Jump to content

Air/fuel Ratio For 2014 Gt500

Recommended Posts

Does anyone know what the correct ratio is for this car? Only mod on car is FRPP muffler

#M-5230-MSVTLB. On page 219 of 2014 FRPP parts catalog http://www.fordracingparts.com/download/catalogs/virtual/index.html#218-219/z

it states that a ratio of 12.5:1 is safe, and 14.1:1 is potentially damaging. The indash air/fuel gauge

fluctuates between 13.0 and 14.7. The car only has 6000 easy miles on it. Should this be checked

or is the 'safe ratio' different for the Shelby??

Link to comment
Share on other sites

Why lean makes more power but is dangerous

When discussing engine tuning the 'Air/Fuel Ratio' (AFR) is one of the main topics. Proper AFR calibration is critical to performance and durability of the engine and it's components. The AFR defines the ratio of the amount of air consumed by the engine compared to the amount of fuel.

A 'Stoichiometric' AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric, A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel. The stoichiometric AFR depends on fuel type-- for alcohol it is 6.4:1 and 14.5:1 for diesel.

So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture.

For Example:
15.0:1 = Lean
14.7:1 = Stoichiometric
13.0:1 = Rich

Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.

Let's take a closer look. As the air-fuel mixture is ignited by the spark plug, a flame front propagates from the spark plug. The now-burning mixture raises the cylinder pressure and temperature, peaking at some point in the combustion process.

The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.

There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power.

Link to comment
Share on other sites

It's going to fluctuate when at idle and cruising, that's perfectly normal. At idle mine is anywhere from 14.0 - 14.3 back and forth, and at cruise it's all over the place, including 20.0 when letting off the throttle. This is normal.

Take the car out, put your display to the A/F gauge, and put the pedal to the metal under full boost and you should see the A/F somewhere between 10.5 - 11.0 A/F ratio. At 15psi, mine is held steady at 10.5 A/F ratio, which is very rich.

Link to comment
Share on other sites

Your ECU will goal seek the proper AFR using the feedback from the O2 sensors (like any good digital fuel injection system). The issue comes when the air flow conditions (both in and out) are changed since that is often programmed into the system rather than measured. That is why CAI and exhaust mods sometimes require a new "tune".

Link to comment
Share on other sites

Thank you for the replies, never too old to learn something new. Did WOT for a second or two

in a 45mph construction zone and the mixture did read 11.0 briefly. Upgrading to a KB3.6l LC

this summer, so I want to make sure everything is working right before I void the warranty!post-42201-0-19470400-1426882342_thumb.jpg

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...