Differences
This shows you the differences between two versions of the page.
| Both sides previous revision Previous revision Next revision | Previous revision Next revision Both sides next revision | ||
|
techtalk:evo:carb04 [2019/03/22 02:27] ixl2relax [Exhaust Pipes & Cams - Power or Reversion] |
techtalk:evo:carb04 [2019/03/22 03:23] ixl2relax [Gaskets] |
||
|---|---|---|---|
| Line 21: | Line 21: | ||
| \\ | \\ | ||
| + | |||
| ===== Exhaust Pipes & Cams - Power or Reversion ===== | ===== Exhaust Pipes & Cams - Power or Reversion ===== | ||
| + | All exhaust systems have harmonic pressure waves passing down and up the pipes. These waves are initiated by the operation of the exhaust valves. The waves move back and forth, with new waves combining or competing with the existing waves in the exhaust system, to either help move the exhausted mixture out of the pipes or back toward the cylinder. In a complicated way, the rpm of the engine is creating a certain frequency of new waves while the length and shape of the exhaust system is reflecting those waves at its own frequency. | ||
| + | This complex combination of frequencies causes each exhaust system design to have it's own characteristics of helping the combustion cycle at certain rpms (making more power) and hindering that cycle operation at other rpms (creating reversion and less power). The trick is to match the right exhaust system and cams so they are working together to efficienctly create the most power. | ||
| + | In the following quotations, of an XLForum discussion about stock 'D' cams versus Andrews 'N4' cams, Aaron Wilson provides a very good explanation: | ||
| **Quoting Aaron Wilson from the XLForum (aswracing):**((http://xlforum.net/vbportal/forums/showthread.php?t=1028905 - Post#12)) | **Quoting Aaron Wilson from the XLForum (aswracing):**((http://xlforum.net/vbportal/forums/showthread.php?t=1028905 - Post#12)) | ||
| Line 33: | Line 37: | ||
| Overlap is the period in time when the exhaust valve is almost closed and the intake valve starts to open. There's a window of time when both valves are open. The piston passes through TDC during this window, finishing it's exhaust stroke and starting it's intake stroke. | Overlap is the period in time when the exhaust valve is almost closed and the intake valve starts to open. There's a window of time when both valves are open. The piston passes through TDC during this window, finishing it's exhaust stroke and starting it's intake stroke. | ||
| - | Imagine what's going on right then. A whole bunch of hot, high pressure exhaust has just left the cylinder. Sitting in the intake tract is a fresh, relatively cool, air/fuel charge sitting at low pressure. Both valves are open.\\ \\ The piston starts down, and starts pulling. Where is it going to pull from? The answer is that it wants to pull from the exhaust. That's where all that hot, high pressure is sitting. | + | Imagine what's going on right then. A whole bunch of hot, high pressure exhaust has just left the cylinder. Sitting in the intake tract is a fresh, relatively cool, air/fuel charge sitting at low pressure. Both valves are open. |
| - | So what makes it pull from the intake instead?\\ \\ It's extremely critical that the exhaust system creates a suction wave right then. If it does, the suction wave will actually pull on the intake tract (remember, both valves are open) and get that fresh charge moving before the piston even starts pulling on it. If the exhaust system does this, it greatly helps cylinder fill. | + | The piston starts down, and starts pulling. Where is it going to pull from? The answer is that it wants to pull from the exhaust. That's where all that hot, high pressure is sitting. |
| + | |||
| + | So what makes it pull from the intake instead? | ||
| + | |||
| + | It's extremely critical that the exhaust system creates a suction wave right then. If it does, the suction wave will actually pull on the intake tract (remember, both valves are open) and get that fresh charge moving before the piston even starts pulling on it. If the exhaust system does this, it greatly helps cylinder fill. | ||
| On the other hand, if the exhaust has high pressure right then, it'll actually push the intake charge back out the intake tract, really screwing up the cylinder fill. What's more, the intake charge will go back out the carb. Then when the piston goes down and the exhaust valve closes, the piston will pull on the intake again. The intake charge ends up passing through the carb three times: in, out, and back in again, picking up fuel each time. | On the other hand, if the exhaust has high pressure right then, it'll actually push the intake charge back out the intake tract, really screwing up the cylinder fill. What's more, the intake charge will go back out the carb. Then when the piston goes down and the exhaust valve closes, the piston will pull on the intake again. The intake charge ends up passing through the carb three times: in, out, and back in again, picking up fuel each time. | ||
| Line 59: | Line 67: | ||
| The shape of the torque curve tells the story. It's a map of cylinder pressure, which for all intents and purposes is a map of cylinder fill. A perfect torque curve is table flat. Where it comes up from that tends to be where the pipe is working. Where it drops from that tends to be where the pipe is pushing back. When it's pushing back, the afr also tends to go rich.\\ | The shape of the torque curve tells the story. It's a map of cylinder pressure, which for all intents and purposes is a map of cylinder fill. A perfect torque curve is table flat. Where it comes up from that tends to be where the pipe is working. Where it drops from that tends to be where the pipe is pushing back. When it's pushing back, the afr also tends to go rich.\\ | ||
| (end quote) | (end quote) | ||
| + | |||
| + | \\ | ||
