"MPGiata" modding thread: aiming for 50 MPG with a 1990 Miata. Update: success, 55 MPG!

[joemac]

Originally Posted by AdrianD

I never knew about the air-flow meter adjustment. Could be different between the 1.8 and 1.6 models.

A new O2 sensor would definitely help.
There are reports of people who turbo Miatas and get better mileage if they keep out of boost, as the stock tune could be leaned out.

I would not recommend touching closed-loop A/F. On a vehicle with a 3-way catalyst, net-lean operation (>~14.7 A/F on E0 or about 14.1 A/F on E10 ) will result not just in high NOx emissions - the Pt in the 3-way catalyst will oxidize most of the NO in NOx to NO2, which is still a problematic pollutant in many urban areas. NOx itself (NO + NO2) is also a major contributor to tropospheric ozone, which is also still a problem in urban areas, and contributes to nitrate secondary particle formation. Let's keep it clean.

[MetroMPG]

2018's emission test will confirm whether the change to the MAF spring tension keeps things within specs. Given the soot in the tailpipe prior to the change (suggesting a rich condition), I'd bet it's still OK.

[kurzer]

I´d bet, it´s better than stock.

[MetroMPG]

So the car is finished for the year!

On the weekend it developed a not-insignificant coolant leak that I first assumed was a hose or clamp issue, since I'd just done a flush & fill. But on further inspection it appears to be coming from the water pump.

Unfortunately, replacing the pump isn't a simple task -- it's behind the timing belt, and the pump housing has 2 belt idler pulleys integrated in it:



Fortunately, "replace timing belt" was already on next year's to-do list.

So the car will go into storage broken, and I'll sort it out next summer.

[joemac]

Originally Posted by MetroMPG

2018's emission test will confirm whether the change to the MAF spring tension keeps things within specs. Given the soot in the tailpipe prior to the change (suggesting a rich condition), I'd bet it's still OK.

That depends on the emissions test. My guess is that we aren't talking about $5000 per day FTP75 testing on an electric dyno. IM240 testing on an inertia dyno is like a go/no-go gage when it comes to NOx emissions and it isn't even capable of measuring NMHC or NMOG emissions, only total HC. Going off of closed-loop operation to lean of stoichiometry will cause a large increase in NOx emissions under those conditions even if the IM240 test doesn't hit those exact speed/load points during testing - in other words, it will only pick it up if it hits the conditions where you are operating net lean. If you plot exhaust lambda vs. engine speed and torque for U.S. or California emissions compliant vehicles produced since 2004, what you will see is closed loop and near constant exhaust lambda (typically slightly rich of stoichiometry) over the entire speed/load map, even for transient operation, except along a line running from peak power to peak torque, and sometimes at speeds below peak torque, where it is often open-loop and net rich for component protection (exhaust temperatures at higher speeds, preignition at very low speed/high torque). This is largely also for NOx emissions compliance. NOx cannot be reduced over a three-way-catalyst (TWC) when the partial pressure of NO or NO2 exceeds the partial pressure of O2 in the exhaust. NOx reduction over a TWC at stoichiometric conditions exceeds 98%, so NOx literally goes through the roof when lean - it blows right past the TWC unchanged other than oxidation of NO to more toxic NO2. A secondary reason for this type of A/F calibration is catalyst protection. A TWC will rapidly sinter its precious metals when operated even at moderate loads in a net-oxidizing exhaust environment (i.e., 600C exhaust temperature - pretty common). Same temperatures at stoichiometry or rich operation has a much lower sintering rate (in fact, slight-net-rich at 900C is even OK). When precious metals migrate through washcoating material and sinter, they rapidly lose surface area and the TWC begins to turn into an exhaust restriction instead of an exhaust catalyst.

So, again, lets keep it clean. There are other ways to achieve higher engine efficiency besides lean operation. The primary benefit of lean operation is pumping work reduction from dilute combustion (i.e., larger throttle opening for the same BMEP non-dilute). The charge dilution doesn't need to be with air. What is becoming more common with new production spark ignition engines is to use cooled (usually engine coolant heat exchanger) exhaust gas recirculation (EGR) for charge dilution (2nd gen Prius, 2018 HEV and non-HEV Camry, Mazda CX9 turbo, any number of recent Subarus) in order to reduce pumping work and improve cycle efficiency while maintaining exhaust lambda near 0.97 - 1.00. Also, any type of charge dilution also benefit significantly from a higher energy ignition system in order to reduce misfire (misfire is more common lean and/or with EGR), so going to a higher energy ignition system, preferably coil-on-plug with multi-strike capability (I think latest MegaSquirt variants support this) helps both combustion efficiency and hydrocarbon emissions. Increasing spark ignition energy also allows an engine to run even more dilute, which improves part load efficiency even further.

[serialk11r]

I think you need a rear spoiler, the trunk is way way low and short on the Miata. Apparently the lexan duckbill/"whale tail" that is pretty popular raises the pressure over the rear enough to add a huge amount of grip on the track.

I unfortunately crashed my MR2 so now I'm thinking about an old Miata. One idea I came up with is to retard the intake cam a bit; do you know if that's easy to do? I heard about people tweaking timing by moving around the cam position sensor (I've only worked with VVT cars so this is hilarious to me), I figured the cams themselves might be easy to get to as well.

I would guess that the cams have too much overlap at low load since there's no VVT. A bonus of that mod would be that maximum hp might increase (and the Miata could definitely use some of that...).

[Stubby79]

Originally Posted by serialk11r

One idea I came up with is to retard the intake cam a bit; do you know if that's easy to do?

Aftermarket adjustable cam sprockets.

[Baltothewolf]

I'm sure whatever MetroMPG has done is fine. I'm sure he isn't polluting any more than my insight, Miata or 4Runner is polluting considering none or them have catalytic converters.

[kurzer]

the thing is, the AFM-mod does not change the closed loop in any form.

i´ve been driving some years with my AFR gauge and i can tell. in CL everything is as usual, in OL it is a lot better, because to much fuel is even worse. but i was aiming at the optimal spot and 4 klicks are still a little on the rich side.

[joemac]

Originally Posted by kurzer

the thing is, the AFM-mod does not change the closed loop in any form.

i´ve been driving some years with my AFR gauge and i can tell. in CL everything is as usual, in OL it is a lot better, because to much fuel is even worse. but i was aiming at the optimal spot and 4 klicks are still a little on the rich side.

That sounds good. Using a UEGO (your AFR) is a great way to confirm wear you are. If you tune open-loop fuel using air-box modification, you may want to tap a k-type thermocouple into one of the exhaust runners near the exhaust port just to keep an eye on things and make sure that the lessened enrichment doesn't get you into exhaust valve or other thermal issues.