A/F ratio meter really worth a darn?

[tpiini]

I'm just getting used to my new toy. It has a very stout 289 with a very lumpy idle. One of the P.O.'s had installed a Holley air/fuel ratio meter on the dash. You know, the kind with the lean/normal/rich lights.

Anyway, just by the exhaust smell I'm guessing that the car is running extremely rich at idle, but the meter shows it to be running lean. When I cruise it up, it'll switch to "normal" briefly, but mostly stays on the "lean" reading.

Anyone have any real experience with these things? I haven't been into the carburetor, so I'm not sure if it's been rejetted or not. Just looking for opinions on the worth of the meters themselves at this point.

Oh, and the O2 sensor is mounted at the collector on the driver's sidepipe. Is there an optimal place to mount the sensor? Will it read differently if mounted in different places in the system?

Thanks, Tom

[VitaminC]

Any unit (like the the Holley meter, the old Halmeter, the Cyberdyne, J&S's unit, etc) that is based on an everyday 02 sensor rather than a true wideband air/fuel meter and sensor, will confound and confuse you rather than give you data. There are now a number of true wideband air/fuel meter packages out there. The Dynojet Commander and the Innovate are two that come to mind, I believe they're under $400, and have lots of neat features way above and beyond just being a meter. Check 'em out.


Hope this helps,

Chris Weisberg
Carroll Shelby Enterprises
(310) 538-2914

[Tommy]

Tom -
I recently installed a similar meter in my car with the sensor mounted about three inches from the head on what appeared to be the leanest cylinder. I did a lot of Internet research to make sure I understood the limitations of the sensor before I tried to use the meter to tune the carburetor. The meter seemed to indicate a lean condition (1:15 FAR) at partial throttle cruise, and richer conditions (1:14 FAR) when the power valve opened. My research indicated that this FAR was not a problem at low power, partial throttle settings. However, I found the meter did not react to some jetting changes the way I expected. Wide open throttle (WOT) initially indicated slightly rich (1:14.4 FAR). After I increased the primary jets by 5 sizes and the secondary jets by three sizes, WOT indicated only slightly richer (1:13.8 FAR). I expected a more pronounced change on the meter.

At this point, I don't know whether the meter is inaccurate or my understanding is incomplete. I hope someone who understands these things responds to your question so we'll both know.

[Excaliber]

How about going to a local gas station and having the AFR checked against their computer (Sun?) diagnostic AFR checker?

I went from 75 jets to 68 based on "smell" and "burning eyes". The car was reported to have run 11.9? WITH the 75's and I ran 11.904 with the 68's. Therefore, I guess I "done good". It "smells" way better at least.

...... helluva way to dial in your jet size though.

[Mohuska]

I agree above with the post about using a normal O2 sensor vs a wideband O2 sensor. Normal wideband O2 sensors are not linear in their response and are very sensitive to temperature changes.

First, for any O2 sensor to read accurately, it must be mounted in the collector, downstream from where your headers join together. Don't every place an O2 sensor in one cylinder, if you are going to get anywhere near accurate readings.

Second, many O2 sensors do not have a heater element in them. A heater element is necessary to ensure that the unit is always within the tolerable bandwidth of temperature for the sensor to respond. Too cold or too hot and it will not be accurate.

Third, There can be no air leaks in your header system. This will draw air upstream from the sensor and show an abnormal lean reading.

Fourth, quite often, a sensor can show lean when it is actually running rich. If you are smelling unburned fuel, and your O2 sensor is showing lean, lean out the mixture until you are showing stoicmetric readings. For normally aspirated engines, the stoicmetric AFR is 14.64:1 for gasoline.

If you really want to get accurate readings, mount a wideband O2 sensor in the collector....either side is OK...or there are systems where you can mount one in each side and use a toggle to flip back & forth. One of the best units I have seen is the FJO Racing unit....there are many options...but this is a pretty good one....

One more thing....when choosing O2 sensors, try to use Bosch ... It is the most linear of the available O2 sensors.

Mike

[RICK LAKE]

TPIini Tom couple of things Unless you are running FI where the computor is adding and taking fuel from the motor, you are not going to get an accurate reading. The reading will change by you pressing the gas pedal. If you run rich for a short time you will foul out an O2 sensor and it will lose it's ability to work properly. A wide range O2 which I have in my Speed-pro system works great. There is no heated O2 sensor but putting the sensor in the best place for the most accurate reads is 6-9" after the collector. The sensor as to reach 600F before it starts to work. Some race gases and additives foul out the sensor and give wrong readings.You should get an EGT module with 2 sensors to check both banks for them both working around the same temp. Low temp means too rich,high temp too lean. Then compare it to your A/F ratioand make adjustments from there. A good A/F is $400.00 min with a wide O2 sensor. You have to weld the bung into the exhaust, EGT sensor can be clamped on to the exhaust pipes. By running the motor lean NO MORE THAN 1 MINUTE, this helps clean the O2 sensor ( operating temp) Make sure that any carb cleaner or FI cleaner is O2 safe. If not it will leave a film on the O2 and wrong reading. Bosch makes the best after market O2 sensor but with factory FI system go with the factory one for that system. It's like putting Autolite plugs in a GM motor. It runs OK but not right. Good luck Rick Lake

[phantom03]

Install a Bosch heated style sensor and you should get good readings. It needs to be heated to keep it clean. Mohuska has it correct.

[Sizzler]

Anyway, just by the exhaust smell I'm guessing that the car is running extremely rich at idle, but the meter shows it to be running lean. When I cruise it up, it'll switch to "normal" briefly, but mostly stays on the "lean" reading.

By definition? (more like by design), a lumpy idle is caused by the engine running lean at idle due to too much exhaust scavenging. The cam is designed an rpm range far above idle, and far above idle only. That's why your a/f meter is showing lean combustion at idle.

And yes, it takes more than you might think to move the a/f ratio much at WOT. Computer controls, where you can actually set the desired a/f ratio in concrete so to speak work best. If you then examine the datalogs after changes to the a/f tables, you'll see just how much more fuel needed to be delivered to get those numbers. Amazing.

[mr0077]

Check out this link, some interesting reading on using O2 sensors for tuning.
http://www.bob2000.com/carb.htm

[tpiini]

Good information, all.

Thank you & keep it coming.

Tom

[Byots]

nm .

[Power Surge]

I don't post a ton here, so many of you may not know who I am, but to keep it short, I own a performance Ford shop in NY and specialize in dyno tuning for both EFI and Carbs. Just a little disclaimer before I type my response, that I know a thing or two about A/F and tuning .

A quality Air/Fuel meter is probably the best money you could ever spend on your car. Obviously proper A/F is essential for a heathly motor at WOT, but knowing your A/F at idle and part throttle can drastically improve driveability as well. There are many A/F meters on the market. The ones you want to stay away from are the ones that tap into stock oxygen sensors. That's probably not an issue for carb guys, and they'd need a complete kit, but I say this for the benefit of the EFI cars here. You want a true wide band A/F sensor setup. Now there are lots out there, and I've used most of them. Right now the hot setup is the Dynojet Wideband Commander. This is the most complete setup on the market, and it's got a great price as well. It comes with a gauge you can mount in the car, but if you don't want an extra gauge, it can also datalog in real time, which you can then download to your home PC and check out the readings. Full info on the Dynojet WBC can be seen at www.widebandcommander.com. I sell them and I did a group purchase for them on the FFR forums not long ago. I can get them for anyone, but I really don't want to get too much into that here because I'm not a supporting vendor. But I'm more than happy to help anyone out if they want to contact me privately.

As for the tuning end of it, tuning carb motors is not hard, just time consuming, since you have to pull the bowls on and off to change jets. And you guys are correct, that big jet changes seem to make small A/F changes. Here is an example... I had a Superformance on my dyno last week. It had a 392 stroker in it. The owner (who posts here and may chime in) had a bunch of head work and new cam and such added to the motor, but never dynoed it since the new work. First pull on the dyno, the car was over 14:1 A/F, which is too lean. Optimum A/F for a pump gas NA motor is about 13.0-13.5. He was running a Speed Demon 650 carb, and it had 70 main jets on the primary side. I switched up to 80s, and the A/F only dropped down to 13.2. Luckily that was the perfect A/F, so we left it like that. The car also picked up about 10hp and 15 ft lbs. Now aside from the WOT tuning, I noticed the car was very "lazy" off idle as well, and it didn't have good part throttle repsonse. A/F showed it was really lean at idle. I adjusted the idle mixture screws until I got about mid 14s on the A/F (this is where EFI cars run when in closed loop as well). After adding in some idle fuel, the off idle throttle response was DRASTICALLY improved, and it was much easier starting the car off in 1st gear now.

This is a few basic ideas behind tuning and A/F meters. I hope I helped shed some light on it for people, and I'm always happy to help you guys out if you have tuning questions.

Sal

[Byots]

nm .

[Buzzmobile]

Are there any systems out there that do NOT have to have a bung welded into the exhaust? And I am hearing an aweful lot about the EGT tuning now...

I want something that I can use to tune my car without going to a dyno all the time. In fact, I am not sure if I have a dyno close here in Tulsa that does this. Would a shop with just an analyzer and no dyno be able to tune my 2X4 rig just as well as a dyno. I know you have to have loads on at times to do it???....

[Loadco]

Check out www.innovatemotorsports.com. The manufacture the LM-1 Air Fuel Ratio Meter that comes with a wideband O2 sensor. Excellent piece of equipment.