911 Racing Motors & Oxygen Sensors

Rhet911 · 04-26-2003, 03:14 PM

I was wondering if there were some expertise on this board with regards to oxygen sensors and their applicability to racing motors.

Specifically, K&N makes a kit to install a sensor on the exhaust system of a vehicle. This sensor is then hooked up to a guage that measures the air/fuel ratio. The advertizement says that it can be used for many different types of fuel with the exception of leaded fuel.

The reason I am submitting this post is that I have a mechanically fuel injected 911 that I am preparing to go racing with. It has been modified so I am thinking that if this K&N works as advertized, I should be able to easily dial in the MFI for my fuel requirements. I can monitor the gauge (at idle or under load) and make the appropriate adjustments. Apparently, they designed the kit for carburated engines so, I dont see where this is really any different.

Any thoughts on this oxygen sensor set up? Any advice is appreciated.

Thanks

Rhet

'90 911C2 & '73 911S

TimT · 04-26-2003, 04:17 PM

I know a guy that uses the K&N to dial in his MFI car. On the street he has a stock muffler, at the track he uses open pipes...

He gets to the track, removes muffler, installs megaphones drives a few laps and then adjusts the mfi..

snowman · 04-26-2003, 09:12 PM

Only problem is you really need a continuous , chart of the performance. Otherwise its still a guessing game. Dyno tuning is the only way to go at this time.

Wayne 962 · Wayne 962's Garage

Not necessarily true. The factory tuning procedures require a portable mixture measurement machine to be placed in the car and driven around. There are procedures to follow in the factory manuals for tuning and adjusting the MFI system based on these readings.

The K&N sensor and gauge is nothing more than an up-to-date version of the original tester. I think it would work very well in your car...

-Wayne

emcon5 · emcon5's Garage

Is this it?

http://www.knpowersports.com/docs/air_fuel_monitors.html

Tom

911pcars · 04-26-2003, 11:21 PM

Many sources say a regular O2 sensor is only accurate in the stoichiometric range - that is, around 14.7:1 air/fuel ratio. These 1, 2, 3 and 4-wire sensors were primarily designed to monitor and adjust the correct mixture for emissions purposes.

For performance tuning where the mixture is decidedly in the richer areas of air/fuel, a so-called "wide-band" oxygen sensor is more accurate. NTK is one brand. These are $200+ and may need special electronics to interpret the more accurate signal.

Sherwood Lee
http://members.rennlist.org/911pcars

jluetjen · jluetjen's Garage

Considering the limited number of adjustments on the MFI (at least at the track) , I would think that this would most likely be a usable application.

Jandrews · 05-06-2003, 02:48 PM

I have an exhaust gas monitoring guage and O2 sensor on my RS Spec MFI motor. I like it. I definitely responds to adjustments on the MFI pump, as well as changes to timing, dwell, air flow screws etc...

Ideally, I would like to have one installed in each heat exchanger, feeding left and right hand guages. I just have the one, installed in the passenger side exchanger.

Mine is not a K&N, but one I bought from some guy who races and designed is own. It seems very sensitive, and seems to reflect somewhere in the powerband, changes to each click on the part-load MFI adjustment. I would think it would be EXTREMELY useful in getting an MFI pump that is way out of whack close.

I also fatten my mixture up a bit at the track. My guage uses LED's to measure mixture, and I can tell from "relative" change in the lights, right about where I want to be.

Precise? No. But have I used it to adjust and do I keep an eye on it while driving? Yes.

JA

snowman · 05-06-2003, 10:20 PM

Guys,
What ever happened to EGT (exhaust gas temperature) guages??? They work no matter what type of fuel is used, they are inexpensive and highly accurate

Wayne 962 · Wayne 962's Garage

Jack, what would an EGT tell you?

JA, I doubt that having parallel gauges would tell you much of anything, except if a throttle butterfly is out of adjustment, or an injector is clogged...

Parallel gauges would be best suited for tuning carbs in my opinion. Although you can certainly get some use out of them with an MFI system.

-Wayne

TimT · 05-07-2003, 12:36 PM

Wayne,

An EGT is a helpful tool to use in conjuction with an O2 sensor. It is like a check and balance to compare both gauges. On cars with aftermarket fuel enrichment, or engine management systems, we tune the car with a wideband 02 sensor, and a EGT gauge. Then when the car is released the wideband sensor is replaced with a regular O2 sensor. Reading both gauges gives a true indication of the state of combustion.

i.e. if the a/f guage is giving an odd reading check the EGT, perhaps the a/f reading is an abberation. The EGT gauge is usually dead reliable, the a/f reading sometimes get crazy.

EGT corresponds directly to mixture lean=hot (power) rich=cool (less power)

Wayne 962 · Wayne 962's Garage

Okay, so the EGT is used to check the proper operation of the O2 sensor - I guess that makes sense...

-Wayne

snowman · 05-08-2003, 11:04 PM

The combustion temp is correlated to the mixture, thats how the old guys used to dial in their race cars in the days before O2 sensors.

ldangelo · 05-14-2003, 05:35 PM

Here is a link to a wideband O2 sensor/meter that I purchased.

http://techedge.com.au/vehicle/wbo2/default.htm

I have not personally used it yet (because my engine is apart for rebuild) but I plan on using it to help me dial in my new TECIII system when I get my motor back together. It even logs the data to a serial port (laptop)...

I hope someone other than me finds this link useful.

-LeoD