Theory: BASIC Tuning Guidelines


This article is being written to answer the most basic questions about what to shoot for when tuning an engine.  This is not intended to be absolutely what you must do – it’s intended to be a starting point for those who don’t know any better.


This article will assume you have read pretty much all of the Education section, particularly the article on Modes of Operation.  This article will assume you have a spark-ignition reciprocating piston 4-cycle (stroke) throttle-body fuel injected or multi-port fuel injected engine.  (If you aren’t familiar with these terms, click them!)

Basic Setup Guidelines

  • Make sure the ignition system is in good shape before trying to tune a vehicle.  Coil(s), wires, and spark plugs themselves must be in good condition.  Fouled plugs will ruin your day.  Improper heat range or gap will cause ignition issues that will ruin your day.  A rule of thumb is to go one step colder on plugs for every point of compression (i.e. 9.0 -> 10.0) OR half atmosphere of boost (7.75 psi)  and decrease the gap by one third (i.e. 0.045″ stock to 0.030″) for every step colder plug.
  • Make sure timing is correct.  “Timing” here means BOTH the mechanical connection between your crank and camshaft AND any adjustment of distribtor, CAS, etc. used to mechanically adjust ignition timing.
  • As dumb/obvious as this may sound, you cannot make adjustments on an ECU to fix a mechanical problem. Things like bent valves, damaged pistons, dead coils, defective injectors,  bad sensors, incorrect mechanical timing, etc. are not things that you can fix with a computer.
  • If the engine is operating in closed loop operation, it’s fueling behavior will be determined by the operation of the O2 sensor.  DO NOT TRY TO FIGHT THE O2 SENSOR.  Use the O2 sensor to guide your tuning activity i.e. try to get the ECM to make zero changes based on O2 sensor feedback
  • Do not try to tune WOT using a narrowband (lambda) style O2 sensor, which is the most common type.
  • O2 sensors can “lie” about the mixture.  LARGE camshafts and misfires are the most common culprits for this behavior because Oxygen sensors measure the Oxygen content of the mixture in order to infer lambda.   Large camshafts and misfires both cause “extra” oxygen to be present in the exhaust, which will cause a false lean reading.  If the ECM is operating in closed loop when this occurs, it will generally add fuel when no such trim is required.
  • If closed loop O2 feedback is working against you, turn it off.  If you have closed loop feedback turned off, you should monitor conditions with a wideband.
  • If you are dealing with a volumetric efficiency type system (i.e. TBI/TPI GM and others) it is a good idea to have your VE values resemble reality.  I.e. if you have 180% volumetric efficiency at idle to achieve stoich, this is bad.  Most “hot” naturally aspirated engines will achieve 85-95% VE, *in a narrow RPM range at WOT*  Some older engines with poor cylinder heads and manifolds will struggle to achieve a 80% VE.  Extremely modern engines will often see a peak VE close to 100% in places.  Motors almost always lose VE at low throttle angles/low MAP sensor readings due to pumping losses created by the restriction at the throttle body.  See the Speed Density article for more.
  • If you are dealing with a Ford that uses Load, it is a good idea to make sure your injector size resembles reality so your MAF transfer function and calibrated load values will resemble reality.  The MAF and LWFM articles cover this as well.
  • Looking at  a graphical representation of your tune should be a “pretty picture” not a bunch of noise.  Things aren’t going to be straight or perfectly smooth most of the time or you wouldn’t be tuning it but you should see trends.  It does not matter whether you are talking about a MAF or speed density or Alpha-N setup.  You should see clear trends.  The absence of trends or unexpected reversal of trends can often indicate a mechanical issue such as a fuel pump that has reached its maximum flow capacity, misfires, reversion, etc.
  • For measuring power, your butt dyno is wrong.  Use a repeatable performance measure, i.e. dyno, accelerometer, 1/4 mile track, etc.
  • Use all your senses particularly SOUND when tuning.

Basic Fueling Guidelines

  • Best emissions are generally achieved close or at stoichiometric.  This is generally around 14.7 AFR gasoline, or 1.0 lambda.
  • Best fuel economy is generally achieved between 15.5:1 AFR gasoline (1.05 lambda) and 16.2:1 (1.1 lambda) for port injected engines.  Newer cylinder heads with fast burn characteristics generally do better with leaner mixtues.  TBI setups generally need to run at least stoichiometric or richer.
  • Best power is usually achieved around 0.85 lambda (12.5:1 AFR gasoline) on modern cylinder heads.  Older heads generally require richer mixtures.
  • Forced induction engines run richer, mostly to combat knock.  How much richer will depend on the engine and conditions.  Except in rare cases, there is no benefit to ever running richer than 0.75 lambda (11:1 AFR gasoline)
  • Oxygenated fuels (Q16, E85, E98/Ethanol, Methanol, Nitromethane) require substantially larger volumes of fuel than “regular” gasoline.  If you have an option for stoichiometric ratio, use it.  If not, it is generally preferable to use injector constants / base pulse width modifiers instead of MAF transfer/VE to tune this out.
  • Almost all widebands on the market read in lambda but convert this to an AFR value for gasoline (where 14.7 AFR = 1.0 lambda) to display it.  If you are burning hexane, this is fine.   If you are running any other fuel, think of the desired lambda you wish to achieve and convert this lambda value to AFR gasoline.  I.e. target an AFR of “11.2 :1” to achieve a lambda of 0.77 with E85 at ~7.4 :1 AFR.
  • Most pump gasoline as of 2012 in the US is at least 10% ethanol, which means that a true stoichiometric mixture is closer to 14.1 than 14.7.
  • Summer and Winter gasoline blends can have dramatically different ethanol contents, especially in colder climates.  Different octanes and brands of gasoline can have a large variation.  Although somewhat outdated, see the gasoline faq for a more in depth discussion of fuel composition and why it matters.
  • If you are tuning the vehicle with closed loop O2 feedback disabled, make sure you tune such that the ECM will not have to make big changes to achieve its targets when closed loop is turned on.  This boils down to shooting for around 14.7 AFR (1.0 lambda) in areas where closed loop will operate.
  • Get AFRs around idle as smooth as possible in open loop without any feedback or idle troubles will happen.  Do not rely on closed loop to maintain fueling at idle.

Basic Ignition Guidelines

  • Your ECU expects the distributor/CAS/other-adjustable-timing-thing to be in a certain spot.  ALWAYS SYNCHRONIZE YOUR TIMING WITH A TIMING LIGHT BEFORE DOING ANYTHING ELSE!@#!#!!!
  • Mechanical factors (mostly combustion chamber volume, shape and design) are the primary factors determining optimal timing requirements.  Optimal timing is often referred to as “MBT” or Mean Best Timing.
  • Most naturally aspirated engines like to run between 24 and 36 degrees of advance @ WOT at RPM-of-peak-HP
  • It is often not possible to achieve MBT due to the engine knocking first.  Knock will destroy even the strongest engine.
  • Higher compression motors need less timing than lower compression motors.  Higher compression motors are more likely to be knock limited.
  • Forced induction motors need less timing as boost increases.  Forced induction motors are more likely to be knock limited.
  • Aggressive camshafts generally let you run closer-to-optimal timing than smaller camshafts.
  • Race gas and higher octane fuels generally allow closer-to-optimal timing.
  • At a fixed RPM, the engine will generally require less timing at higher load.  I.e. more throttle less timing
  • At a fixed RPM and load, the engine will generally require more timing with a leaner mixture.  (One reason to run a slightly richer mix is that you don’t need as much timing to effectively burn it.  There are plenty of exceptions to this and too rich can be a big problem too.)
  • At a fixed load, the engine will generally need more ignition advance as RPM increases until around maximum horsepower where timing requirements generally flatten.
  • Spark at idle is critcally important for maintaining a stable idle and not having stalling issues.  Too much spark will generally result in hunting/surging.  Too little will generally result in stalling or lumpy idle.  Spark control at/near idle is extremely manufacturer (and sometimes even ECM) specific.
  • You can tune ignition timing to some degree by reading plugs but instantaneous acceleration data and/or a dyno while monitoring knock is the best way.
  • The trap speed of a 1/4 mile run will tell you about power output but it will not tell you about specific RPMs, just overall performance.
  • Your “butt dyno” is totally inaccurate.