O/2 Sensor location

[Bill Bogues]

We've bought a racepak data recording system for our stocker. The paperwork from racepak says we need to put the O/2 sensor in the header tube about 12" from the header flange. I was planning on putting a sensor in each collector. It seems to me that it would be hard to tune a engine just getting a reading on one cylinder on each side.Can anyone with experience with this enlighten me about this racepak system. I'm beginning to think that I spent a bunch of money on something that all it's going to show me is how little I really know about tuning a car. Thanks for any replies.

[KRatcliff]

With just one O2 sensor you would be far better to put it in the collector, but not too close to the end. Try to get it right at the south side of the point all four tubes meet in the collector. The further from the end of the collector the better. You don't want the fresh air giving you false lean readings.

The very best would be an O2 sensor in each tube at least 12 inches from the exhaust port/flame travel.

[Tom Goldman]

I disagree..
I have a Racepak Sportsman recorder with O2 and the primary tube location is far easier to tune with than the collector location.
I did move to different cylinders when I first started tuning with it to check cyl to cyl fuel distribution, but once my fuel distribution was confirmed , I went back to just 1 cylinder.
With a few good recordings under your belt ,you will see A/F ratio changes quickly on the data graph and know what it will do to your cars performance.
Call Greg Kelly and ask him about it , he is a wealth of knowledge about these recorders.

[69Cobra]

Bill I'm in about the same boat as you. I've used a FAST wideband which I don't recommend to anybody but that's besides the point. I had it located in the collector right after the four tubes merge. The reading where so erratic going down track you couldn't really get ahold of what was happening. Now again this was just a live reading and not a recorded run to review it after the fact. I've talked to a few NMRA Hot Street guys that use the racepak and they are all very happy with the accuracy of the readings and if I remember right with the RP there is a way to average the reading from start to finish through the run and I don't recall exactly where their sensors was located. I think Tom's got a good idea there. Put a bung in each tube and move it around to fine the cylinder thats the leanest and tune off of that more less.

Now I think my readings on mine would have probably been better if I had a true megaphone/merge collector instead of the big Hookers collector. I think it was mixing in fresh air and making the readings all over the place.

[Ed Wright]

If you see erratic readings with the sensor in the collector, you have something wrong. The only time I have seen erratic readings going down the track, there was an intermittent misfire in one ylinder. Misfire = lean spike. Mine (and MANY others) have all been in the collector (mine are 3 1/2" straight) about ten years now. Never an erratic reading. If not using all 8, I would never put one in a primary tube. It needs to be at least a foot from the end of the collector.

[Tom Goldman]

Kris , you are correct .
you can smooth out the O2 graph so it makes sense on the time line of the run.
Having read and interperted data recordings made for repairs on street vehicles at the JEEP dealership I worked for made it easier for me to use the RacePak O2 graph, but you can get a lot of data with a very few runs, and lean to interpert it .

[Signman]

It probably is better on a carburetor car to have the O2 in the collector if only one. Intake manifold can make fuel distribution uneven cylinder to cylinder. If in the collector you need to have an extension with the O2 as close to the convergence of the four primary as possible. One in each primary within 20 inches of the exhaust valve is best. In fuel injected cars because fuel distribution is pretty equal based on the location of injectors one in primary tube can work well. It does for me.

Understand that everyone has their own opinion /experience and different O2 locations work good or not so with varying assemblies. What is important is that what ever data reviewed is understood for what it really is.

O2 in the primary tube will see less fresh air than when installed in the collector. Am convinced with the cams we run O2 will see some fresh air no matter where located and more so in the collector.

In the collector AF will read more lean than if in primary tube. Check plugs and exhaust ports / headers. They don't lie. Be careful when helpful people say it should be this or that AF. My last motor was lifting the head trying to run it as lean as was being advised. They had O2 in collector while mine was in primary.

If you would like to get in the ballpark quickly spend a day on a chassis dyno with a good tuner. Review sheets from your the best runs. Those AF numbers are real for your assembly. Maintain and test variations on those.

In Div 1 there is a good tuner in Danbury CT.

Good Luck and Have Fun!