Please explain the practical difference between a two stroke engine and a four stroke engine. My initial impression was that a four stroke engine was "better" than a two stroke, but this reasoning appears to be incorrect. This information would help me match airplane to engine and vice-versa. Thanks!!

In a general comparison, 2 stroke produce higher RPMs, while 4stroke will swing larger props.
2 strokes are good for speed, 4 strokes are good for slower more powerfull aerobatics ( torque rolls, hovering, vertical climbs) 2 strokes will allow you to do all of this too, but it's easier with the 4 strokes and the larger props.
It's all a matter of taste, needs and perspective as to "which one is better"
I'm no engine expert thats just my $.01 worth

Sounds right to me! Oh, 4 strokes tend to be more expensive than two, But I am a 4 stroke fan,,,,,,,,I like the torque!
If you are into speed than 2 strokers are for you. High r's and small props.

Two strokes can run bigger props. I routinely run 12x4 props on .46 sized engines.

This website sums up my thoughts on 4 strokes:

http://mypage.yhti.net/~dmcdnld/4cycle.htm

Yes and I run 12 x 6s around 10k on a Saito .56 at close to the same weight as an O.S. BB .46 w/muffler. Still agree that the 2-strokers will turn higher RPMs and the 4s have the torque for larger (includes wider bladed) props at lower RPM. Slower, wider, fewer blades = better prop efficiency. Both types have a place for me and work quite well, if you overlook the differences in sound.

quint

A 4-stroke .52 can turn a 12x6 prop at around 10,000 RPM, right up there with your woefully bogged down .46 2-stroker. That's 50% more, BTW.

The Saito .72 can turn a 13x6 or 14x4 prop, and weighs less than most .46 2-strokes.

Don't let Dave McD's FFFFour Stroke page sway your decision, because most of it is exaggerated or simply untrue.

Wow, it sounds like "Dave McD" has had some problems with four stroke motors. Perhaps trying to run them too lean and tossing some props? Whatever his misgivings, they're obviously based in a lack of engine experience, which caused him to throw a little temper tantrum via a webpage. Some people just never get the hang of some things, but should recognize their lack of skill or knowledge with a particular item for what it is... "Lack of skill or knowledge". Obviously "McD" is challenged in this area.

Did anyone mention fuel economy? 4 Stroke motors are generally more fuel efficient than 2 Stroke motors.

How about cleanliness? Stick a 2 Stroke motor on a plane and bring lots of windex to clean it up. Use a 4 Stroke, and bring a little windex.

As mentioned, 4 Stroke motors can swing larger, and often more scale sized propellers in a more efficient manner than 2 Strokes.

4 Strokes are quieter, and in most cases more realistic sounding.

Don't get me wrong, I am a 4 Stroke fan bigtime, but I also own 2 Stroke motors. Each have their applications and followers.

After experiencing my 1st. 4-Stroke (FS .70 Surpass), will never purchase another 2-Stroke.

4-Stroke :thumbup:
2-Stroke :thumbdown
daven

For me, four strokes have great aesthetic appeal. But, my Webra 1.20 is proving to be cheaper and easier to run than my Saito .72. The larger two strokes run at comparable speeds to the four strokes and turn big props slower. It also depends on the airplane. If you just want to crank and fly, I have to give it to the two stroke. I'm more concerned with improving my flying skills than tinkering with my engine.

Don't get me wrong, I love four strokes. But, for cheap, reliable, crank and forget it flying, I'll buy a two stroke.

I thought Dave's F-F-F-Four-stroke piece was F-F-F-Funny as heck. You can't honest read it as anything but tongue-in-cheek.

I own both 2strokes and 4strokes and like them both. A lot of the genralizations have some truth to them, but there are always exceptions. There are high CR, long stroke 2cylce engines are can swing big lumber and be almost just as fuel efficient as comparable 4strokes. OTOH, there are 4stroke whose raw power puts 2stroke to shame while guzzling as much if not more fuel.

2stroke's operation has an elegance of simplicity to it. It's hard to imagine some other IC engine design that can make as much power with fewer moving parts.

OTOH, 4stroke's unique sound and the ability to swing large props has an undeniable appeal to many folks.

Take you pick, they are all good engines.

My post above was mostly in fun. This can be a hotly debated topic, and we all have our own opinions.

However, lets please compare apples to apples.

Claiming 50% more power from a .56 fourstroke compared to a .46 two stroke is just plain untrue. Please explain this one to me. Most .56 fourstrokes I have seen, are pretty close in power to a .46, and normally most people will use a .72 4 stroke in place of a .46 two stroke.

Another point I would like to make is cost. People seem very willing to spend big bucks on a 4 stroke, but overlook a premium 2 stroke engine. Compare a Saito .56 to a Jett .50, or a Webra .50 and you will see no comparison in power.

I have seen identical planes, one with a Saito 1.80 and another with an O.S. 1.60 and the power difference was remarkably in favor of the 1.60. All this, at $150 less dollars.

I'll agree with just about everyone about the sound of the 4 stroke, but the sheer power per $ numbers of the 2 stroke are hard to beat.

Why do Dirt-bikes use 2-strokes (rrrrr-ring-ding-ding-ding-ding) while Harleys use 4-strokes?

Will a dirt-bike make it up the hill with a 4 banger? Yes

Will a Harley cruise down the freeway with a 2-stroke? Yes... (providing you don't run into a group of bikers who immediately skin you alive and use the 2-stroke as your new suppository)

So it all boils down to what kind of power YOU want to put in your plane.

Two strokes get their power from rpm's. 4 Strokes just have power. example, ever stick your finger in the prop of a 2 stroke running 4000 rpm? "might cut you a little" Try it with a 4 stroke running 4000 rpm. you'll have nubs

"WARNING" DON'T TRY THIS AT HOME! "WARNING"

Thought I would throw a little fuel on the fire

Oh boy, you got me going! I just have to post this link!

http://www.revoh.org:1234/insane.html

"Tounge in cheek"? Perhaps I missed something, but whoever did that page sounded pretty peeved at 4 Strokes in general.

Interestingly, I seem to have more problems with 2 Stroke motors. Once I get my 4 Strokes broken in and tuned, after the initial valve lash adjustment, they are quite reliable and trouble free.

As far as comparing the same planes with different motors, I have 2 Rascal Fortys, one with an OS LA .46 and the other with an OS FS .52. Speed? They both top out at about the same speed. Power? They both climb equally well. Granted, when I drop in the FX .46 I plan on buying today, I'm sure the 2 Stroke Rascal will climb better, but I doubt I will see more speed, as that is typically a function of the particular airframe being used.

However, the only reason I am buying the FX is I don't have the extra $50 for a new .52, and I'm really hoping it is less troublesome than the LA .46.

Regarding efficiency:

My speculation on why 4 Strokes swing bigger props is that they are not wasting part of the power stroke exhausting the combustion gasses midway down the power stroke, thus they are able to make more efficient use of the fuel burned, and *actually* have longer power strokes than an equally stroked 2 Stroke. Make sense? A power stroke only lasts as long as the fuel/air mix is burning in the closed cylinder. I'm sure this is why we see less wasted fuel on the fusalages of 4 Stroke powered planes. I realize that proper pipe tuning can make a 2 Stroke more efficient at burning it's fuel, (Thus more power) but even then the combustion is less complete than a well tuned 4 Stroke.

ilikeplanes, if your Webra .120 is cheaper to run than your Saito .72, I'd venture to say you have a fuel adjustment problem with the Saito.

daven, there is no comparison in the power output between the OS 1.6 and the Saito 1.8. If the plane was designed for a 1.6 2 Stroke, I'd end up with close to a 2.4 4 Stroke in it... Then we'd probably see some equality in performance. On a .60 size plane, we see manufacturers recommend .80 to .90 4 Strokes. Heck, I've stuck .80 and .90 4 Strokes on .40 size planes with great success. I've got a 1.5 Saito on my .60 size GP Tiger Moth that has probably never seen 8000 RPM, but I can't imagine a 1.6 OS on it! Considering that 4 Strokes have smaller, more flexible exhausts, the size difference in the motor is offset (usually) with this difference in pipe size, making the 1/3 increase in displacement doable in most cases.

I guess what I'm trying to get at is that to compare the performance of 2 Stroke motors to 4 Stroke motors, we should not compare the same size motors. Fair? Not fair? Who knows?

We should also keep in mind that model engine horsepower ratings are determined under lab conditions, and do not take into account what kind of plane the motor is in, what altitude we are flying at, what the weather conditions are, what variations in fuel we are using, etc. etc.

The true score is determined by what motor works best in what type of aircraft, and it is not always the motor with the most power in a given size range.

mskyhawk, that's a great example of "power"! Just flip starting, when a 4 stroke fires and hits the back of your finger or nail... Damn! Yeah, that's power. Even using a rubber chicken stick I've busted wood props when a 4 stroke fired too fast for me to get the stick out of the way.

I don't flip start 4 strokes anymore.

Good Call Damnathius!

If have a scale looking plane I want a four stroke in it just because I love the sound of a four stroke.
If I have a plane that want to haul butt it gets a two stroke. And I do like the sound of a two stroke making HP at 15,000 revs.
Try'em both!!!

Hmm... how about sport bikes? Those 4strokes makes all kinds of power spinning upwards of 18,000rpm. I know I'll catch flak from diehard hogs for saying this, but Harleys are a bad example for representing 4strokes, as they are stuck with 1930s technologies.

Look at F1 and SuperBike racing and you'll see modern 4stroke and 2stroke technologies both making boatloads of power, but in general, though, 2strokes do make much more power at the same displacement than 4strokes.

Still, you can design a 2stroke to be a torquer and at the same time design a 4stroke to be a rev-happy high-RPM motor.

gearing is the answer on these bikes. not the case with direct drive model engines

2 stroke pop corn poppin motorcycles have something most planes do not have , a range of gear reductions to better allow matching engine speed to load. YES ,I know 4 stroke bikes have them too!
The problem as it applies to planes is while the two strokes do make more peak horsepower along with a higher noise level (that more people find objectionable) they do it at a HIGHER crankshaft speed. Unfortunately for airplanes the props are a lot less efficient at the higher speeds so the plane cannot make use of the power and the decreased effixciency also slows the planes response to power increases.
Flying wide open from take off to touchdown may not bring out the crucial differences as much as using the planes speed envelope will. There is nonetheless a difference preferrred by many flyers who DO use the left fore/aft stick .

Yes ,for the MOST part a two stroke of a given displacement will make more power,make more noise and burn more fuel than a four stroke. They also usually require BIG ,often HEAVY mufflers and bigger fuel tanks too . You may find some weight savings with the itty bitty prop . In fact in many applications such as weed whips and outboards,motorcycles etc. The dirty little two strokes are being outlawed by the environmental types !

The fact is four strokes are getting a bigger percentage of the new engine sales every day as two stroke lose market share. More people are going to four strokes and STICKING with four strokes exlusively than vice versa .



Two strokes COST less because they are getting to be WORTHless

Hey Daven thats the best LOL I,ve had all evening hilarious site and obviously very tongue in cheek. I cannot understand the lack humor in some of the posters.

I just want to make one point; The very best engine technology for your model aircraft is the one that turns you on at the time!

Yes I have a whole bunch of 2's, a somewhat smaller number of 4's, a couple of diesels, a Co-2 engine, two Jetex and a pulse jet. You know they all run, they all fly and I love them all, and not one of them is 'WORTHLess".

John

Have you heard about the new Sterling Cycle 3D engine from OS?

A question from someone with no 4-stroke experience:

Is it any more difficult to get a 4-stroke broken in or to keep it running/starting well as compared to the 2?

It's been my experience that four strokes are more reliable, as in far less dead sticks, and run reliably with less break in than two strokes, except for diesels, they are the absolute champions for running right out of the box and no dead sticks.

Perhaps, JBuckner, "some of the posters" require a bit more humor to get a laugh. Ya know, it's just got to be funny to laugh. It was not in the best interest of the person that asked a perfectly logical question about the differences between 2 and 4 stroke motors to mislead him with such questionable "humor".

But what do I know? I'm new here.