nite crawler

Okay. I did few searches, and found the same info I always find. Stay away from certain RPM ranges, stay light on the gas, and do not let the engine bog too low.

I want to focus on the lack of speculation on this last statement.

Obviously we do not want to take off in 6 gear. (Even 2nd is shakier than most cars, but thats off my topic anyways.) I do not understand EXACTLY why I will get better MPG in 5th gear at 2500 RPMs, than in 6th gear at 2000 rpms. SOME say that is the way it is, without any DETAILED explanation. The way I figure, at roughly the same % throttle, if I am at 2000 rpms doing 45 (thoeretically, dont go out in your 8 and say 2000 is 39 mph) instead of 2500 rpms, that is 20% less gas that is used for going the same speed (covering the same amount of distance in the same amount of time, just with less turns of the engine)

Basically, since Mazda gives no good ranges (and they should) I am asking someone to explain WHAT and WHY the ranges are where they are. I assume that if you shift TOO high, then you should lose serious power and begin to stall out. If that is not happening, and the engine shows no signs of hesitation, then who am I to keep from being theoretical and mathematical.

THANKS FOR ANY INPUT,

Nate

truemagellen

your engine won't cut out easily...you have to understand that a rotary engine's rotars spin at high speed and so maintains spinning easily...MUCH easier then a cylinder based engine.

I know that I have been easily able to drive in 5 and 6th gear on the road leading to my street...no jerkyness...no engine cut out...no problems (except lack of acceleration that was expected)

I have driven several stick shifts...including a Mazda6, Integra, Maxima, even a piece of **** Ford Zx2 and it was hard to stall ALL of them...that is because they are modern cars...older sticks if you didn't watch out could cut out on the freeway if the clutch wasn't down or gas was being given...but as hard as it was to stall these cars the RX-8 was almost impossible...even if my foot slips off the clutch in first gear I can save the engine from cutting if I'm quick to engage the clutch again

As for MPG...I've come to the conclusion that besides the whole environment damaging aspect, it is only a matter of a few hundred dollars a year if you are sloppy in your gas conserving shifting...so what? the car is worth 30grand...have fun with it

the only way it would matter is if you are logging more than 25k miles a year...then maybe you might want to be smarter about shifting or maybe you should by a second car...or how about a Moped/Scooter...my brother has one for college and it gets 155 MPG...so I figure his lack of gas usage evens out my abundance of gas usage (in environmental terms)

RX-Dreamer

Because the torque is too low... You need momentum to handle that gear ratio.

It's not a 500 tq Viper.

The RX-8 has around 125 tq at 2000rpm.

At 2000 RPM you have less than 50 horsepower.

Momentum = Power with a rotary. It's always going in the same direction.

It's a CRUISING gear... Meaning you've already built up your momentum.

nite crawler

So can someone PLEASE explain to me how the gas mileage is worse in 6th gear than any other gear. Like I said, I am not using 6th gear at 20 mph to accelerate and decelerate with stop and go traffic. I will be in 2nd and 3rd at those speeds due to gearing VS. momentum. My point is simply this: Most speed limits around here are 30 and 40. That means I cruise at 37, and 47 (7 mph over respectively). If I am CRUISING at 37, and I put it in 6th, she cruises just fine. It is not like I have so little "torque" that my car is "struggling" to stay where it is. Mathematically it just seems like 6th is the mpg cruising gear as long as I am at or above 1500 rpms.

Acceleration VS. gearing is an issue we can stay away from. Being light on the gas pedal is another issue I do not want to discuss here. Thanks though for the input.

ANYMORE?

Nate

truemagellen

its not worse

r0tor

iTrader: (1)

to make a long story short, every engine has something called a brake fuel consumption (bfc) curve... bfc is defined as the ratio of fuel needed and hp produced. The lower the bfc number, the better fuel efficiency.

The curve has rpms on the bottom and bfc on the side. The curve on all cars ends up looking like a giant 'U' that starts at infinity, reaches a lowpoint, and then heads back towards infinity. If you step back and think about it it kinda makes sense - at close to 0 rpms you have no power but are still using fuel (so the fuel/hp ratio is very high) and at the other end your putting more and more fuel into it but the engine is only going to make so much power as it can't breathe and gets more inertial parasitic losses (so the fuel/hp ratio again is high).

Well anyway, the lowpoint is the rpm value that you will get best efficiency (general rule of thumb is somewhere about 1/3 up the rpm band)... so to the right or left (high or lower rpms) and your going to get worse milage. Thats how you can theoretically get better gas milage doing 30mph in 5th gear then 30mph in 6th or 4th gears.

bgreene

Even if you're not accelerating, it still takes torque from the engine to overcome air drag and friction within the drivetrain (at low speeds, both are fairly low, but neither is zero).

It shouldn't take 50 HP to keep up with drag at 47MPH, which is why you can cruise that speed at 2k. One thing that might be killing mileage if you're cruising at low revs is that unless traffic flow is at a perfectly even speed, you'll be trying to do some light acceleration from time to time, which might cause the computer to just dump in gas, since you're out of the power band.

Since the intake ports in the engine open at different RPM's, and the flow rate through the manifold runners varies in a sawtooth with engine revs as a result, you'll get different efficiencies at different rev ranges, and that doesn't even get into the effect of the computer's fuel injection maps (which are not yet known even by many of the "tech guys" on this board).

One of the theories, which I've seen some unscientific success with (maybe I'm imagining, though....) is to try to cruise in the range of 3250-3750. This is the range between when the 2nd set of intake ports open and when the secondary injectors start adding fuel. This range also keeps you in a range where power is available fairly readily for the occasional push.

check here:
https://www.rx8club.com/showthread.p...highlight=3250

zoom44

IN YOUR OWNERS MANUAL THEY GIVE THEIR OPINION ON WHAT THE SHIFT POINTS SHOULD BE.

RX-Dreamer

bgreene perfectly expanded upon my post...

It's the drag, and occasional acceleration you're giving your car at such a low power range causing your car to use more gas... :o

wakeech

stop listening to people who've barely given any thougth to it, and next time search (i know what to look for, so don't worry 'cause i know you wouldn't have found these threads).

RX-8 Gearing Ratios?

RX-8 Emissions

in both of these gearing is discussed to the furthest extent it ever has been on this board. remember that these threads are 3 years old, and we were working only with what we knew at the time.

the best rpm to be at while crusing, put non-technically, would be the lowest speed you could go in the highest gear you have. probably around 50 mph in 6th (that's 2500 rpm).

Xyntax

^ I used to do that. I even preached it. I dont if it's just my 8 or not, but I have found cruising above 3000 rpm the best. It's not boringly slow, feels strong, and you can easily accelerate higher. On top of that, MPG has increased. I have been logging data with my Canscan and tried my new method of driving my 8. After this, I will be logging data driving granny style (shift and cruise below 3K rpm)

What I have found so far is that, depending on load and throttle position, my 8 runs leaner between 3000 - 4000 rpm.

I'm not dismissing whatever theories and principles out there, but I will believe the data that I get from my 8. I'll share them some day once I complete my data gathering.

red_rx8_red_int

Wakeech, I need to start stalking (following your posts) you. I've always respected you since I joined this club, but my respect level has just gone up. those are great threads and, because of that, I just bumped both.

nite crawler

OK.

Let us say I am cruising at 1500 rpm from point A to point B. It takes me 5 minutes at those rpm in gear 6. That is 7500 revolutions. If I travel at 2500 rpm (roughly) in a different gear, that makes 12,500 revolutions. You guys are telling me that I will shoot more gas in those 7500 revolutions, than in those 12,500 revolutions. DOES THAT MUCH MORE GAS GET SPRAYED IN.......... SERIOUSLY???!!!

nite crawler

Oh, and about the ranges in the book. Isn't mazda talking about accelerating, not cruising. If not, then I was wrong. I am checking again.

RX8_Buckeye

Actually, in this case, yes. Fuel flow rate is dependent upon both engine speed AND throttle position. Assuming the same vehicle speed, it takes significantly more throttle to cruise at 1500 rpm in 6th gear than to cruise at a higher engine speed in a lower gear. This is because the engine is close to lugging in 6th gear at such a low rpm, and to overcome the lugging the engine will need significant throttle (more air/fuel). In a lower gear, there will be significantly less load on the engine and thus less throttle will be required to hold speed. This is an extreme example, however.

pr0ber's post has done the best job so far explaining the gearing/mpg relationship; however, he didn't mention that there is a different fuel flow rate vs. engine speed curve for every throttle position. To illustrate how these curves can be used, let's assume that we want to know what gear to use to get the best fuel economy for a cruising speed of 60 mph. The engineers first estimate road load (rolling resistance, air resistance, driveline loss) that the vehicle would experience at that speed in all gears. Then, based on the engine output torque map (torque vs. rpm vs. throttle position), the engineers can determine how much throttle would be needed in each applicable gear to hold 60 mph. At this point, we know the engine speed and throttle position required to hold 60 mph in all applicable gears. Now it's just a matter of locating these conditions on the fuel flow rate vs. engine speed vs. throttle curves. The numerical values of fuel flow rate taken from the curves can be compared to determine which gear gives the best fuel economy for cruising at 60 mph.

What this should make clear to everybody is that the relationship between gearing and fuel efficiency is not simple. In most cases, you can't just use common sense to figure out which gear you should use at a particular vehicle speed to get the greatest efficiency. To determine this analytically, you would need the fuel flow rate vs. engine speed vs. throttle position curves for the engine, a good estimation of road loads for all vehicle speeds and gears of interest, and a map of engine torque versus engine speed and throttle position to determine required throttle to hold constant speed in each gear.

I know this might be a bit confusing, so I'd be happy to answer any questions.

TRZ750

The Mazda Owner's Manual shift points are primarily picked for the EPA Emmision test as these are the shift point that will be used during the test. We all know that for the best accelleration we want to rev the engine higher than what the book says. For milage prober is correct about the bfc specific fuel comsumption curve. I would like to see it for the RX8 as it could be strange with all the various valves opening. But it is probably fairly flat between 2500 (may be even 2000) and 3500. I dought if it is really beter at 3750 or higher, but could be.

So for the original question "is 2000 in 6th better than 2500 in 5th?" It is logical that 5th would be better (even though I usually use 6th) as 5th is direct drive in the transmission, so less friction, and not low enough to be at a rising point in the bfs curve. But also it is usually best to turn an engine as slow as possible to get the best milage. Charles Lindberg learned this and taught many WWII pilots this.

TRZ750

RX* Buckeye is correct. It is not a easy topic.

Actually I am tired of all the fuel milage issues on this web site. In did not, and suspect most did not, purchase the RX8 for it's gas milage. I got it for the fun factor!! How may that can afford a exoitic sports car care about the fuel milage. Or a Hummer? Or a power boat (they commonly get less than 2 mpg).

r0tor

iTrader: (1)

i chose not to mention the throttle position because there isn't a whole lot of variability in the bsfc curves at normal cruising throttle positions... 10% throttle looks a lot like 25% throttle in most cases.

RX8_Buckeye

Good point. Do you have a rough idea of what throttle percent is required to hold 80 mph in 6th gear? It wouldn't surprise me if it was close to 40%... however that's just a guess.

nite crawler

I figured it would be almost impossible to answer this question as of yet. It figures that I would ask a question that COULDN"T be answered.

I did not buy this car for mileage, but it is important to know where I am wasting gas and saving gas WHEN CRUISING. If you are sick of all the mileage posts, then why did you come into this one. At least, instead of another boring post about crappy gas mileage, this one focuses more on MILEAGE according to GEAR vs. RPM vs. MPH. This is actually a healthy discussion.

If anyone can get more info on how much gas gets injected at certain times (% throttle, rpm, etc.) then lets get the answers out here.

Thanks Buckeye for giving the an straght answer close to what I was confused about.

LOL

Nate

wakeech

at 3750 (the suggested mazda shift point, i think) the 3rd injectors come online and that's when the duty cycles get much bigger.

also, i completely disagree with the comment about having to keep the throttle open more, say twice as much (as this would somehow use twice as much gas, which it wouldn't... the mass of fuel injected is completely dependant upon the mass of air ingested by the engine) than you would if the rpm was higher in a lower gear, say less than twice the rpm.
with the throttle open wider, the engine is actually closer to its maximum thermal efficiency, and running the motor at a very effecient level at a lower engine speed (like 40 or 50 mph in 6th as opposed to 4th or 5th) is going to be better for your fuel economy.

PoorCollegeKid

I read an article a while back on a test that BMW did to determine how to maximize your fuel efficiency. They found that shifting in to the highest gear possible at any given speed would net you the best gas mileage, even if you had to floor the throttle to get the car to accelerate. It might not be the best thing you can do to your engine, but it apparently works very well as far as netting an extra few MPG goes.

wakeech

i don't see why flooring it at a lower speed would be worse for it than flooring it at a higher rpm... it's just removing the resistance the engine is encountering sucking the air in.

r0tor

iTrader: (1)

i really don't understand what the obsession is with additional injectors comming on or secondary ports opening... theres a fixed a/f ration in closed loop modes, air flow is pretty much entirely dependant on rpm (rpm x engine displacement) not how many ports open (which is just ensuring you efficiently get the right amount of air flowing), and because of an open loop condition an extra injector comming on isn't going to mean any more fuel is going to be pumped in during a closed loop condition.

PoorCollegeKid

Lugging an engine causes combustion chamber temperatures to rise (I'm assuming it's because a large amount of fuel/air is being combusted in the chamber, but since it's moving relatively slowly this hot, burning air transfers more of its heat to the surrounding environment than it would otherwise have a chance to do. This is the best explanation I can come up with, and a quick GIS doesn't bring up a better one) which can cause pinging to occur. Pinging, as we all know, is a bad thing for your engine. Someone else on here probably has a better explanation than mine, but the fact remains that lugging your engine is not a good thing to do as far as durability is concerned.