1994 Miata MPG project B3 STEALTH MK II-Page 13| Builds and Project Cars forum |

Doc Brown Dork
6/2/17 11:52 p.m.

In reply to Gunchsta:

Thanks, I think this is the only fully documented fuel efficiency project on the whole internet. My imposable goal of 50 MPG without trickery may soon be a reality. The little B3 engine has done well and has covered in excess of 60,000 miles without fault. Unfortunately I think its time to put this project to rest and move on to a different and more mainstream adventure. I had hoped to turbo the little B3 but realistically my heart is just not in it.

The Miata will wind away the rest of the summer in its current form and be rebuilt once again this winter. the good news is I'm going to continue to maximize performance on the least amount of fuel... just with a bigger engine....

alfadriver MegaDork
6/3/17 9:40 a.m.

In reply to Doc Brown:

Depending on your plans- one option may be to see what you can do with the 1.8.

I know I've posted this earlier in the thread- but a while ago I did some research about gear sets for the Miata. IIRC, by using a gear set from a late 80's RX7 and a very low final drive- you could pretty much match 1-3 gear ratios from the original Miata and a very nice drop in rpm for 4-5- with 5th being about 75% of the original.

And based on much of the driving I've done- the fuel economy is a little more limited by the cruising engine speed vs. the aerodynamics. So going from 4000 rpm to 3000 rpm at 70 mph should net a pretty sizeable fuel economy gain.

The other idea I had was to combine the above with a 1.6 and a small turbo- so that you had the 1.8l performance, including top gear acceleration, but 1.6 fuel consumption. It would be a trick to do well, but possible.

Doc Brown Dork
6/3/17 10:01 a.m.

In reply to alfadriver:

The RX-7 gear swap was one of my first mods when I built the B3 drivetrain. I did a complete write up with a bazillion photos and step by step instructions on Miata.net. I reckon replacing the 4.10 Torsen with the 3.90 Torsen might be enough to push this car into to 50 MPG zone. The desirable six speed gearbox that came with the parts car is being sold off to help fund the project.

The 3.90 Torsen is going to be installed next week... so results will be soon.

alfadriver MegaDork
6/3/17 12:16 p.m.

In reply to Doc Brown:

I'll have to look that up, as it's still a thought to try that in my car.

DaewooOfDeath SuperDork
6/12/17 3:13 a.m.

I just thought of something that might be fun. What if you tried a tuned length header with super short runners to get optimal scavenging at something like 2800 rpm?

Doc Brown Dork
6/16/17 11:30 p.m.

In reply to DaewooOfDeath:

Not that long ago there was a person over at the Ford Festiva/Aspire camp that was offering tubular header kits in knock down form. I don't believe these kits are still available, however that would have been the easy way to fab up a set of scavenging headers.

Meanwhile at Doc Brown's laboratory, progress is being made...

The 4.10 Torsen has served admirably and gives way to the 3.90 Torsen. The new differential will cause a 5% error in the odometer readings, but nothing a bit math cant fix. If my calculations are correct, the combination of the RX-7 5th gear and the 3.90 ring and pinion will reduce engine RPM by 430 @ 88 MPH. (back to future reference)

The differential swap more or less went without a hitch with the exception of the left axle.... This berkeleying b*stard axle was stuck in the hub and not letting go. I spent about an hour trying to coax the axle out but in the end I gave up and pulled the hub from the parts car. The plan all along was to use the axles that came with the 3.90 so I wasn't really too upset.

The most recent MPG data is not too shabby at 48.7....

More on that soon....

The NB parts car had decent leather interior that allegedly would fit in an NA. Whelp, some of the stuff fits and some of the stuff can be made to fit. The door panels will fit, however its borderline half ass. One of the deal breakers on using NB door panels is I would have to convert the car back to having power windows. Nope, ain't going to happen.

Let's look at some more pictures...

NB leather seats fit perfect. The rear plastic panels had to be modified for hard top mounting hardware. My 94 is also equipped with a cross brace (side impact reinforcement?) anyway, I had to further modify the plastic panels to accommodate the cross brace.

the NA door panels were dyed to match the NB parts. I used SEM 17293 flexible paint/dye to match the parchment color of the 2000 SE interior. Meh, my perception of color ain't all that great but others have indicated this is a perfect match. Looks good to me.

Oh yeah, I also swapped in the fancy Nardi steering wheel and six speed shift knob. The six speed shift knob has a nice feel on the five speed transmission.

The NB door threshold trim can be adapted to an NA, however new holes need to be drilled in the rocker panel. Nope, ain't doing that. I dyed the NA trim to match the rest of the interior.

For now I'm keeping the black carpeting....

Paint stuff.... Dying plastic involves several pre-treatment steps....bla bla bla....

Stay tuned!

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simon_C New Reader
6/17/17 12:24 p.m.

That looks pretty great

Doc Brown Dork
6/17/17 9:38 p.m.

In reply to simon_C:

Thanks!

Dusterbd13 UltimaDork
6/18/17 9:49 a.m.

I really like that interior. Lools very high class.

livinon2wheels GRM+ Memberand New Reader
6/28/18 5:41 p.m.

I love this project and what you have done...I have to wonder what kind of results you would have gotten by doing the tuning and choking down the exhaust on the stock engine and changing the gearing and keeping the torquey, by comparison, engine you had...restricting the exhaust would have helped bottom end torque, reduced fuel consumption, and all the other changes you made would likely have resulted in a more driveable car without the aggravation of the engine swap. But I have to hand it to you for guts and a willingness to go into uncharted waters. Interesting idea and approach for sure.

Doc Brown Dork
6/28/18 8:18 p.m.

In reply to livinon2wheels :

Thanks!

I learned a lot with the little B3 engine. For a street car the power was not too shabby. Anyway I learned a lot and felt the project needed to be taken to the next level.

You probably already know this, but the project is still alive and well........ now with more engine and more power. --------> Miata project with semi-stock engine.