Influence of air temperature on AFR, can it cause lean running?

[S4turbo]

Hi,

I was asked an interresting quiestion yesterday by one of my firends with an AAN engine, GT30R, 7A cams +++. Standalone boost controller. He runs a chipped (mapped for him) ECU, and has installed a wideband AFR sensor.

During summer he recorded AFR ratios down to 10,8 approx, during 20-25 degrees outside temperature. Now during winter he reports AFR ratios into the 13.5 region.

Question is:

Can the drop in temperature affect AFR ratios in this way, caused by more air getting into the engine (colder, denser charge, colder IC)? I guess that during full throttle the Motronic does not use lambda values.

How does motronic use the MAF signal during full throttle?

Any other good suggestion on what can cause the lean running?

Regards, Johan

[Jamo]

I think this is a problem with speed density systems.

Just thinking motronic is uses a MAF which is air weight, aftermarket ECU's general use speed density

To be honest it's the other way round 10.8 is a very rich, you don't want it below 11.5-12.5

I'll have to dig out my info

[S2central.net]

assuming this is a motronic ECU - its just IAT compensation in there... Notionally setup for more richness with high IAT as an aid to knock prevention. 10.8 is too rich in my opinion.

[GazzaG60]

id agree 10.8 is a little rich for these engines. unless boost is very high.

a colder denser chage will have effects of AFR. that is a big flutchuation though and id never expect that much. seems strange.

do you have a graph.

is the car going 100% DC on the injector and showing 10.8 then leaning out after like pump or injector is too weak for boost levels.

[A80Avant]

but if that is the case,would a 13.5afr be good enough for low IATs? that would mean that the EGTs are quite ok with that mixture.

[GazzaG60]

on boost 13.5 is lean regardless. i run mine to 13:1 on the rollers. heat was rising then.

if i rememebr my gas laws properly the difference between a cold intake charge and a warmer intake charge boosted is very little. this is mainly due to the amount of air the turbo is dragging in not being able to get warm in the small amount of time it has before comresssion. ie even with warm ambinet air, air charge temps are similar. where IAT comes in is when the turbo is beyond efficiency hence fireball or the intercooler isn't working too well, the boosted air is too hot and so often extra fuel is thrown in to cool the charge. timing and boost may be lowered by ecu.

its about the right fuel and and timing combo. sometimes fuel is good at cooling othertimes timing is good at cooling.

no mention of EGT has been made above. if we ignore timing for now and presume its good. the 13.5:1 will have a higher EGT and power level than the 10.8:1.

[A80Avant]

so could the motronic be so effective in producing the same amount of EGTs but having such a big difference in AFRs by the change of IATs?

[GazzaG60]

not quite sure what you mean?

i cant see how motronic could or if it would fluctuate the AFR's so much as to target an EGT. from my knowledge the motronic has no way to measure AFR or EGT itself. it can see IAT and target stoich with narrowband but wideband is all map based wioth some maf algorithm calcs.

correct me if im wrong paul.

[A80Avant]

what i meant is that depending on a motronic map,and based on IATs, can the motronic fluctuate so much it's targets(whether that is boost or mixture) so that you can have a certain amount of power AND at the same time keep EGT's stable?

To ask this,im assuming that the above car has the same EGTs in both occasions.

If it cannot do that,then it is 100% certain that the EGTs are different and the car is having higher EGTs in winter

[GazzaG60]

id say the motronic could change its targets but only due to poor feedback from 02 sensor and the like. the ECu wouldn't expect that much change but as you may know a bad lambda can turn cruise into a chugging black cloud.

as there is no EGT feedback i cant see how it would manage to stabilise EGT.

id still go for fuel pump and/or injectors and 100% duty at some point and maybe only when cold.

also is the boost controller calibrated to atmosheric as this will change so maybe its seeing more boost on cold days too.

[A80Avant]

definitelly motronic cannot see EGTs but i thought if it was able to do that indirectly.

Say on high IATs it is making the mixture rich to avoid detonation
On low IATs it is making it lean because it can

Can those facts also give the same EGTs though? just by the differences it measures through the MAF and IAT?

[S4turbo]

Well,

seems like the problem is solved, a new MAF was all that was needed. The wires were not broken, but the values it gave obviously were wrong.

J