RPMs at cruising speed
 

When I'm in 6th gear and cruising at around 120km/h (so around 75 mph) im revving at almost 4000rpm...is this normal? I know rotaries revs high but this just seems wrong...

it is normal, it's around 4k rpm

^ Damn if she keeps drinking that water her boobs are gonna bust!

Um, yeah, normal. It revs to 9k, you know. Really only 3k for the rotors, no big whoop.

I don't think its the water.... but yeah, it's normal

our 6th gear is too short

^and... lol

also...im getting just over 300km to a tank of gas...half the tank was from lat summer because I didnt get to finish it before i stored it but i used stabilizer and filled up the other half with 91...

you're fine

keep in mind that the only part in the engine that is moving at 4K rpm is the eccentric shaft. the rotors rotate at 1/3 that speed 1,333.33---rpm. this why it is so important to run the car to red-line.

but it drinks a hella fuel...tho:yumyum:

^ Yep, even at 9k redline the rotors are only spinning 3k.

I cruise at 4k in 6th which is right at 80 mph (well, actually a bit slower since going with 265x35 tires).

Here's a comparison for you:

1. I own a VW Passat 1.8t which runs at 3000rpm at 80mph.
2. I own an RX-8 which runs at 4300rpm at 80mph.
3. A piston completes a combustion cycle in 2 crankshaft rotations.
4. A rotor completes a combustion cycle in 3 crankshaft rotations.
5. So, a rotary engine needs to spin 1.5x as fast to produce as much power as a piston engine with comparable displacement.
6. That means my rotary engine is roughly equivalent to a 1.9L piston engine, and it's spinning 1.43x as fast as my 1.8L piston engine.
7. Adjusting for the .1L difference in "virtual displacement" between the two engines, my rotary is spinning 1.35x as fast as my 1.8t to produce the same power, which is well below the 1.5x rule.

Same here
I'm running 265/35 as well. Add another 2mph to the already +2 mph the speedo is off already..

How could it not be normal? seriously stop and think a moment. what in the world could possibly make the engine spin at a different rpm at a given speed in a given gear?

unless its an auto

what? how would it differ with an AT? rpm x gear = speed

8. :Eyecrazy:

A 4-cylinder engine has to do it 4 times, while a 2-rotor rotary has to do it twice for one complete engine combustion cycle, if we look at it as a whole.

It seems an apples-to-oranges comparison.

I wasnt aware that a rotary engine had a crankshaft! :Freak_ani

If you are talking about a single piston, it would be more analogous to compare a single rotor face. A single rotor face completes a combustion cycle (intake,compression,power,exhaust) in 1 e-shaft revolution, comparable to a single piston in a 2-stroke motor. The number of power strokes per e-shaft revolution in the 2-rotor wankel could be compared to a 6-cylinder 2-stroke piston motor, if such a thing ever existed.

You really can't compare power output per revolution, rotor or e-shaft, to a piston motor as there are many other differences and variables.

Automatic (6-sp) RX-8s are geared more for cruising (in 5th & 6th) than absolute accelerative performance, so they run almost 120 mph @ 4000 rpm in 6th.

cuz the torque converter. when its not locked up, the rpm woud go up or down (like a sliping clutch but it wont damage).

i herd something interesting the other day, that if your leading spark plugs are gone then the engine has to spin twice as fast as it normally would in order to produce sufficient power to cruise at the desired speed. just saying. ;)

are you serious ?

Yeah, the more I think about this, the more ridiculous a direct comparison becomes. The piston engine is more fuel efficient, generally, but the Wankel has a better power-to-weight ratio, a lot fewer moving parts, and less loss to force vector changes within the engine. And this is post-engine drivetrain aside.
The piston engine has also had over twice the development time as the Wankel concept, and in terms of man-years of development that ratio skyrockets when you consider the vast number of piston engine designers and manufacturers over the last 110 years, at least.

Physics. Not everyones strong point.

btw i was joking im sure somebody got a laugh out of it. but its going to be what i tell everyone body that bothers to ask the stupid question of is it normal to rev at 4k at 120km in 6th gear. as zoom has said how the hell could it not be normal?

Wow, there's a bunch of people here who don't actually know how a rotary engine works. (not to single you out specifically, you just had the longest and most quotable response to my post.) A single rotor face does not complete a combustion cycle in a single rotation of the crankshaft. (or eccentric shaft, call it what you want, it does the same thing.) It completes a combustion cycle in three rotations of the crankshaft. Look at any animation of a rotary engine and you'll see I'm right.

Also, it doesn't matter how many rotors there are, or how many pistons there are in a piston engine; a rotary engine ALWAYS takes three crank rotations to complete combustion cycles in all of its combustion chambers, and a piston engine ALWAYS takes two crank rotations to complete combustion cycles in all of its combustion chambers. (please don't mention 2-strokers, we're talking about car engines, not weedwhacker engines.)

So, since it ALWAYS takes 50% more crank rotations for a rotary engine to complete all of the combustion cycles currently in progress, you have to expect a rotary engine's crankshaft to spin 50% faster to deliver the same amount of power, provided that the engine internals displace the same volume in the same span of time. It's that simple.

if you are getting more than 18 mpg on your 8, its not normal...... lol

Oh, this one is going to go well....

It's sad that having an unhealthy 8 is considered 'normal' then.

I could post 24mpg any time i wanted to when my engine was healthy. Then my cat failed and clogged up, dropped to 9mpg, and probably overstressed one or more seals, cat replaced, back up to 24mpg for about a month, but the stress is catching up to it, and my engine is failing fast. And I have dropped down to 17mpg.


If you can't post 24mpg on straight highway cruise, you have something wrong with one or more of the following:
- plugs
- plug wires
- coils
- O2 sensor
- catalytic converter
- engine compression
- air filter
- MAF sensor
- e-shaft sensor

And possibly:
- transmission fluid
- differential fluid

It's a big list, and if you don't stay on top of it, then your MPG can easily drop off what it should be.

Amen to this.

Yeah, down to 17mpg with clearly failing compression, and the change is noticable to me, including I can hear one or more of my apex seals hitting the housing when the harmonics are right at really low revs (have an appointment tomorrow morning for a compression test for the official verdict).

If this is what people's 8s are like when they are getting 17 or so MPG, no wonder they believe this car is slow. It is, when my 4th gear is accelerating like my 6th used to. Seriously. I could effortlessly pull from 50mph to 130+ in 6th gear. Not pelvis crushing acceleration or anything, but it wasn't a struggle, it wasn't hard, and it was smooth as hell. Especially from ~90 to 120. I could get reasonable acceleration (and faster than most commuters accelerate at) for normal road use from as low as 35mph. In 6th.

If you are getting such poor mileage, and aren't bothering to take correction action, then you have bigger issues than your 8 does, and are leaving alot of power and performance on the table as well.

well when im in 6th gear if i double the rpm's im running at, it will tell me my speed.....so if im at 4000rpm's i know im going 80 mph, 3500rpm's = 70mph, 3000rpm = 60mph and so on.

I don't want to come off as too egotistical (I think just a little will suffice ;)), but I am an "expert". I have an engineering degree -- which involved several years of studying physics, calculus, differential equations, statistics, and all that other fun stuff that all engineers have to live through -- and I use that degree on a daily basis. I am quite confident that what I'm describing is correct, but as always, I'm willing to consider proof that I'm wrong -- assuming that proof can withstand peer review.

- - -

EDIT: I read the thread you linked (or at least the first part), and the issue of measuring displacement is not one I'm disputing; in any four-stroke engine, the displacement is equal to a single combustion chamber's volume at BDC minus its volume at TDC, multiplied by two (to reflect the need to hit TDC and BDC twice each to complete a combustion cycle), multiplied again by the number of combustion chambers. As far as I know, in the 13B and Renesis engines, as far as I know that number is 1.3 liters; that's fine, I wasn't talking about that.

What I was talking about is: in order to complete a single combustion cycle in any of its combustion chambers, a rotary engine's crankshaft has to rotate 50% further than a piston engine's crankshaft does, so to deliver the same amount of torque in the same amount of time (i.e. the same amount of horsepower), a rotary engine's crankshaft will need to rotate 50% faster. The displacement argument is a completely separate issue, though obviously the fact that both apply at the same time makes the overall picture much harder to understand.

It really is best not to try to base your understanding of rotary engines on your understanding of piston engines. As someone pointed out earlier, rotary engines are different from piston engines in many ways, and trying to "adjust" your understanding of piston engine design to accommodate all those factors at once is an exercise in frustration. Better to have two separate understandings instead.

- - -

ANOTHER EDIT: Before someone says "if you know so goddamn much about rotary engines, why aren't you building them?":

1. Knowing how something works and knowing how to build it from scratch are two different kinds of knowledge; I have the former, but I don't have the latter.
2. I don't damn well feel like it. I've already spent years learning how to do things engine designers will never be able to do, and I'm content to be in the same position relative to their skills.

I understand what you’re saying. I need to stop posting to forums while distracted at work. I forgot to additionally take into account the rotor’s orbit around the e-shaft along with the rotor’s rotation, and I had actually intended to compare power pulses (strokes, whatever) instead of the complete cycle, so my analogies were wrong. So yes, I do understand that each chamber’s overall cycling results in 3 shaft revolutions. But this has gotten off track of what I originally wanted to compare, which was individual power pulses per shaft revolution.

Prompted by your original post here which equated power vs shaft revolutions, I was curious to equate the number of power pulses to shaft revolutions in the Wankel. So (now talking about rotors not individual faces), since each rotor generates 1 power pulse per shaft revolution, would it be accurate to say that a 2-rotor Wankel providing 2 power pulses per revolution would equate to the power pulses from:

A 2-cycle / 2-cylinder piston motor
A 4-cycle / 4-cylinder piston motor

I was just kidding lol. Anyway despite having done all the above like you suggested, my mpg has never improved better than 20 mpg since the day i got my car. Its a 2005 that I got, currently at around 46000 km which is pretty low for a 4+ year old car (coils, plugs, air filter replaced around 35000 km), the only upside I could think of is I'm still getting the same mpg I got since day 1.

I had tried babying it, shift up early cruising in 6th at around 3000rpm. Tried various other rpm ranges that seem to work for others but to no avail, still stuck at around 20 mpg. I have given up on mpg, all I hope is the engine don't fail on me before 200000 km coz there are noone around that can rebuild one where I'm from.

^ I rarely check my gas mileage. I suspect it's never better than 20 mpg either. But then, I almost always "drive it like I stole it." ;) An '05 as well with over 25k miles now and it pulls strong in every gear.

btw: I don't know why anyone would say the RX-8 isn't a fast car. I'm NA, although nicely availed of a MM tune, and it never ceases to pleasantly surprise me just how quick it is. Obviously we don't have off the line torque, but otherwise it holds its own quite well.

Well, since the two rotors are offset by 180 degrees, one of the combustion chambers fires each half-rotation of the crankshaft. So yes, I agree that the power delivery of a 2-rotor Wankel has the same pattern as either of the piston engines you mentioned. All three have perfect primary balance, which is why (not accounting for the differences between brands of piston engines) they all tend to run so smoothly.

So I think we were actually arguing past each other, because I think the points we were each defending were along different lines. All I was saying to start with is a Wankel engine, if considered as a "black box" unit, will have to have three crank rotations to return to the same internal configuration as it started out in (thus having completed a power cycle for the entire engine), whereas a 4-stroke piston engine will only need two crank rotations, hence why rotaries have to rotate faster to deliver comparable power. I think that concept can live happily-ever-after with the point you were making.

On a side note, because of this thread I was reading up a bit on direct-injection two-stroke piston engines, and now I'm curious to see how they would compare to diesel engines -- since 2-strokers only need a single crank rotation to complete a power cycle for the entire engine, that means they ought to be able to produce a lot more torque than 4-strokers, unless there's some important caveat I've missed.

^ There's a reason 2-cycle engines are used for gas-portable tools such as weed-whackers, chainsaws, etc. - a lot of bang for the buck (thrust-to-weight, ... whatever you like). Their downside is of course the need to pre-mix and also poor emissions quality, the latter is the reason btw why most marine outboard engines, snowmobiles, etc. are/have gone 4-cycle.

The only thing that improves my gas mileage is hot weather. Near freezing, I'll get 50 miles to a quarter-tank; around 60F/15C, I'll get 60 miles to a quarter-tank. The past few days it's been 80F/27C where I live, and I'm averaging 70 miles to a quarter-tank. This is a hot-weather car if I've ever seen one.

What I can't decide is whether that's because the engine is bleeding less combustion heat, or whether I really really really need to get better gear oil. :uhh:

colder weather, more resistance on the drivetrain
as it gets warmer it gets better to a point where its too hot, then it get worse

Probably the latter :)

I could post 24mpg regardless of temps.

EricMeyer dyno proved that cold transmission or Diff fluid can knock 5hp off the curve easily. Poor fluid could easily have the same result, or else your fluid is staying too cold?

Conventional 2-strokers are dirty for two reasons:

1. Opening the intake and exhaust ports simultaneously causes some of the intake charge to blow straight out the exhaust, unburned;
2. Because the intake cycle is performed in the crankcase, oil has to be added to the gasoline because gasoline alone will de-grease the crankcase, resulting in very premature wear on the crank bearings.

I'm sure you knew all that, I was just stating it for the record. Anyway, direct-injection 2-strokers get around both problems by intaking only air, no fuel -- the fuel is injected directly into the combustion chamber after the piston has started moving upwards and the intake/exhaust ports are closed. There is still some oil induction into the combustion chamber because of the air moving through the crankcase, but it's minimal compared to the oil that has to be pre-mixed to run a conventional 2-stroker -- and of course, not blowing raw gasoline out the exhaust helps a lot with emissions too. Anything that remains can be easily cleaned-up by a catalyst.

Also, since 2-strokers have twice as many power strokes, each power stroke only needs to generate half as much torque, which makes the use of roller-bearings much more practical, which in turn reduces the engine's lubrication needs. It's a shame there aren't more small, high-revving engines using roller-bearing crankshafts, like the old Honda S600 did. Jay Leno has one, and the thing sounds amazing. Not very powerful, but 60hp/600cc is perfectly in-line with the best large piston engines we have today.

Which reminds me...I'm going to have my shop put AMSoil gear oil in when I get the oil changed, but I have no idea what weight to order. What is the stock weight, and is there a better weight to use if I'm going to go with full-synth gear oil?