Razmtaz

When i bought my RX-8, i asked the dealers what the gas mileage was and they said about 18-24 like the sticker said and that on a rotary, if u hang around 5000 RPMs u get better gas mileage. Now on a normal car, the lower the RPMs, the better the gas mileage, my question is, is what the dealers say true, higher RPMs means better gas mileage, or like any normal car, lower is better?

Vertigo-1

iTrader: (3)

From a mechanical point of view, it's simply impossible. The secondary fuel injectors open up at 3750 RPMs. If you stay below that, you never touch those. Personally, I haven't seen better or worse MPG if I run to redline often or keep it below 3750, and I'm just about 100% city driving. Always get around 15 MPG.

VikingDJ

I see the opposite, but it's barely noticable. When I drive 75-80mph in sixth going highway I get around 19-20mpg, and when I drive 65-70mph in sixth I get about 21-22mpg. IN my long commute to work, I do see a few more miles on a tank highway, driving a bit slower. We all have different ECU settings, so I'd imagine this can and can't be true, since you gain mpg running 4-5k. This is also likely different in city driving as well. this basically proves that our cars are different. My STI is really noticable. I get between 25 and 28 mpg driving 60-70mph, and only about 22-23mpg when driving 75-80mph, but i'd imagine that is normal on a FI engine.

himitsu

Not exactly higher RPM...
Every engine has a point where it is most economical true, usually between 50% to 75% of the powerband, but you have to associate it with distance travelled. If you spin your engine to 5000 in the middle of the city, you still do nothing but drain your fuels, you get WORSE millage.

In the highway when you rev high, and are able to speed, thats where the millage adds up.
That is why when you drive in highway where your speed is most likely high and almost constant, you will get more millage than city driving where you stop and go alot.
Its even more apparent in automatic cars.

So simply saying the higher your rev the higher your millage is not true...
Theres alot of factor affecting millage (outside your car), wind condition, road condition, angle of incline, decline, traffic, air temperature, humidity, the fuel mixture, octane count, your engine condition, tire, and so much more...

Only thing sure is, if you want the best millage, drive on a quiet highway.

And I dont get his comment on other cars either... conventional piston engine has their max rev like around 6000rpm, and their peak comes at about 4000 or so... of course its considered low by rotary standard, but I dont see anything out of the ordinary with that in a piston engine...
So yes a normal car in a highway revving at about 3000rpm should get the highest millage, and the RX8 would do that at like 6000rpm... so?

Brice-RX8

It doesn't make any sense to say the higher the rpm the better the gas mileage. The higher the rpm the more fuel that is dumped into the motor, thus more is coming out of the tank. Yes, there is an optimum setting, which someone in another thread did some testing and it was in the 3000-3500 rpm range, that is where you want to shoot for.

joseight

no, the higher the rpms the more fuel you use. its as simple as that.

r0tor

iTrader: (1)

peak efficiency is 3200-3400 rpms

SDB

Yes and no. If each rpm used the same amount of fuel, but they don't. This is dependent upon the ECU and based on many things.

Also, it also depends upon how far each rpm moves the car since we are measuring miles per gallon, not gallons per hour.

fullsmoke

Bingo. It also has to do with efficiency. Our engines are designed to be run at 8500 rpm, where it's peak power exists. This is the point where the engine is MOST efficient. Yes, keeping RPMs at 8500 will eat gas but per gas unit eaten, it produces the most speed (or HP). At other random points in the engine speed, the efficienty at each infintessimal point is different--some higher, some lower. These lower points (compared to other random points) will usually produce less mpg (but again, this depends on so many things).

FS

Butch Brown

If they open at 3750 then when I go down the mountain on my way to work in 4th gear 60mph the engine is holding my speed while the secondary fuel injectors are open. I got the most recent N flash last week and the first tank of gas got 23.7 mpg. I drive 35 miles to work cross 2 mountains (small ones) 2 stop signs and 3 traffic lights on the way. I shift when no one is ahead of me at 5-6000 rpm's only exceed half throttle when passing someone, then drive conservatively averaging about 55-65 mph on country roads 28 miles of trip, 65-70 on the highway 7 miles of trip. I usually got 20-21 mpg on the M flash

valpac

iTrader: (2)

Increasing RPM causes an increase in velocity. Increased velocity increases kinetic energy. Gas is potential energy. All other variable being equal, you must increase potential energy to balance the increased kinetic energy. More gas at higher RPMS

MX6_2_RX8

Has everybody lost their minds? What does peak horsepower have to do with peak efficiency? Peak horsepower means that it can burn the MOST fuel and turn it into power. Frictional losses & heat losses go up with RPM. When fuel is ignited it does two main things, it makes the car move and it makes waste heat that needs to be dissipated by the radiator. For arguments sake lets say you are going 80mph in 3rd at 8000 RPM vs going 80mph in 6th at about 4000rpm. The speed is the same so the energy needed to move the car is the same. BUT at 8000 RPM you are heating up the combustion chamber twice as many times! There are of course dozens of other factors but that is probably one of the biggest. You could make the same argument with friction. The owner’s manual recommends shift points for cruising that are around 2700 RPM. I’d probably start looking for peak efficiency there.

Car salesmen don’t always know what they are talking about, plain and simple. I would also be careful about believing everything you read on this forum.

edit.. Forgot to mention the extra heat going out the exhaust. Duh.

Tigerfootball

geez i hate physics. but you are correct about the conservation of energy. i never would have thought about that. I have been experimenting myself lately on how to imporve my gas mileage. so far, the best i have had for a tank is 22-23 mpg.

rx8wannahave

I've never broken 20mpg but I drive 80% of the time in the city. So far my avg fuel economy is at 19.2+mpg with 2599 (or close to that) miles on my car. I speculate that, since I cruise in 6th as much as the speed allows me to, the cruise rpm is the major factor. I take my 8 over 5.5K daily and hit redline a few times a week so I conclude my "normal" cruise rpm is to credit for my high "mainly" city driving fuel economy.

Try it...and "hopefully" you will see! I can't wait to take my 8 on the highway to test the highway fuel economy.

Brandon

I bet you would burn a lot of gas lugging it around at 1500 rpm or something because you have to open the throttle more, but I would think above 2500-3000 rpm or so (depending on the gear) and at a steady speed, the higher the rpm, the more gas used, plain and simple.

I bet the dealer was just trying to make the car sound special and otherworldly. I test drove one at one dealer and he had me go through this elaborate warm-up procedure which included not touching the gas for 30 seconds, then blipping the gas a few times, then holding it steady at 2,000 rpm, blipping the gas some more, holding it steady at 3,000, etc., etc. until the temp needle rose. All the while saying, "it's really an unusual engine. You have to do unusual things with a Ferrari as well." *roll eyes*

rx-8_or_?

The only time I broke 20 was when we made a trip to the desert and were stuck in convoys of cars through most of the trip. Most of the trip was spent at 3000-3500 rpm. A mostly-freeway trip at around 4000rpm give me about 17mpg.

beachdog

I hope that you set that idiot straight! And by that I mean a swift palm to the forehead. Perfectly legal when idiots say stupid things.

Unfortuantely, too many people MSU (make stuff up) whan they really don't know what they are talking about.

Razmtaz

wow... sounds pretty confusing, so many factors... Well i'm gonna hafta play with my RPMs to see how to get top gas mileage cause i hate having to buy premium gasoline for my car with such high gas prices right now... Which brings up another question, do RX-8's really need premium or is that a bunch of bull too? lol, i hate dealers, bunch of morons -.-

MadDashRX8

No problems here with mid-grade 91. Also, have mixed the cheap stuff 89 with half a tank of premium 93 without problem. That would balance out to 91 anyways, right? :D My $.02.

Sounds like I have to do the test for myself. Most of my driving is at 75 in sixth. Will try one way 75 in 6th on full tank, return at 75 in 5th on full tank and compare. You've all made a very curious driver outta me.

jbrasure

Well, I guess we have not answered the original question: "Will higher RPMs give better gas mileage?" Personally, I tend to think that lower RPMs will be better. However, that's just a guess, and common sense might not apply to the rotary engine.

I think it's safe to assume:

1) If you're RPMs are too low then you'll get terrible mileage. (Imagine trying to lug up a hill at 500 RPMs.)

2) If you're RPMs are too high then you'll get terrible mileage. (Imagine driving at 9000 RPMs where ever you went.)

3) Thus, there must be a magic RPM range that provides the best mileage for a given speed. I'm not sure where that is, but I'm guessing that you cannot figure it out on paper. There are too many factors to consider (wind, slope, energy loss to heat, etc.)

In short, I doubt we'll answer this question without a bunch of experiments.

BlueFrenzy

I did an experiment awhile back based on cruising RPM.

The first tank of gas I cruised at 3750-4000rpm while shifting where ever I wanted. This yielded 14.5mpg.

Second tank of gas I cruised at ~3000rpm and kept shifting the same. This yielded 17mpg.

I take the same route to work everyday, same gasoline, same driving conditions and same driving habits. Granted this is only two tanks worth of data (statistically insignificant). I'm certain if we all tried this, it would yield very similar results.

Sigma

Absolutely not as simple as that.

Every engine is different as to what it performs most efficiently at. Ford 4-Cylinders, for example, have historically widely reported greatly heightened mileage while cruising at 85mph than at 65mph, as much as a 8mpg improvement driving much faster. This of course means they're running at higher RPMs. But RPMs have little do with what the ECU is deciding you need for fuel at that time.

RPMs are only one factor in mileage. And a relatively small one really. There's also A/F Ratio, Gearing, and countless external factors, among other things.

As someone said, it takes the potential energy in the gasoline to turn into the kinetic energy required to move the car. So it has to take more potential energy to generate the kinetic energy requried to rev another 1000 RPMs. But the potential energy of combusion is absolutely not constant inside the engine. Nor is the conversion of potential into kinetic energy linear either.

Vip_C

What the f**k???

The 8 is a guzzler, get use to it!

I wouldn't drive mine like a pansy to get an extra 10/20 miles out of a full tank, besides, I like pulling up next to unexpecting boy racers in small hatchbacks who's cars are only modified asthetically. i can either drive to save on consumption and let him and the next 6 cars through before i push in or i redline and push in 1st. I go for the latter! :D :D

Drive it how you like..