I Want To Run Lean
 

I got MS II going and have a wideband hooked to MS and a 12 dollar panel meter guage, so I can measure, datalog, and watch my AFR. I want to set the car up to get the best gas mileage. I know a Ring and pinion swap would help, but for now I just want to lean it out for best mileage.

I have it set for 12.5:1 at WOT, and 16:1 at cruise. I've run it 17:1, and it ran fine. Should I just keep moving it lean till it starts to stumble, then add a little fuel? Lean at cruise shouldn't hurt anything as far as I can see. Is there a majic number for best gas mileage? I'll be making a 270 mile trip friday and I'll be able to see what my new mileage is running MSII.

Thanks.

good questions, i'd imagine in cruise you'd be able to run it as leans as you can as long as there are no drivability issues.

i'd also like to know what's better for fuel mileage, lower revs or more vacuum?

ive got mine set to 12 WOT, and it dithers between 14.3 and 15.3 at cruise... and upon warm up chills at 17 and slowly works its way down 12 day, 14 night (dont ask me i dont know why its like that.)

I'd keep it richer than 15 anywhere but flat no-throttle cruise or lift-off just because you'll not want to be too lean as the manifold pressure swings from vac to boost.... I'd call that limit 60 kpa absolute, maybe 50. I cruise between 15-16:1 and shut off the injectors below about 17-20 kpa.

Otherwise your only limiting factor is going to be EGT. You dont want to run super lean and end up with 1500F EGT at cruise or you'll burn up valves with all that mileage. Try 1300-1350F for a max EGT during cruise.

+1 IIRC VW tried running higher than 14.7:1 a few fews ago (maybe it was 20) they got 17:1 out of a stock ecu...

I choose a Honda CRX HF..56 MPG back in 1990...

+1 IIRC VW tried running higher than 14.7:1 a few fews ago (maybe it was 20) they got 17:1 out of a stock block/ecu...

I choose a Honda CRX HF..56 MPG back in 1990...

Lean burn, good for mpg, bad for emissions.. We discussed and played with this in the hydra training course.

right. raises NOx like mad.

yep. who doesn't need more nitrogen oxide in their life?

I'm around 16.5 during cruise with a lot of timing to keep the EGTs down. It also tests your cooling system.

isn't 12.5:1 a bit fat a WOT naturally aspirated?

An N/A car will usually make best power between 12.6-13:1 at WOT. If the car is dedicated race (especially road race)... stay closer to 12.6:1 for longevity.

where do you put afr on a turbo car at 8 and 12psi?

Maybe just a little, but not much. My stock ECU on stock injectors used to do about 13:1 at high load.

It really depends on the car, but generally speaking in Miataland-- maybe 12.3-12.4:1 at 7-8psi and 12:1 flat at 12psi. Up around 14-15psi I'm looking for around 11.8:1 or maybe a tad richer.

This is on a properly intercooled turbo Miata, no water injection, etc. Everything factors in... If your IATs are out of control you might have to run stupid rich as a bandaid for poor intercooling, though really you should fix your IC problem. If you're running WI you should be able to keep it in the low 12's as the WI will keep the combustion temps lower and help prevent det.

sadly it is not the one you want.

lol. true story.