1995miata

After reading about the ranges of fuel mileage NA's are getting, I bagan to wonder, "What allows one NA to get 3-5 mpgs more than another of the same basic model?"

So, apart from having tuneup parts up-to-date and running stock configurations in normally aspirated cars, what appears to be the cause for one NA6 or NA8 to squeeze out better fuel mileage?

AFM type or cleanness?
Plug heat rating?
Tire/wheel size?
Average rpm ranges?

I drive very consistently, do not redline or abuse my NA8, keep the mechanicals in tiptop shape - and get between 28 and almost 30 mpg with very accurate mileage calculations at the pump. I'd like to cross the 30mpg mark if there's something I can do. Any ideas?

RedTurboMiata

thought about this, only thing that comes to mind is clean throttle body and maf. maybe add an aftermarket ecu that you can tune get a power bump and also have full control over the fuel and timing table. but 30mpg is really good for a sports car.
Coming from a guy who is shooting for power only...

mytwo

I am not sure how you get any better than the 28. On a good day I get 27 tops
I talked with a guy that has a 93 and he says he gets 40 .I find that a little reaching myself
Driving only 50 mph, wind to your back, top up, headlights down, tires inflated properly and maybe having 15 " tires, a good tuneup and the items that RedTurbo mentions may get you that 30
Then what fun is driving that slow with the top up ?

sixshooter

The '93 and earlier cars had smaller, lighter brakes, differentials, flywheels, and bodies with much less bracing, less displacement, one less airbag, and typically fewer additional factory options. If you were looking to hypermile, those would be the ones to have.

mx5tc

Most 1.6L NAs had batch fuel injection, not sequential. CA 1.6L cars had sequential (instructions in the Link ECU installation kit detail the mods required to convert a non CA car to sequential injection). I haven't seen the OEM 1.6L injectors on a injector test flow bench but I suspect that they have very poor atomization compared to similarly sized modern fuel injectors. Older Jeeps (late 90s early 2000s) often get significant fuel economy gains by swapping the OEM injectors for later replacements with finer fuel atomization. Early NAs also had 4.30 gears which hurts fuel economy.

As another poster also mentioned, the stock AFM and air cleaner are restrictive and the stock exhaust is also restrictive. Best AFR for fuel economy is typically leaner than the best AFR (14.7 to 1) for exhaust emissions. Also, the stock 195 degree thermostat is optimized for emissions, not fuel economy.

I suspect you could get a 1.6L NA into the low 40mpg range at reasonable highway speeds if you did the following:

-MS conversion (eliminate AFM, optimize fuel and timing curves, utilize wideband)
- 175-180 degree thermostat (fan settings adjusted accordingly in MS)
-Wide band tuning of fuel map for cruise to optimize economy (leaner than 14.7 stoich)
- Low restriction air intake & low restriction exhaust
-3.63 or 3.90 gearing
-Replacement fuel injectors with improved atomization pattern over stock
-Tweaking of cam timing (EXIN swap and/or adjustable cam gears) to optimize engine efficiency @ cruise RPM
-Running reasonably sized tires (195 or 205s) at higher pressures (35-40psi)
-Running stock or minimally advanced timing in order to run 87 octane fuel (87 octane has more BTUs per gallon than higher octane gas)

YMMV,

Mitch

sixshooter

More timing advance at cruise will net mileage improvements.