painly

My Feedback: (4)

I would like to know the real diff. i realize 4 strokes last longer and sound better, and less fuel. but some fact bother me, as i just bought a 4 stroke saito.

how can 4 strokes turn bigger props then 2 strokes when there hp rating is lower at a lower rpm then 2 strokes.? im debating on buying a 60 2 stroke for what i intended on my siato 72 to do.

shed some light please

Aero330LX

My Feedback: (-1)

The Saito 72 will be lighter than the 60 2 Stroke. It turns bigger props because of the higher torque it puts out at lower rpms...horsepower means absolutly nothing on a model engine. The Saito won't ring your ears, it will start easier. It will save you money in fuel. It will also have better throttle response, and more grunt. I have a Saito 72 and it is a jewel of an engine...it really is.

lortap1969

"im debating on buying a 60 2 stroke for what i intended on my siato 72 to do. "

I think you may be a bit disappointed in the performance of the 72 if your putting it in a 60 (two stroke) size plane. For the most part .56 to .72 is usually the 4-stroke equivalent of a .40 /.45 size plane, and most people go to a .91 4-stroke for the 60 size planes...some even use the Saito 100 (remember to add glue to the firewall :-)

Mike

Spaceman Spiff

My Feedback: (1)

Horse power is basically torque times speed, so it is possible for two engines to have the same hosepower, but have vastly diferent torque. Basically you can trade RPM for torque. of course the optimum prop for each of those engines will be different, so what you will want to know is how much will it pull and what speed will the plane have, all other comparisons are going to be a bit murky

I have limited experience, but so far i prefer 2 strokes. They do the job just fine, cost less, and for me thay have been more reliable. Your mileage may vary.

A few months ago i replaced a saito 72 with 13x6 in favor of an Irvine 53 with 12.25X3.75 because i was having trouble with flame outs and excessive vibration from the 4 stroke. Some of the issues with that engine were later resolved by using the corect plug, but i still haven't put the saito back on because the Irvine has been doing the job. I didn't take any measurements of static thrust for the saito, but in the air the two engines feel about equivalent (iMO)

Basically what ever works for you is the right one.

Checklst

Yakes!!!! the old HP (high rpm's)verse Torque issue again. I don't know of if at 3:30am if my brain will work well enough to describe why a slower turning 4C swings a larger prop. Due to the way a valved engine is (timed) you get a vary different fuel burn time causing a higher CYL pressure(not to be confused with a high compression ratio) this high Cyl pressure (torque) can turn a larger prop.

See my brain already forgot piston Stroke length and plugs heat ranges(changes the burn time)affecting the CYL pressure/ torque curve..........I'm going to BED

Good night checklst

Spaceman you are a fast typer your post was not there when I started my brain dead post.......LOL

sandal

...and another thing:

when the prop turns, some of the power from the engine is converted into thrust, which pulls the engine forward, and some energy is lost. High rpms and small props "waste" more power than large props at low rpms.

So even if two engines put out the exact same amount of power, but at different rpms, the lower revving engine will produce more USABLE power because more of its output is converted to thrust. This is one of the reasons why most park flyers are geared. The engine puts out the same power, but a large prop gives more thrust per horsepower than a small one.

Flyboy Dave

My Feedback: (21)

Oh boy

Sprink

My Saito 72 turns an APC 13x7 at 9850 rpm, my OS 61 FX turns the same prop at 9950 rpm. Seeing as the 61 weighs nearly 1/2 lb more when you include the muffler, it is not worth it in terms of power.

Now using a 13x7 on a 61 FX is over propping, but necessary to keep the noise down (we have strict noise limits in the UK). So seeing as I have to use a big prop at lower rpm anyway, I now only buy four strokes as they are designed for it.

Richard L.

My Feedback: (24)

Most of the old guys at my field who wear hearing aids are the ones who have been using two strokes for years. I no longer fly two strokes just because of this reason. I tuned my last two stroke (an OS 61FX) at the field one day, and my ears were still ringing the day after. Yes, I was wearing hearing protectors. I certainly don't want to be wearing hearing aids when I reach 50.

By the way, this same topic has been discussed in the past. Just do a search for it in this forum.

painly

My Feedback: (4)

i know this topic has been discussed, but all i have found was comparisons between .40 13 oz engines and .72 20 oz engines, never between a .60 2s and .70 4s, which share closer weights.


thanks for all the input so far.

painly

My Feedback: (4)

thanks everyone, ive decided to try out the .90 size 4 stroke instead of teh 60 2 stroke