locating an O2 sensor in a side pipe

[olddog]

Originally Posted by bobcowan

I have read this before. I think I've even said this before. And it makes logical and engineering sense. The sensor pictured above has been there more than 6K miles. No codes, so I'm guessing they're working OK.

My FFR had the sensors in the same place for about 12 years without any problems.

I suspect it's not as big of an issue as we think it is.

It's not a big deal in a Cobra that is almost never driven in the rain and even less likely to sit outside overnight in the winter. Try it in a daily driver when you can watch water spewing out of the exhaust pipes and you will get a different result.

Originally Posted by bobcowan

The advantage to placing the sensor in the collector is that it samples four tubes/cylinders. Yes, it's kind of a long way from the chamber. But the computer can easily compensate for that and give a good accurate signal.

If you have a good intake and header design, all four cylinders will be the same. If this were not the case the leanest cylinder would get toasted. So the one pipe verses four pipes, although not ideal, is not nearly as big of a deal as too great of a distance. Exactly how far is too far, depends on what criteria you are using.

Let me talk process control in general. The computer can only control the process it is connected to. You cannot make it respond any faster than the process is designed. When tuning a PI control loop the gains can be calculated by: proportional gain Pg = Pk / (Tc * To) where Pk is the process gain (ratio of measurement change / output change); Tc is the time constant, and To is the dead time (time it takes the measurement to see an output bump). The absolute worst thing you can design into any process is a long dead time. With a one minute dead time the control variable is changing for a minute, but the computer cannot see it and react to it until that minute is up. As the controller makes the correction to come back to set point, there is the same dead time issue causing overshoot. This forces you to use a smaller gain to keep the loop from going into an ever increasing osculation that spirals out of control. Thus you have massively increased the response time of the control loop.

Now EFI does not use traditional P or PI or PID controllers, but the concept still applies. EFI control is brilliantly different, and it responds better in some ways. It rapidly makes ballpark corrections and very slowly dials in to spot on. It also handles an extremely noisy measurement from the O2 sensor.