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

http://ecomodder.com/forum/attachmen...1&d=1505756110

You may have already seen my initial Miata thread, which documents the saga of buying and repairing this fun old turd bucket to make it road legal. Time for a fresh thread that focuses on the ecomodding!


The car:

• 1990 Mazda Miata MX-5
• Engine/trans: 1.6L (116 hp), 5-speed
• Drag coefficient (random search result): 0.38 top up; 0.40 top down
• Frontal area:
• Curb weight: ~2k lbs, depending on options
• Odometer: 216k kilometres (134k miles)

MPG potential:

EPA says: 22 city / 28 highway / 24 combined

AntiochOG did a 50 mpg fill using pulse & glide (but P&G is not my plan): 50 mpg! (1990 Mazda Miata)

The top 3 Miatas in EM's garage:

• Miazda - bikin' Ed 1996 Mazda Miata mx-5 44.14
• Cinco The Miata - AntiochOG 1990 Mazda Miata 36.13
• The BRG - ChopsQube 1991 Mazda Miata SE 35.9

Fuelly shows a couple hundred owners of the early 1.6L cars are averaging 27 mpg US. Half a dozen owners of the 1.6L "NA" model report averages of 40-45 mpg.


The goal:

50 MPG (US) / 4.7 L/100 km, in my typical summer driving environment:

• relaxed cruising, almost always taking the scenic route or back roads;
• but fun DWB ("driving without brakes") in the twisties!
• 80 km/h (50 mph) max. posted limit most roads
• primary driving techniqe = driving with load (when not affecting traffic)
• but also engine-off coasting toward some turns & stops, or on steep enough descents that allow me to keep speed.
• top will be down most of the time (despite worse aero), since that's one of the main reasons I got it!

Results so far:

1) 37.5 mpg US = 6.1 L/100 km: first fill last year, cool autumn driving (10-15 C = 50-60 F) with no mods other than tire pressure;

2) 43.6 mpg US = 5.3 L/100 km: 2nd fill (1st fill of this year); warm summer driving with some mods in place;

3) 44.2 mpg US = 5.2 L/100 km: warm summer driving with even more mods in place;

4) 49.9 mpg US = 4.7 L/100 km: so close! Continuing ideal warm weather, still more mods!

5) 51.0 mpg US = 4.6 L/100 km: Success!! 2/3 ideal warm weather, still more mods.

Mods
 

Completed mods:

(in no particular order)

- MPGuino fuel economy computer

http://ecomodder.com/forum/attachmen...1&d=1504021128

See: http://ecomodder.com/forum/showthrea...vss-35550.html

- 50 PSI
- zero-toe alignment

- cleaned & regapped spark plugs (the gap was 0.010 wider than spec!)
- air conditioning delete (removed components)

http://ecomodder.com/forum/attachmen...1&d=1505940906

- power steering delete (belt removed)
- ignition timing advance

http://ecomodder.com/forum/attachmen...1&d=1508874437

- air dam, with...
- partial grille block

http://ecomodder.com/forum/attachmen...1&d=1505756110

- half size radiator, and...
- one of two factory cooling fans deleted

http://ecomodder.com/forum/attachmen...1&d=1492450727 http://ecomodder.com/forum/attachmen...1&d=1492450727

- smooth wheel discs

http://ecomodder.com/forum/attachmen...1&d=1505497330

- taller gearing (larger diameter rear wheels/tires)

http://ecomodder.com/forum/attachmen...1&d=1505309221

- side mirrors delete/relocated inside
- LRR tires on the front, narrower than stock: 165/64R14 Dunlop Enasave

http://ecomodder.com/forum/attachmen...1&d=1506615796

- wiper delete
- synthetic motor oil: 66% 5w30 + 33% 10w30
- weight reduction (goal: 100 lbs / 45 kg is attainable)
- MAF / open loop AFR adjustment

http://ecomodder.com/forum/attachmen...1&d=1508874484

- synthetic lighter weight transmission oil (50% lightweight GM Syncromesh + synth 75w90)
- synthetic differential oil

- WAI - warm air intake

(More info & pics to come.)

Future mods:

- engine kill switch (been using the key)
- coolant/block heater
- reduce disc brake drag (springs are missing; service calipers)
- rear fender skirts
- fairing ahead of rear tires
- fairing behind rear wheels

http://ecomodder.com/forum/attachmen...1&d=1507497234 http://ecomodder.com/forum/attachmen...1&d=1507497398 http://ecomodder.com/forum/attachmen...1&d=1507497516

- fairing behind front wheels
- belly pan
- decklid extension/spoiler?

http://ecomodder.com/forum/attachmen...1&d=1507754694

- weight reduction: power steering pump delete
- lower it a bit?

Dream mods (but probably not)

- targa style aero bodywork aft of the seats
- mini boat tail!
- ???

(post reserved)

possible parts car
 

My mechanic says he's buying a 1990 Miata for its drivetrain (to swap into an MG). The body is shot (rust).

He asked me: "Need any parts?"

What stuff would you grab from an ecomodding angle? Off the top of my head, I couldn't think of anything.

What stuff should I grab purely to have spare parts on hand?

---

Edit: I need a driver's side sun visor!

He's keeping the water pump, but the radiator is a possibility. I already replaced mine, though.

Unfortunately, the rear end was the same for all of this generation. A taller diff would be fantastic. Plus it'd let me run the slightly smaller diameter & narrower LRR tires on the back too.

You should also be able to adjust the idle speed on that NA miata. The bros NB miata is too new for that. AND we can't adjust the ignition advance! Enjoy the adjustability of your old miata! His NB miata idles at 900-1000 rpm.

That's an ambitious goal.

It needs to be aerodynamically perfect. The killer is the top down driving enjoyment. Maybe chop the windshield and add a metal tonneau cover and headrest fairing?

That said, your list doesn't include a rear diverter.

http://ecomodder.com/forum/member-fr...11-silver4.jpg

For the rear wheel fairings, I like what I call the Bonnevette:

http://ecomodder.com/forum/member-fr...bonnevette.jpg

Starting just inside the tire they flair out to flush with the outer tire face at the back. Wheelwell turbulence is separated from the underbody flow and thrown outward into the ambient airstream. They would also form the outer fences of a diverter bell with a 4° angle roof.

48!
 

Diverter bell!

---

4th fill-up tonight: 48 MPG. :) [Correction! 49.9 mpg! -- see post #18]

I would like to be able to cruise a little faster than I have been, though. Mods will continue! Average speed may (will probably) increase.

And it looks like another week of warm weather coming, so the cold won't mess with my progress just yet.

to do the idle, u have to connect 2 pins in the engine bay. this will show the real setting. but there is a trick. if the rev is falling to slow while shifting, u can reduce the idle air. drops it to fast, rise it a bit.

i hope u will set the mixture somewhat leaner, as i suggested in the the other thread. u will love it, i promise. miatas running verry rich in open loop.

Metro, see if you can get your hands on a 4.1 rear diff from a 94-99 car, it will lower your RPM by like 5%. They can be had for cheap. You may he able to trade someone for free who wants your 4.3 for track use.

the early diff is smaller than the 1994 and later. you need a shorter prop shaft and 2 shorter drive shafts. additional are differences in the shafts because of VLSD and open diff at least. the bigger stuff is likely somewhat heavier.

Thanks for the reminder. Does the adjustment only affect open loop? That would be useful (warm-up MPG hit is big on this car). I don't want to change the closed loop setting.

I actually suggested this to a friend who has the taller diff. Unfortunately his has LSD, and he autocrosses, so he didn't want to do it.

But I may fish around for someone else who wants to do the swap. Thanks to kurzer, now I know we would need to swap shafts as well.

~50 MPG in sub/urban driving
 

I covered probably the longest stretch of sub/urban driving I've done with the car, so far. Let's say 6 miles / 10 km with 3 destinations (farthest first, like a good ecodriver!).

The car seems good for ~50 mpg with lots of engine-off coasting. Some hypermilers might call what I was doing "pulse & glide," though I didn't really do more than one pulse/glide cycle in between turns or stops, and there was normal top gear cruising in there as well.

For context, keep in mind my driving environment: small, sleepy city, pretty light traffic, with fairly predictable lights, on mostly known routes. Also: nice, warm weather.

I keep wondering what kind of fuel economy I am getting in this town. I just drive 1.4 miles to school and back and going there I can EoC one full mile. Would driving uphill be the pulse? :)

Yes!

But it would be terrible anyway. Because cold engine! And cold everything else. (Speaking of temperature, is it still too hot and/or inconvenient to bike??)

Now I'm curious to know what numbers you're actually getting on that long pulse & glide.

I once did a 4,214 km long pulse! Airplane to the Azores. The glide: sailing back. :D

It will probably be another month before I feel the need to fill up, although I honestly do not know. Gas is 25¢ cheaper fifty miles from here and it is on the way, so I stop there if I am low. I might fill up my five-gallon gas can on one of my trips in my Accord, but then I will not have any usable data! :)

Correction: 49 mpg last fill
 

I just did the odometer correction on my last fill, adjusting distance to take into account the bigger rear wheels (calibrated against GPS & Google Maps). And the corrected number is:

49.9 MPG US = 4.7 L/100 km

Soooo close. Should have done more mods!

Never have I been so excited to almost get 50 MPG. In the Firefly, getting 50 mpg would indicate a major mechanical problem, or a boat anchor tied to rear bumper, or towing a camper trailer...

there will be no or minor change in CL. i had a extrem lean time with 47% ethanol in the fuel and the first klick at the air flow meter. CL was fine, OL was very lean, but >5000 rpm it was rich angain. in this time the cold start was affected (even with E24), afair the engine didnt take the trottle well for some seconds in the morning. everything else was only at the AFR-gauge seeable.

i learned in this time, that the OL is very, very rich, so i´m not suprised by your warm-up observation.

without ethanol the cold start is fine. i have a -17°C cold start vid with 1 klick from 2012, but since 1 year it is at 4 klicks and runs well.
https://www.youtube.com/watch?v=3lWdD1gshaY

Ac no mo
 

Weight reduction continues!

http://ecomodder.com/forum/attachmen...1&d=1505940906

The internet says the AC system weighs about 38 lbs / 17 kg.

I didn't rip apart the dashboard to pull out the evaporator though, so let's call this 33 lbs.

Also: not having the condensor in the nose will let my 1/2 size radiator work more efficiently because now it's got a front row seat for air flow.

Hey, maybe I can reduce the cooling inlet size further...

Bellmouth the radiator inlet.

The weight reduction and reduced inlet should be good for 0.1mpg. You'll have to adjust your goals upward.

Metro, you sure do fill up a lot. Just how far are you going between fill-ups? and how is this going to mess with accuracy?

(I have taken to only believing a full tank worth...sure, I could make a single trip that might get 60mpg, but over-all I would only see 40 for that tank. That's too much like stacking the cards in my favor, just to see the results I want)

Stubby: FYI, the car's fuel log is up to date.

Some have been short fills of a few hundred KM (smallest fill a result of wanting a "reset" so I could calibrate the newly installed MPGuino from full).

All my fills this year have been at the same pump, and I top it off completely (until I see fuel in the filler neck).

I'm pretty confident in the numbers.

I actually kind of did that: it's a rectangular opening, but the edges are radiused (maybe r = 1").

Also: radiator efficiency is noticeably improved with the AC condensor out of the way! I need an adjustable grille block.

MAF / open loop AFR adjustment
 

Done! Easy modification. Thanks again.

My exhaust pipe is definitely very sooty. Running rich? Or just an old pipe with years of accumulation?

The exhaust gas analysis is the best method to confirm - that will be next year's e-test for me.

Tomorrow morning's "cold" start (~15C / 50 F) will determine if the adjustment (4 clicks) will affect open loop driveability.

thinking out loud
 

Next mod?

http://ecomodder.com/forum/attachmen...1&d=1506047893

ahh, wel;l...it looked liked you had filled up two or three times inside of 2 days with your reporting your MPGs....or is that off of your MPGuino?

http://ecomodder.com/forum/attachmen...1&d=1506047893

Off to a good start. What do you think about follows the side trim line forward to the door and then straight down?

Props to you OP for having the composure to drive steadily and economically. I can't do that in my Miata :(

Regarding differentials and gearing, I installed a 3.9 differential in mine, while it originally came with a 4.1 (94 model, with the 1.8). The new diff is off a 10th anniversary NB model. The rev difference is not a lot but it makes for a tad quieter ride. In town however I used to drive almost everywhere in 5th gear, now I only use 5th above 40mph as if feels that I'm lugging the engine.

If you could live without having the top down, you could invest in a hardtop plus a hardtop spoiler. The hardtop spoilers for the Miata seem to continue the slope of the hardtop nicely for aerodynamics.

Later edit:
About the soot on the tailpipe, as far as I know Miatas are tuned rich from the factory. If you want to get that out of the way, take a look at Speeduino, open source ECU. Cheap, already available with plugs for the Miata wiring harness and tunable with the same software as Megasquirt.

You can also increase timing by rotating the cam angle sensor and see how that helps your mileage.

Oh yea I forgot about adjusting timing! Definitely do this. Bump it to 12°. You can still run 87 (whatever the cheapest is) without pinging and it gives you about 5% economy.

There's also the air-flow meter adjustment that will lean it out.

There's also the air-flow meter adjustment that will lean it out.

Oh and between my cleaning the ground connections and installing a heated O2 sensor, my tail pipe isn't quarter as sooty as it used to get.

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.

You can't adjust the AFM on the 1.8 cars, only the 1.6.

Heated o2 definitely helps. Worth doing, considering how cheap iit is.

Typical gains on a aftermarket ECU wkth a completely stock engine are 10%, sometimes as high as 15%.

Yup: I've done the timing advance:

http://ecomodder.com/forum/attachmen...1&d=1508874437

And as of yesterday afternoon, the airflow meter spring adjustment as well:

http://ecomodder.com/forum/attachmen...1&d=1508874484

Going right now to see how it affects a cold start...

UPDATE: the car has always had a slight stumble/miss on cold starts that lasts 10-15 seconds (plug wires?). It was possibly slightly worse on this start-up, but not significantly. Could be my imagination. I'll leave the adjustment where it is.

Even with just the taller rear tires, I've noticed I use 5th in town a little less than I used to.

Heresy! :D I wouldn't have bought it at any price if it wasn't a convertible. There will be no more talk of fixed hard tops in this thread!

What if you got a really good deal?! :D

this is a common problem with miatas. the heat kills the plug wires, the plug wires kill the coil. dont use the old wires too long, 50.000-60.000km is the time to change.

Heat wrapping the headers helps a lot with the above statement, but you lose warm up efficiency so...

Carson-style aerocap?