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Improved Efficiency

We have seen many Miatas built to go faster, but how about one modified for improved fuel economy? Meet Jim Dowd’s 1994 model. Power now comes from a 1.3-liter engine originally fitted to a Ford Festiva, and Jim reports 40–45 mpg.

“The SOHC Festiva engine bolts right up to the Miata transmission, and the Miata motor mounts bolt up to the Festiva block,” his GRM build thread reports. “I mixed and matched Miata parts and Festiva parts, and this is almost a bolt-together swap.” A taller fifth gear from a Mazda RX-7 and a custom ignition setup also help economy.

“This car can keep up with a stock Miata up to 5000 rpm,” he reports. “Then it falls on its face. The beauty of this swap is the car behaves exactly like a normal Miata during the daily commute, making this a stealth economy car. Given the choice, I would rather drive a detuned Miata than a Toyota Prius.”

Want to read up on how this car was built? Check out its build thread.

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Comments

View comments on the GRM forums
ClutchMeister
ClutchMeister
8/27/16 11:45 p.m.

Wait so your saying that this car is only good for fuel economy? other then that even a stock miata takes it on all angles... if thats the case then its a worthless swap!

Driven5
Driven5 Dork
8/28/16 1:47 a.m.
ClutchMeister wrote: Wait so your saying that this car is only good for fuel economy? other then that even a stock miata takes it on all angles... if thats the case then its a worthless swap!

Trading power for efficiency is no less "worthless" than trading efficiency for power...Not that this suddenly negates any of the other great aspect to owning and driving a Miata either. I say kudos!

alfadriver
alfadriver MegaDork
8/28/16 7:42 a.m.
ClutchMeister wrote: Wait so your saying that this car is only good for fuel economy? other then that even a stock miata takes it on all angles... if thats the case then its a worthless swap!

To you, perhaps. I'd love to get a big jump in fuel economy for my Miata.

Knurled
Knurled MegaDork
8/28/16 10:20 p.m.

OMG THIS POST IS BOLD

I have always wondered if a 16 valve B6 head's valves would clear a B3's bores. A 16v 1.3l sounds kinky enough to try. Sure it might still only make 110-115hp but that is pretty well strung for only 1.3l so it should be a riot to drive.

WildScotsRacing
WildScotsRacing HalfDork
8/28/16 10:31 p.m.

In reply to ClutchMeister:

You will learn quickly (if you choose to notice) that around here, stating a purely preferential opinion as if it were undisputed fact will gain you neither friends nor respect. Especially regarding someone else's preferences for modding their car.

Dietcoke
Dietcoke Reader
8/28/16 10:49 p.m.

nothing like making an anemic car slower.

Doc Brown
Doc Brown Dork
8/31/16 6:55 a.m.

Awesome, my oddball Miata got a full page in GRM! Obviously the car is not meant to be driven on the track, however the performance on the street will surprise a lot of folks.

Anyway, this just made my day!

alfadriver
alfadriver MegaDork
8/31/16 7:20 a.m.
Dietcoke wrote: nothing like making an anemic car slower.

Just as good as car people being judgmental of other car people.

Wall-e
Wall-e MegaDork
8/31/16 7:25 a.m.

Why is everyone using such big letters? OMG my letters are big too!

alfadriver
alfadriver MegaDork
8/31/16 7:34 a.m.
Wall-e wrote: Why is everyone using such big letters? OMG my letters are big too!

David left an extra b in his post. I'm sure that didn't fix it. again.

IT DID!!!

Paul_VR6
Paul_VR6 Dork
8/31/16 7:57 a.m.

It would be interesting to poll the guys with stock or near stock Miatas with 1.6L-1.8L and standalone that can tune "differently" than stock. I would think that with more leeway in tuning it, and a light right foot similar results could be had with the larger motors. That being said I think a small displacement engine swap WITH forced induction could reach both goals quite easily.

alfadriver
alfadriver MegaDork
8/31/16 8:31 a.m.

In reply to Paul_VR6:

I don't think there is as much to be gained as you think. At light loads, spark is almost always at it's best, and then you are just limited with fuel, which will reach the lean limit pretty fast in most cars. EGR changes may add things, but most would take it out, when putting more reduces pumping losses more than losing combustion efficiency.

For Doc's work, I've thought about doing the trans and final drive changes- where a combination of parts would give you 1-3 wheel rates pretty close to stock, but 4th and 5th would be far taller. That would be a big help (to a point).

rslifkin
rslifkin Dork
8/31/16 8:55 a.m.

Leaning things out at cruise could definitely help a bit, provided you don't mind NOx emissions going through the roof. Bringing the cruise AFRs up to around 15.7:1 and adding in some extra timing to make up for the lean burn being slower in the Jeep was worth somewhere around a 5 - 10% MPG gain from what I could determine. I could have gone a bit further, but it's already hard to keep the transition in/out of the now leaner closed loop smooth with it leaner than about 15.5:1, so I didn't feel chasing further gains was worth it due to the tuning abilities of the stock ECU being kinda crappy.

With the lean burn tune, it'll get 17.5 - 18 mpg with the cruise set at 75 now. When the motor was stock and running on a stock tune, it struggled to get that mileage at any speed without drafting or a tailwind, and to even get close, you had to keep the speed to 60-ish.

On something geared more optimally for its engine size, the gains may be a bit smaller (the Jeep would benefit from somewhat taller gearing).

EGR helps too as Alfa said. Basically anything that leads to the throttle being further open while cruising will reduce pumping losses and help mpg. Taller gearing helps too, up to a point. If you gear it too tall, you'll spend more time in power enrichment near WOT, which can negate the gains from reduced pumping losses. I've noticed that with a trailer behind the Jeep, as there's typically less than 1 mpg difference between cruising in 3rd and closed loop / lean burn vs staying in 4th and spending a lot of time in power enrichment.

pimpm3
pimpm3 Dork
8/31/16 12:25 p.m.

If you read doc's build thread he was very thorough in his approach and documented why he did what he did. Give it a read before making assumptions or wondering why he decided to pursue a different performance aspect.

Paul_VR6
Paul_VR6 Dork
8/31/16 12:26 p.m.
alfadriver wrote: In reply to Paul_VR6: I don't think there is as much to be gained as you think. At light loads, spark is almost always at it's best, and then you are just limited with fuel, which will reach the lean limit pretty fast in most cars. EGR changes may add things, but most would take it out, when putting more reduces pumping losses more than losing combustion efficiency. For Doc's work, I've thought about doing the trans and final drive changes- where a combination of parts would give you 1-3 wheel rates pretty close to stock, but 4th and 5th would be far taller. That would be a big help (to a point).

I tuned a 2L VW 20v running high comp on pump gas and he would regularly see upwards of 35mpg tanks with "normal" driving, but much less on the go pedal. I would think that a much smaller motor, even stock, could get there with similar tuning techniques. Lean it out, more timing, etc. This one the sweet spot was 16.2:1 and liked a lot of timing to not surge. I think with real vvt and doing egr with the intake cam it could have done better but this head had all that stripped off.

One of those things I'd never do myself, but cool to see others pursue.

alfadriver
alfadriver MegaDork
8/31/16 12:47 p.m.

In reply to Paul_VR6:

There's calibration and then there's driving light- they are not the same thing to me.

And at light loads, most engines already are at best spark- adding more isn't going to help- going past MBT isn't beneficial. I'd be stunned that at light load, the VCT isn't already at it's best pumping for best combustion. The only real thing that can be messed with is target air fuel. And the lean limit is very different for various combustion chambers- some will limit at 15.5 without surging, others can go to 18:1. None of that is in a stock tune, due to emissions, though. Spark to adjust for that, I can see.

Other than lean burn, hours are spent getting the engine to run it's best.

One of the things I've realized over time- most engines do a bad job making 5-25 hp. Which is really where we spend most of the time. How to fix that, that's a pretty deep question. But for a Miata- according to the EPA model, an NB is about 9hp at 50mph. Which is roughly 18hp at 70mph (not exactly- the model back in '00 wasn't a real coastdown- but ok for arguments sake).

Based on my drive, 70mph is about 4000rpm, and at that engine speed it's just at 30mph. But making 18 hp at 4000 rpm is really easy- and it's likely to be far more efficient at 3000 or 2500 rpm.

Even nominal accelerations are not near the rated power of the engine- which is one of the things Doc has figured out that the 1.3 is nominally good enough.

Paul_VR6
Paul_VR6 Dork
8/31/16 2:24 p.m.

Exactly, we never found the lean limit of this engine just the limit of the gauge. We were spark tuning to combat surge, and we didn't have access to enough funds to care about mbt at those loads. Just trying hard to minimize fuel use at the times we could, with some of the 'normal car' parameters out of the way. Funny but this same car could easily pass sniffer emissions tests configured like that, but obviously wildly more open than a new engine would be allowed to do.

Just to think about all the losses you incur most of the time just to have 100-200% more power on tap. Though put a 5hp motor in my car and I sure won't be happy most of the time... until I'm at speed, then I don't care!

alfadriver
alfadriver MegaDork
8/31/16 2:33 p.m.

In reply to Paul_VR6:

FWIW, sniffer tests are pretty useless. The "standards" for most are barely harder than a good running, unrestricted engine.

as for the low power part, you are missing my point- given what you actually need to normally move a car, which is actually in the 5-25 hp range for most cars, that low of output isn't done that well. Engines are more efficient generating much more power than that. To the point that it's not impossible to get almost the same miles per gallon at 40mph as at 70 mph. The ability for an internal combusion engine to do that range well is why they are used over a gas turbine (which can be more efficient).

Its a really difficult balance.

Knurled
Knurled MegaDork
8/31/16 6:42 p.m.

BSFC can be a weird thing. The Subaru I had got best fuel economy in 4th gear. 5th gear was too tall, it did not like cruising at 60mph at only 2800rpm.

Likewise, I was able to glean quite a bit over 30mpg with a carbureted RX-7 with a bunch of timing tweaks, fuel pressure tweaks, and a cruise engine speed in the 4500-5500rpm range. This isn't that crazy to think about given that there were a few years where 4-speed transmissions were standard and 5-speed were optional, the ONLY difference drivetrain-wise was whether 5th was installed or not... and they both had the same EPA economy rating. 5th gear only served to silence the reactionary drivers opposed to a revolutionary and superior manner of driving a rotary.

Paul_VR6
Paul_VR6 Dork
9/1/16 9:26 a.m.

I am sure some of that was to keep people's ears from bleeding listening to a rotary ;)

Shaun
Shaun HalfDork
9/1/16 2:46 p.m.

I really enjoyed reading Jim's build thread as it was filled clever thinking, building, and is written very well with lots of humor. As I bothered to swap the puter', head, engine harness, intake manifold, and all the other bits and then some needed to turn a plain old 16valve NON Vtak YO! 1.6 D16 96 civic hatch into a lean burn VtecK YO! 1.6L D16Y5 96 civic hatch I can relate to looking for efficiency as sport. Mileage bumped from roughly 30 to roughly 40. Probably saved no money, defiantly had fun.

Knurled
Knurled MegaDork
9/1/16 7:23 p.m.
Paul_VR6 wrote: I am sure some of that was to keep people's ears from bleeding listening to a rotary ;)

You'd think that now, but back in that timeframe, Mazda was advertising the RX-7 as the "Silent Sports Car". And they WERE quiet. Most engine noise you'd hear would be a little bit of intake honk, and some geary mechanical sounds, probably from the necessarily straight-cut rotor and stationary gears. Hardly any exhaust at all.

Son_Of_Toyman
Son_Of_Toyman Reader
9/1/16 8:00 p.m.

I actually think the idea is pretty cool. Would I do it, probably not but it is still pretty cool. Nice build!

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