air/fuel ratio gauge

[fred cook]

I installed an Innovate DB air/fuel ratio gauge in my SC over the weekend. Other than having to source a mounting bracket and build an extension for the LC1 cable, the installation was pretty straight forward and well documented. I've never had one of these gauges in a car before and did not know what to expect. I've got the CIS set just slightly richer than stoichiometric at 13.6. When I rev the engine, the a/f ratio stays about the same up and down the rev range. When I first open the throttle, it leans up just a bit, maybe up to 14 or so and when I let off the throttle it goes slightly rich, down to around 12. I haven't had a chance to drive the car yet, but I guess that I was expecting the system to get slightly richer as the rpms went up. So far, I haven't seen that. Of course, actually driving the car and putting a load on the engine will be different than just revving it up. Anyone have experience with these things?

Thanks,

[Ronnie's.930]

Fred, I have an LM2, and I can tell you that revving the engine at a standstill is going to show you nothing useful - you won't see any enrichment on the meter because without a load, the engine needs/uses very little fuel (even at higher RPMs). It looks like you already realize this, however . . .

[timmy2]

Sounds normal, post what you see when driving, I'd like to see the numbers.

[Reiver]

Are you tied into both banks of the engine? I've an AFR that is only getting results from one SSI so I took it to a shop to check the total CO so I could adjust properly.
I moved the gauge dial face to show the tailpipe CO.
Yours sounds normal...AFR is a moving target based upon air density and the engine temp/fuel metering. I like to see idle at 13.2 or so and run, when hot in 90 degrees about 13.50 to 13.75. on a Euro SC 930-10.

[fred cook]

Originally Posted by Reiver

Are you tied into both banks of the engine? I've an AFR that is only getting results from one SSI so I took it to a shop to check the total CO so I could adjust properly.
I moved the gauge dial face to show the tailpipe CO.
Yours sounds normal...AFR is a moving target based upon air density and the engine temp/fuel metering. I like to see idle at 13.2 or so and run, when hot in 90 degrees about 13.50 to 13.75. on a Euro SC 930-10.

No, only reads one side of a set of SSI headers. However, since all 6 cylinders are fed from a common fuel distributor (CIS) and all of the injectors are (nearly) new, there should not be much variance from side to side. This gauge is reading the exhaust on a newly built 3.3SS engine with twin plug ignition. The cams are the 964 units. Pistons are a set of Mahle 10.1:1 installed with a deck height of 1.1mm. I'm sure that it will seem to take forever to put a couple thousand miles on the engine so that I can put it on a chassis dyno and see what kind of power it makes!

[Reiver]

Originally Posted by fred cook

No, only reads one side of a set of SSI headers. However, since all 6 cylinders are fed from a common fuel distributor (CIS) and all of the injectors are (nearly) new, there should not be much variance from side to side. This gauge is reading the exhaust on a newly built 3.3SS engine with twin plug ignition. The cams are the 964 units. Pistons are a set of Mahle 10.1:1 installed with a deck height of 1.1mm. I'm sure that it will seem to take forever to put a couple thousand miles on the engine so that I can put it on a chassis dyno and see what kind of power it makes!

You would think so but that wasn't the case with my vehicle. I've a recently rebuilt 930-10 that runs perfectly, burns about 8 oz of oil in 3k miles etc.
The AFR gauge registered a co of 4/ 13.00 on one bank when the computerized emissions tester registered 3.2/13.31 out of the tail pipe.
Quite a dif. New AEM gauge with Bosch lamda.

[Vereeken]

My favourite subject !

Measuring CO or AFR levels at the tailpipe is not the best of ideas (certainly not at idle and lower RPMs). Look at the attached graph.

Having said that I have been running around with an LM 1 for years and I would be intrested in Freds observations on AFR at load and different RPMs.
Also at WOT and what happens to the AFR ratio when the throttle is suddenly closed.



Michel

[Flieger]

Well of course if you have a cat it will be different. Cats are designed to oxidize CO to CO2, and clean up the hydrocarbons and such. NO to NO2.

[tirwin]

Well of course if you have a cat it will be different. Cats are designed to oxidize CO to CO2, and clean up the hydrocarbons and such. NO to NO2.

Stupid question but I have to ask... Carbon monoxide is generally bad so I get why converting it to carbon dioxide is good. But isn't converting NO to NO2 a bad thing? Is that just an unintended consequence?

[Reiver]

Originally Posted by Flieger

Well of course if you have a cat it will be different. Cats are designed to oxidize CO to CO2, and clean up the hydrocarbons and such. NO to NO2.

No cat, just 2 SSI's feeding a sport exhaust....why would there be that much dif at the exhaust from the bung slightly upstream?
Every Porsche shop measure CO at the exhaust when setting CO....that's not good?

[Flieger]

Originally Posted by Reiver

No cat, just 2 SSI's feeding a sport exhaust....why would there be that much dif at the exhaust from the bung slightly upstream?
Every Porsche shop measure CO at the exhaust when setting CO....that's not good?

I was just replying to the graph which talks about a cat. I have a 2.7 RS spec so no cat.

[Flieger]

Originally Posted by tirwin

Stupid question but I have to ask... Carbon monoxide is generally bad so I get why converting it to carbon dioxide is good. But isn't converting NO to NO2 a bad thing? Is that just an unintended consequence?

My mistake. They work to put it back to O2 and N2.

[Reiver]

Originally Posted by Flieger

I was just replying to the graph which talks about a cat. I have a 2.7 RS spec so no cat.

I was talking about that too......I had a pretty wide variation in AFR/Co from my one SSI compared to my tail pipe analysis....I have no idea what a cat would do to the mix but it could only muddy the waters some.

[Flieger]

Originally Posted by Reiver

I was talking about that too......I had a pretty wide variation in AFR/Co from my one SSI compared to my tail pipe analysis....I have no idea what a cat would do to the mix but it could only muddy the waters some.

OK, I misunderstood your post. I am not sure why the variation. Cam timing different side to side? Maybe the heater boxes effect things like thermal reactors did?

[Reiver]

Originally Posted by Flieger

OK, I misunderstood your post. I am not sure why the variation. Cam timing different side to side? Maybe the heater boxes effect things like thermal reactors did?

I suspect you should really have a mix from both banks...AFR is really touchy anyway.
The engine has a fresh rebuild and runs like a top...even slight differences in injector performance could make a huge dif.
That was my point btw, if you run an AFR lambda on one bank only get your tailpipe CO analyzed for confirmation or deviance. Just common sense.

[fred cook]

I have a Gunson tester so I will be able to ck at the tailpipe. Will do that "soon".

[Vereeken]

The differences in co or AFR measured between the exhaust pipe and a bung before the cat are the same whether you have a cat or non cat car.

If you look at a 911 SC Euro you will find a Lambda probe hole in the exhaust before the part where the cat comes (on US cars).

If you put the LM 1 or 2 in that hole or in the exhaust you can find the same graph as depicted above.

As you will notice once the revs go up the lines fall in-line. At low revs or no load on the engine the exhaust has a tendency to suck in air at the outer edges of the tailpipe, which increases the air in the total ratio. Once the revs and load go up the problem goes away and the measurement is the same.

Yes most measure at the tailpipe as it is the easiest approach, but you will also notice that some have a tool to insert the sniffer as far as possible into the exhaust. Some exhausts are prohibitive

At the emissions control in Belgium some will ask you to rev to 2.5k when checking the co. They are aware of the problems with measuring at the tailpipe and the positive effect it has on Co.

All this to say that if you measure AFR at idle and you read 13.4, your are most likely a bit too rich in reality.

Michel

[fred cook]

My SC has been retrofitted with SSI exhausts and thus has no cat.