DaleClark

iTrader: (55)

Saw something on FB that made me think about this. I've seen this type of stuff for ages.

"My FD gets 12 mpg"
"If you wanted fuel economy, get a different car"
"Oh, why are you complaining about MPG? Are you a wuss?"
"I get bad fuel economy because I drive my car like I'm supposed to. If you aren't you are a wuss!"

Etc, etc. etc.

The FD is rated at 17mpg city and 25mpg highway on the window sticker. I once drove my FD FOR WORK doing service calls in town for a year or two before I bought a daily driver, I routinely got about 19mpg. Driving to Deal's Gap and back I got about 22mpg on the highway and 18mpg while hot ******* it up in the mountains.

If you get worse fuel economy -

- Your car is broken, or
- You are driving like an idiot.

There's no good reason to waste fuel.

OK, so let's address this if you are getting bad fuel economy.

On a stock or near-stock car, a bad O2 sensor will cause a lot of problems and you will run rich. This is the stock, plain vanilla 1-wire O2 sensor. They are pretty cheap new and not a big deal to replace. There is some troubleshooting you can do to verify that is the problem, look in the shop manual or search.

A bad connection to or a broken water temp sender (the green 2-wire at the back of the water pump housing) will cause problems. The sensors are pretty hardy but the connectors and wiring can be bad. This is typically a car that runs like crap when cold.

Weak ignition systems, either due to worn spark plugs, worn coil packs, etc. can also cause the car to run rich. Plugs need to be changed more often than most cars, probably 10-15,000 miles.

A Wideband air fuel gauge will help identify if something is off. You should be idling around 13 AFR and cruise at 14.7 AFR or higher. If numbers are lower than that, you have a problem.

If you have an aftermarket ECU, you may need to address the tune for fuel economy. Way too many variables there to go into with every different system. Most systems should cruise at a reasonable AFR, though. An un-tuned PowerFC will get good fuel economy, same as stock.

OK, next is how you drive the car. You don't have to do some weird hyper mileing, drafting, etc. to get decent fuel economy.

First off, start the car and GO. Don't let it sit there for 15 minutes "warming up" - the car doesn't need it and you are just wasting gas and fouling spark plugs. Start the car, let the idle settle, and go. Extended warm up does absolutely nothing.

Drive the car in the best gear for your cruising speed. You want to be close to but not below 2000 RPM. There's no reason to be cruising down the highway at 5000 RPM or something.

Also, you can get the same fuel economy with a large turbo or with a ported engine. If you are getting poor economy it's most likely something with your tune.

Dale

NoMiEzVR4

great post

madhat1111

Your 22mpg - what kind of driving was that? I got 20mpg doing highway driving (80-84mph) with the AC on...that seems reasonable. I average about 10mpg for normal driving, but I basically only take it out for spirited driving. I feel like that's low, so maybe I need to run through things listed above and double check.

Natey

iTrader: (19)

If I get more than 15-18MPG or so I feel like I'm not driving the (street-ported) car like it was made to be driven. Take it up to 6k+rpms, open up those 5th and 6th ports if you' have em, get both turbos spinning, and then get 30MPG in the daily driver.

F1blueRx7

iTrader: (5)

I recently pulled about 12mpg on an engine that had a chipped apex seal, way down on compression in the front rotor. I thought that was actually pretty good MPG considering it was only running on half a motor.

iceman4357

iTrader: (2)

Completely ironic timing. I literally yesterday hit the trip button to see how many miles per tank or half tank I would get. I have retuned the car, new plugs, Twin power, etc. Also ran a few bottles of Techron Fuel system through, which helped idle last season. Car idles around 13 AFR and cruises in the 14s. I think more than anything the $5/gallon for 93 is what peaked my interest in keeping track.

gdub29e

iTrader: (16)

Dale, I especially like the one "I get bad fuel economy because I drive my car like I'm supposed to. If you aren't you are a wuss!" you are definitely ruffleing some feathers haha.

On my last big cruise ( Maryland to NC and back ) it was a 1129 mile round trip. Granted that’s a lot of highway but I’m large street port, single turbo with all the gingerbread. I kept track and showed 23.4 miles to the gallon over the whole trip. You can have the performance with the fuel economy if you want. Having a solid tune is paramount. Keeping your foot out of it helps a lot too. I have noticed that I get substantially worse fuel mileage when the needle on the boost gauge goes past zero into the positive end, but that’s to be expected



~ GW

DaleClark

iTrader: (55)

On the highway I typically set cruise control at 75mph and go. I've done a LOT of driving for work and I've found that going 10-15 over the speed limit is just exhausting and really doesn't make up that much time on a trip. You are constantly looking for cops, getting mad when someone is slow and you have to pass them, slowing up and speeding down....it's just not worth it. Set the cruise control and chill out, you'll get there.

That said, I pulled up my logs from the Deal's Gap trip -

22.7, 18.3, 18.1, 20.2, 21.8

The middle part was ripping around the mountains but that whole area is so spread out you drive a half hour of steady cruising, go rip up and down the Dragon for a half hour or whatever, then cruise for 30 minutes again.

That's with AC on most of the time. A properly working AC system on an FD will have NO impact on fuel economy. You shouldn't even be able to tell it's on or not except that you aren't sweating. Oh, that was also with a passenger the whole time.

At bare minimum, fill up the tank then reset the trip meter in the car. Whenever you fill up, divide the miles on the trip meter by the gallons you put in the tank. Don't just go by how many miles you are driving between fill ups.

Next step is an app to track your fuel economy. I use an IOS app called Road Trip and it's fantastic. You put in your odometer reading, the price of gas, and the gallons you fill up with. It will then give you an immediate MPG for that trip and keep a running MPG average and also show how much you are spending on average over time. You can also put other expenses in there and have it remind you when you hit service intervals (ex. every 3000 miles it will remind you to change oil). It also tells you depressing things, such has the fact that I on average spend $9 PER DAY on gas for my RX-8. Fortunately my company pays for some of the fill ups but still.....

Anyhow, the app lets you see trends more easily as well. If you notice MPG sliding down it's probably time for new plugs or see what's wrong with the car.

Dale

DaleClark

iTrader: (55)

This is a false argument. There's no way you will be blasting your car ALL THE TIME on public streets. Light turns green, you may rip 1st and 2nd gears, then you are cruising again, or you may rip up an on-ramp. That's fine, that's good. But it's literally impossible to drive full bore ALL THE TIME on city streets. The only place you can do that is a track. This isn't even getting into the "save it for the track" argument, my car is a street car and I enjoy the power and get on it as much as possible. But, after you get away from that light you'll either be on a stretch of road for a while so you're just cruising or you hit other traffic so you get in line with them.

Again, if you get poor fuel economy, YOUR CAR IS BROKEN or YOU ARE DOING SOMETHING WRONG. I'm all for enjoying my car but I'm not for running 10 AFR's cruising along the street for no good reason. That doesn't translate into having fun with the car, that just means you are wasting fuel. It's literally the same as buying a bunch of gas and throwing it in a dumpster, you aren't turning that gas into a fun experience in the car, you are just needlessly burning it at steady cruise on the street.

Dale

cloud9

iTrader: (4)

My car is still on the factory ecu and the mileage is pretty bad. Actually, it's laughably terrible. Rough approximation I'd say it's between 13-15mpg (and I do not drive like an idiot). The reason I haven't put any effort into investigating it is because I have a Link ecu, fuel mods and a bunch of other bits that will be going in during the next round of upgrades.

To-date I always attributed the poor mileage to the factory ecu but this thread makes me question whether something else is actually wrong.

Pete_89T2

iTrader: (4)

^And this is why road tuning a single turbo FD (or any other high HP boosted car) on public roads is so damned difficult! I've been trying to road tune mine (Link G4+ Fury) now for over a year, and it seems I ALWAYS run out of road, or need to back off because of traffic before I can sustain boost beyond my waste gate spring levels (~8psi).

Anyway, the Deal's Gap trip this year was my first chance to really characterize my FD's mileage. My MPGs are not quite as good as Dale's, but not too far behind - best tank was just shy of 22MPG, worst tank was 16MPG, overall average was 19MPG. Looks like I can stand to lean out more of the vacuum/cruise regions of my fuel map to get closer to stoich. Only reason I haven't done so yet is because I currently lack the ability to measure/log EGTs, and sustained cruising (i.e., several hours worth) a rotary at stoich AFRs can lead to EGTs that get high enough to cause problems.

I use the attached spreadsheet to track my FD (and FC) fuel mileage, and also to track my pre-mix oil consumption rate with my RA OMP adapter setups (i.e., pre-mix is fed to the OMP from a separate tank, so I need to track that to ensure my ECU is delivering enough lube).

ZE Power MX6

iTrader: (157)

Interesting that you started this thread Dale, I think the same FB post also sparked a discussion between me and my buddy about gas mileage lol.

We went to an autox event together couple weeks ago, and I clocked in 22.83mpg on the way down to the event, which is about 3.5 hours away. That was probably 99.9% highway cruises with a couple WOT pull to check things out, because that was the first time I started the car in about 4 months The next fill up I clocked in at 20.5mpg, that included the autox and part of the the drive back. BTW I bought this PFC from you nearly 10 years ago, beside the tuning Steve did everything else should still be the same as when you sold it to me

BLUE TII

iTrader: (9)

Hahaha, this thread is silly.

Best I have gotten was 26mpg in FD driving like an absolute grandma.

Any tank that has had racing on it is usually single digit gas mileage. If its all track I have gotten as low as 5mpg.

Its not broken...

Further-
If your making 2x stock power with the same conservative tuning as stock and using that power, you are goimg to use 2x as much gas.

Cruise gas mileage should be same as stock or better.

dabigesii

iTrader: (4)

Very true. I also feel like all the "flame" tunes hurt economy also. People want to be all flashy, then complain that the car gets horrible gas miles.

j9fd3s

iTrader: (3)

the stock ECU switches out of closed loop at 3200rpm, and i think peak economy is right below that, like 3199rpm. my friend and i did a Sevenstock run (~450 miles each way), and we cruised at 3100rpm, and i got 23mpg, and he got 25. my car was stock, and his was ported with no cat.... opening the exhaust always helps mileage, lol

DaleClark

iTrader: (55)

I would resolve that problem on the stock ECU before moving to the aftermarket ECU. The stock ECU is a known place to start with, heavily documented with lots of guides and troubleshooting out there. Once you go standalone ECU many times you are on your own trying to figure things out.

Could be as simple as a bad O2 sensor. That's worth the $30 or whatever for the part and a half hour to swap out.

Factory ECU really should get good fuel economy, emissions was one of the primary goals in the design since they can't sell a car that can't pass emissions standards and you don't pass emissions by dumping fuel.

Another thing to think about is if the car is over-COOLING. A stuck open thermostat will do that. If the car never gets up to temperature it will continuously run rich.

Dale

j9fd3s

iTrader: (3)

my REW swapped FC used to get 12-13mpg, but its really fun to drive it like an idiot, and i never go anywhere so i wasn't too bothered

but then i hooked up the idle air bleed for the injectors and suddenly i was getting 17mpg, and then the next tank was 21...

fc3s-ty

iTrader: (11)

Interesting thread Dale!

My 2c and experience (strictly speaking for mpg)

The rhd is getting mid to high teens- checked my map, and logging, and seems to be cruising right around your noted afr’s. Most of the emissions equip is still on, bolt on mods, new plugs.

It’s a riot to drive, haven’t cared to TRY to get max econ LOL. Seems the tuner from overseas really set the car up well. I am certain I can easily be in the low 20s.

my FC was always a solid mid 20’s mpg (n/a at the time though).

FULL tune up, cruise @ 70-75, stay mild with the boost, and, wow such a surprise, decent MPG. 🤷


FWIW (in case the rotary vs ### comes up) Every “sports car” (20k-60k price range, 300-400hp, varying engines, give or take some values), or performance car in general, gets exactly this mpg (20s)

Friend’s cars, from Evos to Bimmers, my old 370z, my old Q50, and a few domestics, Mustangs, Challengers, etc. Modded, stock, mixed bag.

The best economy I got in my Q50 (VR30DTT, no mods) was 28mpg, and yes, that was a dedicated no pull, no boost cruise on the freeway @ 75mph. Best the Z ever got was 25. Boring, mundane driving.

Unless someone specifically is out to prove they got “30+ mpg in my V8”, one magical trip on highway only with no pulls, they average low to mid 20s it seems.

It’s not a bad thing. Its amazing ANY performance-oriented car can push these mpgs. Let’s enjoy the cars for what they are, and know that the FD can be just fine. Get your car tuned up!

gdub29e

iTrader: (16)

I actually laughed reading this. I keep thinking of all the people who’ve capped this off over the years, stock injectors definitely benefit from this.


~ GW

j9fd3s

iTrader: (3)

+1 i was surprised at the difference it made.

Natey

iTrader: (19)

Nope, not false and c'mon now. Doesn't that window sticker you mentioned fall right into the 18 or so mpg I get?
I live 45 min from a race track and use my car mostly for that. I'm not too worried about MPG on the way down the Rainey Curve. And, no my feathers aren't ruffled nor do I think I'm an authority in all things RX-7..
I'm speaking from my experience driving my FD for the last 2 decades. 30MPG on a street ported motor doesn't happen in my car. No need to capslock me, get your undies bunched up or tell me I'm throwing gas in a dumpster because I like to go drive the **** out of my car. That's what it was made for if I'm not mistaken.

Maybe I'm wrong. Do you think Mazda had MPG in mind when they put a twin turbo rotary engine in their halo sports car?

Pete_89T2

iTrader: (4)

^Am I correct in thinking this only applies to the original OEM fuel injectors Mazda used back in the '90s because they didn't atomize fuel nearly as well as more modern top feed FI's do these days? I'm using the current versions of the ID 1050XDS & ID 1700XDS injectors FWIW.

DaleClark

iTrader: (55)

I think so? Don't know if anyone has really done much MPG testing or anything with aftermarket injectors.

Dale

DaleClark

iTrader: (55)

In your initial post you. said 15-18. If you are on the bottom end of that scale in typical city driving even with getting on it when you can that's really on the low side.

Also no one ever said 30mpg. That's not gonna happen unless you are doing some crazy hyper mileing and that isn't what this is about. It's about having the car running right and getting normal MPG.

Yes, Mazda did have MPG in mind when they made the car. They also had emissions in mind. One of the stories from the FC development was the prototype was going to get terrible fuel economy. and get the gas guzzler tax. Mazda didn't want to have their halo car labeled as a gas guzzler so they started "Operation Gram Per Head" where they took every part of an FC and laid it out in a warehouse and had each engineer find at least 1 gram of weight they can remove from the car. That's where the lightweight seats, aluminum hood, lightweight engine harness, cast aluminum suspension, etc. all came from. They carried over that philosophy to the FD.

Your initial post left out a lot of what you have just said. Again, it's flat out impossible to drive full throttle in any car in a street driving situation. Period. Race track is a totally different thing. When you're not fully on the throttle you are cruising and should be burning just enough fuel to move the car down the road, not dumping gas for no good reason.

My comments are moreso directed to the MANY comments over the years I've seen on the forum, FB groups, and even the old mailing list that seem to equate MPG with "you're a wuss" or "drive the car like a real man". That's an idiotic argument.

Dale

gdub29e

iTrader: (16)

Pete, the general consensus around the forum was the mixing port is not necessary with new modern injectors due to their atomization / spray pattern. I have not seen any concrete information whether or not it makes a difference with modern injectors. Unfortunately when a lot of people went simplified sequential they removed the mixing port hose and capped it not realizing what it actually did. Mazda went to great lengths when designing the primary injector air bleed area, the primary injector location and those pesky plastic diffusers. Clearly they did extensive testing with it and as you see from Mike’s experience, that makes a noticeful difference in fuel economy.

~ GW