cliffyk

The short term fuel trims are only generated and used in closed loop operation, so yes it is always relative to 14.7:1 AFR. Our cars do not provide any injector flow rate or pulse width data via the OBD2 port, therefore the injector duty cycle cannot be read or calculated.

Your formula is a bit confusing to me, the flow rate per injector is:


where MAF = airflow in lb/h, so airflow/AFR = lb/h of fuel, then divided by 8 injectors.

This is the actual flow rate per injector. That divided by the injector's rated capacity = the injector duty cycle.

Keep in mind that Recon displays and logs the MAF flow in kg/h, you need to multiply those values by 2.2046 to convert to lb/h.

So assuming Recon says the the air flow is 800kg/h, and the AFR is 12.5:1:

800kg/h * 2.2046 = 1764lb/h, making per injector rate 1764/12.5/8 = 17.64lb/h (easy math because 12.5 * 8 = 100).

If you have 19lb/h injectors then the injector duty cycle = 17.64/19 = 0.928 = 92.8%.

This is why Ford switched to 21lb/h injectors in 2002, with the PI engine the 19lb injectors were running at 95+% duty cycle. With the 21s that dropped to 88%+, still a bit high but better.

swarthyfellow

So really what u are telling me is that if i want to know my A/R for sure i should be getting a meter. I have been doing some analysis of my 0-60 mph run last night, which was my fastest at just under 5.4 seconds, and for a A/F 14.7 i am at 80% injector duty. So if it is trying to make it anymore rich... which it was, it will be going over 80%. So it confirmed that i need bigger injectors. Just out of curiosity how much air will a bone stock non-pi motor pull?

cliffyk

You have to have a wideband O2 system to properly tune your engine. There's not really much more to talk about regarding fuel settings in SF until you get one.

At WOT, at which I presume you were, your engine was running in open loop mode and was likely running an AFR of around 12.0:1 to 12.5:1 if you have not changed the stock fueling settings in the tune. They tend to run a bit rich from the factory to minimise the possibility of knock with crappy fuel.

However, once again I will say that you should not be mucking about with fuel settings unless you have a wideband O2 system.

Here is some info about how much air our engines can consume. Using the 475cfm @ 6500 rpm number¹ for the 1st paragraph, at STP air has a density of 0.0749lb/f³ so 475cfm = 35.58lb/m = 2134lb/h.

It would take 2134/12.5 = 170.7lb/h of fuel to run a 12.5:1 AFR. 170.7/8 = 21.34lb/h per injector, so you would need injectors rated 21.34/0.8 = 26.7lb/h for them to be operating at 80% duty cycle.

But...

170.7lb/h of fuel would support 341fwHP, or 290 or so HP at the wheels.

The 19lb/h injectors used up until 2002, at 90% DC, can support 275fwHP(232rwHP); the 21lb/h units used in '02 through '04 would max out at 300fwHP(261rwHP).


All that said, the bottom line is that if you are running the original, orange, 19lb/h injectors they are indeed likely running at 95+% duty cycle at WOT and at peak HP.

At 262rwHP the 21s in my car were running at 90+%, I recently installed a set of 24lb/h EV6 injectors and the engine is much happier at the top end.

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¹ - An NPI engine would never come near 90% VE, 80% VE would be more likely. Also, all of the above are only very fundamental "rule-of-thumb" assumptions and formulae.