Quote:
Originally Posted by stonebreaker
Reducing the diameter of your exhaust pipe will do nothing for your torque at low rpm. That's a myth.
The myth got started because back in the carburetor days, adding headers and a performance exhaust to a car leaned it out, causing a loss of torque at low rpm's. Thus the myth that if increasing the size of your exhaust lowers low end torque, then decreasing the size of your exhaust must then increase torque, right?
The real solution to the issue of lowered torque with a big exhaust was due to the headers actually doing their job and lowering backpressure, which leaned out the air/fuel mix. Once you re-jetted the carb to account for the headers, you INCREASED torque all the way across the rpm band.
With modern fuel-injected cars, the computer will compensate for the reduced backpressure automatically so there will be no loss of torque.
About the only thing you could do that would hurt your torque output would be to reduce the diameter of your exhaust system. It will increase your backpressure, causing your engine to work harder to pump the exhaust out the back.
|
That's not entirely true, but for the most part, it is.
Reducing the diameter of the pipe can allow for less turbulent flow through the tube
if the volume of flowing gasses isn't enough to "fill" the tube at it's current temperature/density.
There's alot more to it than that simple phrase, but basically, exhaust tubing follows the same set of rules as an external flow, except with a much thicker boundary layer attached to the walls of the tube.
In other words, you'll get more bottom end if you have a properly sized pipe, as opposed to an obnoxiously large one.
Regarding carbs, once you retuned the carb, you were adding more fuel to the new amount of airflow to acheive more power across the board, which still means that you're burning more fuel at a specific RPM than you were. If the amount of fuel you're burning extra is less than the VE increase, you get a net gain in efficiency. If it's not, you're either breaking even or worse for efficiency, even though you could still be making more power than you were before.
Our purpose isn't normally to increase low-end torque above what's already there, it's to make the engine more efficient at creating it. That means that anything which requires additional fuel, with few exceptions, is going the wrong way for our intentions. Most vehicles already have plenty of torque to do what they need to do, so adding more isn't really going to help, unless it's a byproduct of increased efficiency.