FJ40 Mark/Downey Header

[Fe3]

The collector is not a good place to supply an O2 bung. That’s where I used to install mine before Holley changed its guidance to around 8” past the collector.

One thing I do know is If the flange gasket goes and the bung is right there then it takes the O2 with it. That’s one reason I know I moved all mine.

We were talking a little bit about this last week...

From the Holley Sniper Installation Manual: https://documents.holley.com/199r11031r.pdf

This bung location needs to be verified before using it with the Oxygen sensor included in the Sniper EFI system. Ideally the bung will be 6-10 inches after the collector for a true reading of all cylinders and have a minimum of 18" length past the sensor location. If the vehicle is equipped with a catalytic converter, the bung must be between the engine and the catalytic converter. The bung also must be on the top side of the tube so moisture cannot collect on the oxygen sensor.

And:

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 6-10" after the collector. You must have at least 18" of exhaust pipe after the sensor.

We know this header is a 6-2-1 design. So where/when do all the cylinders merge? Is it as soon as the 2-1 merge? Or is well after the two have merged into one?

Either way, our header falls right into the 6-10" guidance from Holley.

If we look up into the header, the two collectors combine into one at about eight inches:

 

[Fe3]

The stub we provide for make-your-own exhaust positions the O2 bung right at two inches:

 

[Fe3]

By those dimensions, the O2 sensor would be at 10 inches.

If you feel better giving the two collectors some room to merge, no problem. By that school of thought we have the O2 sensor at six inches:

We hope that helps folks who are running, or plan on running an O2 sensor with this header!

 

[rlong]

I’ll buy that location. At least it comes with one! If one questions if it’s working properly in that location for their setup, one could always weld/ clamp one on further back on their exhaust. Point being better to have one bung than none, and two don’t hurt.

 

[wngrog]

On the old style 6/1 JTO, I used to put them before the gasket on the header. I used to often eat O2 sensors. Since I started moving them downstream it's almost stopped completely.

 

[gonzopancho]

On the old style 6/1 JTO, I used to put them before the gasket on the header. I used to often eat O2 sensors. Since I started moving them downstream it's almost stopped completely.

Sounds like a leak

 

[wngrog]

Sounds like a leak

Dunno. I've learned a lot after doing about 40 Snipers. O2s used to haunt me. Now it rarely comes up.

 

[gonzopancho]

Dunno. I've learned a lot after doing about 40 Snipers. O2s used to haunt me. Now it rarely comes up.

Drum brakes next?

 

[wngrog]

Drum brakes will always haunt me

 

[cstav86]

On the old style 6/1 JTO, I used to put them before the gasket on the header. I used to often eat O2 sensors. Since I started moving them downstream it's almost stopped completely.

How far back are you putting them now?

 

[cstav86]

How do you like the white lighting coating? Mine just got back from Swain Tech but it will be another month or two before the engine rebuild is done. Does it do a good job of keeping the heat down in the engine compartment?

It's hard to say, I dont have a data point of how hot they were prior to the coating as I was running a stock manifold. I can hit it with a temperature gun and see what they run at temp.

 

[wngrog]

How far back are you putting them now?

About where it's illustrated above. 3" or so past the gasket.