Safe A/F Levels?

Hi, I recently upgraded to a T78 with 850 cc injectors and I'm trying to tune my car using the Innovate wide band along with an Apexi SAFCr. Can someone please tell me what range of A/F readings I should stay within when tuning the car? I realize that 14.7 is stoic at idle.

AT WOT I've been seeing A/F readings as low as 9.6... very rich...

What is worse for the car, being too rich or leaning it out?

Thanks,

Both are bad.

Too rich is better though. Here is why.

A lean fuel mixture will cause pre-ignition or detonation and excessive heat. Both are catastrophic. If you go lean at 2000 RPM's, it is not as deadly as going lean at 6000 or 8000 RPM and under boost. Lean either way is bad, but under boost and you are almost guaranteed to crack a ring land, or crack a piston.

If you are too rich, like over 10:1 rich, especially at higher loads (more fuel), it will overload the cylinders and possibly bog down. Good news is fuel cools heat, so you shouldn't load up on heat unless you have another issue at hand. You can dial in the fuel here and be good to go. The bad that can come from too much fuel is ringwash. This is a condition where fuel washes past the rings, and also can ruin the rings causing low compression. When the fuel washes past the rings, it gets into the oil supply. Fuel does not have lubricicity. It will not lubricate. So, if fuel instead of oil gets to a bearing surface, you will have bearing to metal contact, instead of an oil film protective layer, and will cause damage to a bearing. Once damage is done, bearings must be replaced or they will further deteroriate and likely cause a failure.

If you are tuning and have been running over 11:1 rich for some time, you should change the oil supply before any hard driving is done. Dial in some fuel, get it closer to spec, switch the oil, and continue tuning. Do not run the motor hard on oil with fuel in it. Do not!

I hope this helps.

[ChrisB]

Your plugs are probably also fouled now. Get some new Iridium IK24's and gap them to around .028". Under boost try to be at around 11.5:1 with the reading on a wide-band plumbed in your DP. With a reading at the end of the exhaust pipe you should be between 11.5:1 and 12:1. Under cruise you can run as lean as 17:1, but that will typically cause bucking. I try to tune for 16:1 on cruise.

Take care,
Chris.