Why Do Rotaries Suck?

[JAS-25T]

Thats a shit theory, considering the motion you need is round and round (output shaft - diff - tyres)

[astrotristan]

Thats a shit theory, considering the motion you need is round and round (output shaft - diff - tyres)

agreed - but dont rotors and piston engines both have diffs and tyres?

it was a comment regarding just the motors themselvess

[2BNVS]

i dont think they suck as such coz my bro inlaw had a series 4 13bt with a gt35/40 on it ( sold it and got an r34gtt), weighed about 900kg lol... so much respect to their performance, i just cant stand the sound. it drives me mental. i prefer a deep growl over the buzzing sound

[FineLine]

When the Americans see a rotor powered pickup win a tractor pull they'll take notice.

[Johnny]

Rotors only suck when they have a turbo and are on boost lolz

i like rotors, i mean most people who buy them are weekend cars so they do little kms per year; rebuild every 20,000-30,000rpm highly tuned aint too bad so would be about 5+ years for someone like me.

ive drive a 12a turbo with 180kw atw and it was impressive, can only imagine a 13b with a t04z, they produce heaps of gas flow to spool up big turbines so torque is quickly made with FI.

they do burn lots of fuel cos of their design but thats what makes them able to flow alot of air to produce power and exhaust gas for lower turbo lag; they also burn oil...but so what lolz

id buy an rx7 series 7 or 8 without regret

[mad082]

well the fact that people have used rotaries in aircraft means that they can't be that unreliable

personally i hate the sound of a NA rotary. don't mind the sound of a turbo one on song though.

[910trx]

i like rotaries

i like old school 4 bangas

i like straight 6 turbos

i like v8s.

heck i love my 49cc 2 stroke on my pocket bike

if it draws in fossil fuel, makes loud noises, propells me and my beer gut without the need to walk, and warms my surrounding climate so its like summer all year - then its alright by me.

better than walking...

[Sydneykid]

They suffer from the lies that Mazda spun back in the 70's.

Lie #1, they are a 4 stroke engine, rubbish they are a 2 stroke. Every one knows it, but Mazda lied because they didn't want the "lawn mower syndrome” attached to rotaries. The fact remains they are really a 2 stroke. The fact that they need oil in the petrol is just another nail in the 2 stroke coffin.

Lie #2, they rev to 9,000 rpm. no they don't. The rotors only do 3,000 rpm they use a step up gear ratio to spin the eccentric shaft at 3 times the rotor rpm. Why? So they could use normal piston engine gearbox and diff ratios. They could have easily used say 6 to 1 rotor to shaft ratios, then claimed 18,000 rpm. The rotors would still only be doing 3,000 rpm, their true rpm.

Lie #3, the biggest lie, the small engine capacity eg; a 13B is 1.3 litres. What a whopper of lie that was. Their true capacity is 3.9 litres, 2 x 3 sided rotors x .66 litre each side = 3.9 litres. Everyone knows it, but Mazda persisted with the lie.

As with most things, the lies came back to bight Mazda. They use a lot of petrol for a 1.3L 4 stroke. But not so much for a 3.9 litre 2 stroke. They take a big turbo for a 1.3L 4 stroke, it’s not so big when the truth is 3.9 litre 2 stroke. They produce an impressive amount horsepower per litre when using the lies of a 1.3L 4 stroke. But when the truth is used, it’s a shit house power output for a 3.9 litre 2 stroke.

So put the lies away and look at rotaries in the true light of day. As a 3.9 litre 2 stroke (in the case of a 13B) they have OK horsepower output, but lousy torque output due to their poor combustion chamber shape. That’s why they need 2 spark plugs. Another reason for their poor torque output is the compression ratio, which is very low, another result of that poor combustion chamber shape. Their fuel economy is OK for a 3.9 litre 2 stroke, but they don’t use the fuel efficiently, hence poor power output per litre of fuel used. They respond well to turbocharging as you would expect from a 3.9 litre 2 stroke with a low compression ratio.

Add it all up and you find the truth is a pretty average engine, with only one advantage and that’s small external size. But the fact is it’s not user friendly size. They don’t fit well in a front wheel drive application compared to a compact inline 4 cylinder. They don’t offer much space saving in a rear wheel drive application as they need a radiator, oil cooler and gearbox big enough for a 3.9 litre 4 stroke. A compact 3.9 litre V6 doesn’t need any bigger engine bay and yet produces more horsepower, more torque and gets better fuel economy.

But back to real reason for rotaries tiny niche market, the Mazda lies from the ‘70’s. Take them away and they become a much more believable engine alternative.

Cheers

Gary

[TALBO]

Some of the NA bridge and peripheral ported rotaries I've heard sound equally as good as a turbo rotor.... Love them.

[Birds]

Very good points Gary, however one could argue the eccentric shaft is the rotary's crankshaft. Piston engine RPM is derived from the crankshaft...it's just the nature of piston engines that the combustion parts move at the same speed. I can certainly see the misconception where people might think the rotors spin at the same speed as the engine's "output" shaft. But this is indeed one of the reasons rotaries do not develop a good amount of power until high up in their rev range.

[JAS-25T]

They suffer from the lies that Mazda spun back in the 70's.

Lie #1, they are a 4 stroke engine, rubbish they are a 2 stroke. Every one knows it, but Mazda lied because they didn't want the "lawn mower syndrome” attached to rotaries. The fact remains they are really a 2 stroke. The fact that they need oil in the petrol is just another nail in the 2 stroke coffin.

I wouldn't say they are 2 stroke, or 4 stroke, as there are 3 parts to the "stroke"

Lie #2, they rev to 9,000 rpm. no they don't. The rotors only do 3,000 rpm they use a step up gear ratio to spin the eccentric shaft at 3 times the rotor rpm. Why? So they could use normal piston engine gearbox and diff ratios. They could have easily used say 6 to 1 rotor to shaft ratios, then claimed 18,000 rpm. The rotors would still only be doing 3,000 rpm, their true rpm.

Not entirely sure about this one.................

Lie #3, the biggest lie, the small engine capacity eg; a 13B is 1.3 litres. What a whopper of lie that was. Their true capacity is 3.9 litres, 2 x 3 sided rotors x .66 litre each side = 3.9 litres. Everyone knows it, but Mazda persisted with the lie.

Capacity of any engine is combustion x cylinders.

0.66 x 2 = 1.3L. It is the number of of combustions per revolution that changes this compared to a piston engine.

A piston engine only fires (combusts) half of the cylinders per revolution, the rotary uses both combustion chambers per revolution. It's easier to double rotaries capacity in this sense instead of halving pistons, when comparing capacity

As with most things, the lies came back to bight Mazda. They use a lot of petrol for a 1.3L 4 stroke. But not so much for a 3.9 litre 2 stroke. They take a big turbo for a 1.3L 4 stroke, it’s not so big when the truth is 3.9 litre 2 stroke. They produce an impressive amount horsepower per litre when using the lies of a 1.3L 4 stroke. But when the truth is used, it’s a shit house power output for a 3.9 litre 2 stroke.

I've seen some 1000+hp 13B's. Thats pretty good power. Even for "3.9L"

So put the lies away and look at rotaries in the true light of day. As a 3.9 litre 2 stroke (in the case of a 13B) they have OK horsepower output, but lousy torque output due to their poor combustion chamber shape. That’s why they need 2 spark plugs. Another reason for their poor torque output is the compression ratio, which is very low, another result of that poor combustion chamber shape. Their fuel economy is OK for a 3.9 litre 2 stroke, but they don’t use the fuel efficiently, hence poor power output per litre of fuel used. They respond well to turbocharging as you would expect from a 3.9 litre 2 stroke with a low compression ratio.

2 Spark plugs is for pollution, it burns the excess fuel/air mixture that doesnt burn from the first spark, an inefficiency.

Kinda like the Z-series motor in the nissan family

Add it all up and you find the truth is a pretty average engine, with only one advantage and that’s small external size. But the fact is it’s not user friendly size. They don’t fit well in a front wheel drive application compared to a compact inline 4 cylinder. They don’t offer much space saving in a rear wheel drive application as they need a radiator, oil cooler and gearbox big enough for a 3.9 litre 4 stroke. A compact 3.9 litre V6 doesn’t need any bigger engine bay and yet produces more horsepower, more torque and gets better fuel economy.

But back to real reason for rotaries tiny niche market, the Mazda lies from the ‘70’s. Take them away and they become a much more believable engine alternative.

Cheers

Gary

A standard box for a 13B isnt that big. But when they quite often match up a celica/supra 5sp or alike to handle the power, it is rather big.

[Sydneykid]

I wouldn't say they are 2 stroke, or 4 stroke, as there are 3 parts to the "stroke"

In one revolution of the rotor they inlet, combust and exhaust. That's a 2 stroke, no argument.

Not entirely sure about this one.................

Check it out, you'll find I'm right

Capacity of any engine is combustion x cylinders. 0.66 x 2 = 1.3L. It is the number of of combustions per revolution that changes this compared to a piston engine.

There are no cylinders in a rotary. The capacity of an engine is simply how much does it pump in one cycle (360 degrees) and in the case of a 13B that's 3.9 litres.

A piston engine only fires (combusts) half of the cylinders per revolution, the rotary uses both combustion chambers per revolution. It's easier to double rotaries capacity in this sense instead of halving pistons, when comparing capacity

We don't take a 6 litre V8 4 stroke and say it only fires every second stroke so it's really a 3 litre engine. They pump 6 litres in 1 cycle of the pistons and a 13B pumps 3.9 litres in one cycle of the rotors. So why should we treat rotaries any different?

I've seen some 1000+hp 13B's. Thats pretty good power. Even for "3.9L"

A 2.4 litre F1 engine make 700 bhp, with no turbos required. A 2.6 litre Indy Car V8 makes 800 bhp at 7psi. There are plenty of RB26’s at 2.6/2.8 litres that make a 1000 bhp and at 3 litres lots of 2JZ’s regularly make 1200+bhp. Put in to that context, a 3.9 litre turbo rotary is pretty average at 1000 bhp. Then there is the fact that it’s a 2 stroke, that means twice as many firings per cycle, so it should really be compared to a 6.8 litre 4 stroke. In which case 2500 bhp is more like it.

2 Spark plugs is for pollution, it burns the excess fuel/air mixture that doesnt burn from the first spark, an inefficiency.

Two plugs are used for more than just emissions, try turning of the trailing spark plugs and see how much horsepower you lose.

Kinda like the Z-series motor in the nissan family

If I remember rightly they fired simultaneously, not consecutively like a rotary. Regardless, they stopped making them 20 years ago, technology removed the need. But the good old 2009 RX8 still needs 2 plugs per rotor, just like they did 40 years ago in R100's. Not a lot of progress there.

A standard box for a 13B isnt that big. But when they quite often match up a celica/supra 5sp or alike to handle the power, it is rather big.

Compared to a 1.3L engine's gearbox it is. Of course, since we know it's really a 3.9 litre, then it's not so big.

Cheers

Gary

[Sydneykid]

Very good points Gary, however one could argue the eccentric shaft is the rotary's crankshaft. Piston engine RPM is derived from the crankshaft...it's just the nature of piston engines that the combustion parts move at the same speed.

But it's not, there is no gear ratio between a crankshaft and its attached pistons, it's a one to one relationship. Why should we multiple the rotor rpm by 3 just because Mazda, rather Felix Wankel, chose a 3 to 1 ratio? If they had chosen 6 to 1 ratio, would we be saying they reved to 18,000rpm? The fact is the rotors are turning at 3,000 rpm, that's the bottom line.

I can certainly see the misconception where people might think the rotors spin at the same speed as the engine's "output" shaft.

That's the Mazda lie and some people believed it for 40 years, some even still do. Certainly proves the old addage, you can fool some of the people some of the time, but you can't fool all of the people all of the time. This people isn't fooled by the lies and never was.

But this is indeed one of the reasons rotaries do not develop a good amount of power until high up in their rev range.

But a 13B is a 3.9 litre, 6 cylinder, 2 stroke. Even at 3,000 (piston) rpm a 3.9 litre, 6 cylinder, 4 stroke piston engine would develop a ship load more horsepower. At the same time use a heap less fuel and generate a heap less emissions. The problem with rotaries isn't horsepower as such, it's torque or rather the lack of it. What if we compared it with another 2 stroke with somewhat similar rpm, say a 3.9 litre 2 stroke diesel? That's when the true rotary torque hole is really revealed.

Cheers

Gary

[scathing]

Add it all up and you find the truth is a pretty average engine, with only one advantage and that’s small external size. But the fact is it’s not user friendly size. They don’t offer much space saving in a rear wheel drive application as they need a radiator, oil cooler and gearbox big enough for a 3.9 litre 4 stroke. A compact 3.9 litre V6 doesn’t need any bigger engine bay and yet produces more horsepower, more torque and gets better fuel economy.

Also, with the housings being cast iron, they're also not particularly light by modern standards.

My understanding is that due to the inefficient shape of the combustion chamber and its larger internal area, a lot more energy is absorbed by the engine as heat rather than generating pressure to spin the moving parts around (compared to a piston engine). Hence why the fuel efficiency isn't as good, it needs such a "serious" radiator, and why Mazda haven't been able to use an alloy housing.

A drove a friend's stockish FD and I absolutely adored the way it does everything, and I still think its the best looking Japanese car made in the modern era, but I don't think I could live with that engine. That engine has got just over 100,000km on the clock and is starting to make horrible noises. Well, horrible-er (I don't like the rotary brap either).

They don’t fit well in a front wheel drive application compared to a compact inline 4 cylinder.

SK, do you know why they don't work too well in a FWD configuration? I'd have thought such a short block size would lend itself well to a transverse mounting position. They could have gone the Audi route and mounted the engine longtitudinally in front of the struts or, given its compact dimensions, mounted it "backwards" (longtitudinally with the transaxle in front of the engine).

[910trx]

Two plugs are used for more than just emissions, try turning of the trailing spark plugs and see how much horsepower you lose.

If I remember rightly they fired simultaneously, not consecutively like a rotary. Regardless, they stopped making them 20 years ago, technology removed the need. But the good old 2009 RX8 still needs 2 plugs per rotor, just like they did 40 years ago in R100's. Not a lot of progress there.

just to clear up - nothing more

The z (especially z18et) ran 2 plugs one ran to combust and one ran after the big bang to spoil left over hydrocarbons.

infact the 2nd ignition system no longer functions after about 3500rpm - simply cuts out

a good upgrade is to graft an early 13b or 12a dizzy on to get both plugs sparking at the same time when you are dumping in mega amounts of boost and fuel.

infact many weld up the 2nd spark hole and reshape the combustion chamber

rotaries are not that bad, if i had my dream club weekend race car, it would be a bug eyed sprite with a series 13b turbo in it, its light, fits where the old 4 banga was, more power than you know what to do with, absence of torque hardly effects a light car and reduces wheel spin because you rely on the revs to get you there. i'll keep dreaming...

But i dont know about calling it either a 2 or 4 stroke - it doesnt really stroke, more of a spinning bike tyre you keep tapping once its spinning.

[2LV8ETR]

rather Felix Wankel

He said wank......

[Johnny]

Is it just me or would every1 like to see sydney kid post this info up on the mazda rx3/4/5/67 forum or whatever lolz.

He just came in and took the whole 1.3 litre thing and tore it to shreds; no wonder people compare the rotors sound to 2-stroke = cos it is a 2-stroke.

may i ask, are rotory engines light? how much do they weigh?; compared to an rb or sr20 etc.

[Birds]

But it's not, there is no gear ratio between a crankshaft and its attached pistons, it's a one to one relationship. Why should we multiple the rotor rpm by 3 just because Mazda, rather Felix Wankel, chose a 3 to 1 ratio? If they had chosen 6 to 1 ratio, would we be saying they reved to 18,000rpm? The fact is the rotors are turning at 3,000 rpm, that's the bottom line.

We should multiply the rotor RPM because it's the eccentric shaft's speed that is measurable and relevant to anything we plan on using the engine for. It is still very much a part of the engine's static internals too, given it is the last link before the clutch/gearbox. When we dyno a rotary we aren't going to pit a variable against rotor speed, which is near impossible to measure; it will go against eccentric shaft speed, which is also useful for comparison to piston engines. Also, rotors don't exactly rotate in a perfect 360 degrees...they orbit more than anything.

Were it the nature of pistons to operate at different speeds to the crankshaft, we would still take engine RPM from a crankshaft, it just happens that piston engines don't operate like that. I very well see your point that rotors don't move as fast as pistons but rotary and piston engines are so different in the way they operate that they cannot be compared beyond the only thing they share in common - an output shaft that spins courtesy of whatever happens inside the rest of the engine, and bolts onto a gearbox. And this is what Mazda marketing, quite rightly (despite any other lies), would base their comparison of engine RPM on. If rotaries were the mainstream version and piston engines were the Mazda underdog, then would you make the same argument that piston engines should be multiplied by three to equate rotary engine speed? Because we'd be adding a variable in there that need not exist.

And yes, if old Felix Wankel had chosen a 6-1 ratio instead of 3-1 we would be saying the engine spins at 18,000rpm and that it has significantly less torque as a result. It's no different to changing the bore and stroke of a piston engine...the product of a piston engine revving to 18,000rpm is a very short stroke and a wide ass bore with a ton of power and next to no torque, e.g. an F1 or motorbike engine. Perhaps think of piston stroke (as per crankshaft lobe travel) as the equivalent of the eccentric shaft ratio.

[JAS-25T]

Because each combustion event lasts through 90 degrees of the rotor's rotation, and the output shaft spins three revolutions for each revolution of the rotor, each combustion event lasts through 270 degrees of the output shaft's rotation. This means that a single-rotor engine delivers power for three-quarters of each revolution of the output shaft. Compare this to a single-cylinder piston engine, in which combustion occurs during 180 degrees out of every two revolutions, or only a quarter of each revolution of the crankshaft

[fuzzy_scuz]

At the end of the day a Rotary always gets PIST 'ON