Attention: TDR Forum Junkies Tim said:
For once I have to set the record straight about this notion of pipe sizing and loss of HP. Tim's comments are close but as usual not entirely correct and in the case of Turbo motors, very incorrect.
In a normally aspirated engine, you have exhaust waves traveling down the header pipe close to the speed of sound. When this wave reaches the collector, it expands, reverses and goes back up the primary pipe. If the pipe is small in diameter, the wave travels quicker with more energy, and therefore will help scavenge the exhaust for the next exhaust event. It is a very common mistake to think that an engine needs BACK PRESSURE to be efficient. This has nothing to do with it at all, but a loss of back pressure can disrupt exhaust tuning, cam selection and overall mixture ratio changes. In all cases, no back pressure combined with correct primary piping, proper lobe center selection and carb adjustments will always make the most power and volumetric efficiency.
What happens when someone installs too big of an exhaust primary or secondary pipe, is at the same RPM, the exhaust wave travels at a much slower speed, and does not efficiently scavenge the exhaust event. In fact at slow speeds, the weak exhaust pulse can actually force exhaust back into the cylinder, whereas the smaller pipe will actually pull intake mixture from the intake port during intake, exhaust overlap. Now, crank up the speed of the motor, and the larger primary pipe will make more power when the exhaust wave once again reaches close to the speed of sound. Big pipe primarys will move the torque curve upward, longer primary pipes will not increase TQ, but instead rock the torque curve.
Now comes to TURBOS. Turbo motors do NOT work as a normally aspirated motor, as there is almost no exhaust waves scavenging the exhaust port due to the simple fact they are converted into mechanical energy to drive the turbo. Exhaust waves are very weak, and do not travel down the exhaust pipe, reverse, and scavenge the next exhaust event. Instead, the turbo forces mixture into the intake port and during overlap will blow some through the exhaust port (which is why most turbo and SC cams have very wide lobe centers (114-118 degrees) vs. aspirated motors built for strong torque in the 104-106 degree lobe center range. .
A large exhaust diameter on a turbo motor will normally not cause the loss of torque by itself unless you have a cam with too close of a lobe center or excessive retarded timing which allows a blow through of the mixture and loss of boost, We have dynoed numerous SC and turboed motors over the years and never found that a change in the pipe behind the turbo made any low RPM difference unless the cam was extremely retarded or had a tight lobe center. Proper cam timing and lobe center selection will always benefit from larger primary piping, period (due to lack of back pressure and reduction of blow through).
Again, the turbo kills any exhaust pulsing that could effect cylinder scavenging and takes this sound wave energy and uses it to drive the impeller. However, remove some back pressure (nothing to do with pipe sizing) the opposite can occur, where loss of back pressure can result in over scavenge of the mixture and loss of actual boost at low rpm.
Thus, the real reason the only possible way it could effect a loss of low end HP and TQ in a turbo motor is a possible over scavenging of the cylinder and loss of net boost. But again, this can be fixed either by advancing the cam (to close the exhaust sooner to build boost faster), or widen the lobe center (to again close the exhaust valve sooner).
Simple?
Last edited:
