In the past with normal small chevy 18436572 firing order always used #5 as place for oxygen sensor but with 4-7 swap its 18736542 so I was thinking #4 as to be leanest hole any thoughts?

If one O2 sensor, I’d say in the collector would be best. That would provide an average of the bank at WOT.

Using EFI they recommend collector location with 15” of pipe behind sensor if wanting accurate readings at lower rpm (idle).

quote:

Originally posted by markemark:
If one O2 sensor, I’d say in the collector would be best. That would provide an average of the bank at WOT.

Using EFI they recommend collector location with 15” of pipe behind sensor if wanting accurate readings at lower rpm (idle).

I agree with M & M.

My previous set-up on our motor had the 4-7 swap cam. I checked the plugs here & there, never paid attention to find the leanest cylinder. That combo stayed in the car for 13 years. Anyway, we reconfigured the motor for a ProCharger & EFI. I needed to put in an O2 sensor & we placed it in the left collector. The cam is the standard firing order now & the #7 cylinder is the leanest. We tune to that that cylinder.

2BKING

I run 8 O2 sensors on my small blocks. The O2 is same on all. Now number 5 may be hotter (EGT) but can not say any difference on the O2.

I threw all my EGT stuff away years ago it was not reliable information or repeatable.

Had sensor in #5 at collector just before dump into open area always read steady there at WOT question was should it be moved to similar spot because of firing order change -as in 4-2

One of the theory's surrounding the 4-7 swap is "smoothing out" the firing impulses and improving the traditional thought of #5 being lean(er).
IMHO, assuming it does improve that issue, I wouldn't bother moving your WB sensor to a different tube. If you are going to do anything I would move it down into the collector more to get a broader reading of more cylinders.

All great advise thats the third vote for put it in collector ****her down changing several things on race car at once -cam- car itself-its a little heavier- header collectors so just looking for advise-thanks

quote:

Originally posted by vetman:
Had sensor in #5 at collector just before dump into open area always read steady there at WOT question was should it be moved to similar spot because of firing order change -as in 4-2

This is what I followed from my Holley EFI instructions:

"Locate a position for the oxygen sensor as close to the engine as possible. The oxygen sensor should be mounted at a point
where it can read a good average of all the cylinders on one bank. This would be slightly after all the cylinders merge. If you
have long tube headers, mount the sensor approximately 1-10” after the collector. You must have at least 18” of exhaust pipe
after the sensor. Verify that the Oxygen sensor is tight in the bung, if the gasket is not seated properly on the O2 sensor exhaust
leaks and poor engine operation will occur."

My O2 is on top of the left collector on about a 45* angle & about 6" after the primary tubes end. I have about 22" of pipe (muffler) after the sensor.

IMO, I would follow the same basic instructions & have a bung installed in the left & right bank. Just swap the sensor & plug the bank not being used.

2BKING

^^^^^^^^^This X2. If you’re not wanting/ needing accurate O2 readings at idle, you do not need a pipe extension after the sensor if in collector. It will read properly at WOT without extension.