I’m having trouble understanding the 2 Stroke vs. 4 Stroke issues. The HP rating is lots lower on the 4 Stroke then the 2 but how can I compare them to know what size I want? For example I have a OS .46FX at like 1.6HP (16.45oz) vs. a FS70II at 1.1HP (20.65oz) but people say they are about the same. Or my OS Max .50fx is 1.7 or 1.8HP and weighs in at 13.76oz vs a FA-91S at 1.6HP which is better performance? The weight is similar to each other. Where can I get more comparative information?
Thanks
Sheldon

Sheldon,
I read somewhere that the conversion is .6 to 1. (2vs4). i.e. a .61 2 stroke would need a 1.00 4 stroke to be equal as a rule of thumb.
My personal experience is I'd take a YS 91 over a .61 2 stroke any day.
(Keep in mind we own 6 YS-120's and 2 YS-91FZ's,,,in addition to the big gas engines)
FWIW,
Steve

Puckett Model Aviation

Published HP figures are almost always misleading because they're typically calculated from exaggerated RPMs.

While 2-strokes like to run at high RPMs, four-strokes are typically happier at lower RPMs where they have much more torque than 2-strokes.

HP = torque (ft/lbs) * RPM / 5252

So it's easy to see that the 2-strokes, while producing more horsepower than the 4-strokes, do so because although they produce less torque than the 4-strokes, they operate at much higher RPMs. On the other hand, a four-stroke with more torque swinging a larger prop will perform similarly.

Is this as clear as mud now?

Yes that would sum it up. Clear as Mud.

How about this analogy:

Comparing these glow engines to carrying water up to camp from the river (no snickering...)

The 2-stroke engine will carry a lot of small buckets.
high rpms = lots of buckets
smaller prop = smaller buckets

The 4-stroke engine will carry fewer large buckets.
lower rpms = fewer buckets
larger prop = larger buckets

I understand the theroy. It is in the application that things fall apart.

So, rubber meets the road, (prop hit the air) is the .70 4-stroke going to give my Tiger 2 Unlimited vertical as the .46 will not? (very close but it won't)

If the weight and cost is so similar, and I can always throttle back, should I just put in the .91 four stroke in it?

Good question. The .91 is only a couple ounces heavier. I don't know, but I'd guess that the OS .91 should be fine in that plane if the firewall can stand the "pressure".

A 4-stroke gives you more torque, more power off the line, better vertical performance, and slower more scale flight speed because it turns larger diameter, lower pitch props. A 2-stroke is generally lighter and more powerful for the same displacement (not always) and goes faster, turning higher pitch lower diameter props.

4 Strokes tend to be quieter if you don't compare them on a db meter and the have the set'em and forget'em capability ...

Meaning: once you get it adjusted ... you usually dont need to fiddle with it for a while ... unlike a 2 stroke ... which seems to be tinkered with every flight (over-exageration).

on the Down Side ... 4-strokes cost more than 2 strokes.
on the plus side 4-strokes drink less fuel ...

the main difference is power vs speed ... you wanna fly fast ...
use a 2 stroke ... 4-strokes are more for stump pulling.

Personnaly, I would take a 4-stroke over a 2 stroke any day ...

but it would have to be one of the big 4 ... OS, SAITO, YS or ENYA

OS & Satio are simialr in price and YS & ENYA cost more for the same sized motors ...

All others ... well ... I would buy Chevy/Dodge or Honda/Toyota over a Yugo or a Kia ...

Well, don't know about Yugo, but, whether you like them or not, Kia/Hyundai (same company) are getting pretty good these days. Much can be said about "the other 4stroke makes" you didn't want to mention.

Hi!
Correct me if I´m wrong but I think comparing twostroke and fourstroke engines of similar size then the the twostroke both has more hp and torque!!??
The reason for chosing a fourstoke engine before a twostroke has to do with sound ...not that the foustroke is more powerfull or has more trust!
Most modern twostrokes will outperform a similar size fourstoke engine ...both when using a big prop at slowspeeds and naturally when using a small prop att high speeds.
Let me take an example. I have several OS .26 fourstokes (open exhaust) and they all rew appr 10000 on a 10x5 Scimitar Master airscreew prop (5% nitro and 15% Motul "Micro" all syntetic oil)
Comparing that with my MVVS .28 (same fuel as above) using the MVVS super quiet silencer which rew appr 11000 on a 11x4 APC which gives much better performance and quieter sound than the fourstroke in the same airplane.

Regards!
Jan K
Sweden

Generally, the two-stroke will be more powerful. So four-strokes of a similar class will be larger (60/90, and 46/72 etc.) The question besides "so do you want that cool sound?" is: Where do you want the power? Two-strokes have their best power at the higher RPMs. Four-strokes tend to be less "peaky" with plenty of torque all across the board.