Whats a looper?

[tsubaki]

Thanks MJ.
I never knew or paid attention to the different types of engines, just wanted to get the ones I had running.
While we at it, how about the difference in the XP or GT or anything else supposed "hot rod"?
Crossflow-scavenged

In a crossflow engine the transfer ports and exhaust ports are on opposite sides of the cylinder and a deflector on the top of the piston directs the fresh intake charge into the upper part of the cylinder pushing the residual exhaust gas down the other side of the deflector and out the exhaust port. The deflector increases the weight of the piston and exposed surface area of the piston, also making it difficult to achieve an efficient combustion chamber shape. This type of two stroke has been largely superseded by loop scavenging method (below). With smaller size and lower piston speed the deficiencies of the cross flow design become less apparent. The last of the OMC (Outboard Marine Corporation now Bombadier Recreational Products BRP) V4 and V6 two strokes produced up to 1995 in their mid range were still cross flows. These were produced in the 90-115 horsepower V4 configuration in a 1.6 litre as well as the 2.4 Litre 150-175-200 Horsepower V6's. These engines remained extremely competitive on fuel use compared to their loop charged competitors due to advanced exhaust tuning by the manufacturer. These Crossflow engines produced more torque and horsepower by burning less fuel than the Japanese loop charged competitors. Eventually OMC shifted to the Spitfire series Loop charged V4 and V6's in their mid range.
The 235 horsepower 2.6 Litre crossflow V6 (1976 - 1986) still remains today as a very high output low weight engine compared to its much heavier loop charged 2 stroke and 4 stroke replacements.
The Crossflow design produces far more low down engine torque than the slightly more fuel efficient Looper design. Many a boater replaced their Crossflow 2.6 Litre 235 (Flywheel rated) for larger (propshaft rated HP) 2.7 Litre and 3 Litre 225 horsepower V6's only to be disappointed with the lack of low down torque offered.
During the late 1970s 1980's OMC successfully raced the OMC CCC engine. This was a crossflow, carburettored V6 2.6 Litre that out ran many of the oppositions Loop scavenged, fuel injected larger displacement competitors.
It lived on with the XP 2.6 until 1986 in a much more civilized form.
Cross flows are still to be found in small engines because it is less expensive to manufacture and allows a more compact design for multiple cylinder configurations. BRP still offer the 9.9 and 15 HP twin cylinder two stroke available through their Johnson brand as there is still no alternative to this popular lightweight high output engine.


Loop-scavenged
This method of scavenging uses carefully shaped and positioned transfer ports to direct the flow of fresh mixture as it enters the cylinder. usually a piston deflector is not required conferring considerable advantage over the cross flow scheme (above). Often referred to as "Schnuerle" (or "Schnürl") scavenging after the German inventor of an early form in the mid 1920's, it became widely adopted in that country during the 1930s and spread further afield after World War II. Loop scavenging is by far the most common type used on modern engines.
01-23-2007, 12:07 AM
Skools Out
God
Join Date: Jul 2007
Location: Graham / Emerald Isle, NC
Posts: 4,556


Re: Crossflow vs. Looper
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.


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1978 V20 Cuddy w/ 225 Johnson or Twin\'s maybe???

[tsubaki]

So who be got pictures of each?

[macojoe]

There are pic's, need to do a search and find them, The problem we go off topic so much when talking its hard to find the right thread.

[tsubaki]

LOOPER PISTON


CROSSFLOW PISTON

[spareparts]

yeah, I had a brain fart there, I got them backwards, newer motors are loopers, not cross flows, the heat must be getting to me

[tsubaki]

Bad part is, I didn't even notice.
Is it possible to tell externally from the rear which is which?

[cterrebonne]

Originally Posted by spareparts

yeah, I had a brain fart there, I got them backwards, newer motors are loopers, not cross flows, the heat must be getting to me

i know the feeling, before i got into sales. i would dread the summers, it would easily get into the high 90's in the shop.


to tell the difference between the two engines, just look at the heads, two piece heads mean cross flows, one piece means looper. they came out with the loopers when they started to use the lost-foam casting technique.

[tsubaki]

This is what they had available in V4

TYPE YEARS
crossflow flatback 78-02
crossflow bubbleback 78-98
60 degree carb 95-06
60 degree FICHT DFI 98-06
90 degree looper 85-01

This is what they listed in V6

TYPE YEARS
crossflow 76-91
60 degree carb 91-06
60 degree FICHT DFI 97-98
front oiler or rear oiler 60 degree FICHT DFI 99
water supply block HP 60 degree FICHT DFI 00-05
90 degree looper carb 86-01
90 degree looper FICHT DFI 99-03

Obviously you will need a lot more than the year of the motor to do an analysis of a problem. Always list your model number.