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L18 tuning questions - EFI and lean cruising.


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I'm looking for very detailed advice here about L series motors, specifically L18s with A87 heads (I know there are different sorts of A87). I'm in the throws of properly tuning a dead stock L18 from a 74' 610 in my 69' 510. I've installed my own homebrewed EFI version of a twin hitachi manifold only with a plenum chamber and single 70mm GM throttle body (yes, I know that's too big). I'm using a Megasquirt Microsquirt ECU using batch fire and wasted spark with a crank trigger.




What I'm wanting to know is if the L series has any specific characteristics around mixtures and combustion?


I ask this because I'd like to try a sort of tune that I used very successfully on an aircooled VW 1600DP motor. Basically it involves running the motor at 17AFRs above 2600rpm and below 92kpa. There's a hard switchover point at 92kpa to much richer 'power' mixtures. To do this, I'd need to run a lot more timing and throttle in that area of the graph. However, once tuned up, the car ran really well and the heads were always cool (unusual for turbo ACVWs). Keeping in mind this wasn't a new, efficient twin cam motor, but a nasty old pushrod air-cooler. Here are the AFR and timing tables from the VW. I dropped the AFR back up near the redline as a precaution but I never had issues.




 I'd like to try this on the L18, but I'm not sure if it will run properly at 17AFR. I'm wondering has anyone had experience with this sort of tuning on an L series?




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As I said, it's generic.  It's been a while since I've done any scientific testing on a Datsun motor, but 14.7 is a good, safe number and you can tune around that.


Here's the article - https://cartreatments.com/air-fuel-ratio-at-various-conditions/#:~:text=The ideal air-fuel ratio,as the “stoichiometric” mixture.

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On a high compression high performance engine maybe below 12-12.5 but for a stock L18 it's just fuel out the exhaust. When I first had my motorcycle carbs on it ran 9.6 (and it may have been richer as 9.6 might have been as low as it gauge would read) You could see back smoke at full throttle on the highway. It was actually faster at 12.5 after several re-jettings because the extra gas was over cooling the combustion chamber.


At 50MPH cruise it's 14.7 and just ever so slightly richer into the low 14s and into the 13s as you go faster because more throttle input and more load. Around town no load, low 15s. When you let off at speed it jumps to 22 and drops back to 16-18 but this is likely a characteristic of constant velocity carburetors. 


Perfect a/f is 14.7 but it's a bell curve with 13.7 to 15.7 really not much different and dropping off steeply on either side. Generally to be safe under load, rich is better than too lean. With no load at cruising you can run leaner but you would have to run several tanks to see if you are actually gaining any mileage from this.

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