Crossflow vs. Looper

[randlemanboater]

OK, who will explain the difference in a crossflow and a looper.

And what do you think that a 1986 or 87 V-4 seadrive will be?

Which is better?

[phester]

skools is the OMC man on this site ;)

[Skools Out]

86 crossflow most likely but 87 Looper Small bore

Post pic of power head.

[Skools Out]

http://www.wellcraftv20.com/cgi-bin/...8488;start=0#0

[Skools Out]

TWO STROKE DEFINED
The two-stroke engine completes its power cycle in only one crankshaft revolution with two strokes of the piston. There are no valves, camshafts, springs chains, etc. so the engine is much less complex and lighter. Instead of valves There are a series of strategically located transfer ports - intake and exhaust, cut into the sides of the cylinder wall. The ports are on opposite sides of the cylinder. The transfer ports are opened and closed by the up and down movement of the piston. To accomplish a complete power cycle both sides of the piston are used; consequently several events occur simultaneously during each stroke. They are:

Up Stroke - Intake and Compression:
On the up stroke the top side of the piston is compressing an air/fuel mixture in the cylinder. At the same time the BOTTOM side of the piston pulls another fresh charge of air/fuel mixture into the crankcase thru a one way valve called a reed valve. Near the top of the stroke the compressed air/fuel above the piston is ignited by the spark plug and begins to burn. The rapidly burning fuel expands and begins forcing the piston down.

Down Stroke - Power and Exhaust
On the down"power"stroke the piston is forced towards the crankcase reducing its volume and creating a positive pressure. As it continues downward travel it starts first to uncover the exhaust ports. Exhaust gas begins to rush out of the cylinder. Then the intake ports are uncovered. The fresh air/fuel charge in the crankcase is forced into the cylinder and continues to push the remaining exhaust gases out.

The 2 stroke process of purging exhaust gases from the cylinder and filling it with a fresh air/fuel charge is called scavenging. Two stroke engines use 2 different scavenging methods, cross-scavenging and loop scavenging. Both differing designs have particular advantages.

TWO STROKE CROSS-FLOW
two stroke cross-scavenged engines can be identified by the irregular shape of the top of the piston called a deflector. This deflector directs the incoming air/fuel up, towards the top of the cylinder. This creates a wall or column of fresh mix that sweeps across the cylinder towards the exhaust ports. As the column advances it pushes the spent exhaust gases out of the exhaust ports.
See Picture Below Hope this makes since to all and helps all.



TWO STROKE LOOPER
Pistons in loop scavenged engines are generally near flat. They do not rely on deflectors to aim the fuel/air mix, rather they have shaped intake ports and combustion chambers to control the scavenging of the cylinder. Several intake ports are aimed upwards and arranged such that their combined streams flow upward and then LOOP down toward the exhaust ports.

Cross-Flow engines are better performing at idle and low speed. All older motors of any horsepower are of this design. Until the late 60's it was not economical to try to produce this design in quantity at a reasonable cost.

Looper engines, although having poorer idling characteristics are more fuel efficient and perform better at higher RPM's than crossflow as they have lighter pistons. This lowers the strain on the connecting rods, bearings and crankshaft. OMC created the first US production looper in 1968 with the 3 cylinder 55HP.

[randlemanboater]

I don't have pic's, havent seen the boat in person yet, just trying to figure stuff out from afar.

Thanks Skoolie.

[Airslot]

Skools, great tutorial, thanks. Having on offline converstaion earlier, I realized that I don't understand exactly how the DFI's are lubing the bottom end. Care to shed any light on that?

Airslot

[Skools Out]

they have lower crank case oilers that are run from theon board oil pump and mixer.

[THEFERMANATOR]

LOOPERS tend to pull better up high and get slightly better milage. CROSSFLOWS pull good off idle but run out up high. OMC increased the bore on the loopers for 88 due to the lack of bottom end grunt(no replacement for displacement). OMC offered the 1.6L(crossflow) sea-drive all the way up to 89 that I've seen. 1.6L and 2.4L and 2.6L were crossflows, 1.8L 2L 2.7L 3L 3.6L and 4L SEA-DRIVES were loopers.

I personally can attest to the power potential that a small-bore looper can make though. The 19' COBRA bowrider in my sigg has run 53 MPH and was still climbing(but chine walking severely and no steering control). The same boat with a 150HP SUZUKI super six BASS pro edition ran 53 as well, and they were the same engine as a 225HP SUZUKI with a lower gear ratio foot.

[Skools Out]

well you can't base a hull on HP the hull design will only go so fast you could put twin 300's on a boat and it still run the same as the same hull with a single 250. all is based on hull design