DiscoWings

My Feedback: (76)

This is one of those questions that gets tossed around. Depending on the person it may go either way but I think we really need to have a definate answer...
The question is: Do Equally displaced 2 strokes have the same torque as 4 strokes?
Compare these 2 engines: OS 91 FX vs OS 91 Surpass II
According to tower they both have the same prop range, wouldn't that suggest they have the same torque?

Sport_Pilot

They may have the same prop range, but I am pretty sure the FX will spin the prop faster. That means the FX will have more torque. This is usually true. It's only when you compare a four stroke to a smaller two stroke of equivelant power that the four stroke will have more torque, spinning a larger prop at a slower speed.

Hobbsy

My Feedback: (102)

This is hard to say, if a .40 two stroke can turn a 10x6 at 14,000 rpm and a .40 fourstroke can turn a 12x5 at 9,000 which has more torque? The fourstroke can't turn 14,000 because of mechanical limitations, the .40 two stroke will get hot trying to turn the 12x5 so you're down to accepting each engine on its own merits. A .40 Diesel will turn a 12x6 three blade at 8,400 which neither of the other two .40s could do. This is good talk material though.

DiscoWings

My Feedback: (76)

well this pharse really bothers me:
4-Strokes have more torque
what the heck does that mean? More torque then what? A smaller engine? Thats like a 30 year old man telling a 10 year old I have more muscle then you... well no duh...[>:][sm=tired.gif]

STG

My Feedback: (26)

While it is true that 2 strokes and 4 stokes of similar displacements usually have the about the same peak torque the curve on the 4 stroke is flatter. That is to say that the 4 stroke will typically have more torque at lower rpms.

What does that mean for our motors...very little as these props do not impose a significant load at lower RPM's. Most people incorrectly attribute spool up with torque. The real key to an instantaneous spool up is correct fuel metering. 4 strokes seem to be much more tolerant to less than ideal needle setting than 2 stroke motors. With the correct needle settings(and sometimes a pump) 2 strokes transition just as well.

I like to compare motors by weight and power rather than displacement. The top performers are:

Weight with muffler
18.5oz -- SA82 & YS63
21.5oz -- SA100
27.5oz -- YS110
33.5oz-- YS160, YS140, SA180
41.5oz --SA220

It is hard to find a similar weight (with muffler) 2 stoke that will run with these motors unless it has a tuned exhaust.

downunder

As hobbsy said, this is hard to say because it's a case of comparing apples to oranges. The 4 stroke's mechanical limitations (valve bounce, small valve areas) mean that they get optimised for peak torque at lower revs. The 2 stroke is far less efficient as far as the mean pressure goes (BMEP) but it's less efficient twice as often . However, because they don't have any mechanical limitations (within reason) they get ported for terrible efficiency at very high revs. This gives good HP but doesn't do much for the ability to swing big props which is what torque is all about.

However, given that a 4 stroke has those mechanical limitations and can't use revs to get HP, there are some 2 strokes designed to run in a similar rev range which is more like comparing apples to apples. These 2 strokes can swing props of similar size to what a 4 stroke can turn so their torques must be very similar.

One slightly interesting thing about 2 strokes happens when they're run so rich they fire every second time (4 stroking). You'd think that firing every second time would halve the torque (and HP) but it doesn't. The torque drops to about 70-75% of what it would have if 2 stroking because the firing stroke is more efficient than normal. This is why you don't see much jump in revs when you transition from 4 stroking to 2 stroking.

Motorboy

4 stroke engines has better cylinder filling than 2 stroke engine and get more energy for each combustion therefore more torque. 2 stroke engine are difficult to get complete fuel/air mixture in the cylinder and there are still rest of exhaust.

Jens Eirik

Harry Lagman

Every time I've tested them, two strokes of the same capacity and performance envelope have had more peak torque than naturally aspirated four-strokes.

Let's look at an example... A flying buddy and I were running in his brand new OS .91 FX (15cc) the other day in my workshop. Alongside, I had my Saito FA100 (17cc) that I was retuning to a 20% nitro mix, after running it on 15% from new (it's done about 7 or 8 engine hours now).

We tried various props on his FX once his engine had done a few rich runs. Bad news for the Saito... The FX, with its stock muffler, consistently out performed the Saito prop for prop. Even a heavy load like a 15x8 was spun faster by the FX - on less nitro too. Calculated torque curves at these datum points on the same props obviously show that the 91 FX has more torque and therefore, at that rpm, horsepower. If you were to try loading the engines down ridiculously, you may find that the Saito could start producing more torque than the FX at really low revs, where the two-stroke's blowby is starting to severely pollute the fresh crankcase-resident fuel/air charge.

The news gets even worse if the two-stroke is fitted with a tuned pipe. I recently saw a guy's OS SX .50 with a long header and tuned pipe pull 11,000 rpm with an APC 12x8 prop - 400 rpm more than my YS .63S, and that's a supercharged four-stroke!

NM2K

My Feedback: (14)

Umh, not necessarily so, regarding valve float.



My point is that four-strokes are not necessarily limited because they are four-strokes. I am sure that the model engine manufacturers could have sold us really high rpm capable four-stroke engines if they felt there was a market for them.

However, the world of R/C flying has been moving toward quieter engine operation. This requires lower rpm and larger props. Just the opposite requirements of what model four-stroke engines are capable of being designed to produce.

While I would enjoy the little four-banger motorcycle as a hoot, I do not want my models to be running at 17k rpm. Just the headache of obtaining and maintaining safe props for that range of rpm is not something that I want to bother with at this stage of my life.

Obviously, a 17k rpm model four-stroke engine would not be using pushrods to actuate the valves, although I wouldn't rule it out as being impossible. Titanium pushrods, anyone? Webra and Conley had overhead cam four-stroke model engines for a while, IIRC.

Ed Cregger

Hobbsy

My Feedback: (102)

DOHC ABITAR 1.20 turning an APC 14x6 on 10% WildCat Premium Extra.

Harry Lagman

What's with the editing Hobbsy? Are motorcycles a dirty word here?

pete913

If you take a look at the weight vs torque/ hp vs fuel consumption, I don't think a 4 stroke can be beat. 4 strokes don't turn the same size props the same rpm's as a 2 cycle because that isn't what they were designed to do. They were designed to turn a larger more efficient prop at the slower rpm the larger prop requires and develop their maximum power at a lower rpm.

They could just as easily have been designed to turn maximum rpm with the same size prop as a 2 stroke. 14000 rpm for a 4 cycle that small, say a .120 should be no big deal.


I think the real limitations of model four strokes come from mostly from the limitations of where the carburetor can be placed, due to fuel draw issues, the farm tractor like updraft manifolds full of 90 degree bends,, the tiny size of the entire intake side of the engine, carb to valve, the wimpy cam timing necessary to creat max torque at low rpm, not because of any limitation in the 4 stroke cycle compared to the 2 stroke.

Motorboy

Webra engines never had over head valves. Rotary and aspin valve only, these engines are not hi rev engines cause limited in timing.

OS FS 26 C (car engine) can rev up to 22000 rpm...

Hi rev 4 stroke need stronger valve spring, bigger valve diameter and more timing.

Jens Eirik

Hobbsy

My Feedback: (102)

I have three OHC 1.20 fourstrokes, a Conley, the ABITAR pictured above and an OPS 1.20 SOHC two valve engine barely visible behind the ABITAR. I tried a 15x8 on the ABITAR and it simply could not rev up beyond 6,700 rpm and sounded awful, the OPS on the other hand easily turns a 15x8 just as well as a Saito 1.20. It's a shame that my Dynamometer can't take engines larger than a .91. My Webra 1.20 turns a 16x8 better than a Saito 1.20 does so it would win any torque comparo there, when it comes to engines and life, timing is everything.

B.L.E.

Just because an engine can turn a large prop does not mean that it makes a lot of torque. It takes less torque to spin a 12X6 7500 rpm than it does to spin a 10X6 10,000 rpm.

PlaneKrazee

My Feedback: (14)

I think a lot depends on valve and port timing. A two stroke engine like the OS 160FX is designed to run below 10,000 rpm and makes more power, can spin bigger props, on lower nitro and less fuel than a Saito 180. You won't see the Saito spin a APC 18X6 at over 9000 rpm or a Mejzlik 20X6 at over 7800 rpm on any fuel. The drawback to the OS is the potential size and weight of the muffler, however it is still the most powerful engine for it's weight including the muffler.

STG

My Feedback: (26)

.

Are you equating efficiency with fuel mileage? If you are you are incorrect. In theory it may be true, but in real life the top performing 4 strokes do not outperform the top performing 2 stokes on mileage. They also do not turn larger props faster than their 2 stroke competition (displacement wise), but they do have a slight advantage if you consider only weight and are willing to run 30% nitro.

I see people talking about displacement to displacement comparisons. As it applies to us on our planes displacement of the motor does not matter-- it is the weight of the motor and the power. Usually the 2 stoke has a much heavier exhaust system and that has to be included in the motor weight if you want to be fair.

BTW, there has been very little development with 2 stroke glow motors in the last 15 years. All the development money has been going into the 4 strokes and started when the pattern rules changed to allow 2X the displacement of 4 stokes.

STG

My Feedback: (26)

Yep, it the weight of the muffler that kills it on power to weight. Really kills it if it is not a tuned exhaust. If you are just talking power to weight and not considering the cost of fuel the top performing 4 stokes have been given another advantage as they have ben optimized to run on 30% nitro.

You put the 1.60FX with muffler (at least 39.5oz with slimline pitts) on top, but I think there is 1 four stroke that clearly wins on power to weight. -- The YS1.6 at 33.5 oz with muffler.

The SA220(41.5oz) is also right with the OS160(39.5oz) and I would bet that it will spool a 20!QUOT! prop up a bit faster -- much larger displacement

Depending on the bird it can be argued that the SA180 or YS140 (on 30%) beats the OS1.6 on power to weight as both these motors are 6oz less with muffler and not far off on static thrust.

Don't get me wrong, the 1.6FX is my favorite glow motor.[8D]

donkey doctor

Is torque the ability to swing a big prop and have clean carberation right from the bottom of the rev range? Or is that Just good carberation? Maybe it's the depth of the power band we're talking about. If the power doesn't start until 5000 rpms is that still a torquey engine?

STG

My Feedback: (26)

Donkey, the load these props impose on these motors is very low below 5000 RPM so torque is not much of an issue.

16 x6 prop
2000 RPM -- .022 HP
3000 RPM -- .079 Hp
5000 RPM -- .351 Hp
6000RPM -- .607 HP
7000rpm -- .963 HP
8000rpm --- 1.43hp
9500 rpm -- 2.4hp

You see there is no "down low".

If you have a 2 stroke that you don't think has a snappy spool up because lack of torque try this....adjust your high speed needle for transition only. DO NOT HOLD THE THROTTLE FULL OPEN BECAUSE YOU WILL DO DAMAGE TO YOUR MOTOR. Just bump it fast and get back off it. You will be surprised to see the 2 stoke jump like a 4 stroke with the mixture set optimum for transition. Now that is how my 2 stokes run (OS108 and OS160FX) with high speed needle set back 100 or so back from max rpm on pump.

I have a recording(.wav) of my 108 revving up from 2000 to 9500 on a 16 x6 if anyone is interested. The motor goes where I put the stick (up down middle) almost immediately. Drop me your email address and I will send it to you.

You can also see a film clip of it on my web page(found in my signature)

DiscoWings

My Feedback: (76)

well here is the deal,
generally 2 strokes weigh less then 4 strokes. Now I don't understand why people compare a light 4 stroke to a havey 2 stroke? e.g. Magnum 91 4 strok is about 30 oz with muffler but the os 91 is about 24 oz

Jet 76L with muffler is 17 oz, saito 82 is 20 oz with muffler? who has more torque?

pe reivers

general comment:
I thought this horse was beaten to death a long time ago;
On a per revolution basis, the two stroke only has 60% of the four stroke power. Too bad for the four stroke which fires every second revolution, the two stroke fires every revolution, so is wins by about 20%

Bodge

Generally two strokes do not weigh less than four strokes. Check the Saito, ASP, Magnum four stroke weights with their equivalent sized two strokes. In fact check the weight of most 120 four strokes with two strokes. Four strokes are lighter.

There is really a lot of rubbish talked about torque (sorry) and horse power. What we should be looking at is Brake Mean Effective Pressure - bmep - which shows how efficient the engine is at converting fuel into turning force, or how efficiently the cylinder is filled. Manufacturers tend not to give that figure, but it used to be shown on the old engine tests. It would be a meaningful comparison of performance. Torque is the turning force (radians per sec (per sec)) developed directly by the engine; horsepower is that force developed over time, i.e. revolutions per MINUTE. Horse power figures on their own give one very little information on engine performance.

Another fallacy is that two strokes are higher revving than four strokes.

It was mentioned earlier that four strokes running at 17,000 would not be good model power plants. Absolutely true in most cases - but it does not mean it cannot be done. At the end of the day, the potentials of four and two strokes are similar - it's how they make that power that's significant. Currently four strokes seem to have a slight lead, but I couldn't say exactly why...

carrellh

I think the real question is: WHO CARES?

Run what you want and don't worry about what others think.

Hobbsy

My Feedback: (102)

Obviously nearly everyone who posted to this cares and most said something worthwhile.