How to select a proper AFR for your engine

Posted by andre perras on

What AFR is appropriate for my engine?

This article assumes a basic understanding of AFR Fundamentals and will focus on applying the basic knowledge to practical application.

It is our opinion as well as strong recommendation that tuning only be performed by qualified individuals, using proper tools and training.  This information is to be taken and used at your own risk.  You are solely and entirely responsible for any damage done to your engine through the use of this technical article as well as verifying whether or not is is applicable to your set-up.  Each and every engine as well as the unique requirements of each application are different.  Understanding the fundamentals of AFR is essential to yielding satisfactory results.

Some important considerations need to be taken before even beginning to map an engine.  Prior to investing time into mapping an engine, the tuner must understand the main variables affecting the target AFR.

  1. Fuel Type;  Depending on what type of fuel is used, you may see different mixtures being ideal.  For this article, we are focusing on Gasoline (91-94oct and our AFR charts are taken directly from a running RB26DETT)
  2. Camshaft Overlap, Duration;  Camshafts with very little overlap, short durations, and gentle ramp rates allow very high vacuum (Suction) forces to be attained at low engine speeds.  This equates to smooth idle and low engine speed operation.  High overlap, long duration and aggressive ramp rates are suitable for high engine speeds as they promote proper filling of the cylinders at high engine speeds - of course this also means that idle quality and low engine speed is sacrificed.  Engines with low vacuum have poorly filled cylinders as well as disruptive airflow in the intake, and thus require more fuel to idle smoothly.  (Big cams = Richer idle AFR)
  3. Engine Power;  Engine power will also have a fairly large impact on what the appropriate air fuel ratio would be.  Engines can be leaned at idle with no negative effects other than poor idle quality.  Engines making high power running lean can lead to melted pistons, knock, blown head gaskets, cylinder wall damage, and all kinds of things you really don't want to happen.  It's very important to understand where this limitation is so that we can extract the most power out of the engine without damaging anything, all while remaining fuel efficient.
  4. Knock Threshold;  Understanding knock is fairly complex for the typical enthusiast.  We won't get into the fundamentals of knock in this article, but we will say that the air fuel ratio will have a very important relationship to the knock threshold on knock-limited fuels.  Knock will destroy your engine, without a doubt, so it's important to understand how the air fuel ratio can be used as a knock deterrent.
  5. Engine Speed;  Engines turning at low speed can run at leaner air fuel ratios than engines at high speed.  This is simply because the engine isn't exposed to as many combustion cycles at lower engine speed as it would be at higher engine speed (assuming time is the same in both cases).  Less combustion equals less heat, and thus we may run leaner air fuel ratios without much adverse affect.
  6. Engine Load;  Engine temperature increases significantly as engine load increases assuming all other variables are constant.  Higher engine temperatures can be kept under control with richer air fuel ratios (cooler burn)  Engines running at cruising speeds can be leaned significantly to the point where we can see about 5% fuel economy with no loss of power or drive-ability in most cases.
  7. Engine Coolant Temperature;  Cold engines require quite a bit more fuel to run smoothly.  This is mostly due to the fact that fuel atomization is poor at cold temperatures - Fuel tends to want to condense when it's cold, and evaporate when it's warm.
  8. Cylinder Wall Temperature;  This is really only important during cold or hot starting.  When you read coolant temperature, you are getting a rather delayed reading of the engine's temperature after the heat has soaked through the cylinder walls, been absorbed into the coolant, and then into the coolant temperature sensor.  One thing that most people really don't ever consider, is that an engine's cylinder wall temperature changes extremely rapidly and significantly in the first 15 seconds of a cold start.  The AFR required to idle and run properly will also change just as rapidly as a result.  Cold cylinder walls need rich mixtures.
  9. Idle Stability;  In many cases, the idle stability will determine where we set the idle air fuel ratio.  This is largely dependent on the camshaft design, the valve overlap, and the ability of the injectors to deliver very short pulse width injections (Somewhat difficult with large injectors), as well as the injector flow balance.  If one injector has 10% less fuel injected at idle, we may need to enrich the mixture for that one injector or the entire bank / set to get a satisfactory idle.

Below is about what you would expect to see for a gasoline (94 octane) powered RB26DETT Skyline GT-R.  For those of you reading that aren't familiar with metric scales, about 100kPa is atmospheric pressure (Zero for you guys using vacuum/boost gauges in imperial scaling), and about 240kPa is 20 PSI Boost.

As you can see, there are a few trends in this particular map.  We see a rich spot around where the engine will idle (20-50kPa and 750-1500 RPM), and then a lean spot above idle at approximately 3000 rpm while the engine is in vacuum (Not running in boost) - This is what we call cruising speed.

The rich area at idle in this engines' case was necessary to get the idle smooth.  You'll often find that engines running at idle tend to sputter, misfire, shutter, and just generally not feel right.  This is incredibly obvious in almost every RB26 with factory individual throttle bodies (For reasons we will get into in another, yes another article)

You'll want to be careful you don't enrich the idle mixture up too much, otherwise you risk fouling the spark plugs and getting the inside of your engine  contaminated with carbon deposits - left behind due to rich mixtures.  We seem to see this limitation around 12.5:1 for idle, any richer than that for smooth idle operation and you need to be careful.  It's not really something that can damage your engine, but it will cause problems. There really is no adverse effects to leaning the engine at idle, other than potentially losing the idle stability, and reduction in power (we don't really need much power at idle), so we basically set the idle as lean as it wants to run with the upper threshold being ~ 15.5:1, and give it a little buffer.  Example; If the engine begins to idle poorly and stumble at 14:1, we will set the target just below that at 13.3 or 13.5:1.

The area of this AFR table that shows the cruising area of the map (3000 rpm row, columns 50 to 90 kPa) show slightly leaner than stoichiometric AFR.  We typically will find the RPM at which the engine will be at cruising speed using the dyno and will adjust these cells.  The engine speed at which this occurs will be determined by the road speed.  In an R32 GTR, because of the transmission gearing, the rear differential gearing, we typically see these engines cruising at 2500-4000rpm - or about 90km/h to 130km/h.  Once we've identified the engine's cruising speed, we then adjust these cells to ~ 15.2 AFR (for gasoline) where we find the fuel efficiency is fantastic.  It is also important to note that engines cruising slightly lean will benefit from carbon deposit cleaning inside the engine.  If your cruising AFR is rich, you will likely have issues with carbon deposits as well as fuel dilution of your engine oil.

 

The area above cruising engine speed shown below is set to be slightly richer than stoichiometric, as we described in the numbered list above - engines spinning faster benefit from richer mixtures.

It's important to note that as much most of you enjoy hearing backfires, pops, bangs and the likes on deceleration, it is commonly known in the tuner world that this phenomenon will likely cause valve-train damage.  These explosions occurring during deceleration are caused by unburned fuel igniting during the exhaust stroke.  The rise in pressure can cause valve shims, rocker arms, and even retainers to be dislodged and result in serious engine damage (Example; dropped valve)

This is the reason why we lean out the mixture to near stoichiometric ratio when under high vacuum (The far left side of the fuel map)

As we approach atmospheric pressure, we enrich the mixture to about 12.5:1 as this is where we yield very good power.  We can certainly agree that our priority is engine power at 8000RPM, and not fuel economy - but we must also now consider the knock threshold as we increase engine load.

The highlighted section of the map below shows what we call "Low Boost".  This area of the map is where we begin to see the trend change significantly.  We can see the air fuel ratio becoming richer and richer quite rapidly as engine speed increases as well as engine load increases.

We do this because we're now getting to the point where one of two very bad things can occur.  

1.  Melting pistons

2.  Engine knock/detonation (91 - 94 octane fuel)

As we stated in the list at the beginning of the article, we know that lean air fuel ratios burn hotter.  35 degrees Celsius hotter inside the combustion chamber per degree.  While this doesn't necessarily mean your piston temperature will increase by the same amount, it will significantly increase.  As we approach the temperature at which aluminum melts, we must be very cautious and enrich the mixture to prevent that from happening.  

The other phenomenon we must avoid at all cost is engine knock.  The term detonation is also often used, which we find helps people understand it a bit better.  As cylinder pressures increase during the compression stroke, (more boost, higher compression ratios, etc...) the compressed air/fuel mixture becomes increasingly volatile.  When the spark plug fires, the air/fuel mixture ignites and expands - it's actually this rapid increase in pressure that instantly detonates the remainder of the air and fuel inside the sealed combustion chamber.  This phenomenon is extremely destructive and will often result in broken ring lands, damaged pistons, cylinder heads, bent connecting rods, damaged bearings, cylinder walls, and head gaskets.

One of the ways we mitigate detonation/knock is by enriching the fuel mixture.  This simply reduces combustion temperature and thus cylinder pressure.  

So - Basically, if you're confident your pistons aren't near melting, and you aren't knocking, you can run slightly leaner air fuel ratios.  If you're encountering knock, you can enrich the mixture to deter the knock.

Lastly, High Boost.

This area is where all of the fun stuff happens.  It's also where all of the engine damage occurs.  Having the incorrect air fuel ratio in this region for any amount of time in excess of 1-2 seconds will most likely cause damage.  Prolonged operation of any engine in this area will cause catastrophic engine failure.

We usually set this area of the map to be richer than the target when tuning, so as to not run it lean under any circumstance, as melted pistons and knock amplitude are much more likely to occur in this region of the map.  

So, to be conservative, you might want to set your targets to be about a full point below the values in this map, with the lowest value being 10:1, and then make small changes to reach the appropriate AFR.

There isn't much change in targets in this area of the map.  In fact, the only reason there really is a change in the higher load cells of this map is because pump fuel (91-94 octane) is prone to knocking at high boost levels.  If you are not using a knock limited fuel (example 116 octane) then you may run significantly leaner mixtures.  Be very careful in this portion of the map.  Any mistakes will certainly cost you a lot of money, frustration, and time.

 Once you're done building your AFR table, you should have a look at the 3D model to see how "smooth" it is, and see if the trends follow.  You shouldn't really have any unusual dips or high spots, other than the ones explained in this article.  

 


Share this post