Syonyk

Colder outside air makes sense - lower intake temperature, less fuel vaporization, worse burn. I've actually debated a "hot air intake" that I could switch to for highway cruising.

Also, I've noticed the engine isn't happy running below about 3000 RPM on the highway (as have other people who have driven the car & received tickets in it). It *really* wants to run hard - it feels happiest somewhere around 90mph. I think this is due to the lower speeds being outside the torque band (where the engine is most efficient).

-=Russ=-

Mint87RX7

i think i have a weird problem with my car. my O2 doesnt go to closed loop unless i am slightly accelarating, if im cruising it just runs leaner than the guage can read. does the same at idle, so lean the guage doesnt light up. think this has anything to do with the afm?

ramses666

iTrader: (1)

Interesting subject. I was under the impression that running under 3.5kRPM was going to be bad for the intake components due to EGR valve being open & the 5th & 6th ports being closed, which would contribute to carbon deposit build up & ultimately premature failure due to busted seals from carbon build-up falling into rotors!?! If I just "chug along" between 2 & 3k RPM's I am going to regret it in about 45K miles when the seals get damaged. I could be totally wrong. I don't know 4 sure about anything really. This was apparently the case of my "88 SE N/A which I inherited from my father. It had a new motor after 45K miles. I don't know why & never will know why the engine was replaced, but whenever I rode with him in it he was very tame with the throttle & RPM's. I remember riding in the car one time & there was this terrible loud clunking noise.. I was like.. what the hell is that? seemed like the air conditioner pump was screwed but maybe it was a rotor seal or something.

As far as mileage goes mine has been consistently 250 miles per 13.5 gallons(18.5 MPG) average city & 315 to 360(23.3-26.6) on the highway @ 80 to 100 MPH Bone Stock. I am 500 miles into a new R/B roadRace exhaust & K&N filter kit. The first tank was low miles because I really had my foot in it - 215 miles @ 14.5(OMG!) gallons (14.8 MPG) second tank was 215 @ 13.1 Gallons (16.4) both mixed driving, but my foot is still in it & I'm not running under 3k rpm's. Haven't run a whole tank on the highway yet since the upgrades. My SE seems to be comfortable around 100 MPH on the interstate but best economy seems 2 b around 80 MPH or so. Hope this was informative & not 2 rambling.

Ramses666

Adam

tire pressure, New plugs made a HUGE diffrence on my NA (47 psi)also got a pressure regulator for the fuel line, cheaper than safc But seems to work. i went from 11 mpg too 19mpg and raised it to like 21 mpg with the regulator set to the lowest setting without it stalling out, its an na and i dont even have a problem with pre detonation, however i noticed the kill switch for the fuel pump started getting hot, not sure if that is related to the regulator, but maybe the fuel pump is working harder now, But timing is a big thing too, also cleaning injectors will big time help ifthey leak! , i drive 60 miles a day and went from spending 15 bucks a day in gas too like 8 or less.

Snrub

iTrader: (1)

At least with my TII there's no way 80mph produces better millage than 70mph. All engine's maximum efficiency point is with a substantial amount of load, hense why highway millage is better than city, despite exponentially increasing wind resistance with an increase in velocity.

I didn't find an improvement with the exhaust, but I did find an improvement with the intake. (I would have through it would be the other way around)

coldfire

iTrader: (1)

on my S5 NA, i found an slight improvement going with a straight exhaust (stock manifold, no cats, straight mufflers). maybe on a non-turbo the exhaust makes a bigger difference for fuel consumption??

i will be putting in a cone intake in the near future, and i have an SAFCII that needs to be installed/tuned.
last summer i was getting good gas mileage on the highway though, around 28 mpg. it definetly goes down in the city.

after SAFCII and intake i will try for over 30 mpg, maybe even around 35mpg.
even before i could already get 600 km per tank (about 373 miles), and i shoot flames!

as for improving the economy? a lot of what has been mentioned helps. also make sure you have the lowest grade of oil as possible, your transmission and differential fluids are in proper order, tire pressure, etc.

WAYNE88N/A

What is that "maximun efficiency" gibberish? Simple physics- it takes far more energy to accelerate a given mass than it does to keep the same mass at a constant velocity, notwithstanding wind resistance...We accelerate at stop lights/stop signs in the city, hence the decreased mileage...

Actually, 80-85 mph cruise is about the point that fuel economy starts to suffer again, according to my projections. If you have a vac gauge on the car, you can follow this yourself- I can run damn near idle vac (15-16") cruising at 75, but at 80 it's closer to 13-14" (signifying that the air resistance is putting more of a load on the engine at that point- which will use more gas). Also depends on which way the prevailing winds are blowing, too...

coldfire

iTrader: (1)

i had discussed the physics earlier somewhere on fuel consumption and drag due to air and increasing your velocity...

i forget if it is a square or cubic relationship...but in any case, the faster you go, the smaller the difference has to be to use the same amount of fuel...for example, the increase in power needed at 120mph compared to 110mph is the same as the increase for 70mpg compared to 40mph.

Snrub

iTrader: (1)

Well you see, there's this thing called the internal combustion engine. One of the characteristics of it is that it is most efficient under a certain amount of load. (I'm not going to get into the 'whys') Efficiency is when something is able to perform a task by using the least amount of inputs. It's not a perfect relationship though and at a certain point it becomes less efficient. Driving at a constant speed of 35mph for a given distance will consume more fuel than traveling at 60mph over the same distance. Primary credit for this characteristic does not go to "flywheel effect" or momentum, the same applies with a conventional piston engine. Sorry for the sarcasm.

Marcus_F

If you want better fuel economy in both city and highway driving PLUS more power at WOT, buy a CD ignition box. Mine was under $100 and it easily increased fuel economy by 15%. If you understand the difference between what a capacitive discharge ignition with MSD does in comparison to the factory inductive system, it makes perfect sense.

torean

dont put a load on the engine will result in the best milage.... rolling up ur window when ur going fast helps also........ u save more gas when u drive in 5th gear then when ur driving in 3rd....its all common sense....just drive smoothly and ur mpg will go up.....also.....the coefficent of drag thing doesnt really play a big part until ur going above ~63mph.....dont ask me how to solve it....i prove it in math class awhile back.....

Tofuball

Not everyone's car is like that. Mine will keep pulling that same vac (15-16) crusing at 100.

IaMtHeRuThLeSs1

hmm...I have a suggestion no ones come up with.

How bout you bust out a freagin map and find the shortest distance to where you're going

Try weight reduction too, less mass to accelerate, get a skinnier gf or something.

Also, when you're coming up on a red light, slow down early, and wait for it to change instead of coming to a complete stop, every little bit helps.

My next suggestion would be to buy a bike...55mpg ALL day son!

Aaron Cake

iTrader: (1)

It's a mileage post, so I have to chime in. I just skimmed the thread, so some of what I write will be a repeat.

As one who regularily achieves 70MPG city and 90+ MPG highway in my Insight, I have learned the mileage tricks. Some of this can be applied to the FC, some cannot.

First and foremost, make sure the car is in good tune. This is a given. Fresh plugs, wires, fluids, filters, etc. Make sure the TPS is set, there are no vacuum leaks, etc. All the stuff that you SHOULD be doing anyway. And don't forget that O2 sensor.

Don't warm up the car in your driveway. There's absolutely no point, and it's bad for mileage, the car and the air. Start the car, do up your seatbelt, then drive easily until it warms up.

When driving, maintain smoothness at all times. Don't jackrabbit off the line at green lights. Gradually get on the gas while letting out the clutch, and use a VERY light throttle input. Shift at under 3 grand. If it takes you less then 20 seconds to get to 40 MPH, then you are driving too fast.

Conserve momentum. Don't fly to 40 MPH at each light if you just have to stop again, and when you approach a light, put the car into neutral and coast the rest of the way. Not only will this use less fuel, but you will be replacing brake pads far less often.

If you have the space, engine brake instead of using the friction brake. Downshift, then allow the engine to slow you down with NO throttle input. When the TPS reads that the throttle is closed, you get fuel cut until the engine reaches 1000 RPM. So you are burning no fuel by engine braking, which is more efficient then allowing the engine to idle while you coast.

You will also want to add some gauges. An A/F gauge will tell you when you are in closed loop, and make you learn to use very light throttle inputs to maintain closed loop as long as possible. A vacuum gauge will be a good indicator of the load on the engine. You want to drive for the highest vacuum possible.

If you want to mod, then a properly tuned S-AFC can give you an impressive mileage improvement. Once I had my S-AFC dialed in, my highway mileage went up by almost 100KM per tank. A CDI box helps as well, but the most bang for your buck will come from tuning your fuel curve.

Syonyk

Aaron - would would you suggest in terms of tuning a S-AFC for economy? Obviously lean it out throughout the power band slightly (at least on a NA), and more at cruise bands, but do you have any specific values you could share?

Also, what would the advantage be of taking out the O2 sensor and using the S-AFC to tweak things lean at cruise? Many of us don't have any catalytic converters to babysit.

For those without access to an A/F meter, a very simple way to check closed loop/open loop mode is to tie into the diagnostic connector (this works for the S4s, I don't know enough about S5s to provide this info). Two of the pins are for the ECU diagnostic readout, one is a +12v source (for powering LEDs), and the final one is an O2 sensor diagnostic. Build an appropriate LED device (LED, resistor, spade connectors) and connect it to the O2 diagnostic connector. It will be lit up when the O2 reads rich, and off when the O2 reads lean. Run the wires up the side of the engine bay so you can see the LED from driving (I tucked mine under the windshield wiper). Now go for a highway drive and pay attention to the light (bloody well pay attention to the road too). If it's flashing, you're in closed loop mode. It should be cycling 4-5 times/second. If it's flashing lazily (1-2 times/second or less), then your O2 sensor is bad and should be replaced. If it's lit up constantly, the engine is running rich (meaning the ECU is in open loop mode). If it's off, the engine is running lean (closed throttle fuel cut above 1500 RPM).

You'll notice that the ECU drops out of closed loop mode very easily on the highway. Digging into the gas when approaching a hill will drop it out of closed loop in a heartbeat. Feathering it in might stay in closed loop. Also, as near as I could tell, closed loop only exists up to 3500 RPM or so - beyond that, you're in open loop fuel sucker mode. It's actually possible to hear the difference as the engine drops out of closed loop mode. It's very, very subtle, but if you have a blinking light or AF meter to tell you when the changeover occurs, you can figure out what to listen for when you don't have the light installed.

Also, the TPS will affect fuel economy slightly due to the fuel cut at higher RPMs. If it's not reporting closed throttle to the ECU, the ECU won't cut fuel.

Another thing that hasn't been mentioned is injector cleaning. In addition to affecting power, a stringy spray won't fully vaporize, and will require more fuel to do the same work.

-=Russ=-

soundwavetsunami

If your car doesn't see any improvement from a better designed intake (whether it's cold air, ram air, or just a better flowing airfilter) you've got some bigger problems than gas mileage to worry about.

Think about it:
Your engine is getting more air than it's used to getting. It's like its naturally getting leaner. If your running leaner that means you're starting to lean out the pig-rich stock maps, which means more available horsepower and, in this case, more importantly, better mileage/efficiency. Higher efficiency means your car can do more (power, miles, whatever) with less gas. Which is what we're looking for. Efficiency is the name of the game when it comes to cars, whether your trying to break 10 seconds at the strip, trying to break 35 MPG, or a combination of those two. It's not impossible, just hard.



My waffles are done, so I'm gonna go take care of those...
-Billy

Syonyk

Ehm... you're right on the 3rd gens, but 2nd gens have a MAF (Mass Airflow Sensor) that will report the increased airflow to the ECU, which will add fuel accordingly.

A SAFC will allow us to lean things out by modifying the signal going into the ECU.

-=Russ=-

Aaron Cake

iTrader: (1)

As lean as you can go. 10-20% is not uncommon. Values are specific to the car. You would set your "low" throttle map as you lean-cruise map, and keep the high map tuned for performance.

Doubt it. The ECU uses the O2 sensor to try and maintain the optimal mixture in changing conditions. You couldn't compensate with a static S-AFC tune.

Thats total poop. The ECU knows exactly how much air is going into the engine via the AFM. It will look at airflow and the intake air temp sensor and make the mixture RICHER when you are sucking in cold air.

sub9lulu

iTrader: (7)

aaron thats what i've been doing !!!!
and i got a great 10 mpg avg

Syonyk

Hence my statement about taking the O2 sensor out of the loop. Based on my analysis from the O2 "status checker light" flashing, it's only used under light cruise. Removing it completely won't affect any part of the operation except light cruise, and if it's no longer feeding data at cruise, the ECU will stay in open loop mode, which *is* subject to SAFC modification. Lean it out as far as it will still run smoothly at light throttle on the highway, and see what kind of mileage I get. I should be able to comfortably run lean of stoich - I've heard reports of many people running 16:1 or 16.5:1 at cruise without any issues at all (except higher EGTs).

-=Russ=-

jgrts20

dont run so rich, thats how i got my mpg at a reasonable rate. I was getting 170 miles per tank before, now i get alittle bit more running leaner. But hey its an rx7 they never had good gas mile, hehe!!

ddub

My ported and fairly modified N/A (untuned) gets 17.5 mpg with ALL city driving stop and go to college every day.

The biggest thing I found was that a full tuneup really helps. Also, set the TPS, that was HUGE for me in changing things. And for those that keep rattling on about the O2 sensor, that doesn't effect mpg hardly at all, I've ran mine unhooked for.... 2 months or so now I believe. Still getting 17.5 mpg.

For tune-up, check this thread https://www.rx7club.com/showthread.p...hlight=tune-up

Read what RXSevenSymphonies posts, that's basically everything you need to do/check to do a full and complete "good" tune-up.

Syonyk

dDub - the O2 sensor won't affect city driving much at all. It only really starts doing it's thing during highway driving (sustained steady light load driving).

Take it out on the highway with it connected/disconnected & see what kind of difference you get.

-=Russ=-

ddub

I hardly ever go on the freeway in my rx7 so I don't really care. I still say it wont affect it much at all.

WAYNE88N/A

Man, if I had to drive most of the time in the city, stop n' go traffic, and crap like that, I'd get rid of the car, lol...