Back Pressure

[awaikart]

Anybody out there have information on the theory of back pressure in relation to 2 stroke outboards?

[DennisG01]

I don't know about "theory", but from real world experience they usually need back pressure on the exhaust side (prop) to help them run smoothly. That's why they often run a little crazy when out of the water and only on muffs.

[boscoe99]

With water flowing over the gear case at speed, tending to create a negative pressure area aft of the exhaust outlet, how might that affect back pressure?

[pstephens46]

With water flowing over the gear case at speed, tending to create a negative pressure area aft of the exhaust outlet, how might that affect back pressure?

Add thrust from prop and who knows?

[DennisG01]

With water flowing over the gear case at speed, tending to create a negative pressure area aft of the exhaust outlet, how might that affect back pressure?

Hmmm. I dunno. But does the load on the engine and the increased exhaust flow out the prop have anything to do with it?

It seems like when on the muffs, the only real issue (that I've noticed) is low/idle RPM. Advancing the throttle seems to be fine and the engine runs normal.

[ausnoelm]

One of the inherent "features" of a simple carb 2 stroke is the fact that they need to be under load to run smoothly (HPDI/DI excluded) and one of the main reasons they were never further developed for auto use (plus oil costs were high, old 2 strokes were around 16/1 ratio) because of the "raw" method of exhaust in a 2 stroke, unlike a 4 stroke where there is the head, valves and a manifold system for exhaust, a 2 stroke has a simple "hole" (called a port) that when uncovered the spent gas just rushes out in an uncon*****ed condition, making the distinct "dak,dak,dak" cackle of a 2 stroke not under load, an exhaust system of sorts is required for better operation, this exhaust controls the outlet by "back pressure" (in very simple terms) and in a very simple way, is like valve timing in your 4 stroke, because at times, the ports can overlap, creating inefficiency.

[TownsendsFJR1300]

I don't know about "theory", but from real world experience they usually need back pressure on the exhaust side (prop) to help them run smoothly. That's why they often run a little crazy when out of the water and only on muffs.

+1 ^^^.

Out of the water on muffs, the idle tends to be higher than if the water.

Once under way, (and above idle), I don't believe the back pressure, or lack of affects performance at all... IE, on plane, (higher RPM's) it's pretty much a moot point..

Four strokes, (FI'ed at least), they idle down to normal on muffs and WAY quieter...