A/F Ratio

[mrjezza]

Does anyone know what the optimum A/F ratio is for a stock internal 318iS?

I'm guessing that due to emissions standards there is a bit of leeway to tune the A/F ratio to gain a bit of power.

Also, how would you go about adjusting the A/F ratio?

[D. Clay]

It's called "stoichiometric" and is a theoretical construct that may or may not actually exist at any given time. The fuel injection electronics attempt to maintain the ratio through various temp, airflow, an O2 sensors. It's about 14.5-15:1. There's a heater plate on the intake and some other odds and ends. To change this you need a chip or know how to trick the electronics or buy an aftermarket engine management sysytem. I don't really know much more about it and even less on how to repair and/or tweak it.

[mrjezza]

Yeah, I know that around 14.7:1 is the optimum when you're looking to optimise emissions but I'm also reading that the best ratio for power varies from engine to engine but in general is somewhere around 13:1 for NA engines.

Just wondering if anyone has had any experiences doing this kind of tuning on a fairly stock M42.

[tim_s]

on my car (UK car remember chaps), the standard setup was at mid-high 13s throughout on WOT and full load, with just remapping the fuel, around 13 was optimum, varied at different rpm between around 12.9-13.2 or so.
hth
tim

[mrjezza]

exactly what i wanted to hear thanks

[tim_s]

exactly what i wanted to hear thanks

thought i'd just add, even though my car's a uk model and so dunno how relevent it is, changing the AFR to optimum for performance (with no timing adjustments) was worth 4-5rwhp at most sites.

[dino245]

I thought I would add my 2 cents.
The factory computer will vary the fuel mixture above and below 14.7 to feed the cat converter a proper diet of gasses to perform the cat function. So at a steady sate condition the fuel mixture will go from rich to lean at a high rate something like 20-40 times per minute, I may be wrong on this number. What I have done to tune my SMT6 is to disconnect the O2 sensor to the DME which causes the DME to work in open loop. Just my two bits.

[tim_s]

I thought I would add my 2 cents.
The factory computer will vary the fuel mixture above and below 14.7 to feed the cat converter a proper diet of gasses to perform the cat function. So at a steady sate condition the fuel mixture will go from rich to lean at a high rate something like 20-40 times per minute, I may be wrong on this number. What I have done to tune my SMT6 is to disconnect the O2 sensor to the DME which causes the DME to work in open loop. Just my two bits.

yes and no. the motronic will operate open loop at WOT and max load, hence why the AFRs are under stoich on dyno runs. its only closed loop on part-load etc off the NBO2 sensor. it would be nonsensical to be closed loop on full throttle etc. if you had an SMT7 you could use it to alter the lambda signal as well as the load signal to run a little richer throughtout closed loop operation. as it is with your SMT6, with the NBO2 still hooked up you can just tune the timing on part load and the motronic will alter the fuelling to run at stoich, and tune both the load and sparks on open loop. do you throw a fault code with your O2 unhooked? my experience with lambda controlled cars is mainly from e36s onwards as here in the UK most e30 m42s run without O2 sensors. on later bmws removing the lambda probe causes headaches running-wise.

[dino245]

I have had no problems tunning with the O2 unhooked. I do get a check engine light on but it does not alter the running of the engine as far as I can tell. I have the O2 signal going to the SMT6 and use this to adjust the fuel mixture by running the data aquisition function in the SMT6 software and then I read through it and make adjustments to the map and then drive it some more with the O2 hooked back up. Seems to run pretty good but it still needs more work.

[tim_s]

I have had no problems tunning with the O2 unhooked. I do get a check engine light on but it does not alter the running of the engine as far as I can tell. I have the O2 signal going to the SMT6 and use this to adjust the fuel mixture by running the data aquisition function in the SMT6 software and then I read through it and make adjustments to the map and then drive it some more with the O2 hooked back up. Seems to run pretty good but it still needs more work.

yeah that doesn't surprise me really, dont think our ecus are as complex as the later ones. why did you choose to ditch the o2 though, i quite like the idea of still running closed loop but having timing control and just using the SMT to run a MAP or MAF and for open loop fuelling adjustments?

[dino245]

Oh I do run the O2 sensor. I just disconect it as I tune the SMT6 and then reconnect it after. I am using the smt6 to tune the Mustang injectors and the Ford MAF

[tim_s]

Oh I do run the O2 sensor. I just disconect it as I tune the SMT6 and then reconnect it after. I am using the smt6 to tune the Mustang injectors and the Ford MAF

ah ok, that figures. so you just tune the fuel to 14.7 on part load?

[dino245]

Yes, I drive the car with the O2 disconnected and save the data and then readjust the mixture and repeat until the A/F ratio stays with in an agreable range and then reconnect the O2 and enjoy. I still have some more tunning to do but have not had the time. This process would be eaier to do on a dyno but I dont want to spend the coin.