GTM supercharger DYNO

SAM@GTM

David,

Thank you for posting this information up and being part of the solution.

I'd just like to make some comments on your graphs. Your power and torque look outstanding, but the afr looks odd. The target afr is to be 11.8 on average from the point the car goes into open loop. I did notice on your graphs the afr on each run got progressively worse toward the top end. Usually that is an indication that the exhaust sniffer coming off the tailpipe was being pushed backwards. It is very difficult to secure the sniffer in the factory tip. Ideally, it is best to collect afr data right before the cat. Collecting after the cat is ok, so long as you realize that it will read leaner than it actually is. Based on the logs I have from you from the factory widebands right before the cats, the afr was on target (11.8) all the way to redline, which is exactly where we want it to be.

It has been our experience as well that the bends right before the tip on the factory muffler significantly throw off the readings at higher power levels.

At stock power levels, the exhaust gas velocity is low enough to make the turn and still flow somewhat evenly (at least unnoticed by the sniffer). However, as exhaust gas velocity increases at higher horsepower levels, the exhaust gas has more momentum and therefore, doesn't flow through the bend evenly. Therefore, at higher power levels, the sniffer is reading a small quantity of fresh air that fills the low pressure zone created on the inside of the bend between each pulse of exhaust gas.

We've tuned hundreds of Z's and witnessed this phenomenon, time and again.

Here are some visual aids to help you guys understand what we're saying here: