Hi guys. I would like to take the opportunity to discuss a little about "backpressure".
"Backpressure" is a huge misconception that has spread to a lot of Malaysians. Im not an expert, but have studied many technical articles that has made this apparent.
"Backpressure", aka reverse pressure / congestion in the exhaust system, is *always* bad. When most people say "backpressure" they are actually referring to a more restrictive exhaust system, i.e. smaller diameter pipe. What this creates is *exhaust velocity*.
There are two aspects to exhaust flow, velocity and volume. A big pipe will flow more volume, but at a slower speed (less velocity). A small pipe will flow less volume but at a higher exit speed / velocity. Remember our physics lesson where if you squeeze the water hose, the water shoots out further? Thats because velocity has increased at the small opening.
The common misconception comes, I guess, from the exhaust in the smaller pipe having a higher air pressure. Technically however higher pressure is not "backpressure".
A higher velocity in the exhaust flow will allow the exhaust system to make full use of the scavenging effect, i.e. the exhaust pulse from one cylinder actually creates a vacuum which "pulls" out the exhaust gasses from the other cylinders, reducing engine pumping losses (parasitic loss). This then improves torque.
However, there is a limit as to how fast the exhaust can flow, and when the engine hits this limit, the smaller pipe will be restricting the exhaust flow, causing "backpressure". This is bad because the engine is then pushing out the exhaust gasses, whereas a good exhaust system will actually help the engine pull out the gasses.
It is very important then, that someone who tunes his exhaust chooses the correct piping sizes. Like extractors, there is no 100% good or bad size, there is only compromise. On a 4G93 for example, a 2" will create more torque at lower rpms, but lose horsepower at higher rpms, while a 2.3" will lose torque at low rpms but gain horsepower at higher rpms. A smaller diameter pipe will also always benefit cruising, since your engine will not be pushing large amounts of exhaust gasses at this time and would greatly benefit from exhaust scavenging (reducing pumping losses).
Keep in mind however that you have to weigh the benefits against the sacrifices. On a stock 4G93 for example, a 2.3" system will give you very little benefits, while sacrificing a lot, when compared to a 2" (meaning you gain very little high end power across a small range, but lose a lot of low end power across a wide range).
The above is however only applicable on N/A engines, turbocharged engines have a whole different physics lesson to their exhaust systems.
My 2 cents from my understanding of exhaust systems, do correct me if Im wrong. :)
"Backpressure" is a huge misconception that has spread to a lot of Malaysians. Im not an expert, but have studied many technical articles that has made this apparent.
"Backpressure", aka reverse pressure / congestion in the exhaust system, is *always* bad. When most people say "backpressure" they are actually referring to a more restrictive exhaust system, i.e. smaller diameter pipe. What this creates is *exhaust velocity*.
There are two aspects to exhaust flow, velocity and volume. A big pipe will flow more volume, but at a slower speed (less velocity). A small pipe will flow less volume but at a higher exit speed / velocity. Remember our physics lesson where if you squeeze the water hose, the water shoots out further? Thats because velocity has increased at the small opening.
The common misconception comes, I guess, from the exhaust in the smaller pipe having a higher air pressure. Technically however higher pressure is not "backpressure".
A higher velocity in the exhaust flow will allow the exhaust system to make full use of the scavenging effect, i.e. the exhaust pulse from one cylinder actually creates a vacuum which "pulls" out the exhaust gasses from the other cylinders, reducing engine pumping losses (parasitic loss). This then improves torque.
However, there is a limit as to how fast the exhaust can flow, and when the engine hits this limit, the smaller pipe will be restricting the exhaust flow, causing "backpressure". This is bad because the engine is then pushing out the exhaust gasses, whereas a good exhaust system will actually help the engine pull out the gasses.
It is very important then, that someone who tunes his exhaust chooses the correct piping sizes. Like extractors, there is no 100% good or bad size, there is only compromise. On a 4G93 for example, a 2" will create more torque at lower rpms, but lose horsepower at higher rpms, while a 2.3" will lose torque at low rpms but gain horsepower at higher rpms. A smaller diameter pipe will also always benefit cruising, since your engine will not be pushing large amounts of exhaust gasses at this time and would greatly benefit from exhaust scavenging (reducing pumping losses).
Keep in mind however that you have to weigh the benefits against the sacrifices. On a stock 4G93 for example, a 2.3" system will give you very little benefits, while sacrificing a lot, when compared to a 2" (meaning you gain very little high end power across a small range, but lose a lot of low end power across a wide range).
The above is however only applicable on N/A engines, turbocharged engines have a whole different physics lesson to their exhaust systems.
My 2 cents from my understanding of exhaust systems, do correct me if Im wrong. :)
Last edited: