Nitro Racer 18

Would you need a 60cc 4 stroke engine to match the power of a 30cc 2 stroke engine?

cooldriver385

Um, that's kind of just street knwledge and assumption. Yes two strokes normally make far more torque and horsepower. But, say your taking a mid-performance, simple 2-stroke, putting it up against an extremely higj-performance high RPM 4-stroke, it may be different. It's hard to really say.

savagecommander

yes. a two stroke of a given design will match the performance of a 4 stroke with the same charicteristics. as in, if you match both engines VE the 2 stroke will still make more power.

sloppyG

now if you could only find a single stroke engine you'd have 4x the power

hasek1616

how about rotary engines! 1/3 stroke! i think...

378

My Feedback: (4)

Two strokes make less torque, but rev higher. The reason for that is they have a shorter power stroke than a four stroke with the same stroke, due to the exhaust port venting before the piston hits BDC. With a four stroke, you have valves controlling when things enter and exit the cylinder, so you get a longer power stroke.

That being said, the two strokes rev way higher, and make more HP. Those very valves that give a four stroke the longer power stroke and better torque also cap off it's RPM potential. HP is simply the output of an equation which factors in a constant, torque and RPM, so if you can rev higher you can make more power. The OS FS-26S-CX, for example, only makes 0.65HP or so. BUT, it makes way more torque than a comparable smallblock two stroke, so if you up the pinions appropriately, you can go faster. You can supercharge the four strokes if you want to recoup some of those losses. While that RB Innovations supercharger is of dubious use at best on two strokes, with a four stroke, it should boost power significantly.

Personally, I feel the only real reasons to fit a four stroke in a car are fuel economy, noise and engine life. Being four strokes, you can put mufflers on them without crippling them, they have steel sleeves and rings so they last forever compared to an ABC engine, and since they have half as many power strokes, they get roughly double the time per tank than a comparable two stroke. That's kinda why I want to get one for my CEN, if I'm honest. That thing flies with a two stroke but I think a four stroke would be better suited to it.
Rotaries are a bit of an oddball. OS does make one, displacing roughly 0.50ci and for use in aircraft, but from what I hear they suffer the same reliability problems their larger cousins suffer. Namely, the apex seals wear out pretty damn quick. The reason they aren't used in cars is twofold, IMO. One, their shape and the location of the output means they don't fit a chassis well, and two, cooling them in a car will prove to be a bit of a bother.

proanti1

My Feedback: (2)

Not exactly double the power... a 4 stroke nitro engine of the same size has more torque, but a tad less overall horsepower because it revs out a lot sooner. There are a ton of characteristics in play, not all of which can easily be explained by members of a forum. Are we talking ignition? Or nitro engines? Flat head, ohc, or ohv? Reed valves, rotary valves, or piston port? Does the two stroke motor have a tuned pope? Is the 4 stroke made by rotax? All these things are to be taken into consideration, and all these factors do not really allow the broad "which is better?" question to be answered.

The_Shark

4stroke- more torque off the line
2stroke- more horsepower at high rpm.

i would stick to 2stroke.

moe7404

this is my idea, if iam off its fine if you say so. iam not a engine expert, i just play one on tv. take a 2 stroke engines 60% of the torque curve is fewer rpm span than a 4 strokes 60% torque curve. like i said i might be wrong. but note i have been around engines from 1965.. iam NOT trying to say anyone is wrong, just my idea.

cooldriver385

I don't see how a one-sided piston can make that possible.

cooldriver385

Well I don't really know about nitros much but in motocross, in the Lites class you can ride up to a 125 2-stroke or 250 4-stroke and in the 450 class you can ride up to 250 2-stroke or 450 4-stroke. And they are all high performance race engines that of course put out more power than these nitros do.

Justin B

My Feedback: (8)

There are some interesting answers.
Technically, yes you would need double the displacement with a 4 stroke to equal the power potential of a 2 stroke. Every time a two stroke piston comes up in the cylinder it is a power stroke and it exhausts on the down stroke. A four stroke engine's piston goes up and down twice before a power stroke. So for 2 engines with the same displacement (2 vs 4 stroke) the 2 stroke will have twice as many power strokes as a 4 stroke. Four strokes make more torque sooner and can rev every bit as high as a two stroke. (think formula one engines) 2 strokes are lighter, have less moving parts and are easier to work on.

proanti1

My Feedback: (2)

Not exactly, double the power strokes doesn't mean double the power. Two strokes have a ton more compression to fight against (on the up stroke, as well as the down stroke), four strokes have a heavier flywheel (or crank) that can hold a lot of momentum, which is one of the reasons they are so torquey.

378

My Feedback: (4)

Two strokes don't generally have more compression than four strokes. Compression ratio is dictated by the fuel used most predominantly, so a gasoline engine is going to have between 7 and 12 to 1 compression regardless of the cycle used. Where in that range a given engine falls is determined by the desired output. Our glow engines run at between 12 and 14 to 1, somewhere between gasoline and diesel. Reason for that is methanol, the primary component of our fuel, has a much higher octane rating than gasoline, and if you can raise compression you can raise power.

sloppyG

either way, it's a very rare application when you have the option to choose 2 stroke vs 4 stroke

4 stroke engines are way more expensive

378

My Feedback: (4)

Yes, they are. My LHS actually has an OS FS-26S-CII on the shelf.


260 bucks, and that's not counting the bump box I would need to get it going.


My recoil start 18 CV-RX makes twice the HP and cost me 109.99 brand new.

cooldriver385

Well, I am again gonna use my knowledge on motocross engines and normally two-strokes have a compression of 8-10:1 and 4-strokes normally have 12.5-13.5:1.

Justin B

My Feedback: (8)

Not exactly, double the power strokes doesn't mean double the power. [/quote]


Agreed, but it does mean you can make the same power from half the displacement.

savagecommander

4 strokes dont have any more torque than 2 strokes. an engine reaches torque peak when the cylinder is filled the best, and it happens at a lower rpm in 4's than 2's. and compression ratio has no effect on power lost- it makes up what it loses.

and yes, if an engine produces twice as many power strokes in a given time, it will produce twice as much power-and burn twice as much fuel. lets not try and clog the thread about who's smarter, or has been modifying engines longer or whatever- its not a contest.

savagecommander

sloppyG

in a rough sense sure, that's a good approximation but the two stroke doesn't fill and empty the cylinders as efficiently as a 4 stroke so you can't burn twice the fuel (because you're not getting twice the oxygen into the cylinder)

depending on how rough we're being you are correct

Justin B

My Feedback: (8)

It's a discussion on a public forum, not a clog the thread contest.

dustin7837

My Feedback: (1)

2 strokes are cheaper and rev higher since they don't need any valves or a cam and they fire every time they hit TDC. In small engines they also generally use simpler carburetors.

4 strokes are more expensive and require some type of valve setup whether it be cam driven or whatever. A 4 stroke only fires every second time it hits TDC.

It's really hard to compare, because they are two different beasts. If I had to make a generalization about 2 vs 4 stroke peak power numbers, I would be inclined to say that for a similar displacement, a 2 stroke makes roughly 1.5 - 1.75 x the power of a 4 stroke. That being said, I still don't feel that it is an accurate comparison.

The thing to remember is that HP = (Torque x RPM) /5252 5252 is just a conversion factor.

So a two stroke will always have a rated horsepower advantage as long as it can keep making power in the high RPM ranges. Also, Peak HP and torque numbers are highly over rated and mean very little. The shape of the Hp and Torque curves are much more important. The shape you want depends on your application.

savagecommander

case in point- 250cc 2 stroke in the same class as 450cc 4 stroke.

cooldriver385

My motocross engine knowledge agrees with you. The carb on my 250F is a pain to work on compared to the 125 2-stroke carb.




Exactly.





2-strokes of the same displacement have a ton more torque.