And I still stand by that statement I have a couple of 4-stroke engines but I am not a 4-stroke only snob by any means. It's the myths and misinformation that I don't like.

I have more 4-strokes than 2-strokes in my hangar and I have to say that the 4-strokes sound like tractors.

There is no way they sound anything like a full-sized aircraft, and I should know, I work at a full-sized airfield most days.

The reason I have 4-strokes is because for the type of flying I prefer (3D) they have very real advantages over a 2-stroke (able to swing a larger prop, better spool-up and lighter per pound of thrust -- yes, that's true with Saitos).

I also have a 4-stroke in a P51 mustang and despite pushing it to 10,800 RPMs and running without a muffler, and despite it flying just as fast as a friend's 2-stroke-powered identical model, it's just not a scale sound.

In fact, his ST90-powered P51 sounds just insane with an MAS K-series prop with its tips sizzling and crackling as it does a low pass. I may pull the 4-stroke out of my P51 and put a 2-stroke in it -- much more realisitic.

However, the *most* realistic sounding planes are those powered by gas motors. A good 35%-45% model with a 100cc-150cc twin sounds *very* realistic (to me anyway).

Two cycle gasoline engines sound like a chain saw and I have never a full scale aircraft that sounded like a chain saw. Yes sir those little Cox 15 two cycles really sound like a ROLLS Merlin. And the 049's really do. Love that realistic two cycle sound. Just like a Piper Cub or Citabria.

Gotta agree, they sound about as much like a Merlin as a Saito 100 sounds like the Lycoming AEIO 540 in an Extra.

yeah, but my Saito 1.00 with a straight pipe sounds like a Ducati racebike comin' down the front straight at Daytona W.F.O.

Yes, and my SS piped Jett 50 sounds like----
That's one of the things I like about a high revving two stroke, they sound like nothing else. [8D]


All a matter of personal taste I guess.

Now you take one of my OS 4 cylinder 4 Cycles in a Cub or my Five cylinder OS FF300 in a Fleet Biplane and you get a more realistic sound than a scrreaming 2 cycle or gasoline engine sounds like a Rolls merlin. By the way someone awile back mentioned a Packard Merlin. Now THAT would be a rare engine. Rolls Merlin or Rolls Griffin I will believe but a Rolls packard?? Nope.

Oh and turbines sound like a small vacuum cleaner.

A Jett sounds like a big Cox 049. Two cycles sound like two cycles. Whiney obnoxious noice. That's why we got kicked out of so many flying sights.

Here's a question, would you rather wake up Sunday morning to the sound of some guy using a chain saw or a guy moving his lawn with a Briggs and Stratton powered lawn mower?

I've got both and I don't understand what all the fuss is about. They've both got applications that they're more suited to.......just depends what you want to use it for. I *love* the torque and the sound that my YS 4 strokes produce though......I can't imagine flying my Ultimate Bipe with anything else

Here's a little item for you. A four cycle engine does not have more torque than a two cycle. It has a higher BEMP, brake effective mean pressure. A two cycle firing every stroke has more torque.

Actually it's BMEP, BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

ORIGINAL: loughbd


Actually it's BMEP, BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

Doesn't the 2 stroke have to be doing more revs to produce the same torque? I was always lead to believe that the 4-stroke would produce better torque at lower revs, making it more suitable for swinging larger diameter, lower pitch props.

I know that there are examples of the opposite, for example my MVVS .91 is real torquer but my ABITAR 1.20 fourstroke is a revver, it will turn a 14x6 APC at 12,000 plus but can't handle a 15x8.

As you say the general rule is that the fourstroke will have a broader torque band.

BHP = torque (ft/lbs) x rpm / 5250

If a 2 stroke delivers more torque - why do they need to rev higher to produce the requisite horsepower to suit the engine load (created by the propeller)?

It would appear that the original question has worked. For most people there is very little practical difference - it is mostly personal preference based on the sound. The exeptions being true 3d (4 stroke) - or genuine top end speed (2 stroke) otherwise it is probably much of a muchness. -

1 lb of fuel will only contain a set amount of potential energy - it's how efficiently the engine transfers that into kinetic energy that makes the difference. - This is why, displacement for displacement, different engines have different power outputs - it's all about engine efficiency which is controlled by many factors.


BTW - BMEP is controlled by a large number of factors - most of which are to do with engine design - I would just about guarantee that, size for size and cycle for cycle, an older style of engine will produce a lower BMEP than one with later more efficient designs (Schneurle porting for instance). this also applies to combustion chamber design, valve timing atc and is a fascinating science all of it very own.

In most 2 strokes the torque is going down well before it reach's peak HP. The four stroke is limited in top RPM because of valve float, and no company as yet has tried the stiffer springs, roller tappets, overhead cam, etc, so that they will reach the same RPM. At the lower RPM many of the two stroke will attain a higher torque than an equivelant displacement four stroke.

BTW I think Hobbsy's formula is an approximation as it doesn't take in account all of the possible rod, crank, stroke and length combinations. Also I think that displacement is for one power stroke. For a four stroke the power stroke is followed with a no power intake stroke so the torque measured by a dynometer will be about half of his calculated figure.

Yeah Dave,

I knew it was BMEP. Just didn't engage brain at the time. It will stir up a can of worms though, you watch.

Well put, Steve.

I love ALL IC engines, even a Ueda .55. You had to be there.

Ah, now there's a name I haven't heard in a *long* while.

My most cantankerous engine was an original OS Pet. They were horrible things -- really loud (when you could get them to go) and difficult as hell to start. They'd either be dry or flooded -- with little ground inbetween.

I like both two and four strokes. The small two strokes are a little to pitchy but the big ones with the right muffler sound excellent and more like a full scale aerobatic engine. I have a HP VT49 in a SE5a and it sounds good. The OS 160FX sounds great with a Jtec dual Snuffler muffler in a Extra 300L, very close to full scale sound in a high speed pass.

Multi cylinder four strokes sound great. My OS FT-160 sounds wonderful and running at 5000 rpm sounds like a true Cub engine, so does the sewing machine Saito 90TS.

The Enya VT 240 is insane with a Mejzlik 20X6 prop spinning 9000 rpm, extremely loud. About as close as you can get to V-12 sound!!!!!!!!! I have thought about installing it in a 88" spitfire that was given to me. The plane needs the nose weight but the heads will stick out of the cowl.

Someday I hope to get a OS 3.20 Pegasus and install it in a 1/4 scale Super Chipmunk.

The Enya four stroke engines were designed to run at up to 12,000 rpm with push rods and overhead valves.

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My Enya 80-4C liked to spin up too. Climbing knife-edge in a Goldberg Cub. Now that's realism! <G>

I have two OS pets and they both ran fine.

Knife edge in a Cub is realistic. I suppose they flew in the Reno air races too.

I agree and have been saying this all along. The torque needed to turn a prop is exponential to rpm, to turn a prop twice as fast you need to quadruple the torque delivered to it. Turning a prop twice as fast also quadruples the thrust BTW.
Because of the exponential relationship between RPM and torque, it actually takes more torque to turn a 10x6 at 14,000 rpm than it takes to turn a 12x6 at a somwhat lower rpm.
Not understanding the exponential nature of prop load leads to the popular misconception that 4-strokes make more torque than 2-strokes.