hobbsy
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| posted on 13/6/10 at 01:13 PM |
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Air to fuel ratios - what is optimal? 16:1 on light cruise ok?
I'm just about to sort myself out with a logging setup, all Innovate kit. An LC-1 for the wideband lambda and a SSI-4 to log throttle position,
rpm and injector duty cycle.
I've also got an XD-16 gauge to display AFR.
I've got the LC-1 and XD-16 gauge running for a couple of weeks now and although the higher rpm and mid to high throttle openings fuelling seems
ok (between 12 and say 13.5:1) at idle and some light throttle openings I'm seeing 16:1 or maybe even 17:1 which seems really lean.
I know stoich is 14.7 and max power is to be had at around 13.2 to 13.5:1 but I thought you shouldn't see leaner than 14.7:1 apart from on the
overrun.
That said I did a search and found this Autospeed article:
http://autospeed.com.au/cms/title_Tuning-AirFuel-Ratios/A_1595/article.html
Which says the following under its "Cruise" section:
"Light-load cruise conditions permit the use of lean air/fuel ratios. Ratios of 15-16:1 can be used in engines with standard cams, while engines
with hot cams will require a richer 14:1 air/fuel. If a specific lean cruise function is available, air/fuel ratios of 17 or 17.5:1 can be used,
normally at the standard light-load ignition advance."
I don't have the luxury of an EGT sensor to log how hot it is but then I suppose that isn't particularily expensive to add. K-type
thermocouple and some control circuitry (anyone know a good source - I might start a new thread after doing a search).
Anyway once I've got the logging up and running should I up the fuel in all the areas that come out leaner than 14.7:1 back to that figure?
Now I think about it at low rpm, constant small throttle openings you can feel the engine suddenly come alive - its tricky to keep one eye on the AFR
and the road but I *suspect* this coincides with the AFR going from 16:1 ish (where the flat spot is) to say 14 or 13:1 (where it suddenly picks
up).
Your thoughts please gents.
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RazMan
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| posted on 13/6/10 at 01:42 PM |
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I would be inclined to enrich your small throttle openings a little, bringing the AFR down to 13-14. In the absence of a EGT gauge it is better to be
a little rich than a little lean.
I find mapping with my LC-1 is really easy and I have been fine tuning for a few weeks now - it has transformed the driveability and saved me a
session to the dyno (about £200) after fitting lumpy cams. It can be a bit addictive though 
[Edited on 13-6-10 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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turbodisplay
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| posted on 13/6/10 at 03:22 PM |
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You can run lean at low throttle openings as there is not enough energy to cause an issue.
The problen with lean at high power is that there is an excess of oxygen so the piston starts to burn, so it gets hotter, and hotter until the piston
has a large hole in it.
My logs show very lean running of 17 - 18 : 1 at low throttle.
Darren
[Edited on 13/6/10 by turbodisplay]
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flak monkey
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| posted on 13/6/10 at 04:27 PM |
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Cruise is normally good around 15-16.
Max power is 12.5AFR BTW.
David
Sera
http://www.motosera.com
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turbodisplay
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| posted on 13/6/10 at 05:48 PM |
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Also just remembered to mention that if you tune the engine for, let say, 14.7:1 afr, this is the average of all the cylinders. As some cylinders can
get more air due to manifold design, etc you need to think how much richer you need, to ensure all cylinders do not risk being lean in the cylinder
with more air.
Darren
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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