/3,456*(.9) = engine airflow requirement
((blower CFM/airflow required)*14.7) – 14.7 = boost
So, as an example if you have a 350 and a 1,000cfm blower and want to make peak boost at 6,000rpm:
350*6,000/3,456 = 607*.9 = 546
1,000/546 = 1.83*14.7 = 26.9 – 14.7 = 12.2psi is the approximate max boost at 6,000rpm with 1,000cfm.
You select pulley sizes to spin the blower to close to max impeller speed at the rpm where you want the power peak (primarily determined by the cam). Anything which increases VE will increase power but decrease boost. Lower VE will increase boost but decrease power. Anything that interferes with the blower output (especially inlet restriction) will decrease blower CFM and decrease boost and hp. An engine with a larger CID will see less boost and make about the same peak power as a smaller motor if the same blower is used and maxed out in each case. However, the larger motor will have a fatter torque curve and go quicker. Increased air density (cooler, drier, higher barometric pressure) will increase hp, lower air density will decrease hp.
Manufacturers web sites provide blower specs, including maximum CFM. Here are links to the Procharger and Vortech pages. Kennedy’s is a dealer for all the major supercharger and turbocharger brands but recommends the Procharger line!