1 (edited by duthehustle93 2022-05-18 10:15 AM)

Topic: Miata AFM mod for fuel economy

Good morning,

I've heard of early miatas and RX7's modifying the spring tension on their AFM for more power(maybe?) and better fuel economy. Miatas run pig rich at WOT. From what I've heard, you basically modify the spring tension on the AFM to adjust how much the flapper opens and therefore tricking the ECU into "seeing" more or less air. We have a 1.8 swapped NA6 miata so it still has the AFM/flapper. I don't care about the power gains, but with gas at $6-7 a gallon in CA, I'd love to see if we can save $ on gas at races. Our current burn rate is about 5.5-7 gallons/hour depending on whose driving.

Has anyone (ideally with a miata) modified their AFM and adjusted their AFR ratios, and was there a noticeable increase in fuel economy? I also don't know if it ignores the AFM at WOT, I think it just ignores the o2 sensor? Anyways, and advice or input is appreciated... I'm good at some things, but engine management and general engine stuff isn't one of them!

Thanks,
Brandon

Full Ass Racing
#455 Piñata Miata
#735 BMDollhÜr 7Turdy5i

Re: Miata AFM mod for fuel economy

You don't want to run lean at WOT and high RPMs, that leads to melted pistons.  Adding additional fuel beyond stoic of 14.7 protects against knock as well as provides cooling.  There also is a delicate balance between fueling and your timing.  An auto-generated table for a 1.8L with similar specs as a Miata engine suggests about 13.0 AFR at WOT 6k RPM.  My turbo car is in the 11s and 12s.  A wideband O2 and gauge will tell you were you actually are at.

I believe at WOT many ECUs just use their base tables.  It needs the AFM as one of its inputs, probably ignores the O2.

If you really want to optimize your fueling, it is best to do on a dyno with an ECU where you can change your fueling and timing tables.  Also get some det cans so you can hear when its pinging.

If you want to save on fuel, put a block of wood under the accelerator pedal.

Team whatever_racecar #745 Volvo wagon

Re: Miata AFM mod for fuel economy

Yes, that's a thing that is or was done to increase power.  It is or was generally done on a dyno.  There are or were also folks selling tuned AFMs on an exchange basis.  I guess the assumption is or was that there is a systemic tuning issue and that the individual engines didn't vary enough to matter.

Reducing the spring preload on the flapper can cause driveability issues at low throttle, issues with idling, and (if I recall properly) failure of the fuel pump cutout to work properly.

Fun tangent: Miatas can exhibit differences in AFR in turns.  Like different in left turns than right turns.  Because of G forces moving the flapper.  People used to rotate the AFM out of the horizontal plane to try to tune on this.  The best one could hope for is what one would call "marginal gains".

I really wouldn't bother messing with the AFM.

Re: Miata AFM mod for fuel economy

Best way to get more MPGs is keep your foot off the go fast pedal. I have heard of people shifting early but I'm not sure how effectve that would be.

"get up and get your grandma outta here"

Re: Miata AFM mod for fuel economy

Thanks all for the input! I'll just go the "stay out of WOT" route...

@hoverducky: Good point about the flapper moving during corners, I never thought about that. Tuning it to avoid it running lean in a corner would be way more effort than it'd be worth for us.

Full Ass Racing
#455 Piñata Miata
#735 BMDollhÜr 7Turdy5i

Re: Miata AFM mod for fuel economy

Is your vehicle running reliably?

Regardless the cost per gallon, the difference is 1.5 gph x #hours driving, depending on who is driving, yes?

I'm convinced any issue here isn't an engine management issue.

This seems like a resource / results,  cost / benefit, funding / sharing, who pays for what breaks, issue.

Re: Miata AFM mod for fuel economy

duthehustle93 wrote:

Thanks all for the input! I'll just go the "stay out of WOT" route...

@hoverducky: Good point about the flapper moving during corners, I never thought about that. Tuning it to avoid it running lean in a corner would be way more effort than it'd be worth for us.

Not sure how much weight you can get off the car, but that helps. Also keep you "unsprung"  as low as possible, meaning use the lightest wheels you can find. Aero can also help a little at speed.

"get up and get your grandma outta here"

Re: Miata AFM mod for fuel economy

@Lemon_Newton-Metre: Yes, it generally runs pretty reliably *knock on wood*. My lizard brain kinda lost what you're trying to convey... I believe you're pointing at a driver correction rather than a AFR correction, right? Most of us drive with a pretty heavy foot so you're definitely not wrong here. We could probably do some analysis and discover that we'd place X positions higher if we can run X% longer stints by conserving X% more fuel... but I don't care about winning. The drivers with a higher burn rate have more fun just driving fast (safely/cleanly of course) until we're out of fuel. The drivers that are more efficient prefer driving slower and longer stints... I basically just give my drivers a tank per stint... how they choose to spend it is entirely up to them as long as they're driving clean. Anyways, my point is I was hoping to make our miata run more efficient without changing our driving habits. Might be a big ask, but I stumbled upon this mod and it sounded like a good way to maybe save 1 gallon/hour for a few hours of time and <$200 for an AFR gauge/wideband.

Mr.Yuck: Thanks! The PO already stripped this thing clean... other than stripping paint and pushing the boundaries of safety/Lemons body "complete-ness" rules, we're pretty much there. We also have a front air dam which helps a bunch with drag. We do have an open top cage which is probably super draggy but none of my drivers are willing to invest in a cool suit and the air flow helps a bunch at these hot CA tracks. If they eventually buy a cool suit then you've got a good point on making a ghetto fast back from a sheet of HDPE... that'd save a bunch of drag.

Full Ass Racing
#455 Piñata Miata
#735 BMDollhÜr 7Turdy5i

9 (edited by Lemon_Newton-Metre 2022-05-19 10:32 AM)

Re: Miata AFM mod for fuel economy

duthehustle93 wrote:

[snip] I basically just give my drivers a tank per stint... how they choose to spend it is entirely up to them as long as they're driving clean.[snip]

^ This makes perfect sense to me - yes.