fiery

My Feedback: (2)

I have a few ringed .40 to .60 size glow engines

I have always wondered why compression feels stronger turning them over backwards (clockwise) than standard rotation (counter-clockwise).

Can anyone explain this phenomenon?

downunder

Derek,
Apparently this is because when the engine has been fully run in the ring seal against both the bore and the ring land in the piston groove takes a particular "set" because of combustion loads and side force from rod angularity. Turning the engine backwards simulates these loads. Sounds good doesn't it?

fiery

My Feedback: (2)

Thanks for the explanation Brian. It is the most plausible I have ever been given to date.

aspeed

Maybe the base compression is more when going backwards, and that pushes out the rings a bit? Maybe this is not plausible. The intake port window is closed a bit longer when going backwards.

Sport_Pilot

I have heard it described that it is the result of the asymmetrical crankshaft placement. On most engines It is actually off center a bit to change the timing on the upstroke VS downstroke. This is not done on four strokes and I have not noticed this effect on a four stroke.

downunder

Crankshaft offset from the cylinder centre line is called DeSaxe and is fairly rare although all Mercos used it and the Fox 35 (at least) and SC 120 had it. It's not one of those things that engine testers seem to look for though.

1QwkSport2.5r

The K&B Sportsters are DeSaxe also.

gerryndennis

G'say Fiery and all,

I've noticed this effect as well, and also only on two strokes.

I assumed it was due to the ring being pinned so that the gap is on the transfer side (halfway between the rear schneürle port and the boost port on my engine)

When you flip the prop in the correct direction the rod angularity pushes the piston toward the exhaust side exposing the ring gap and allowing it to leak. Vice Versa when you flip backwards.

Obviousely when the engine is running in the correct direction the piston is pushed to the transfer side (covering the ring gap) during the power stroke when cylinder pressure is highest.

I have no idea if this makes any difference when the engine is up to temperature when running (ring expanded by heat and seated by cylinder pressure) but the designer has to put the gap somewhere.

Sort of what Downunder said?

Do all engines have the gap on the transfer side? If not do these engines have better compression flipping forward.

I presume the effect is absent in four strokes because the rings are not pinned and the gap could be anywhere?

Dave H

Hobbsy

My Feedback: (102)

On Fox engines, the ring is not pinned and appears in the exhaust port on occasion, the Fox ringed engines don't have this directional variance in compression. My Fox .50, .60 and .74 have ABC like compression holding ability, they don't leak when flipped slowly. My ancient SuperTigre 3000 I bought from Mr. Davis about 22 years ago has excellent compression holding ability as does my old 4500. My ST .51 and .90 neither have very good compression when flipped slowly. (forward) The 2300 is pretty good. The ST's all show softer compression when flipped CW.

Sport_Pilot

Most two strokes have at least a small amount of offset. Not rare at all.

aspeed

Only way to check the DeSaxe effect (new word for me, although I knew about the practice.) on different compressions would be to take off the backplate and check the compression again both ways. I doubt anyone will bother.

Lone Star Charles

Sometimes the wrist pin is offset, giving the same effect. Also, it is not limited to two-strokes.

Mr Cox

The crank timing on the intake creates a vacuum when the engine is turned in the reverse direction, this then feels like a stronger compression although the actual compression at the TDC is the same (appart from a possible de-saxe then). After TDC the timing again lets the crank turn easier in the reverse direction (no compression build up in the crank-case), creating the feeling that there is more pop in that direction.

At least that's my take on this topic, it has been up on this forum a few times before I think....

gerryndennis

G'day Mr Cox,

I had a play round with a couple of my non ringed engines today. I tried flipping them with the plug in and out, and the carb blocked and clear.

You are right the primary compression does make a difference but not very much. While rotating the engine backwards you can feel a very slight restriction after the transfer ports close but the effect is hidden very shortly after when the exhaust port closes and the normal compression starts to be felt. The crankshaft port opens well before TDC (releasing the primary vacuum) and just as the compression is starting to build up. There is very little difference in feel oved TDC in either direction (certainly not the difference you can feel in a ringed engine).

You can feel a difference after TDC in the forward direction (rotating slowly) as the primary compression builds up, but the bounce over TDC is nearly the same.

In other words whatever it is that the ringed engines are doing, the non ringed ones don't do, which suggests there's something about the ring itself I guess.

Dave H

Kweasel

My Feedback: (29)

Its the ring gap in conjunction with piston clearance. The leak is not as as great with fourstrokes because they typically have less piston/liner clearance than twostroke do. Once an engine is running, the dynamics of temperature, speed, and oil take care of any leaks.