Bob,
The arrival of the negative pressure wave at the exhaust port, what you identify as pulse vacuum, will still occur, it's timing will just be too early to provide any improved midrange scavenging. The early arrival is exclusively due to the short length of the header pipe and the reduced time to travel the distance between the valve and the newly created O2 sensor chamber between the header and the side pipe. While that sounds bad, and in fairness it is not good, the length of our side pipes tunes that pulse to a very low rpm because of their extraordinary primary length.
That low rpm tuning will enhance low speed engine torque at the expense of mid range and higher rpm torque. That said, it is possible our side pipes may produce no more power than a good low restriction under car exhaust. The reason that they may not, primary length not withstanding, is that the original side pipe mufflers that came with our cars tend to be fairly restrictive.
There are several aftermarket side pipe alternatives that look less restrictive than most original mufflers. As good as our side pipes look, a good, large diameter undercar exhaust with more traditional muffler geometry may produce a net power increase.
Bottom line I
suspect, but don't have data to support the suspicion, that the chamber for the O2 sensor (between the header and the side pipe) will not materially, effect engine power enough that the driver can feel it.
It is in all likeliness because of what Rick (olddog) was suggesting about usage frequency. The more frequently we operate the car the more frequently the sensor gets exposed to water condensation from a cold engine start.
At first glance, this does not seem to be a big deal however most O2 sensors do not just fail they tend to fade or drift with age getting increasingly less accurate until they finally give up the ghost and we need to replace them.
Along the way to giving up the ghost the AFR or lambda reading they communicate to the ECU results in a progressively incorrect AFR/lambda performance compared to the commanded target in the tune. On a high horsepower engine this mis-communication can bring on incipient detonation which the driver cannot hear but the pistons can feel.
The pic below is of a piston from an engine with 30K+ miles on it's tune that did not smoke but had become lazy. I am not trying to be cute, the engine still made significant power but it was becoming tless responsive to small changes in throttle position — it wasn't as much fun to drive.
When it was torn down all eight pistons looked the same. All pistons had all three rings still in place although the top rings were pretty beat up. The top ring looks like it is missing in the photo above. It is not, it is actually still there! Notice how nice the skirt still looks, not withstanding the damages to the ring lands.
Although they are bit pricier, the NGK sensors have an interesting attribute that depending on your perspective is either good or bad. That attribute is that the NGK sensor will not drift. There is more to this story than what it appears to be. As the NGK sensor begins to deteriorate instead of drifting it totally fails sending you a clear signal it is time for replacement.
One of the nice attributes of the 14 Point 7 wide band sensors, other than their stunning speed, is that they continually compensate for the sensor drift attributable to age, water, over heating etc. Eventually they will also need replacement but your AFR/lambda readings will be accurate all the way to the end of the line — sort of a poor man's NGK.
You're right about the newer ECU's being able to offer some drift compensation, Bob. However, as Rick has already pointed out this delayed reporting of the presence of oxygen in the exhaust (associated with a sensor that is far from the exhaust port) will produce a less desirable driving experience because the engine is not being properly fueled in real time.
Virtually all EFI systems today use some variation of fuel trim tables that over time essentially "learn" what the engine wants for fueling and eventually produce a pretty good driving experience. This is true until something like the O2 sensor begins to drift and the fuel trim tables are off because the O2 sensor is mis-reporting the actual oxygen content in the exhaust. This is further aggravated by a tardy sensor that is late in reporting how rich or lean the exhaust is. Both whoopses affect proper injector pulse width calculations and the ECU's ability to properly fuel the engine becomes progressively worse.
One of the really nice attributes of the 14 Point 7 sensors is their stunning speed. Properly placed in the exhaust system they produce the best real time reporting of oxygen in the exhaust allowing the EFI system to track the commanded AFR / lambda the tuner has programmed into the EFI system at the highest fidelity level the EFI system is capable of.
In general EFI does some pretty amazing things operationally, compared to carbs and distributors — even when we hobble it by not optimizing the installation. When we do optimize the installation the engine performance is for all intents and purposes, untouchable.
Ed
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