1NSIGHT

iTrader: (3)

Ive been trying hard to get my mpg up on the highways. Im running as lean as i can cruising and i cannot crack 16mpg. Anybody have any tips?

t-von

iTrader: (8)

More info on your setup will help? Engine configuration, porting, body kit ect! Aerodynamics play a big part too. My Fd NA 20b has been sitting at 16mpg on the highway cruising at 80 mph also. I'm trying to get is closer to 19 mpg.

R_PROWESS

iTrader: (3)

Have you tried negative split?

j9fd3s

iTrader: (3)

the factory is around 32-35 degrees of timing at cruise (we can monitor the Rx8 directly), split is generally small. the ECU is in closed loop so AFR is around 14.7:1.

rpm depends on the model, but its generally 2400-3200 @65mph. i've found that lower rpms are not always better for mileage. for instance my Rx8 gets best mileage around 70mph, which is like 3500+ rpm. my 79 also got best mileage between 3500 and 4000 rpm, which was about 65 in 4th or 80 in 5th. since its MILEage, and you're going faster, i think that has something to do with it.

tires should be at least inflated to whats on the sticker, 32psi, a little more usually helps. alignment should be reasonable, too much toe in or out will scrub tires and kill mileage. bearings should be decent, brakes shouldn't be stuck.

lastphaseofthis

iTrader: (8)

i asked mazda maniac this question during a webinare and was told that lean as possible is not best for cruising, tune for maximum vacuum while holding a steady speed. you'll find the afr's a little richer results in less air, and therefore fuel, and higher vacuum in the intake.
you want to make the most power, with the least throttle.

t-von

iTrader: (8)

Hmm I need to try some of this stuff. I just ordered a new aem 30-5130 wideband sensor kit to replace my innovate lc1. It's has been acting to erratic even with a new calibrated sensor. I've been running negative split and only 20degrees at cruise. We'll see if I can get these numbers up.

t-von

iTrader: (8)

Could you clarify your explanation? "and therefore fuel" part is confusing me.

diabolical1

iTrader: (3)

yes, my first thought would be to ask what you're driving, what mods (porting, etc.) and how you're calculating the 16 MPG, particularly what kind of actual highway mileage are you covering during your calculation? over the years i have tracked mileage on a few of my cars and what i found was VERY similar to what J9fd3s described above.

my '84 (13B/Dell'Orto combo) was getting 24+, but i'd be damned if i could tell you what timing i ran on it, but probably stock or close to it. the Rx-8 gets 25+ (RB exhaust, stock timing). faster definitely help, to a point. overall, i did/do most of my highway driving between 70 and 80 MPH and i think it worked out to be around 4000 on both cars. however, the distance also makes a difference. i have/had the luxury of running anywhere between 50 and 250 miles at a time. i took a short reading just for the hell of it a few times (maybe 10 or 20 miles or so) and the readings got skewed upward to like 27 or 28. i would tend to think any significant mixing of city miles in your data would skew it just as ridiculously, only downward.

diabolical1

iTrader: (3)

i didn't quite get this either, on a few levels.

this, i do get though. funnily enough i learned it fairly recently (maybe 2013) while researching to do some work on my Impala. it seems counter-intuitive at first, but when you think about it, it totally makes sense.

j9fd3s

iTrader: (3)

traditionally i've liked the sevenstock drive, its 380miles each way, all freeway.

Tvon, i would try more timing in the cruise area, 20 is ok, but sounds low, even the distributor is running more than that. i don't think it will be a night and day difference, but put some small pieces together and you can make a pretty big difference.

Mr Phase; MM has a point, tune for best BSFC. i've never tried it myself, my best mileage setups have been an exhaust with the stock ecu, and the feedback is cycling between the engine being too lean to run, and running.

1NSIGHT

iTrader: (3)

Ok. Im running a street port on a fresh motor. stock primaries stock turbo Haltech ECU.

I get about 10-8in of vaccum at 70ish and 2800-3000 rpm and about 14.8 AFR.

My tires are alil grippy and I probably could use another alignment its been over a year. But I have never broken 17mpg since I went turbo. I was really hoping to get 20. LOL

Are there any critics on the map overall?

j9fd3s

iTrader: (3)

looks ok to me.

EDIT: since you ask, how much vacuum does your engine pull at full decel? i think you could probably remove the -28psi column. instead i think i'd make a -6psi column, you could have both, but i like that you can make the map really small, makes it easier to start with.

lastphaseofthis

iTrader: (8)

try a 13.8 to 13.5 afr, and monitor your fuel pulsewidth before and after. the lower your injector pulsewidth the better your milage. after you are at 13.8 afr give it more timing and see if the pulse width moves.

j9fd3s

iTrader: (3)

there is a point here, AFR = air to fuel RATIO, its relative. pulse width = fuel volume, sorta.

lastphaseofthis

iTrader: (8)

if your tps shows 10 percent then your giving that much in fuel, if running richer enables you to close the throttle( and increase vacuum) to say 7 percent, then your running less fuel, even if it's more fuel per air, its less fuel over all.

lastphaseofthis

iTrader: (8)

minimum pulsewidth at given SPEED. Ie 70 mph.

t-von

iTrader: (8)

This seams backwards in so many ways but I'm starting to see your point. So what it all boils down too is cruising at the same speed with less throttle than before to increase MPG. I got a nice long 700 mile round trip to Dallas next weekend to test this out.

Edit: My bone stock fd with cold air intake and downpipe averaged 25-26 hwy mpg with A/C on before I did the conversion.

t-von

iTrader: (8)

Definitely will be changing the timming.

Gilgamesh

On street ported FD, i have noticed at 80mph timing is around 30 (zero split) and 20% duty on 550/1600 combo.

when i retarded the timing across the entire map via the commander, i noticed it needed a little more gas to make the power to stay at 80mph. so inj duty was 23-25% at 25 degrees or so.

lOOkatme

iTrader: (19)

I have played around a lot with mine for various reasons. The only thing you need to monitor really is what is your input to get a desired output. you want highest MPG, so you want the least input of fuel.

You can measure how successful you are by cruising and watching your injector duty cycle. If you are really picky about MPG and want to maximize it, you will need to run a hair richer in the lower rpm region, say 2000-3000rpms and cruise at 60mph or so. run the car with an AFR that can maintain this speed in this region. I have found a good AFR is 13.7. This has a good amount of power there to maintain speed. the slower the RPM's the less fuel is used. I have gotten in the mid 20's MPG on my single turbo rx7 and doing so while doing spirited canyon runs while boosting the turbo. The thing is, I am running the car between 8000-10000 feet above sea level. less air density and my 1 bar boost is only 24.7 absolute manifold pressure, the same as running 10PSI at sea level.

as rpm increases you can decrease your AFR for better mileage. I like 14.3-15.3 AFR depending on speed of engine rpm. I only run this lean for light load cruise cells, I then ramp around this to 14.0 and 13.0 when load increases to help spool the turbo. on super light downhill throttle or flat light load throttle run a little richer 13.7-14.0 afr for smoothest cruising.

side note. I pull -42 in-HG vacuum when off the throttle cruising down a mountain. I idle around -21 to -26 in-HG in the mountains as well. when not in the mountains I am still 6300ft above sea level.

I run stock timing in the cruise cells.

t-von

iTrader: (8)

Wow your post basically proves that my timming is way too low at cruise. 5% is a big difference with only 5 degrees. What Im gonna do this weekend is 1st increase the timming to 30degrees and not change my a/f to see what I get on the way to Dallas. Then I will change my a/f to around 13.8 on the way back and see if that makes a difference also.

j9fd3s

iTrader: (3)

that sounds like a good plan!

peejay

What engine do you have? I didn't see that anywhere in the thread.

With 9.4 rotors (12A late style) and late 12A rotor housings I saw no advantage to cruising over 22-25 degrees leading, but I used a lot of negative split under cruise. With S4 N/A rotors and GSL-SE rotor housings (and a bridge port) there are negative gains if timing goes over 22-24ish at highway cruise. Either way excessive timing means negative work being done tryign to compress a burning charge, and that is wasted fuel.

Leaner is not always better. If the engine misfires 5 times out of 100 you won't really feel it but that is 5% of the fuel wasted right there, and the engine will be making less power so you will have to up the load. Don't be afraid to try running richer. I've run as rich as 12.5:1 under cruise for best fuel economy. I am running a bit leaner now because I wanted to trade off a little economy for not having to change the oil every 1000 miles.

LOSE THE TIRES. Sticky tires is the difference between 16mpg and 23mpg for me. There's no point to having ultra sticky tires on the street, you will never be legally allowed to use that much grip, so all you are doing is heat-cycling away your fun rubber and wasting fuel.

peejay

HUH???

Absolute vacuum is a shade under 30 inches of mercury, 42 inches of mercury is like negative degrees Kelvin

Barry Bordes

A couple of years ago I did an experiment with AFR vs speed at a fixed Injector duty.

Most successful AFR would be the highest cruise speed at 20% injector duty.

Tires are a big factor as Peejay said. Of course my top speed changed with wider/ taller 18's.

Four of our emergency spares at 65 psi would probably be good.
The low rolling resistance tires on my wife's Accord seems to pick up speed when I click it into neutral trying to optimize fuel economy (best 42mpg going to Houston).

Anyway, it seemed that about 15 AFR gave the highest speed of 80 MPH (stock 16"225/245 tires)- 22MPG to DGRR
or about 76 MPH on the taller/wider 18" 245/285 combo. - 20MPG to DGRR

Once AFR is optimized you would then do the same with timing... adjusting +/- degrees looking for highest MPH with 20% duty cycle as the constant.

Ah, the things we do on a long drive!
Barry