Whats the difference between 4 stroke engines and 2 stroke engines? The price jumps almost 200% even when they both are rated a .70 engine.

Thanks
Electrolight

I assume that you already know the difference between 2 and 4 in terms of basic functions...

e.g. Like 2 cycle versus 4 cycle engines

A 4 stroke engine has a nicer sound and you'll hear everyone say "it can turn a bigger prop".

That is somewhat true...

However compare the output power of a .70 2 stroke engine verus a .70 4 stroker and you'll find that the 2 stroker engine is far more powerful.

The reason for this is that the 2 stroke engine ignites the fuel on every movement ( compression ) stroke of the cylinder. The 4 stroker, does it every other time.

As a result the 2 strokers build higher RPM's, but they feature ligher cylinders and less moving mass.

Conversely the 4 stroker uses a larger cylinder with more mass to help carry the engine through the non firing cycle.

The 4 stroker then tends to burn less fuel too. They also sound much nicer, more like a real plane engine.

So let's say you are looking at a .60 sized plane, and the instructions call for at most a .60 sized 2 stroke engine.

You could put in a .60 to .70 2 stroker or to get the equivalent output you'll need about a .91 ( yes this is inexact ) four stroker.

The four stroke engines cost more because they are more complex.

I've had about equally good luck with 2C and 4C engines.

Usually 2 stroke engines are used on "smaller" planes. As you move up fuel consumption rises quickly on the 2 strokers, making the 4 strokers a bit cheaper in the long run.

I typically go to 4 stroke engines in planes around the .90 and above size ( with a few exceptions ), and use 2 stroke engines on planes smaller than that...

Between .70 and .90 it's a real toss up for me.

Okay thanks.

Could you answer one more question. On a plane that says it flys nicely on a 4 stroke .70 engine, could I use a 2 stroke .60 engine?

Thanks
Electrolight

But ... You can use a .70 on a .40 size plane. The 4S will swing a 14X6 propeller about 9500 RPMs where as the .40 2S will turn an 11X4 about 12,000 rpms (comparitively speaking). On lighter planes like profiles and 3D, the extra weight of the 4 stroke is offset by it's ability to swing a large prop. On warbirds, people like the 4 strokes because of the sound (sounds more like a lawnmower as opposed to a weed eater). I think it's just a matter of personal taste. Power wise, the same size 2 stroke is more powerfull, but they weigh more.

Sure. I have an UltraStick 40 for which a .40-.46 two stroke engine would be used. I'm instead installing a four stroke .62 engine. Strangely, the .62 four stroke is lighter weight than my .46 two stroke. And it really does sound nice...

This is the typical marketing hype mantra.

However it misses the other HALF of what should be added to that statement..

2 strokes, when compared to 4 strokes of equal displacement produce more power.
The "larger prop" really only factors in when you move to a larger four stroke engine.

As it is with equal displacements the difference between the recommended props moving from 2C to 4C varies by only ONE increment if it is not the same.


Weigh more?

Depends upon which vendor's engines and sizes you are looking at.

Since you have to offset the decreased power of the 4 stroker at the same displacement size, by taking the engine size up several notches... typically the 2 stroke engines end up being about equal or in many cases weighing less... As you get into bigger engines such as the O.S. 1.60 2C, things tend to reverse and indeed a 2.00 four stroker is "lighter" in that range..

e.g. I can put in a .75 2C into my Skybolt, and because the engine is lighter than the recommended 4 stroker with muffler, I would have to add more weight to the nose.

I've flown my Skybolt ARF with both engines, and it is more powerful with the .75, but I prefer the O.S. .91 Four Stroker in the plane for other reasons... And yes I'm swinging a bigger prop, but that is in large part due to the fact that the O.S. .91 is a larger engine.

I do LIKE 4 strokers though and if they were a bit cheaper I might run them exclusively.

Yes.

With the two stroke .60 engine, the plane would be more powerful.

A .60 2C engine ( assuming a typical non-bleed carb engine, UNLIKE the anemic O.S. LA series engines... ) is roughly the equivalent of a .84 to .90 four stroke engine. ( That's not to say you can find an engine in the .84 to .90 range... this is just explanatory... ).

A 2C OS 1.20 AX would be akin to something like a 4C 1.60 - 1.80 in terms of output power.... but the 1.20AX would consume fuel at a high rate ( well over 1.3-1.4oz/min at WOT ).

Fact. A saito .72 weighs less than an O.S .46.

Fact an O.S. 46 weights less than a Magnum .52 RFS 4 Stroke, let alone a .70 sized four stroker.

To paraphrase: " It don't mean a thing... ".

Your opinion don`t mean a thing to me.

Awww....

You did put up the Saito info... so it must have meant something... least you would not have graced us with the response.

The data I posted on the .52 RFS was not an "opinion". Just another fact.

If you fly your .46 OS without a muffler it will weigh less than a 72. Even a Saito .82 is less wieght than an os 46 ax with it's muffler. Do you weigh your engines? OS publishes thier engine wights without including a muffler. As far as prop size goes, displacement is displacement. 60 size 2 or 4 strokes like the same size props. The 2 cycle will typically spin the same prop faster though.

Saito's are manufactured to be light engines so they do weigh less than an O.S. 46.

I turned around and mentioned that the relatively small Magnum .52 RFS weights more than the O.S. 46.

But then that is not comparing apples to apples at all.

Of course there are also 2 strokers that weight less than the O.S. 46 too.

"The 2 cycle will typically spin the same prop faster though. "

The engine that produces the higher RPM's with the same prop, produces higher power.


In the smaller engine sizes, to obtain the same output power on a four stroker, compared against a 2 stroker, I need to increase the displacement of the 4 stroker. THEN you can swing that "bigger" prop.

It's not that I have anything against 4 stroke engines... but quite often the marketing hype is taken to be gospel, and in turn bantered around to newbies too...

A .60 four stroker doesn't compare to a good .46 2 stroker in terms of output power.

Which answers the original poster's question...


You can save the money if you wish, and use simply use a 2 stroke engine.

In the OP's shoes I'd use the 4 stroker when I desire more "scale like" sound, looks and performance.

2 strokes = 2 stroke to complete a cycle. ( intake, compression), (ignition, exhaust)
4 strokes = 4 stroke to complete a cycle., intake, compression, ignition, exhaust

A 2 stroke will also generate more RPM due to a longer rod and less strokes to complete a cycle.
The piston dose not have to travel as far as a 4 strokes.

A 4 stroke will generate more torque.

R/C intro engines have sleeving that are ported different by different manufacture.
The porting also effects performance.

The size of the carb also effects performace,of course.

obviously a .90 4stroke is lighter than a .72 4 stroke if it uses the same casing and the .90
has just been bored out. less material less wieght of course.

a .52 is lighter than a .40 of couse if it use the same size casing..bascially if you bore it out.lol

nitro heli 2 stroke engines also has a shorter rod than an airplane 2stroke engine.

Theres other factors such as prop pitch and size...air frame

You can have high RPM ,but if it's not moving air, it's just making noize.
mmm...kind of like getting on a multi speed bike and pedaling the heck out of it and hardly moving.

There's also a threashold or power band. Peak RPM dose not equal peak HP.

don't go crazy assuming you can use a car Nitro engine for air craft ..Car engine are in the 30000+range.
R/C props are not designed to spin that fast and will disintregrate.

The difference is how the cylinder is reloaded for the next firing. They both compress the fuel air charge, ignite it, and then allow it to expand to make power. A two stroke reloads while the piston is at the bottom and every revolution of the crank has a compression and expansion stroke. On a four stroke engine, there is a compression and expansion stroke that makes the power, and then the engine coasts for one revoulution during which the cylinder is reloaded for the next firing. It takes two revolutions of the crank to complete a power cycle.

Because you only hear an exhaust eplosion every other revolution on a four stroke engine, they sound like they are reving slower than two stroke engines. Actually, their ability to rev is not that different. As you get to the larger sized engines, it is the four strokes that can outrev the two strokes, this due to the two stroke engine's inability to reload the cylinder completely during the extremely short time of the reloading phase at higher rpms.

However you are confusing prop stall with output power.

Given higher RPM's on an unstalled prop, the engine which produces higher revs produces more work = power.

A 2 stroke .50 turning a 12x6 prop at 12-13k RPM's is not stalling the prop.

Take that prop and put it on a .60 Four Stroker which will turn only 8k to 10k and you get lower output power, hence the need to increase the displacement to compensate.

That is why we don't use a .46 four stroker in place of a .46 2 stroker.

Back to the original poster's question, he gains no power advantage in his plane going for the four stroker, just the added costs. At that size the fuel offset is not that great so as to justify the 4 stroke engine. This of course changes as you move up to the larger size engines, and typically by the time you hit the 1.20 sized engines, you start saving significantly in terms of fuel consumption going to the four strokers.

The small four srtokers may sound nicer though.

Yes, this is yet another reason that as you move up, the four strokers gain an advantage.

At the smaller size the OP is talking about however, he is better off saving the money, which really was the original question.

I saw a Fokker that recomended a OS .70 4C. What would be the equivilent in a 2 stroke engine? would it work with a 2 stroke engine? The only reason im asking is because if I can save $80+ than It would be worth it to me.

Thanks
Electrolight

There is more to this switch than just raw power. It's a different flight characteristic. A 4-stoke .70 can spin a larger prop that a .60 2-stroke; but slower. You'll have more of a mid-range power and torque curve. For things like pattern flying this is nice because you can play the throttle for equal speed in the nose-up and nose-down vertical legs of maneuvers.

But for just general flying and basic aerobatics do you think it would work?

Electrolight

PS also the plane calls for a 14 X 10 or 14 X 6 prop (I think. I can't remember exactly.) what would I spin to keep it close to the same strength.

Bottom line, YES, he'll have no problem what-so-ever using the 2C .60 engine.

He does not have to worry about the difference in flight characteristics on his .40 sized Ultra-Stick.

It's not going to be noticable as the extra power from the 2C .60 will offset any torque curve differences in the 4C .70.

At worst he can merely use a bigger lower pitch prop.

You would be better off matching the prop to the engine first. The recommendation is usually related to the engine used.

Both those props are way too much load for a .60 two stroke or a .70 four stroke. A 14x6 might work with a .90 four stroke. The 14x10 would probably work well for a ".60" size electric motor.

Sure. You may lose some flat out speed and acceleration. You'll also probably have to add some nose weight (further slogging down the performance) if the model is designed for a 4-stroke. I have an OS-70II in my Contender 60 and it still scoots along.

As far as prop? Sometimes a personal preference. 11 x 7 would work on the .60. A 13 x 6 would probably give a more scale performance for a Fokker.

Ugh... I'd hate to put in a 2 stroke engine in a Fokker.

That plane BEGS for a good 4 stroke engine.

Electrolight, consider spending the difference on the 4 stroker.

For this plane you'll be very happy you did.