MFI AFR Guidance

[Sway Bar]

I need a little guidance as to what are acceptable AFR measurements for MFI. I'm in the midst of tuning my first ever build and I can't quite get a handle on what I'm ultimately aiming for.

Quick back ground:
3.0L MFI
9.5:1 JE Pistons
Early E Cam
1973 MFI T pump

Use is 98.85% spirited street motor.

My understanding is the T pump and space cam will suffice and later if one wishes more specific, precise mapping one would have to get a space cam ground accordingly.

That being said for the time being I'm just trying to find what are acceptable targets. So far in absorbing some posts from a few years ago this is what I know:

1) WOT under load aim for highs of 12.9 to 13.0?
2) Cruising speeds/RPM's (3000rpm - 4000rpm) 11.2 to 11.4?
3) What would be an acceptable rich end under idle (after warm up) and under load?

I have an LM-2 data logger and I think I'm getting close to main rack adjustment and understand to wait before I adjust the idle speed.

4) Does the Idle Speed adjustment have any influence on driving RPM's or is it exactly as decribed "idle speed"?

Any guidance would be greatly appreciated. So far in the 180miles that I put on this creation have been a blast. I'm just paranoid I may cook or wash out something.

[cmcfaul]

your numbers look good. You will be an expert MFI tuner when this is all done. Idle does not matter much. 10-12 would be my target. Don't, Don't go too lean. You may not hear it but it will detonate above 15 with pump gas and that compression ratio. The bottom of the JE piston valve pockets are very thin and can break.

You will see different AFR numbers when the engine is cold, warm and hot. Compromises will have to be made to get it close where it matters most to you.

There are 4 primary places to adjust the MFI.
1. idle screw
2. main screw
3. main linkage
4. individual butterfly's (check each plug to ensure one is not leaner than the others and adjust as needed)

Do your best to keep temps down as high temps and hi compression don't mix. Twin plugging is recommended.

Chris

[mattC2993]

an AFR of 10 would be eye watering. Cruise at 11 would be gas guzzling. Here is a resource from the Innovate web site Race Engine Jetting - Tuning a Race Engine :: Engine Tuning Resources & Tips. I hear that the MFI systems do run rich. Perhaps that is just erring on the safe side.

[cmcfaul]

Lean is fast, too lean is expensive. My car has a similar setup, though 2.4 liters. Runs great and relatively clean at an average AFR of 12.

Partial throttle goes lean when warming up (will hit 15 briefly)
Rich when cold (by design (MFI thermostat))
about 12 all the time when fully warmed up (a little richer at idle)

It took a while to get it to where I want it. Always tempted to go a little leaner but burned two JE Pistons that way and keep one out to remind me.

MFI like to be rich and dont seem to mind it

Chris

[Sway Bar]

Thanks for the info Chris,

I think I'm getting close on the main rack and understand it will be a compromise due to displacement over the 2.4 and a T space cam. Being primarily a street engine I'm not worried about 6200rpm pulls but wanted to make sure I'm safe on the lean and rich side running 93 octane.

So far I'm hitting mid 13's for short spikes at 5500 + rpm and cruising at the 11.2 range. I think I'm nicely synchronised and pulling just under 7kg/hr at 1000rpm across all 6 throttle bodies. Only thing is I'm really rich on the idle/low throttle but I haven't touched the idle adjustment as of yet and goes to figure since I'm dialing up the fuel volume on the main rack.

Warm up thermostat is doing it's thing and dumping in more fuel until up to temp.

When I get to the 400 mile oil change, re-torques etc. I will give the plugs a look as well.

The fun was to make this look exactly like my original 2.4 but have a little more torque

[Sway Bar]

So for interest sakes...when I'm seeing these great monsters that are 3.2, 3.4, and even 3.6 MFI engines besides of course the intakes, throttle bodies and/if twin plugging, what is being done to the pump? The term recalibrated is kind of open ended.

[mattC2993]

My MFI pump is at MFI Werks. He is going to recalibrate because the engine displacement is increasing 10%. What does he do? I'm not sure if it is just a cam change and initial adjustments or if there is more to it than that.

Pelican Technical Article: Mechanical Fuel Injection Information MFI