 |
PM |  | |
Quote | | |
Reply | | |
Full Topic | |
| Joined: | 04-07-2022 |
| Location: | Oregon USA |
| Posts: | 87 |
| Status: |
Offline
|
Having read this VERY old thread, I wanted to clear up some misconceptions that are woven into the discussion. First and foremost, lobe centers (that is, the timing of the intake cam relative to the exhaust cam) affect three things; Power, idle, and emissions.
With tight lobe centers (e.g. both cams set to 110 degrees) the following occurs:
1. HC emissions go up. The amount of time BOTH valves are open is relatively high. This means that while the exhaust gasses are exiting, the fresh intake charge is trying to move in. Remember that air has inertia, so as the exhaust gases continue out they tend to pull the intake charge along behind them. Some of the intake charge goes out the exhaust, giving rise to unburned hydrocarbons in the exhaust. This is pollution. It also causes fuel economy to go down (unburned gas is wasted gas).
2. Peak power goes up. In an ideal engine, when the exhaust valve closes there will still be exhaust gasses present in the combustion chamber. Then the intake valve opens and a fresh charge enters the cylinder, DILUTED BY THE REMAINING EXHAUST GASSES. In a 10:1 compression engine, this means a 10 % reduction in power because there will be 10% less room for the intake charge. By holding both valves open longer, the exiting exhaust will pull intake charge behind it, "flushing" the remaining exhaust gasses out of the cylinder. Theoretically, it is possible to flush ALL remaining exhaust out, such that there is no dilution of the intake charge and more fuel is burned for each power stroke. A larger amount of fuel burned means more power. The driver may notice the engine feels "peaky" as the power comes on suddenly which is more exciting, but there is a commensurate loss of torque (see number 3, following) because combustion is compromised at low RPM.
3. Idle gets rougher. At low RPMs, there is sufficient time for the for the exhaust to actually reverse direction and push some of the intake charge back out the intake valve. This causes worse conditions for combustion, and a rougher idle. As the RPMs increase there is not enough time for the exhaust to reverse direction, and so the engine smooths out. This is why aggressive cams give a choppy idle, but make more power as RPMs increase.
Depending on the engine, as lobe centers get tighter power goes up until the lobe centers get to about 106 degrees (e.g. 106/106 cam timing) after that the power begins to fall and the engine begins to run ever more roughly with decreasing power.
When lobe centers are wider (e.g. 115/115 cam timing) the following occurs:
1. Emissions go down. There is less time for the intake charge to follow the exhaust charge out through the exhaust valve. That means less unburned fuel in the exhaust, and lower HC emissions. Fuel economy also improves since there is less wasted fuel going out the tailpipe.
2. Peak power goes down slightly. The intake charge will be more diluted by remaining exhaust gases since little "flushing" of the exhaust gasses occurs. Coincidentally, NOx emissions go down, since peak combustion temperatures go down by virtue of inert gasses being present which reduces peak combustion temperature. Low end torque improves, so the engine feels less "peaky".
3. Idle smooths out, since the exhaust has little or no chance to reverse direction and push fuel out the intake valve.
So, what is "best" - 110/110 cam timing or 115/115 cam timing? Well, it depends what you want. Good fuel economy, good torque, smooth idle and good emissions or more peak power, less torque, less fuel economy, higher emissions and rougher idle?
You can't have it all, it is the nature of the beast. You pay your money and make your choice.
Vance
Last edited on 04-14-2025 01:48 am by vnavaret