What are the advantages of a 4-stroke as compared to a 2-stroke? Why would I want a 4-stroke when a 2-stroke is available?

And now for the other side of the coin.....visit the F-F-F-Four-Cycle Page

4 strokes have more torque but less speed where as the 2 strokes have the exact opposite

The best thing about 4-strokes is they'll swing a larger diameter prop than a 2-stroke. This isn't a benefit for all R/C airplanes but the scale ones really shine when they are 4-stroke powered. A good example is Great Planes' 1/5th scale RV-4. The real RV-4s are usually powered by a Lycoming O-320 that spins a 70" - 72" diameter prop. At 1/5th scale the RV-4 needs a 14" prop to be accurate. A .46 2-stroke just ain't gonna swing one of those!

From a previous post;

>>4 strokes have more torque but less speed where as the 2 strokes have the exact opposite<<

Actualy this is not correct. A four stroke has nowhere near the torque or horsepower of an equivalent displacement two stroke. Think about it. At 10K rpm a two stroke has 10K power strokes/ min, a four stroke would have 5K power strokes. The other half would be exhaust strokes. Torque is not produced on exhaust strokes.

For a two stroke to reach max horsepower it has to rev higher, which requires a smaller prop. It's max HP and max torque are quite far apart in the RPM range. A large propeller would lug the engine and it would overheat.

A four stroke on the other hand develops max HP and max Torque much closer together in the RPM range. That is the reason it will swing a larger propeller quite happily. This is purely a characteristic of the four stroke engine.

So the advantages are that a four stroke will swing those larger propellers at a lower RPM.

The lower rpm and firing every other stroke makes them quieter. They do not need those big clumsy mufflers. They also run cooler.

They burn fuel more efficiently, which makes them easier on fuel. Although this is usually cancelled out by bigger engines being used.

Ed S

very interesting...

The less Dead Stick is good!!!

Also - do they idle more dependable? Last longer?

It's been my experience that they do idle more dependably than two strokes, I know with Saito's that if you set the idle a tad rich they would idle a full tank out, I have two new Ys's, a .91 FZ which idles very well and a .53 I haven't run yet. The only dead sticks I've had with four strokes were with a Saito .80, probably my fault for setting the high speed needle too lean before take off.

Quote:
"Actualy this is not correct. A four stroke has nowhere near the torque or horsepower of an equivalent displacement two stroke. "

Well actually that is not absolutely correct.
While the fourstroke does only develope power on every other stroke ,the four stroke fills the cylinder better and compresses the charge more effectively and burns the charge more completely.
This allows for higher cylinder pressure which in turn produces more torque for a given displacement. They are able to swing larger props at lower more efficient speeds. Horsepower is a function of Torque AND RPM . Propeller efficiency falls off at higher rpm and four strokes are able to make power at lower RPM taking advantage of the increased efficiency.
The more efficient burning allows for a huge savings in fuel and a lighter take off weight from the lower fuel load.
The two strokes also need to run huge mufflers to bring the noise level down . The size and weight often overcomes the weight advantage of the bare 2 stroke engine. And the mufflers themselves usually do very little for the asthetics of the model.
4 strokes are less prone to loading up and accellerate more cleanly after extended periods at low throttle.
I know among flyers there are a few who like the sound of a two stroke better than the 4 stroke. Among the NON flyers who put forth the effort to shut down fields the two strokes are the noise that makes for most of the complaints. We have been involved many times with neighbors and almost without exception the four strokes were far more acceptable from the sound standpoint. If all we had to fly were two strokes our flying field would be a Soccer field by now.

The YS 91 rivals most 91 size 2 strokes power wize.
And the planes perform very well.

For WOT from take off to landing where noise and fuel consumption are of little concern the two stroke may have a slight edge

Another quote :
"And now for the other side of the coin....."

Thanks for the offer but I know 2 strokes, I hve been around them most of my life. Will I go back to them ?

A four stroke will not develop as much torque as a same displacement two stroke.


You are correct that it burns the charge more effeciently. A four stroke will achieve a higher cylinder pressure, due to the fact that it's cylinder is more completely filled with combustible gas during induction. However it's actual mean torque is halved compared to the engine's brake mean effective pressure because of the alternate firing.

That is my last word on the subject.

Ed S

You are confusing Torque and HORSEPOWER. A very common mistake.
Torque is a function of cylinder pressure and stroke. Horsepower is a function of torque and rpm.
And lately the four strokes have been closing in and in some cases exceeding many of the 2 strokes in the horsepower race as well.

Any given propeller running in a test block will present a specific torque reactive mean LOAD varying with RPM . Fixed at 9000 rpm a 15-8 Zinger will present the same torque reactance wether being turned by a steam engine , diesel engine, 2 stroke glow ,four stroke glow or electric motor. Because the torque and rpm are the same,the rate of doing work (DEF:Horsepower ) is the same .AT THAT SPEED>

Most 90 size 2 strokes don't have enough TORQUE at 9000 rpm to turn a 15-8 at that speed. THEY WON"T PRODUCE THE REQUIRED TORQUE. To make an equivelent HORSEPOWER they must turn more RPM at lower TORQUE.The Ys 91 4 stroke has no problem swinging a 15-8 at 9000 rpm.



The increased efficiency negates a large part of the 2 stroke advantage.
The one efficient power stroke can make more usable power than two very inneficient strokes. And do it with less wasted fuel to boot.
The 2:1 ratio is far from accurate.

WOW! All that and she's gorgeous too!

Also the 4 stroke puts the pressure on almost all the way to the bottom of the stroke . power to weight, i'll still take 2 stroke.

If you were to put a timing wheel on a 4-stroke model airplane engine you would find that the exhaust valve opens pretty much at the same point after top dead center as the exhaust port in 2-stroke sport engine. About 100 degrees after TDC.

Horsepower is calculated while torque is what's actually measured. Horsepower = 2 times pi times torque(ft lb) times rpm all divided by 3300.

OR:

HP = 2(pi)(T)(RPM)/3300

As you can see, if you raise your torque you can make the same horsepower at a lower RPM. Conversely, if you raise your RPM you can make the same horsepower with less torque. In a nutshell this is the difference between a 4-stroke and a 2-stroke. A sport 2-stroke will make less peak torque but will turn a much higher rpm than a similar displacement sport 4-stroke and put out more horsepower.

Now, if you compare a 2-stroke R/C engine with a similar displacement 4-stroke at the SAME RPM you will find that at higher rpm the 2-stroke will make more torque simply because the 4-stroke will have fallen off it's torque curve. Unlike the full size 4-stroke engines in the auto and motorcycle racing world our R/C 4-strokes are very rpm limited due to their valve and port configurations.

All the dyno graphs I have seen of 2-stroke motorcycle engines show that peak torque and peak horsepower come at very near the same RPM. This is very different than the way 4-strokes behave and contrary to what Ed Smith posted. In fact, if you read Mr. Smiths post it is a bit contradictory itself. Y'all pay attention to what Lady Flyers posts. She knows what she's talking about

All said and done lets see what the actual results are, I have flown 2 strokes for 10 years before I had a 4 stroke. Fact, The 4 stroke will take off a plane faster with more authority than a 2 stroke will. The 2 stroke does not have the pulling power of a 4 stroke as soon as the throttle is advanced. Here are the reasons I fly nothing but 4 strokes now. #1 they have a better throttle response than a 2 stroke, #2 They are more reliable than a 2 stroke, If set up right they will idle forever and throttle up without hesitation. #3 they fly the model at slower speeds and still have good vertical while a 2 stroke flys fast but soon runs out of vertical when took up. #4 Landing a model with a 4 stroke is a whole lot easier than with a 2 stroke. the 4 stroke has a braking effect that the 2 stroke does not have. plus I love the sound of a 4 stroke. The 4 stroke does cost more but I am willing to pay the difference to get the performance I want. I just enjoy flying a 4 stroke far more than a 2 stroke period. Everyone does not agree and the only way to find out is to try one. I did and I will never go back to a 2 stroke. These are some of the things I found out by flying them, forget the theory and all the arguements and fly a 4 stroke and the difference will slap you in the face. there is that much difference between the two. Forgive my spelling it is late and I ready for bed. Just my 2 cents.

[QUOTE]Originally posted by Ladyflyer
And lately the four strokes have been closing in and in some cases exceeding many of the 2 strokes in the horsepower race as well.
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Pretty interesting comments/opinions among the different flyers between 2 stroke vs. 4 stroke. However, there are incorrect information that needs to be corrected to clarify the facts.

As Ladyflyer stated above... " And lately the four strokes have been closing in and in some cases exceeding many of the 2 strokes in the horsepower race as well. " Well, this information is based on assumption. The trueth is... The best 4 stroke (which is a YS's) doesn't even come close in horse power to a 2 stroke in comparable size/displacement. There are .40's size 2 stroke that produces horse power in the range of 2.5-3.0 HP depending on application and tuning done by the manufactures (.40 Jett and Nelson engine). So to say a 4 stroke exceeding 2 stroke in horse power would be of false information. If it's torgue, then I would agree.

Yes. There are some benefits to a 4 stroke vs. 2 stroke, but those benefits must be weighted by the individual to make that decision based on what he/she will be doing. For example, if an individual want/need horse power, light weight and speed for his plane, then a 2 stroke is the way to go. But if the individual's looking for pulling power (run bigger prop), quieter run, less fuel consumption and to lift heavier airplane, then he's better of with a 4 stroke. That's the real benefits of a 4 stroke vs. 2 stroke.

As for a 4 stroke lasting longer or better engine to have... Well, that's just personal opinion and preference. The actuality would be the quality of materials that it's made out of. So, to say 4 stroke last longer would be incorrect also. For example, a Sport Jett .40 made out of the best quality materials would out last any 4 stroke out there guaranteed. Especially if it's tuned down a little. If one doesn't believe it, then prove me wrong if you will. Do it by counting how many flights you will get before your engine give out. So the answer for longer life on a 4 stroke is irrelavant. It really depend on how's the quality of the materials and how it's made.

Don't get me wrong, but I do like both (4 stroke and 2 stroke). Except, I use it based on the application (i.e. bigger and heavier air plane 4 stroke. Smaller, lighter and faster speed 2 stroke).

There are no real perfect engine's for all applications, wether it's a 4 stroke or a 2 stroke. So the answer would be... Buy what you think would be best suit your needs/application and enjoy. Hope this help answer yours/all others questions.

Sam S.

The bottom line here is that all the info except the longevity factor are useful, even if made totally of unobtanium they will wear out. By the same token I have an Enya .46 MKII, a Saito .80 and Saito 150 all about 10 years old and none has lost anything performance wise. Even my lowly Merco .50 which was supposed to be a poor engine and is older than all the above will still turn a 12x6 at 8,800 on 5% nitro fuel and fly all day without so much as a hiccup. Try a four stroke and you'll love it, quaranteed.

The reason I started this thread is to learn about 4-stokes - which I think I have done.

I've had problems with 2-strokes idleing smoothly & accelerating smoothly and quickly in flight.

DO 4-strokes run better inverted and in rapid climbs?

Now another stupid newby question - is 4-sroke the same as 4-cycle and 2-stroke the same as 2-cycle.

Yes to your second question and someone else will have to respond concerning the inverted engine question. Did you get my E-mail about Raceway 7? If the weather is fit I'll be there Friday night for test and tune night, wringing out my brothers new car we built for this year.

Sam ,You need clarification before you pronounce information as incorrect.

What I said was carefully worded it was TRUE . I said... in SOME.. cases exceeding ..MANY... (as in NOT ALL).
The point being the old wive's tale of halving everything is FAR from true. The (YS 91 Does put out more power than SOME two strokes. I never said it was the MOST POWERFUL 91.
And putting usable power at usable speeds are what 4 stroke do well.
Yes in many cases four strokes are competitive size for size with 2 strokes. Also in many cases they fly better too.

Please NOTE , "MANY" is not an absolute ,as in "ALL"

The amount of development work done on model aviation 2 strokes in the last 60 years dwarfs the work done on 4 strokes since they started getting serious attention about 20 years ago.
The latest developments are starting to pay off and we all win.
Maybe we can save a few fields in the process.
As Hobbs said his aged Merco 50 still puts out a very respectable 8000 rpm on a 12-6 . The YS 53 puts out 10,000 rpm under the same conditions. I know this is best to average comparison . It does demonstrate how much the gap has closed .


Put that 3 horse Jett on an Aeroworks Edge and you will be making my point. The Jett's 3+ Horsepower won't fly it near as well as a 2.5 Horsepower Saito 100. Iam referring to the HORSEPOWER here forget the displacement.
By the way how much does 3 HP cost in a Jett package?

I have already said there are applications where one is better suited than the other.

It is also true the percentage of planes piston powered,running four strokes is growing all the time

Longevity is an issue I have never weighed in on. I haven't worn one out yet so it's not an issue with me. I do know in competition motorcycles 2 strokes get rebuilt at least twice as often as their 4 stroke counterpart.
Your ASSUMPTION a JETT will outlast ANY four stroke ????. That IS an opinion /ASSUMPTION and a real stretch as well

As to question of stroke or cycle , The proper term is 2 stroke cycle or 4 stroke cycle . Referring to the number of strokes to complete a full power cycle. The common vernacular uses one or the other interchangeably if not accurately.

Well here we go again. I will start another argument with the "Experts"in which I will not participate.

The term 4-cycle (or 2) is incorrect when describing engines. A cycle is a complete set of events. Kind of like the seasons in a year, get it?

A four stroke engine has four piston strokes and two crankshaft revolutions per cycle. It does not have four cycles.

If the engine is called a "Four cycle" engine does this mean that it cycles only four times? That is sixteen piston strokes, eight revolutions. Then what?
It can be seen how incorrect this description is. Or is it too complicated for the "Experts" to grasp.

It could be claimed that every reciprocating internal combustion engine on the planet is, in fact, a one cycle engine. Although I am sure there must be some exceptions!!

Ed S

Lady flyer Quote:

"As to question of stroke or cycle , The proper term is 2 stroke cycle or 4 stroke cycle . Referring to the number of strokes to complete a full power cycle. The common vernacular uses one or the other interchangeably if not accurately."

Does that somehow disagree with what you said?
It looks like we both said the same thing !

Funny how words get changed around
The proper term is ______-stroke cycle . Either two or four. Most people understand that we say 4 cycle or 2 cycle interhangeably but it does throw a curve at those just getting started.
If we call it a 4 stroke or 4 cycle to them it's

Sure if you want to be ridiculous after four cycles ,16 strokes then what ?
By the same token and convoluted example :4 strokes ,TWO revolutions THEN WHAT ? DUHHHH !

I hope this doesn't confuse the poor guy's question any further.
What he asked was are strokes and cycles the same .The SIMPLE anwer is YES,in the common way we use the terms they are TAKEN (by REASONABLE people) to MEAN the same thing.
For the sake of accuracy the proper reference is Four or Two STROKE CYCLE . It describes the number of STROKES required to complete ONE CYCLE ! As Ladyflier said.

It may still confuse SOME ,but I'm no expert and I got it the first time.

Maybe cabin fever sets in up North and they jump just to be jumping

To Ladyflyer,

Madam, we said the same thing, almost, at nearly the same time. It was just a question of which post made it to the board first.

To be strictly correct the terminology should be :- Four strokes per cycle.

Ed S

"Put that 3 horse Jett on an Aeroworks Edge and you will be making my point. The Jett's 3+ Horsepower won't fly it near as well as a 2.5 Horsepower Saito 100. Iam referring to the HORSEPOWER here forget the displacement.
By the way how much does 3 HP cost in a Jett package? "
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Ladyflyer,

Per your statement above... " Iam referring to the HORSEPOWER here forget displacement." As to my understanding, you're refering to horsepower. So to if that's the case, you know that a 2 stroke will make more horsepower in comparable size. But if you would of just mentioning torgue, than I would agree with you.

Each engine designed have it's strong and weak point. To compare them two in same manner would be insufficient.

In term of a Jett's 3+ horsepower not flying as well as a Saito 100. Well, lets reverse that to putting a Saito 100 2.5 horsepower to a .40 size Pylon Racing plane. Let see how well that flys. So therefore this arguement is irrelavant.

As for the cost of a Jett's package... Cost is $275.00 to $351.00 depending on which engine design (standard investment cast or BSE series-machined Bar Stock Edition).

Sam S.