- Mar 3, 2005
- 8,180
- 3,678
- 5,213
I think everyone is sharing based on experience and trial and error....well here I want to share mine...this is based on observation and my theory....
From the exhaust port to the primary runner down until the bottom of the engine. This section the exhaust gas will be at the hottest since it's the closest to the exhaust port. That's why you see most headers on turbos or NA they start to glow hot 1st around this area....so as the exhaust gas travels further away from the engine it starts to cool down. The whole exhaust acts as a cooling conductor and in physics "heat will always conduct to the a cooler area"
So to improve scavenging, flow or velocity of the exhaust out of the port is very important because you want the exhaust to exit the chamber as much as possible before the valve closes and the intake stroke starts.
So we go back to the temperature 1st, hotter gas have higher pressure and higher flow in a confined space. So at the start of the primary runner you want the runner diameter to be bigger because the gas is super hot at that stage and it will need as much space it have to flow out of the chamber as much as possible...but after it gets under the engine it starts to cool down and in order to maintain the flow you reduce the piping diameter...this forces the hot exhaust that trying to cold down won't have a too steep temp drop...you force hot air into a smaller area it's velocity increase and it's temp drop will reduce....(just like heater use in winter, the pipe where the heat is given out is small instead of big) By doing this, you maintain the flow of the exhaust in a control area. But this does not mean you reduce the size to 1 inch is good because it will cause pressure to build up also....the reduction is size needs to be calculate and done in tandem with the intake.
That's why, when you use a bigger pipe at the rear, the exhaust gas have much more area to expand and cool down (cool air is heavier) and when you use a smaller exhaust the temp drop won't be too steep....a very good actual test is, you put your leg at a small medium size muffler, you can feel the exhaust gas pushing and blowing against your leg at a distance...compare that to a big muffler, the exhaust that exit is slow and sometimes you can feel it blowing on your leg.....
For those drag car that is posted by member here especially the 3rd picture, you can see that the exit diameter is smaller. Some what tapered...this maintain the exit velocity of the gas....
From the exhaust port to the primary runner down until the bottom of the engine. This section the exhaust gas will be at the hottest since it's the closest to the exhaust port. That's why you see most headers on turbos or NA they start to glow hot 1st around this area....so as the exhaust gas travels further away from the engine it starts to cool down. The whole exhaust acts as a cooling conductor and in physics "heat will always conduct to the a cooler area"
So to improve scavenging, flow or velocity of the exhaust out of the port is very important because you want the exhaust to exit the chamber as much as possible before the valve closes and the intake stroke starts.
So we go back to the temperature 1st, hotter gas have higher pressure and higher flow in a confined space. So at the start of the primary runner you want the runner diameter to be bigger because the gas is super hot at that stage and it will need as much space it have to flow out of the chamber as much as possible...but after it gets under the engine it starts to cool down and in order to maintain the flow you reduce the piping diameter...this forces the hot exhaust that trying to cold down won't have a too steep temp drop...you force hot air into a smaller area it's velocity increase and it's temp drop will reduce....(just like heater use in winter, the pipe where the heat is given out is small instead of big) By doing this, you maintain the flow of the exhaust in a control area. But this does not mean you reduce the size to 1 inch is good because it will cause pressure to build up also....the reduction is size needs to be calculate and done in tandem with the intake.
That's why, when you use a bigger pipe at the rear, the exhaust gas have much more area to expand and cool down (cool air is heavier) and when you use a smaller exhaust the temp drop won't be too steep....a very good actual test is, you put your leg at a small medium size muffler, you can feel the exhaust gas pushing and blowing against your leg at a distance...compare that to a big muffler, the exhaust that exit is slow and sometimes you can feel it blowing on your leg.....
For those drag car that is posted by member here especially the 3rd picture, you can see that the exit diameter is smaller. Some what tapered...this maintain the exit velocity of the gas....