S50 + freeflow exhaust + TMS chip = AFR concerns

[static]

So, in preparations for converting the car to Megasquirt2, i decided to datalog the stock (TMS chip) ECU AFR curves against RPM and MAP (manifold absolute pressure used by MS instead of MAF/HFM) I use Innovate LC-1 with it.

The results were shocking: i get about stoich 14.70 at 4600 rpm and 90% throttle, where it should be more like 13.00. Good thing i run 94 octane, otherwise i'd probably be pinging like crazy.
People who tune MS suggest that it could possibly be the non-standard free flow exhaust, and i know from talks with MarkD that free flowing exhaust requires more gas to maintain good AFR numbers especially at WOT.
(i have 2.5" mandrel single pipe with magnaflows). The log shows believable 15.00 AFR when ideling, so looks like nothing's wrong with the WBO2 sensor/signal.

If it won't ping, it definitely ain't so great for the power at the torque peak, so you guys better dyno your cars with some good WBO2 probe to see what's happening. :hitler:

EDIT: just looked at some stock e36m3 AFRs and it's somewhere around 14-13.50 between 4krpm and 5krpm

[matt]

OK, finally have some time to reply to this.

Bottom line: Everything you're seeing is totally normal, you don't have a probem.

Everywhere except WOT, you are operating at "part throttle" as far as the ECU is concerned. You're using part throttle fuel maps and part throttle ignition maps, which are tuned to work together to produce optimum emissions and fuel economy. This occurs at stoich (14.7:1 AFR) and relatively conservative ignition timing (to make damn sure it doesn't knock at stoich which will throw your NOx through the roof)

What are your WOT AFRs? If they are off, you may have a MAF configuration issue that's throwing off the readings.

Exhaust makes very little difference *in air fuel ratio* to a chip tune on a mass-air fuel system. The ECU has reads every bit of air that goes in... more air goes out, more air goes in, more air gets measured, more fuel gets injected.

[static]

just dug up JimC's response to exhaust change concerns along with others (back in 2002)

http://forums.bimmerforums.com/forum/showthread.php?t=50147

so basically he says that if exhaust had been substantially reworked, volumetric efficiency of the system is changed and a car requires a retune (for best results of course). Just changing a catback should be fine.

here is another one where NickG says that anything above 14.70 AFR at WOT is death to the motor

http://forums.bimmerforums.com/forum/showthread.php?t=15005

[cheades]

Stock S50 injectors are undersized when you start modding the motor, upgrade to 19 or 21lbs.

[static]

Stock S50 injectors are undersized when you start modding the motor, upgrade to 19 or 21lbs.

got that covered (24#), it's the tune, not injectros

[matt]

Bump because I edited post #2.

got that covered (24#), it's the tune, not injectros

So you're running 24lb injectors with a chip that's not tuned for them? That's a big no-no.

[Jason89i]

i agree with matt. i would check what the afr is at wot. my stock s50 is 14.6 at 80% throttle around 4-5k. full throttle at 4-5k is 13.4 and slowly drops below 12 above 7.5k

cheers, jason

[static]

i am running 24# , 3.5" HFM and TMS chip for this config

I will do some more datalogging and report back.

Matt: it does make sense that HFM should account for exhaust changes as it measures the amount of airflow (and MAT corrects for temperature to find real air mass).