4 to 2 exhaust O2 sensor

[redmt]

I'm setting up a Mega Squirt system on an 8 stack injection. The headers are 4 into 2 to the exhaust side pipes on a coupe. There are 4 individual side pipes, 2 per side. I understand for an accurate reading the O2 sensors need to read a mix of all cylinders. In this case there would need to be 4 sensors on the 4 to 2 cylinder collectors. Am I heading the right way or is there something else a little less expensive ?

[bobcowan]

The more cylinders you can monitor, the better off you'll be. But, for a lot of applications, it's just not reasonable.

Some systems (like Megasquirt) will allow 8 sensors. But most systems will not allow more than 2. Some people have placed the sensor in a single header pipe, and it will work just fine.

The risk is that if you have a problem with that one cylinder, it will effect the entire system. Of if you have problems with a different cylinder, you won't know until something bad happens. But those situations are pretty rare.

Some sensors need to be fairly close to the head, like 18" or so. If yours is one of them, then you'll need to place it in a single pipe.

[Gaz64]

If your ECM will allow it, run 2 wideband sensors, (one per bank).

Hopefully you have the primaries gathered in a 180 degree order.

[redmt]

Originally Posted by Gaz64

If your ECM will allow it, run 2 wideband sensors, (one per bank).

Hopefully you have the primaries gathered in a 180 degree order.

Thanks for the advice but you missed the part about the 4 into 2 collectors. I wish it was 4 into 1 and then I could run only 2 sensors, 1 per side.

[vector1]

you could weld multiple bungs into wherever and when not in use plug them. use one as primary efi mang. and move the sensor to other positions to check readings, or whatever your imagination comes up with. i used one sensor in a primary pipe and asked a fairly knowledgeable tuner if i need to make any compensation to the efi programming to acct. for the pulses, etc. in reading only a primary pipe and he said no (not definitive by any means), but still take into consideration other cylinders may vary.

[Gaz64]

Originally Posted by redmt

Thanks for the advice but you missed the part about the 4 into 2 collectors. I wish it was 4 into 1 and then I could run only 2 sensors, 1 per side.

No I didn't.

With your 4 into 2 per bank, you would not want the O2 sensor fitted to a pipe with successive exhaust strokes and then nothing, (90 degree spacing then nothing for 540 degrees).

Ideally you would want 360 degree spacing but that would make for a difficult pipe layout.

Personally I would have 180 degree 4 into 1s.
2 from one bank and 2 from the other bank into one collector, with only side pipe per bank.

[redmt]

With the cost of the O2 sensors from Mega Squirt, I'm considering a different exhaust layout. This is the Coupe originally Built by the "professional Cobra Builder".

[Luce]

With megasquirt, it's far less critical than anyone will lead you to believe. As long as your injectors are matched and your butterflies are well synced, you can use just one header pipe... so long as you're not running on the ragged edge of lean. Your challenge will be the plumbing to get a good smooth vacuum signal, or are you running only Alpha-N.

If you want to put your mind at ease, you can put a bung in all pipes and move the sensor around to be sure they're all uniform.

I have mine welded into a pocket parallel with the pipe. It reads a little slowly, but you can filter that out with the tuning software. I'm really not concerned with running closed loop though. Closed loop really isn't so easy with a lopey cam anyway.

Megasquirt rocks. You can do all sorts of extra things with it that you can't with the others. Especially if you're running boost.

[redmt]

I ordered another sensor for the opposite bank. I'll figure out which cylinder goes to which collector and decide where to put them relative to the firing order.

[Gaz64]

Something to think about.

If your O2 sensors are only looking at one cylinder per bank, and that cylinder misfires intermittently, the O2 sensor reads lean (high oxygen from no combustion).

If the ECU retrims the fuel accordingly from a momentary lean condition, the whole engine will be trimmed in the rich direction.

A permanent miss in that cylinder will have the engine go rich as the ECU trims will allow.

[redmt]

4 into 2 means that there are 4 pipes on one bank going into 2 collectors instead of 1. The sampling would come from one of the 2 collectors on each side.

[Gaz64]

Originally Posted by Gaz64

Something to think about.

If your O2 sensors are only looking at one cylinder per bank, and that cylinder misfires intermittently, the O2 sensor reads lean (high oxygen from no combustion).

If the ECU retrims the fuel accordingly from a momentary lean condition, the whole engine will be trimmed in the rich direction.

A permanent miss in that cylinder will have the engine go rich as the ECU trims will allow.

Please reread my post above. "One cylinder per bank" are the key words.

Originally Posted by redmt

4 into 2 means that there are 4 pipes on one bank going into 2 collectors instead of 1. The sampling would come from one of the 2 collectors on each side.

I know what 4 into 2 means. Can't see the point really, especially if a negative pressure wave is not put to use.

Yes, you would fit an O2 sensor in a collector on each bank, or you could fit both sensors in both collectors on the same bank.
Either way, you'll only be sampling half the engine, rather than sampling all of the engine to get mean average A/F ratios.
Any misfire will then "average" higher O2 content, and then the whole engine will richen up momentarily to compensate.

[Luce]

Lopey cams misfire all the time at idle. With megasquirt, you can dictate how much authority it has to adjust the fuel based on O2 readings and how quickly it can make adjustments. I'm not even allowing it to run closed loop, but rather just using the O2 for tuning so I can make sure the data is valid by hand.