This is a question that has been boiling in my mind for quite some time, and I figured this would be the best place to ask it.

I understand after installing LTs and a free flowing exhaust it takes away some backpressure built up from ill-flowing cats/mufflers/resonators/etc. The factory has a certain tune that works well with the set-up and I understand the O2 sensor allows for bit of tweaking due to difference weather conditions/altitudes.

But what affects will the free flowing exhaust have on the A/F ratio and how do tuners make the most out of it? Does it have anything to do with the A/F ratio? Is timing the main tuners variable for freeing up the exhaust?

If a good intake system will lean out the A/F... will good L/Ts do the same?

Thanks!

 

Anything that will get the air in and out of the car quicker is going to have a slight affect on air fuel ratio. Additionally, with a set of long tubes, your o2 transport delay is going to need to be modified in order to keep from having problems with fuel trims, cold starts, hesitation, etc. There are alot more variables than just air fuel. To answer your main question, yes installing long tubes with high flow cats is definetly going to affect your air fuel ratio. Timing will be much more affected by the octane fuel, and performance map or race map that you choose than the headers. Once you go with a supercharger or boosted application, then the headers will have a small affect on boost, and in turn... spark.

CR
Tillman Speed

 

Thanks CR. I think what I was getting at was how the A/F ratio was affected by freeing up the exhaust. If I was going to tune the vehicle myself (I'm not going to, starting out), would I richen up the fuel or try to lean it out a bit?

The reason I ask these questions is I'm going into an Automotive Tech school after I get out of the Army and trying to arm myself with some basic knowledge before hand.

I didn't even think about the O2 sensor delay from placing it further away than stock.

I appreciate the help, CR... I understand you're a very busy man and it's especially cool of you to help me out with this small question. I plan to order my exhaust/CAI and tunes from you in the future.

 

would I richen up the fuel or try to lean it out a bit?
Thats what I was wondering also

 

Probably you would need to richen the mix. If there is more air flowing, you will need more fuel.

 

already calculated via mas

 

Richening it up. The more air that comes into the car, the more fuel you'll need to stay at an optimal 12.8-13.2 air fuel ratio for these 4.6L 3V motors.

This is calculated using the MAF, which reads how much mass of air is coming into the car. The tune has a base fuel table, but needs to be fine tuned via the MAF transfer function.

At WOT, your o2 sensors become useless and rely on the fuel calibrations that you enter. At idle, the upstream o2's figure out how to keep your car at optimal idling AFR and cruising AFR.

So keeping your fuel trims at 1.0 will set you at as close as possible to what the base fuel table is commanding.

So basically, in this situation you would want to richen the car up. This accounts for the air intake, but still does not address timing, or any driveability concerns such as throttle, etc.

CR