I'm just getting ready to go the URD route, and as per Gadgets advice I'm trying to get a real picture of how my rig currently runs to help with future trouble shooting isues.

I havn't ordered a true wide band o2 sensor kit yet, I'm just using a pretty high end OBDII scanner (OTC unit) to monitor my o2 voltages. When the scanner hits the Lean light I'm showing .05 - .290V. According to this site: Graph at bottom

That puts me at 16.2:1 - 14.7:1. I'm just curious when does the lean condition get too hot to handle for our poor little pistons.

Is this graph even accurate?

 

Post weekend bumpy....

 

TOO LEAN!!!!!!!!!!!

 

I am not sure anyone has ever identified the exact A/F ratio that can cause detonation. If you find this out, please let us know. I think every vehicle and weather (altitude) conditions will change this measurement. For the most part ping is the identifier of an air fuel ratio that is too lean. (signifigantly beyond stoich)
Note: load will signifigantly change your readings.

Do you hear ping?

 

First off, yes, that graph is accurate.

I figure that you're watching the O2 voltages during normal driving? If so, that's not going to tell you a whole lot with a narrowband sensor. The norm during closed loop (cruising speeds) is that a narrowband A/FR will swing wildy, like you're seeing. This is because the ECU is constantly adjusting the fuel mixture to hit the Stoich mark (14.7:1). A wideband sensor has a lot more resolution (about 5x as much) and you won't see as much of a swing.

But even so, with our engines, you can't really run "lean" or "rich" in closed loop. The ECU will always adjust the fuel mixture to create the 14.7:1 ratio. With piggybacks such as the SMT-7 or FTC-1, you can only make the ECU's job easier during closed loop.

Now, open loop (WOT) is another story. THAT'S where you can affect the mixture, and also where reading O2 voltages off of a narrowband sensor will make sense.

 

Something else to add....

Some gauge manufacturers say you should tune to achieve maximum or peak EGT for maximum performance. This is incorrect. Peak EGT generally occurs at an AFR of around 14.7- 15.0 to 1 on gasoline. This is far too lean for maximum power and is dangerous under continuous WOT conditions. Many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry- about 15 to 1 for our purposes. If you go richer than 15 to 1, EGT will drop and if you go leaner than 15 to 1 EGT will ALSO drop. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will occur at an EGT somewhat colder than peak EGT.

As you can probably figure out by now, leaning the mixture from the target 14.7 to 1 will NOT cause a hotter exhaust nor will it cause you to burn your valves. This is not to say that leaning the ECU's program under all conditions will cause a cooler exhaust. There is one condition that could be hotter and that would be running at WOT (wide open throttle) at 14.7 to 1 instead of the programmed 13 to 1. A continuous running at this condition might indeed burn your valves.

 

Make sure you're using a wideband setup to figure out if you are lean or rich.
Typical O2 sensors are 0-1V reading while widebands are typically 0-5V



The AF chart above looks a little steep in the center because it shows large variance in voltage with no change in AFR.
This seems a little more realistic.

 

Thanks for all the info guys. I have actually been one of the blessed few who hasn't experienced ANY ping or any other bad manners form my SC, and its been in for 10k ish miles. I just suspect lean out from my throttle response/feel and dis-coloration of my exhaust.

I just wanted to see what kind of lean out was occuring to get a true before and after picture. Sounds like I should just bite the bullet and get my LM-1 now and start getting some true readings. This OTC scanner seems to jump around way too much in open loop to give me any idea at WOT.

Either way I feel the the URD kit is worth it for the power/peace of mind factor. (Plus it will be hella fun to tweek! )

 

oh ya, forgot to add LOL!