Porsche-O-Phile

Okay, title says it. I'm interested in learning/knowing as much as I can about ways of measuring the air-fuel mixture. As I'm sure you all know (or should) too-rich or too-lean mixtures come with their own subsets of problems and the ability to check for either condition is useful and a necessity for proper, high-performance tuning.

There's direct measurements based on so-called "lambda" sensors (measures the amount of O2 in the exhaust gasses). More O2, leaner mixture (unburned/unreacted), right? There are also indirect measurements such as CHT and EGT temps, although in my experiences these are more heavily relied upon in aircraft, where one is constantly re-adjusting mixture to varying altitude conditions. Most manuals actually have procedures to tune for either "best economy" or "best power" based on EGT values. I've never seen an aircraft with a lambda-type A/F gauge.

On dynos, there's often a "thingy-in-the-tailpipe" used to measure A/F ratio. How reliable is this? Does it work on the lambda principle or some other principle?



Share you knowledge guys, let's learn the truth about these things. I've always been a bit skeptical about the off-the-shelf A/F "lambda" gauges and would prefer to use EGT measurements to help tune, but let's discuss the advantages/disadvantages to both methods.

SoloRacer

The tail pipe sensor used on a dyno is a lambda 02 sensor. Tuning with an 02 sensor is critical in my opinion. EGT's don't change fast enough to tell you at what RPM/Load Point that your air fuel is off. If I was to guess why planes use EGT's I would think it is because they are more of a steady state environment or they use carbs. It's old school thinking and EGT's were the only thing you could measure and respond to. You have to datalog your 02 readings and then have the ability to make small, accurate changes at exactly the right places to make a difference. If you don't have the ability to datalog or make the changes then there is no reason to use an 02 sensor.

jgporsche

Not a fan of the tail pipe sniffer. It's not accurate at all. I have a 4" tip on my 944S. While dyno testing different chips we were trying to perfect the a/f ratio, but were starting to lose power after tuning. The sniffer said we were really lean. So we kept richening it, and it lost us power. I decided to go with the Zeitronix WB O2, and their was a HUGE difference between the Zeitronix and tail pipe sniffer. Problems are now fixed.

Porsche-O-Phile

Kind of what I thought about the tailpipe sniffer - I attended a dyno session recently that was getting massively inconsistent readings from one run to the next and between an onboard A/F gauge and the dyno's measurement (one of the reasons I wanted to bring this up in a thread).

How accurate and instantenous are the zeitronics-type A/F gauges? I'm trying to decide between a Zeitronics or a LM-1. The LM-1 is nice because it's useful for tuning any car (plug & play) although the benefit of an on-board system might be nice too. Is there a way to steal the data from the on-board O2 sensor and display it as A/F either on a gauge or datalogger?

What about the guys running open-loop (no lambda O2) mode on race cars?

jgporsche

My guess is they are pretty accurate.

at idle, it jumps around........

But under load it is stable.

jgporsche

here is video of what it looks like at idle on my 944S..... right click, save as

Ski

I've installed two LR UEGO units and we have one on the track car. That sensor that comes with the UEGO, with the control box, offer great and quick readouts. I personally like WBO2 kits, place the sensor in the test pipe or cat section, about 14-18" from the turbo. I have the Techedge and what I like about it is that I have a calibration that I can do with a DVM - some others may have that I've just not seen it.

The LM-1, that goes in the tailpipe - Jim L has one for his 930 and it works excellent as long as you use the supplied bracket. If you lay it in there or try to put it on a stick, pipe, or anything else, crazy readings.

Bri Bro

The O2 sensor should be jumping around (dithering) at idle because it is running closed loop. The catalytic converter needs to be run a little over lamda=1 then a little under lamda=1 to work correctly.

The reason planes use EGT is because they are bulletproof, unlike WB O2 sensors. Planes are not tolerant of malfunctions while in the air, normally bad things happen so everything is done to improve reliability. WB sensor will fail over time, EGT sensor last a very long time.

Diver944

I have recently started to use the Dynojet Wideband Commander and I highly recommend it. Not only does it have an analog gauge with a needle so you can see instantly where you are on the rich/lean side of things but it is easily connected to the USB port of a laptop to get meaningful data plots compared with RPM.

It enables you to instantly see the affect of changes to the fueling or boost so you can safely setup your car. I haven't yet compared it's accuracy with a professional WBO2 seup at my local Dyno but I will be shortly