Interesting learning point on AFR`s and MAF`s


Active Member
Afternoon all,
Just thought i`d share this experience with you lot, and mabe you could check your intake system if you don`t have access to a wideband sensor.

I`ve done a fair few changes to the intake / fuel system so i wanted to see if my air fuel ratio`s were still ok.
Checked them and found them to be a little lean but also fairly unstable. The plot below shows wide open throttle from ~2300rpm:

This morning i was going to map out the lean spots but comparing to some older plots i had, i wasn`t happy as it used to run richer when initially mapped.
i`ve had experiences with air filter adaptors not fitting the maf right in the past and causing lean spots in the map.

i checked out the maf filter adaptor on mine and holding up to the light, you could see daylight through the join, albeit a very narrow gap.

I found an old maf on the garage and it had a much better adaptor on it, one that matched up perfectly to the contour of the maf and completely sealed it along with a cheeky bit of instant gasket.

Done a few more logs and the afr is now as stable as it used to be and richens up nicely.
did realise what a big difference a gap less than 0.5mm wide would make, but i must have changed the airflow through the sensor enough for more air to get in than what it thought.
Heres the comparison showing the difference just changing the intake adaptor and fully sealing it made:

It maybe just worth checking how your adaptor is, as if you don`t have a wideband sensor, and it is not fitting/ sealing correctly then your motor may be runner leaner than you think.

Hope its useful,



Active Member
No probs mate. Still going to work on the map but atleast now I know the cause for it running leaner


New Member
Great work Ollie.

Just for a better "before and after" afr graph, you should have done both runs with minutes between each other. There are many non constant parameters which will change the AFR's such as ambient temperature and humidity.


those afr's look pretty good ollie to me in the second graph

in the first graph where its very unstable and leaning off around 3000 when turbo really starts to pull more air

just goes to prove that any unmetered air past afm can cause havoc with irregular afr's
think il check mine out:-D

good info for people there:thumbsup:


Active Member
Trip, I always try to use the same piece of track and the same points to accelerate etc but as you say, the temperature and humidity can also factor. The results are that substantially different and the temperature of both runs was nigh on the same, that its fair to say that the adapter change was the major factor to the difference between the runs.

Bob, its wierd, I actually enjoy doing this stuff, mabe I need to get out more :)

Glad its being useful, it definately has been for me
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Bob, its wierd, I actually enjoy doing this stuff, mabe I need to get out more :)

your correct ollie it is interesting and very handy to learn as you know exactly how your engines running which in the longterm will save you a wad of wedge (plus mapping fees).

mapping is something ive never had the patience or time to learn tbh, i understand the theory of how it all works but it all gets a little more complicated when you have to throw in ign advance and balance that out with good afr's in order to avoid det or borewash.

i wish sometimes i had more time to play around with this myself but i dont understand all the computer jargon / parameter readings etc to enable me to set the thing up initially.
im sure it would be quite easy if you can understand that part though.