StewC625

Which brings us right back to "3250 - 3750 rpm" - search for that post and you will see the light, Grasshopper.

I agree with Batmobile - there is a direct coorelation between the amount of fuel burned and the broadness of the silly gring pasted on my face when driving the -8.

Stew

Gord96BRG

There's the problem - that assumption is completely wrong. In the olden carburetor days, the gas pedal was connected to the throttle - a blade in the airflow of the intake. You decreased vacuum, the engine sucked more fuel (aside from the accelerator pump giving an extra squirt when the throttle was moved). Air/fuel ratios were determined by the jets that controlled the amount of fuel that got sucked in to the engine in relation to the amount of air flowing by. The throttle controlled the AIR entering the engine, NOT the fuel. The fuel was determined by the carburetor jets in response to the air.

On a modern fuel-injected engine like the RX-8, the gas pedal is NOT connected in any way, shape, or form to the throttle. The gas pedal is connected to, and only to, the computer. Of course the computer is "tampering" - it's got 100% control! Even then - the computer controls the throttle, and fuel is injected in proportion to the air entering the engine. ALWAYS in proportion. There are feedback loops to keep the air/fuel ratios precisely controlled - there is absolutely no way that extra fuel is being dumped into the engine just because RPMs are low, it can't happen.

No and No. The lag for fuel to arrive in response to air is measured in milliseconds. Timing refers to the ignition system firing the spark plugs in relation to the position of the piston or rotor - has absolutely nothing to do with fuel control.

No. There will be differences in efficiencies at different rpms, but not for the reasons you've listed. Those differences will come down to tuning (intake/exhaust system optimizations) and mechanical (friction vs. rpm), and will make only a few % difference in the overall mechanical efficiencies. Give it up - your theory is falsely grounded, it's wrong! Sorry.

Regards,
Gordon

Paul_in_DC

Gord (or anybody else) answer me this... I'm going n mph in my 8 in 4th gear. I shift into 5th and maintain the same speed. Which (if either) of those is more efficient in terms of mpg?

rx8wannahave

I'll talk more about it later but with my FIRST tank of gas I got 19.73mpg (city driving) or 252.5 miles in the city on 12.80 gallons of fuel.

DrRockin99

I took a trip over the holidays , on the way back mainly highway and one tank of gas I used about 1/3rd of a tank intown I got 21 mpg ...I cruized anywhere from 75-85 mph . The wierd thing is , I filled up last Monday night , I usually get right at 200 mpg , I have 210 on this tank and have just under 1/4 of a tank left ...must be that oil change I made last Sunday !

dannobre

iTrader: (3)

Lower RPM, same speed = better fuel mileage,(less air flow+ fuel flow)

zoom44

we need a lesson here about the difference between "open loop" and "closed loop" a/f mapping. luckily there is an excellent thread about this by Hymee here. go read that and then come back here with questions. basically here is the way i understand it.

below a certain trottle position and/or rpm (i have to re-read the thread myself to be certain) the pcm operates in closed loop which means it is always hunting for 14.7 to 1 a/f ratio. so if at a higher rpm you will be sucking more air so the pcm will add fuel to stay at 14.7. higher rpm = less mpg. in open loop it is entirely left to the maps in the pcm to determine a/f ratio. most of those maps are not looking for 14.7. those ratios can be tuned with a piggy back to what ever you want but leaning it out from our stock maps generally will increase power and mpg from what they were stock. so staying in closed loop at a constant speed and as low rpm as you can results in the best mpg. go into open loop and you will have a/f's that were best described by Paul Yaw as "PIG RICH" and your mpg will suffer as a result.

dannobre

iTrader: (3)

I guess I was being a bit simple.......:D

We could go into all kind of difficult discussions about the open/closed loop, target A/F ratios at different load, throttle positions, MAF Flow etc....and as a generality I would still say the same thing

All things being equal... my simplistic reply should suffice.....except for this tidbit i noticed on the weekend that my CZ was running in open loop above 77MPH because it thinks the TP is zero when cruize is on ?????

zoom44

dannobre you were of course right i just wanted to point out to some otehr poster that there was much more to it than they were thinking.

because it is measuring throttle positon at the pedal and not at the throttle body? the pedal is linked to a potentiometer like device right? so anytime it in cruise it thinks trottle position is 0? but you are saying above 77mph. are you saying that below that it reads a throttle position?

dannobre

iTrader: (3)

No...It's still 0...but it's in closed loop below 77. It was running an A/F of about 14.0:1...in open loop at that RPM, same load.....14.6ish in closed loop 2MPH lower,. Why......... I guess that it was reading a different part of the table that didn't like something...so it went to open loop??

sometimes I wish it was as simple as the PCM in my MX-6

Gord96BRG

Not quite - a gallon of gas contains a certain amount of energy, and a certain fraction of that energy makes it to the wheels to move a car down the road. Generally, gasoline engines and vehicle drivetrains are roughly 40% efficient in turning fuel energy into rear wheel power (Diesel engines are ~45% efficient, which is the simple reason why diesel cars get better gas mileage). To move a vehicle at a constant speed takes a constant amount of energy.

say you're driving a steady 60 mph. As a guess, it takes 25 rear wheel hp to keep an RX-8 at a steady 60 mph on level ground. It doesn't matter what gear you're in, 60 mph needs 25 rwhp. 2nd gear at 8700 rpm, you need enough throttle to generate 25 rwhp (would be a relatively small throttle opening). 3rd gear at 6000 rpm, you need enough throttle to generate 25 rwhp (a bit greater throttle opening). 4th gear at 4800 rpm, 25 rwhp, and a yet larger throttle opening. etc, etc.

As a gross generality, it takes a specific fuel consumption rate and associated air flow rate to generate 25 rwhp - again, that's regardless of what gear you're in to generate it! At high rpm in a low gear, you'll have a small throttle opening to only produce 25 rwhp, but at low rpm in a high gear, you'll have a larger throttle opening to allow the same amount of air to pass to produce that same 25 rwhp.

Say it takes 2 gallons per hour to generate 25 rwhp in general (which would be ~30 mpg for our example). Now to remove the generalities, if you factor in specifics like varying friction loads in the engine at different rpm, varying gearbox mechanical efficiencies in different gears, varying efficiencies in the intake and exhaust systems at different rpms, and varying fuel/air ratios at different rpms, and throttling losses, then you could see how it's likely that an engine will be slightly more efficient at lower rpm and higher throttle opening than at a higher rpm and smaller throttle opening to generate the same amount of power.

Testing generally shows this to be true - so from a fuel economy perspective, you're almost always better off to use the highest gear and lowest rpm to maintain your speed. The only time this wouldn't apply is if you need more than ~80% throttle opening, when engines typically richen the A/F ratio to avoid detonation (so fuel consumption will be relatively higher at full throttle at a lower rpm than part throttle at higher rpm to produce that same power demand).

Regards,
Gordon

Batmobile1

what a guy

therm8

with Shell Vpower i get 24-26 at 80(long trips), with anything else I get 20-22. about 16-18 around town no matter what gas I use. 'L' flashed AT.

spool up

I always feel like I'm hurting the engine crusing at 90+ mph since it's over 5,000rpm. I know that it's not really bad for it, but it just feels strange crusing w/ the rpms that high. My last car was a 3rd gen rx-7 which at 90 mph was only doing around 3500rpm. Guess it will take me a while to get used to crusing at such high rpms.

rx8cited

Stock transmission/gearing ?

JM1FE

Remember, that's the output shaft speed. The rotors are only doing 1/3 of that.

http://auto.howstuffworks.com/rotary-engine7.htm

rx8wannahave

I'm still breaking in my 8 so I have not pushed her, but I agree...I tend to be a little scared considering 7K RPM was the most in my other cars. It will take a bit to get use to the 9.1K shift point...I'll learn fast I bet.

DrRockin99

The best part about driving this car is running around town doing 6000 rpms !

mysql101

even better part is letting off the gas and hearing the "turbine" engine spin down.

MarthaStewart

I pretty much do mostly city driving. So I thought I'd share some mileage numbers with you and see if anyone else has the same numbers.

After filling up with a full tank of gas, and doing city driving I got 88.7 miles before the gas needle reached the half-way point.

Would anyone like to comment.

Thank You.

scottmhr1

Sounds like you got about 11 miles a gallon. Do you do a lot of real short trips?

Racer X-8

Fill it back up tommorow and do the arithmatic, then let us know what it really is ok?

legokcen

19.4 mpg since I've had the car. This also includes a track day which gave me 7.2 mpg Hell of a weekend at Road Atlanta. Max was 23.7 mpg. You can't really tell anything from "half a tank." Consistency is several tanks.

My standard deviation is 2.7 mpg, so I'm pretty much in control. Only out of control was one high (long road trip which was the high) and a track day which was the low. I'm relatively consistent around 19 - 20.

G-ReX

Posted by DavisRx8 in November and is right on what my car does (~16mpg)

rodmeister

You can't measure MPG by using the fuel dial since it might not be linear. The only way to measure is by filling up, drive, measure miles and divide into the the gallons needed to refill. I get 15mpg.