Author Topic: AFR's and Emissions - A little bit more info.  (Read 2696 times)

Ash-Lee

  • Global Moderator
  • 200BHP Senior Member
  • *****
  • Posts: 4208
    • View Profile
AFR's and Emissions - A little bit more info.
« on: 13:31, Tue 22 December 2009 »
This guide is based only on my experiance and research.

15+ (0.00 CO)  Cruise mixture .
14.7 (0.25 CO) Stoichiometric mixture .
14.5 (0.50 CO) Standard EFI idle.
14.0 (1.50 CO) Standard carb engine idle.
13.5 (2.50 CO) Richer idle might be needed in cars with longer duration camshafts.
12.6 (6.00 CO) Full Throttle.

Anything richer then 12.6 then power will start to fall. You can make this slightly leaner (12.9 for example) for better economy but i prefer to run 12.6. If only for extra cooling.

EDIT - This isn't specific to the R5, it's general 'rule of thumb' regardless of application. 12.6 to 1 is known to be the ideal mixture for maximum power.

Considering how it's almost impossible to get a carb to hit those targets 'bang on' (anyone with one of those digital wideband readouts will know, they flicker all over the place) i recommend trying to aim for between 12-12.5. There is no reason to go lower then 12, this mixture is to rich.

You can aim for 12.5-13 if you wish for better economy, i can't comment on how this effects higher boost users because the extra heat becomes a factor which might cause pinking, but it shouldn't if there is adequate cooling.


CO and HC readings, CO shows how much fuel is being burnt where as HC shows how much fuel isn't being burnt. If both are to high then the mixture is to rich, is CO is correct but HC is to high then there is a problem with the ignition system or some other engine fault (worn bores), cam timing might be out or there is a problem with carb jets not atomizing the fuel correctly (blockages).

Ignition advance, i myself leave it standard. It's a bit of a myth that more advance = more power and it can end in tears.