458 Full exhaust system - AFR imbalance without tune?

Full exhaust as in cat delete?

If you are installing a real more free flowing exhaust then you'll need a tune for proper AFR.

 

I doubt an exhaust can impact your AFR in any material way (how much more VE could you possibly get with a decat?)

The engine operates on closed-loop most of the time and even running straight pipe won't significantly push flow beyond the point where the engine can trim itself out (there's significant leeway in the fueling map)

A tune is "neccesary" to prevent a CEL because the engine monitors the pressure in the exhaust to check if the catalytic converter is working properly. Removing the catalytic converter obviously results in less pressure across that section of the pipe which on a factory tune would be interpreted as a cat failure. It's not "neccesary" to the running of the engine.

Where a custom tune would be beneficial is adjusting the variable valve timing to account for the lower backpressure in the exhaust. If this isn't done it wouldn't be that the engine is running "poor", it just wouldn't be at the optimal level.

 

anyone know 100% for sure if or how much the stock ECU will adjust with a more free flowing exhaust? At the top of my mind would be to be sure the ECU does adjust and AFR is not lean and timing is correct ect. You should call a custom tuner company and talk to them...Mase or HD to start with but remember everyone is out to sell you something.

I think if you just remove the rear muffler you'll probably be Ok. You can definitely pull the stock exhaust valves and be fine

 

Pulling the exhaust vacuum lines is probably a quick and easy way to make it sound great and unrestricted.

 

Pulling the factory exhaust valves for free.

 

running straight pipe or decat won't affect your AFR man

Your fueling is determined by the fueling map and oxygen sensor.

If you get better flow the ECU will adjust fueling to account for this to maintain target AFR.

The way fueling works is there's some base expectation of Volumetric Efficiency (VE) where the computer, knowing the bore/stroke, throttle angle will "expect" a certain amount of airflow (which is further checked by the MAF) and then it will look up how much the target AFR should be in a map and inject the corresponding amount of fuel. Then it will use the wideband O2 sensor to check if the real AFR is matching with the target and tweak the fuel injection accordingly.

All of this is standard and there's a massive amount of margin in the system to account for the fact that these cars are shipped all over the world at different altitudes, temperatures, climates, etc.

Changing an exhaust isn't going to materially affect your volumetric efficiency enough that you start operating off the edges of these maps.

What changing an exhaust WILL do is mess with your catalytic converter monitoring system resulting in a CEL.

What changing an exhaust WILL do is mess with your VE enough that the perhaps the variable valve timing needs optimizing. Modern engines run with a lot of valve overlap, that overlap was determined based on balancing how much flow there is on the intake and exhaust. If one or the other suddenly flow more freely or is obstructed then the valve timing should shift to account for this. This process is not done on the fly (at least for high load operation, there idle logic could use vvt in addition to ign timing but this is neither here nor there). The only processes which are done on the fly are ignition timing and AFR when we're talking about on throttle operation.

 

I recommend a 360Trev reprogram as opposed to a "tune". Not sure if tuners rewrite factory programming or shut off monitors. Trev's tune rewrites factory parameters so when you smog or hook the car up to an SD3 or the like, it looks stock. Just the programming has been optimized for your car and any mods. I've had a 360 and a 430 done and the improvements were very noticeable. Waiting for his US rep to return to the states so I can get my 458 done.