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

[kurzer]

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.

[MetroMPG]

Originally Posted by kurzer

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.

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.

[MetroMPG]

Originally Posted by Baltothewolf

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.

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.

[MetroMPG]

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.

[Xist]

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?

[MetroMPG]

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.

[Xist]

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!

[MetroMPG]

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...

[kurzer]

Originally Posted by MetroMPG

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.

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

[MetroMPG]

Weight reduction continues!



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...