Air Fuel Mixture

Arghhhh!!!!!!!

Have you ever worked on a project so long and so many times that your threshold for patience is exceeded? That threshold seems to be when the number of four letter words used far exceeds the number of non-four letter words. Well, I am there. I would rather remove a tooth using shards of glass than continue to adjust my carbs. #103 uses three 2″ SU carburetors which are absolutely beautiful to look at but it also means three times the hassle!

Triple 2″ SU Carbs

There are big issues to tackle like trying to get the fuel level at just the right height in the jet and keeping it there reliably, determining what needle size not only works but is optimal, and adjusting mixture.

These items are all related to the goal of feeding the engine with just the right amount of fuel. To much fuel (the car is said to run rich) and power will suffer. To little fuel (run lean) and the temperature of engine components could rise to the point where devastating problems can occur…like burned valves or melted bits. If this happens you might as well buy roses for your spouse before breaking the news, get out the checkbook for the engine builder’s house and boat payments, and start crying in your beer. Just the right amount of fuel creates the most power without damaging the engine.

But how do you know when the engine is running rich, lean, or just right? One approach is to examine temperatures. A cylinder head temperature senser can consist of, for example, a little ring like device that goes around the base of a sparkplug. Another is exhaust gas temperature (EGT) sensors placed in the exhaust manifold. From this point all you need to do is gradually lean out the carburetor mixtures’ until the sensors detect significant temperature gains…but don’t go too far lest you need to think about flowers, checkbook, beers and tears.

Example Cylinder Head Temperature Sensor offered by AIM Sports.
Example Exhaust Gas Temperature Sensor Offered by AIM Sports.

Reading Plugs

Another approach is to “read the plugs” which means pulling the plugs after a race session and looking at the color, texture, and patterns on the ground electrode, center electrode and connected ceramics, and on the threads and ring nearest to the center electrode. The below figure from TuningMatters is a good start but there are so many other aspects to look for it boggles the mind.

It is an understatement to say that reading plugs between super rich or super lean can be compared to the black arts, voodoo, or spiritualism more than it is closer to science. When someone reads plugs it’s like a tarot card reading experience! The person holds the plug up and turns it with a critical eye, pulling it away and then closer and then away again, turning it around and ultimately looking at the entire plug, and then uttering “Hmmmmm”…but in a way that sounds neither overly positive or negative…just largely uninformative…but it sure sounds like they have decades of experience behind that largely uninformative utterance.

By this time several more people have surrounded the “plug whisperer” waiting on baited breadth for the eventual verdict. After the verdict is read aloud, the plug owner responds with either an equally uninformative “Hmmmm” or “Oh good” depending on the mysteries solved during the plug reading ceremony. I’ve never had my tarot cards read, but I do know there are a lot of charlatans out there, and this is the case too with spark plug whisperers. There have only been two people that I trust to have a plug reading…and I am not even one of them!

But here is the rub and the thing that really stresses me! Racecars are typically run at full throttle for as much time as possible per lap. By the time you get a plug reading after the session is over to know if the car is (was) running lean, you may have already caused damage. All the plug reading does in this case is let you know how big of a bouquet to buy, magnitude of the check to write, how big of a beer you need, and how many tissues to grab. I am always amazed at the callousness of plug whisperers…they see the pain in the car owner’s face from their message and are rarely ever sympathetic and, worse, never ever even offer up a beer as consolation!

Wham, Bam, Thank you Lambda

What is a shade tree mechanic to do? If you’ve read my other investigation articles, you will see that I have a genuine lack of appreciation, patience, and enthusiasm for ambiguity. I often hear “That is just the way we do it and that’s how we have always done it.” I say hogwash to that…let’s figure this out! Why not measure the air/fuel mixture directly? How can I measure air/fuel mixture directly?

A lambda sensor does just this. The sensor consists of a probe that is positioned in the exhaust gas flow and measures residual oxygen content. Not surprisingly, these are also known, more appropriately, as oxygen sensors. Modern vehicles use these to detect air/fuel mixture and then automatically adjust the fuel system to optimize the mixture.

Drivers did this by hand in the past but now the vehicle automates this process using a lambda sensor and a fuel control system. Lambda sensors require a little bit of control and need to operate within a specific temperature range…a “lambda control unit” does exactly this.

Bosch 4.9 Lambda Sensor.
AIM Sports Lambda Control Unit (LCU).

What data can a lambda sensor provide? Generally, sensors will indicate the air/fuel mixture measured as parts of air to parts of fuel with an optimal mixture being about 14.7 to 1 (it takes 14.7 parts of air to properly and efficiently burn 1 part of fuel…we are talking gasoline and not other types of fuel like methanol and diesel). More air than needed (higher than 14.7) results in the dreaded lean mixture that causes much crying. The figure below shows an air/fuel mixture gauge with a reading of 14.64…just a touch rich. The lambda value expresses this mixture on a different scale with 1 being equal to 14.7 and values greater than 1 indicating a lean mixture.

Example Air/Fuel Mixture Gauge.

Garage Testing

Well, maybe there is light at the end of the tunnel after all! I set the jet height at .08″ below the bridge, adjusted the float level such that the fuel level in the jet was about 1/8 to 3/16″ below the jet height, unscrewed the slow run valves 2.5 turns from the bottom and, then after warming up the car, set the idle at about 800.

I installed the sensor bungs on exhaust header pipes 1, 3, and 5…so that I can get data on cylinders 1, 3, and 5…which are fed by carburetors 1, 2, and 3, respectively. This sounds like an easy step but actually involved removing the headers and welding in the bungs. I installed lambda sensors in exhaust header pipes 1 and 3, LCU1-1 and LCU1-2, respectively. I will get data on exhaust header pipe 3 a little later.

The figure below shows the data provided by the lambda sensors when I ran the car in the garage for a few minutes. What is not seen is the earlier data where I adjusted the carbs to get both lambda values just below 1…about .98.

Two items are clearly visible. At idle the front carburetor is still a bit rich compared to the middle carburetor. Second, when RPM is raised and held there for a short period, the lambda value drops a bit which means the engine tends to go into a slightly more rich state, at least up to the ~1800 RPM tested here.

Results from Garage Testing of Air/Fuel Mixture

The scatter plots below compare lambda values against RPM for LCU1-1 and LCU1-2, including the data collected when I was adjusting the carburetors. The data generally show above ~2000 RPM the front carburetor is running about .84 lambda while the middle carburetor is running at about .9 lambda.

It is interesting to note that when checking jet height after adjusting the mixture, I had raised the jets to .07″ below the bridge to achieve the above mentioned lambda values at idle. A little research found that the best power is likely found with lambda values between .85 and .9, so for now I will adjust the carburetors to be similar to each other.

LCU1-1 Lambda by RPM Readings.
LCU1-2 Lambda by RPM Readings.

Garage Testing II

Just a little more fettling and tuning but now on the rear carb. The below data plots exhaust header 1 and 5 lambda values (LCU1 1 and LCU1 2, respectively) after I adjusted the mixture of the front carb and the rear carb to get them very close to 1 at idle. The engine was run up to 4200 RPM.

The data indicate the mixture becomes richer with an increase in RPM, just like before, with lambda values for 1 and 5 very close to each other. The great news is that both become rich with an RPM increase which means I am moving away from flowers, checks, and beers and tears. Data collection at the track in a few weeks will be really valuable for seeing how the mixture may change across the RPM range and at full throttle.

Preliminary Findings and Next Steps

So what does this initial garage testing indicate?

  • The initial jet and fuel level heights were pretty good, albeit a bit rich, but I prefer this over being too lean.
  • I am looking forward to testing throughout a greater range of RPMs, particularly between 3500 and 5800 RPM which is the normal racing range. Plotting lambda values by RPM will indicate how well the needles are metering the fuel across that range.
  • If the mixture is too rich in general (particularly at race RPMs), there are several choices to lean the mixture out. The easiest is to raise the jet slightly. A challenge with this approach is the little jet adjusting screws become more and more loose as the retainer spring tension is reduced. If the screw drops out the jet will raise all the way and, you guessed it, flowers, checks, beers and tears. A second approach is to lower the fuel height in the jet. A third approach is to use leaner needles.

I feel like I am further away from wanting to find shards of glass. Stay tuned for further testing results.

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