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Old 11-21-2016, 08:02 AM   #1
Join Date: May 2010
Location: El Segundo
Posts: 19
M10 Air Fuel Ratio Question

I recently rebuilt my 1984 318i M10 engine with the following upgrades:

Early block
92mm pistons
2.0 crank
Metric Mechanic "sport" head
Ireland Engr 292 camshaft
Ireland Engr Shorty ceramic header stainless downtube
2 1/4" exhaust, performance muffler
Stock new injectors
New 02 sensor

I now have 500 miles on the engine and just did the first oil change, engine runs fantastic, idles really well at 1000rpm and just passed smog here in Socal - whew.

My A/F ratio and Performance question is this.

I left the stock intake, AFM, Air filter and fuel injection system in place and have been concerned about running too lean. I installed an innovate wideband (new bung) over the weekend and was surprised to see that I am running at 14.7 at idle and part throttle and at 13 (roughly, varies from 12.5 to 13.5) at WOT.

With the new feedback from the wideband, it seems as if I have everything right where it needs to be and am happy with the current performance. Because I believed that I had to be running lean, I was planning on an M20 throttle body (and adapter, jeep TPS) and a microsquirt system. And the question: Am I leaving any additional performance on the table by not going M20/microsquirt? I seriously thought the current setup would've been too lean and put the engine at risk.

Thanks in advance for feedback.

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Old 11-21-2016, 10:55 AM   #2
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Join Date: Oct 2011
Location: Carnation WA usa
Posts: 3,771
Amazing how even the earlier factory efi systems are pretty darned good when they work right, huh?

Technically, yeah, if you spent the money and time on the dyno to really tune an aftermarket
injection system, you'd probably find a few ft-lbs and bhp. A few.

But you passed smog, which is big, and you'd have to tune the aftermarket to try to do that.
Which might be hard, depending on how critical the visual inspection is, and how you
package it.

Me, I'd drive it. If it keeps making you happy, don't dick with it! Spend the money you
save on a nice set of shocks, springs and sway bars. Be even happier.

what I'd do.

now, sometimes I just mess with people. It's more entertaining that way. george graves
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Old 11-21-2016, 11:07 AM   #3
Join Date: May 2010
Location: El Segundo
Posts: 19
Air Fuel Ratio

Thx Toby.

Yes, that is my initial thought too - that I'd only get a bit more gains. I already have the Bilsteins and HR sport springs, time for some sway bars and other goodies then!

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Old 11-21-2016, 11:55 AM   #4
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Join Date: May 2009
Location: Hacienda Hts, CA
Posts: 12,111
13.5 is stepping into lean territory, but you're right at where it's going to make the most power. You're running 91oct right(Sorry had to ask!)? 13.2 is sort of the magic safe number for NA, but I think you're fine.
Going MAP/Megasquirt is going to grant you a few extra ponies indefinitely, give you a snappier 'newer car feel' throttle response with the variable TPS, and take full advantage of those .2 extra liters and cam, but you just have to make sure the tuner takes a real good time tuning part-throttle/everyday driving scenarios. WOT tuning is and fast to do.

My advice is similar to the one above - just drive it for now since it's nice until you have the itch to tinker with it more.
Next time you fill up, throw in a bottle of octane booster first and pay attention to your WB gauge to see if the AFR drops a bit in the areas you're noticing it hit 13.5.
1991 325iS turbo

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Old 12-18-2016, 01:11 PM   #5
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Join Date: Nov 2013
Location: Macomb, MI
Posts: 34
You can trick L-jet to run richer or leaner by adjusting the wheel inside the sensor on the airbox. Try that or maybe slightly larger injectors, any bosch ev1 injectors fit. The stock ones are around 14# I believe

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