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 Post subject: O2 sensors
PostPosted: August 30, 2017, 6:57 am 
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Location: Green Bay, WI
My O2 sensor instructions are telling me 24" after a collector for placement. Who has that kind of room? Is this normal? Looking at redoing my entire exhaust to try and shorten it up at least 14". What's everyone else's experience? Any false lean mixtures experienced?

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 Post subject: Re: O2 sensors
PostPosted: August 30, 2017, 8:16 am 
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Steve, I think the reasoning is to 1) allow the individual runner's gasses to mix well and 2) allow the exhaust gasses to cool down a bit. In mine, the wideband O2 was only around 5 inches or so downstream of the collector. Rotary exhaust gasses are much hotter than piston engines. I never had a problem located there. I could have put it between the resonator and the muffler, but there, the wiring would run in a route that would have made it very susceptible to being damaged by a passenger.

I think you'll be OK in a similar location. Just be sure it is installed above horizontal (maybe 45 degrees above?). At least that's what I read.


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 Post subject: Re: O2 sensors
PostPosted: August 30, 2017, 10:29 am 
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I've seen just about everyone like yours so that's why I questioned it. I understand why they want it 2' down stream. You get the best laminar flow. These are such short systems.

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 Post subject: Re: O2 sensors
PostPosted: August 30, 2017, 11:00 am 
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You can shorten the primary's to gain length. That will raise the RPM of your possible peak power. If that is important to you, then you haven't got too much other choice. As you say, there isn't much room to move things about. If your O2 sensor has a heater, maybe put it after the muffler? Not sure if anyone has done that before. it wouldn't react much different than an emission sniffer. The added distance might affect your tune during transients if you plan on running O2 closed loop. I recall that the Megasquirt can change the delay time for that. However delay only works at some given volume flow rate (RPM) maybe the newer MS firmware has that covered by entering a distance rather that time delay.

Some have had room to run the exhaust pipe between the tire and chassis and place a transverse muffler across the back. Can you do that? It would open up a world of opportunities for locating the O2 sensor, not to mention reduce the exhaust noise to the driver & passenger. If I were building another, that is what I'd try to do.

FYI, here is where the stock 02 sensor is located on my RX7. Certainly not 24 inches downstream. Maybe 12 inches from the engine port? It didn't seem to bother Mazda. Not much laminar flow there either. although the #1 and #2 rotor gasses were certainly mixed. So I think mixing to get an average of all exhaust ports is more important that Laminar flow. :mrgreen:


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 Post subject: Re: O2 sensors
PostPosted: August 30, 2017, 1:09 pm 
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Whoa after the muffler. I am checking that out, great idea. I can make that work. I'll check my microsquirt for the controls on the O2 if it's a time vs distance. Nice ideas Chuck thanks.

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 Post subject: Re: O2 sensors
PostPosted: August 30, 2017, 1:28 pm 
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I guess it always pays to call the maker. They said don't sweat it. 8" or so after the collector will be fine. After the muffler is also fine. Whew. I can proceed as planned

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 Post subject: Re: O2 sensors
PostPosted: August 30, 2017, 1:53 pm 
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Good news. Now go out there and make some more car parts. :cheers:

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 Post subject: Re: O2 sensors
PostPosted: August 31, 2017, 2:00 am 
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My neighbor was a vehicle development engineer for Ford and GM for 40+years. He said that it should be located aft of the last tubing intersection a distance equal to five times the diameter of the secondary tubing. 3" secondary, 15" aft of the last split. He said that minimizes flow turbulence over the sensor. For what it's worth...


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 Post subject: Re: O2 sensors
PostPosted: August 31, 2017, 6:20 am 
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In this case it would be 12-1/2". That 5x pipe diameter is the same in my job to for sensor placement after pumps.

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