3% CO on LM-1

[PorscheGAL]

I've attached an LM-1 and I have found my idle to be rich at 11.6 AFR. I know the conventional wisdom is 3% CO and a AFR of 14.1 at idle.

My question is how do you determine that the CO is set at 3%? Is it found by reading the LM-1?

[911nut]

On my car 3% CO resulted in a 13.5/1 AFR on the LM-1 (measured at 900 rpm).

[mark houghton]

Originally Posted by 911nut

On my car 3% CO resulted in a 13.5/1 AFR on the LM-1 (measured at 900 rpm).

That sounds about right. I usually try to run at around 2.5% or so, and my AFR's at idle are around 14+ a tenth or so...depending on the mood of the CIS.

[PorscheGAL]

Thanks. I have a follow-up question for the replies to better understand the relationship between AFR and CO.

If I set my AFR to 14, does that mean my CO is at 3%? Or can there be different AFRs results at 3%?

[911st]

My two cents for what it is worth.

Two reasons to run CO fat at idle. One is to try compensate some what for the lean surge that comes with applying the throttle as CIS dose not have anything like an accelerator pump to keep the AFR's near an ideal 13.2 for max torque. Some CIS motors do have provision for lowering control pressure with acceleration to try to minimize this. The 3.0 turbo through 77 did.

The other is it adds some fuel to the AFR's on boost to help minimize the lean section that comes in the later part of the RPM's.

I would not be to concerned with a fat idle unless one spends a lot of time at stop lights. I would be more concerned with my AFR's on cruse if it is a car that is going to see a fair amount of street driving.

It might be possible to dial in the AFR's for max TQ off idle and off curse with acceleration and then plug the O2 back in to bring them back for better fuel economy. I it just depends on how far one goes as to if the Lambda computer can compensate that much. The when the TPS indicates acceleration, the 02 would be pulled out of the loop and one would have there proper acceleration fuel.

I built my own electronic WUR using an Andial Frequency valve that lowered control pressure with acceleration to moderate the lean surge when still keeping my idle and cruse AFR's in the 14's. The new D-WUR should be able to do the same thing. This can also be done by converting the WUR to vacuum sensing.

[WinRice]

One of the many handy charts out there:

The MG & MGB Experience: Library: CO% to Air Fuel Ratio (AFR) Table

FWIW around 14.0 seems to be the sweet spot for mine, like others mentioned.

Any richer and seems to get hard to start occasionally, with a little flooding.

[Chuck Jones]

When I had RarlyL8 re-do my exhaust system with one of his custom builds, after reading as much as I could on the subject, I installed the Leaske WUR along with his solenoid kit....some say you really don't need the solenoid. Regardless of your view on the matter I've been able to get an AFR of 14.5/7 at idle (after proper warmup of course), and a cruise AFR of between 14.2 and 14.7 since it does tend to fluctuate a bit. I was unable to attain these AFRs prior to the installation of the Leaske WUR....not that the WUR is the key to the variety of problems I see some folks encounter here...but it was for me.

I also run an LM-1 in the cockpit just to keep an eye on things, but it's not a permanent mount. I am informed by my indy that I really need to adjust the settings twice a year...once as summer comes on and again as the fall and colder weather approach. Here in California, we don't get the wide disparity of temperatures that you folks do in the northern states, so I am fortunate that I can drive the car year round.

[beepbeep]

Originally Posted by 911st

The other is it adds some fuel to the AFR's on boost to help minimize the lean section that comes in the later part of the RPM's.

Idle CO screw doesn't do much on boost. It's primarily there to adjust idle emissions. Once on roll, AFR's aren't affected by it much.

[Chuck Jones]

WRice: Thanx for that link to Townsend's AFR/CO conversion table. I've saved it off for future reference. You can also find the link by Googling Townsend AFR CO conversion. Good info to have.

[911nut]

Originally Posted by beepbeep

Idle CO screw doesn't do much on boost. It's primarily there to adjust idle emissions. Once on roll, AFR's aren't affected by it much.

Adjusting the mixture screw changes how much of the slits on the control plunger are exposed in the control plunger barrel, which routes fuel to the distribution chambers. End result is that more or less fuel is is flowed to all injectors for a given deflection of the air flow plate.

[mark houghton]

Originally Posted by 911nut

Adjusting the mixture screw changes how much of the slits on the control plunger are exposed in the control plunger barrel, which routes fuel to the distribution chambers. End result is that more or less fuel is is flowed to all injectors for a given deflection of the air flow plate.

Boy oh boy, has this topic been hashed out a zillion times before. For what it's worth, general consensus is that the mixture screw has only a small impact on AFR's across the band. Larger impacts are the result of WUR control pressure settings and ambient temperatures. If, for example, you're too lean at cruise or under boost, no amount of screwing in that mixture screw will solve the problem.

Set your CO%/AFR at idle using the screw, and set it at or near 3% for best idle to low end power response. Start there, listeng to and feel how she runs, and adjust as needed. So much also depends on how stock your motor is.

[911st]

Agreed, a fatter CO is more about throttle response.

My heavly modified 3.3 CIS used to lean to above 15/1 on first accel off idle. Setting it up so I went instantly to near 13/1 helped my throtle response.

It is interesting that per the reference above, 3% CO is about 13.3 AFR. I believe a 911 motor makes it's most power at about 13.2 so a 3 to 3.5% CO is near perfect for normally aspirated acceleration on a CIS motor. Not good for cruse MPG, emmissions, or motor life.

If the O2 system can pull things back with a fat CO it might be the best of both worlds.

However, increasing CO has been a traditionally accepted first step toward helping to extend fueling a bit on boost for a mildly modified motor (muffler, ports, inter-cooler, .9 bar boost, stock or K27-7200 turbo). I forget what stock CO setting is. I suspect reseting the CO to 3-3.5% also increases the fueling about one half to one full point in the mid range up to until the metering plate starts to stall at which point the motor will still go lean quickly.

Not an expert, must what I believe so far.

[911st]

I love that MG CO table listed above. Great resource.

Looking at it closer approaching 3% SO seems a good compromise setting.

.5 to 1.0 CO would be a best fuel efficiency setting.

3.5 to 4% would probably provide for about the best normally aspirated acceleration. After the first lean surge it should maintain close to a 13.2 to 13/1 AFR pre boost.

However, doing this might exaggerate the early on boost enrichment AFR's if not addressed.

Just a guess.

[mark houghton]

Originally Posted by 911st

Agreed, a fatter CO is more about throttle response.

My heavly modified 3.3 CIS used to lean to above 15/1 on first accel off idle. Setting it up so I went instantly to near 13/1 helped my throtle response.

It is interesting that per the reference above, 3% CO is about 13.3 AFR. I believe a 911 motor makes it's most power at about 13.2 so a 3 to 3.5% CO is near perfect for normally aspirated acceleration on a CIS motor. Not good for cruse MPG, emmissions, or motor life.

If the O2 system can pull things back with a fat CO it might be the best of both worlds.

However, increasing CO has been a traditionally accepted first step toward helping to extend fueling a bit on boost for a mildly modified motor (muffler, ports, inter-cooler, .9 bar boost, stock or K27-7200 turbo). I forget what stock CO setting is. I suspect reseting the CO to 3-3.5% also increases the fueling about one half to one full point in the mid range up to until the metering plate starts to stall at which point the motor will still go lean quickly.

Not an expert, must what I believe so far.

What ever became of the dixie cup add-on to the metering plate, to help with early on enrichment? I seems somebody machined one and tried it out. Any concrete results beyond opinions and guesses?

[911st]

I wonder to. It is difficult to get a basic car with before and after test results.

Adding volume above the metering plate is concept I learned from a CIS consultant I paid a retainer to when developing my car. That "velocity cone" as borrowed from I believe an AMG tuned MBZ, may or may not work as such. The maker's sale was not increased fueling as I suspect but was increased air flow. Not sure if that flies unless it reduces turbulence around the metering plate. Been wondering about that for about 7 years now. But what do I know, I am just a want to be engineer I guess.

The best.

[mark houghton]

Originally Posted by 911st

But what do I know, I am just a want to be engineer I guess.

There's a little bit of engineer in all of us. You've got to at least have that mindset in order to drill down into the finer points of tuning. One thing for sure, all things automotive operate within sound logical patterns; it's all cause-and-effect. Understand it, put it all together, and become a valued resource. Building knowledge will never end and how we apply it is what really matters. Sometimes I sure wish to crap I knew what I was doing.

So much for Saturday night philosophizing

[cole930]

The dixie cup is on my CIS at Turbokraft in AZ. I just sent Chris an E-mail to get it back to me. If Keith had a turbo to try it on I'd send it to him when I get it back.
Brian was making a CIS test set up he might trial it for us. Chris broke a piston before we could get any comparative data.

Cole

[beepbeep]

Originally Posted by 911nut

Adjusting the mixture screw changes how much of the slits on the control plunger are exposed in the control plunger barrel, which routes fuel to the distribution chambers. End result is that more or less fuel is is flowed to all injectors for a given deflection of the air flow plate.

Yes. A milimeter of difference on plate on cone is going to make major change on idle CO. To the point of motor not idling at all. But it won't put a dent on AFR curve on boost. Don't believe me? Try it with WBO2 sensor...

[mark houghton]

Originally Posted by cole930

The dixie cup is on my CIS at Turbokraft in AZ. I just sent Chris an E-mail to get it back to me. If Keith had a turbo to try it on I'd send it to him when I get it back.
Brian was making a CIS test set up he might trial it for us. Chris broke a piston before we could get any comparative data.

Cole

I knew someone was hording the goodies....just forgot that it was you, Cole.

[PorscheGAL]

Originally Posted by WinRice

One of the many handy charts out there:

The MG & MGB Experience: Library: CO% to Air Fuel Ratio (AFR) Table

FWIW around 14.0 seems to be the sweet spot for mine, like others mentioned.

Any richer and seems to get hard to start occasionally, with a little flooding.

Thanks for all the input guys. Also, this chart is very helpful!