MPG has stalled at 35 - I need help!
 

'91 Miata with a full replacement ECU (i.e. I can set my own A/F ratio - no need for even an O2 sensor). I have detailed a few of the changes I have made to the car here:
https://www.lightweightmiata.com/mpg/index.htm

Anyway, the big changes so far have been to driving technique, pumping tires up to 50 PSI, and changing the A/F to 15:1. I've increased my MPG from 20 (a very rich mixture with a supercharger) to 35. I've been at 35 for the past few weeks even though I have:
- changed from 10W-30 to 0W-30
- dropped my A/F ratio down to 15:1
- changed from a 180 to 195 degree thermostat

Anyone have a good idea on where to go from here. I'd like to try a partial grill block next but I was really hoping to hit 40 MPG on this last fill up. I really leaned the mixture out (and the car feels kind of jerky at times) and thought I would see some good results but I didn't. Help!

I like the partial grille block you show on your website!

How fast are you driving? What kind of driving are you doing--all freeway, all city, what mixture of the two? How fast? I usually do pretty well hanging around 55-60 mph.

Don't know what it's like where you live, but being a desert dweller, any kind of grille block scares me...

You should be running fuel cleaners at all times. 99% of them are snake oil so find the 1% that aren't. You should have your injectors cleaned, blueprinted, and balanced if you can find spares. No need to run half your cylinders at 14.5:1 and the other half at 15:1 when you can have them all run at 14.7:1.

The Bosch EV6 injector is a really good atomising injector when compared to the standard bosch pintle EV1. They really help across the board with leaning out fuel mixtures and delaying lean misfire. If they are a straight fit there is one guy on youtube that has a video of the spray pattern of different injectors and also sell them (he refers to them as gen3 injectors)

Another thing I have done is to put 18volt booster to the primary side of the coil. This helps alot also as it helps fire lean mixtures much better. You may have to reduce the dwell on the EMS to not overheat the coil but it definately helped in my car. MSD6 is crap for lighting lean mixtures if you have ever thought of that.
With these two mods allows me to run 16:1 on a warm engine with no lean misfires, but not just on cruise but 15:1 on nearly all other parts of the map with no drivability issues (except WOT where you want richer mixtures but I havel used 15:1 on that also but you dont need to cause you never use that part of the map on economy runs)

you're doing quite well for a car that is not super efficient. do try a partial grill block. do you feel comfortable with(or are you already) doing EOC?

might need some radical aero mods to achieve 40mpg and beyond.

Why not use an oil with a lower operating viscocity to reduce pumping losses?

Hot air inlet? Some people report benefits from them. I put one in recently, but have not been able to evaluate it well because my wife has been driving it recently. (Her idea of P&G is hit the gas, hit the brake, repeat as necessary until tank is empty. And don't forget the air conditioner! <sigh>)

I did the injector reman just last week on the Caravan. Got injectors out of same make/models in junkyard ($4.50 ea), sent seven to Witch Hunter Performance with instructions to rebuild six (seventh was in case they found one inoperative). Stuck them in as part of a tune-up.

agree with SD26: using 0w30 instead of 10w30 is only gonna help MPG when the engine's cold. on the other hand, I think oil is a bigger concern to the engine than gas (easier to fix gas related problems and they usually don't cause lasting harm). in all my testing using different size tires, the increased distance traveled BY FAR outweighs the decreased torque to the road. My truck came with 205/60/14s and adjusting for that, got 21 mpg combined that would have been 23.5. 5 gallon fillup, 120 miles instead of 132.

a HUGE thing that will help you out is new gearing in the rear end. it's not as expensive as you think unless you want to get ONLY the gears and put them in your differential. Since you have IRS it's a lot easier to get just the differential housing with the gears you want already in it from a junkyard or another member and install the whole thing in a bolt-up affair. I know I can get diffs for my Cressida for under $150 but shipping can be as much as the part so find one nearby (My cressida's diff housing and gears is about 80 lbs, your's is probably a little less because it's a smaller car with a 4-cyl).

Yes either get a different rear diff (but that will affect all gears though), or simply swap a much lower 5th gear (and I know mazdas are so interchangable).

That would cause to only drop your fifth gear and reduce rpm's by a lot.

Also, once your going to extremes of leaning out the mixture, get water injection and spray a minumum (and i mean MINUMUM) amount of heated water into the intake. This is act like a warm air intake, occupy space in the combustion chamber so less fuel is used, and possibly allow you for even leaner mixtures as the hot air will make it easier for fuel to burn.

Lastly, heat the fuel as well (just as you would with water) by wrapping a cooper wire through the exhaust manifold.

This should easily get you at 50mpg, and I AM NOT KIDDING, I dont know why people dont go more into heating and leaning mixtures out. And if you can control it so it only does it at low throttle openings (low load) then you will be completley safe when accelerating and under load when detonation occurs.

Oh, yes, and aero mods, that will help A TON!

-Tito

We have a '94 Miata. There are a few things you can do.

Free flow air cleaner, but you probably have one with the supercharger.
Header and cat back exhaust.
Lean it out at specific rpms, say you cruise at 3000rpm then only lean it at say 2900-3100 rpm and normal at other rpms so you can take advantage of the supercharger. You will pass thru that rpm when 'having fun'.
Smaller engine pulleys.
Lowering springs and a front lip spoiler or full ugly front spoiler.
Remove wiper blades depending on where you live.
Lighter wheels, alumn instead of 13" steel wheels.
Synthetic oil in engine, trans, diff.

Good luck with your project.