When we discuss exhaust, please understand that what makes the exhaust gases leave the combustion chamber isn't the piston "pushing it out" rather the inception of the exhaust gases leaving is due to thermal expansion. When the exhaust valve opens, the hot / expanded gases are looking for somewhere to go about the same time the exhaust valve cracks open. The exhaust races out the cylinder head and are attracted to atmosphere pressures and temperatures, where the pressure and temperature can then be equalized. We refer to this time as  “blowdown”; Or, as a relationship between cylinder pressure and exhaust system pressure, beginning when the exhaust valve opens and ending when the cylinder pressure equals the exhaust-system pressure. When the blowdown period begins, residual combustion pressure overcomes exhaust-system pressure and forces gas out into the atmosphere. When this period ends, the remaining exhaust gas must be pumped out by piston action or any pressure excursions (tuning) remaining in the exhaust path. Whatever can be done to reduce blowdown pressure tends to increase the amount of work done on pistons to produce raw power.

Mechanical compression ratio is also a factor in blowdown pressure, as is timing of the exhaust valve opening, ignition spark timing and heat losses to combustion-exposed surfaces. As blowdown pressure decreases, there is an increased dependency on the exhaust system’s tuning characteristics to rid cylinders of unwanted gases. Therefore, the importance of selecting proper headers, collectors and mufflers increases if power is to be optimized. In other words, increased combustion efficiency and power output requires a critical examination of all exhaust system components.

Even though it’s possible to document “pulsating” flow in an exhaust system, the total process may be more easily understood by considering the exhaust gases “flow” from the cylinders to the atmosphere. If we do this, then it’s reasonable to accept that the volume of exhaust gas an engine must expel is a function of rpm and piston displacement. Even though friction horsepower and pumping losses are part of the equation, it is interesting how accurately headers can be sized based upon engine rpm and piston displacement.

There is a significant body of information supporting the notion that peak torque (volumetric efficiency ) can be associated with a specific flow rate in an exhaust passage. This value is in the range of 240-260 feet/second. Since our assumed exhaust “flow” passes through primary header pipes, its rate is influenced by the pipe cross-section area. This phenomenon then becomes a “tool” for relating torque output to rpm to piston displacement. By first determining the rpm span in which a torque boost is required and knowing a value for piston displacement, primary pipes can be sized. Even though primary pipes are typically joined in some fashion at their exits, resulting in a sharing of pressure pulses among cylinders (X or H pipes) primary pipe sizing does not always need to be equal. “Stepped” pipes and multiple pipe sizes can also be used to pinpoint rpm points at which torque boosts can be created. 

Our customers may also consider material used in their pipes, thermal coatings, as well as these sizes of primary tubing and clearance issues that may be of concern with a daily driver. The Frank Racing Inc line of headers were specifically made for consideration to where power is made but also with regards to fine workmanship, clearances, and material used to give our customers the finest part available.