[turnsignalslol]

I've heard so many things from different people saying that downpipes (catless or 200cell) will cause your car to run rich/lean and I'm looking for a technical answer as to how this could be.

I would assume that by running an O2 spacer, you could run rich/lean because you're attempting to fool the ECU into thinking you're running cats when you're really not, so the ECU gets false data and makes false corrections which then messes up the AFR.

When running downpipes with NO spacers, you'd be giving the ECU real data and while the ECU will fail the catalytic converter efficiency check, shouldn't your car still be able to run safe AFR ranges because the ECU is automatically adjusting for less backpressure which leads to increased boost? The CEL that usually pops up when installing downpipes is for catalytic efficiency so I don't see how that would affect fuel trims/overall engine performance.

Could this information all be outdated from earlier times where engines didn't automatically learn and recalculate parameters? This could explain why some people insist that a tune is necessary for downpipes. I've also read online that the engine can support up to 25% past its original fuel trims table and while my personal theory is that downpipes won't really add too much power and just increase the amount of overall boost (which the ecu then wastegates since it's going to make the car run lean), this could be another explanation as to why cars run weird on stock tune with downpipes.

Looking to start a discussion and get some real answers since everyone is apparently a master tech online lol.

[Tag]

My understanding is that the 02 sensors post cats should have a certain value after the exhaust has been 'cleaned' by them. Those numbers are used to determine AFR. If you have downpipes the pre and post cat 02 sensors would read the same values causing issues. That's the extent of my knowledge on it; never ran downpipes and never would. I can't stand the smell.

[mike@x-ph.com]

Without the spacers, the downpipes won't cause the car to run rich or lean. The DME automatically adjusts to keep your AFR within acceptable limits.

The problem is when you trick the DME by feeding it incorrect values, which can push the AFR outside the acceptable limit.

[Tag]

Originally Posted by mike@x-ph.com

Without the spacers, the downpipes won't cause the car to run rich or lean. The DME automatically adjusts to keep your AFR within acceptable limits.

The problem is when you trick the DME by feeding it incorrect values, which can push the AFR outside the acceptable limit.

Excuse my ignorance, but isn’t something required for lambda so it tricks the DME to think the cats are working as normal? Achieved via dongle or coding? I thought having the same reading for both O2 sensors would cause a CEL and issues?

[mike@x-ph.com]

Originally Posted by Tag

Excuse my ignorance, but isn’t something required for lambda so it tricks the DME to think the cats are working as normal? Achieved via dongle or coding? I thought having the same reading for both O2 sensors would cause a CEL and issues?

Correct, im responding to the question regarding changing downpipes and not adding any O2 spacers.

I dont comment on O2 spacers because we dont offer them, and it's got implications with emission-related issues that I'd rather stay away from.

[turnsignalslol]

Update for anyone interested: I was talking to a guy on reddit who had downpipes on his Supra (B58) and logged the car before and after getting tuned. He reported that AFRs didn't look strange. I took a look at the logs and while the AFRs certainly did seem a little high, people are saying this can be attributed to cars with DI generally running a little leaner. I'd overall say that downpipes on stock tune with NO spacers won't really give you any issues other than the annoying CEL you'll have to clear every 30-50 miles if you want to use remote start. Still, I think it's better being safe than sorry and took them off until I'm ready to give the car a proper tune.