Quote:
Originally Posted by aerohead
If we had the EcoModder chassis dyno we could test your car under load.Your decibel reading might be different if the engine was really 'working.'
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That is correct. It would also be louder at peak power output. I don't believe SAE J1169 is intended to be a measure of the peak dB that might be heard by a person, but is rather a specific test to compare one car to another. While it would be louder under load, it is unlikely that anyone will have their head 0.5m from my exhaust pipe while I am accelerating under load.
So, cars that measure 95dB and lower under this specific test are considered to be acceptable under all conditions even thought they may exceed 95 dB in other conditions.
Quote:
Originally Posted by aerohead
Some messengers have reported a need for back pressure for scavenging at low load,and less restriction as you climb the power curve.
Some motorcycles have dyno-tuned 'Smart' exhaust systems which incorporate a butterfly valve which is computer driven,always holding the valve in the 'sweet-spot' according to test cell development mapping.
I raced a Honda in high school and it's power fell off without the factory muffler.A reverse-cone megaphone exhaust provided a nice balance between low and high end performance.Some of my friends fathers were rocket scientists and they could explain the physics involved.Kind of tricky.  |
There is never any condition where back pressure is desired.
Scavenging is the process where a pulse of exhaust gases from another cylinder (or an earlier cycle in a single cylinder engine) will cause a vacuum behind it, and if the timing is just right to coincide with the exhaust valve opening, it assists the evacuation of the cylinder. This is a "tuned" system and only works well in a narrow rpm band.
Large diameter exhaust tubing will improve scavenging at higher RPMs resulting in more peak horsepower. However, it reduces the scavenging at low RPMs and reduce low end torque.
Small diameter exhaust tubing will improve scavenging at low RPMs resulting in more low end grunt. But, it will be restrictive at high RPMs and reduce peak power. This would generally be considered to have "high backpressure" but the reality is that it reduces the pressure at the exhaust valve at low RPMs
Improved scavenging means less pressure at the exhaust valve. In no case do you want more pressure, but you have a tuned system and you can adjust where it is most effective at reducing pressure. Ideally, you could have great scavenging at all RPMs, but it doesn't work that way.
To put it another way: an appropriately designed header has less backpressure than if you had no header/exhaust manifold at all...
So, while the loss of the muffler may have moved the RPM band for peak scavenging, I don't believe it moved it much considering the long length of the overall system. Certainly not enough that I could measure, and there was no detectable change in my fuel economy.
Your old Honda motorcycle would be more susceptible to exhaust modifications for two reasons. One, the short length of the exhaust system. And two, the carburetor's inability to adjust the fuel mixture for it.
P.S. I happen to have a Mechanical Engineering degree and took courses on Internal Combustion Engines for what it is worth.