Racing exhaust for laker 49 OMC

[Tim Kurcz]

Merc used 3 + 3 exhaust tuning on the inline 6's starting with the first gearshift motors. Generally the firing order was alternating pairs for balance with odd/even exhaust coupling (1, 3 & 5 in one group and 2, 4 & 6 in the other). The disadvantage was length between these 3 cylinder exhaust groups favored power in the 5,000 rpm range rather than the 8,000 rpm range.

Its not that it wasn't there, it just wasn't there for super top end.

On the T-2 and T-2x race motors they welded 2 3cylinder 49ci cranks together to get 3 + 3 exhaust tuning with close spacing, 1, 2, 3 in one group and 4, 5, 6 in the other exhaust group. Again there was a problem with length ... the original T-2 set up put them TOO close for 7,000-8,000 rpm. Like these OMC side exhausts ... the T-2X used an extended exhaust chest to drop the tuning into the 7,000-8,000 rpm range and get almost 20 hp over the T-2.

As Sam points out, Mercury did use 3+3 for inline sixes, a vast improvement over the 2X3 exhaust tuning; in fact they actually patented the concept. It improved power, but was not as effective as late model triples as the cylinders were too widely spaced - every other cylinder was used - inteconnect distance was still too far apart.

Sam's continues his explantion for the T2 series Mercs which correctly indicates that closer interconnects make another huge improvement. So close interconnects are the ticket for power delivery from close spaced triples (and multiples thereof).

The question stands: Why the power difference between right and left exhaust porting as seen in the OMC FR19S -vs- FR31M, or the opposite banks in any late model V6?.

[Powerabout]

piston rock and hence the piston sealing the exhaust port when the port is below the piston top so the exhaust leaks into the crankcase

[Tim Kurcz]

piston rock and hence the piston sealing the exhaust port when the port is below the piston top so the exhaust leaks into the crankcase

Indeed, torque rock causes exhaust pressure from the "open chest" to leak past the piston skirt into the crankcase, polluting the intake charge and reducing power for the port side exhaust bank.

The problem can't be overcome in a V engine unless dual exhaust paths are used (both banks using starboard side exhaust ports). The solution is simple for port side engine lovers: Run the engine backwards and locate opposite throw wheels.........

Way to go Powerabout!

Tim

[Fastjeff57]

Ah, wouldn't that simply transfer the problem to the other bank?

Jeff

[Mark75H]

Not on a triple

[Powerabout]

Its a shame all the late 500cc jap engine tech has been kept out of the public eye so we have never seen the top end of 2 stroke technology.
I heard Honda worked out the bores should be oval anyone know about that one?
( I dont)

[Bill Van Steenwyk]

that competed in bikes some 10-15 years ago, possibly longer. I don't remember now whether is was a four stoke or not, but for some time the four stokes could compete against two strokes with a "spot" on displacement, i.e. the four stroke could be larger displacement wise compared to the two stroke because of a power stroke every revolution versus one every other revolution with a 4 stoke. I don't remember now whether this was the F-1 class of bike racing or the next class down, but the engine was definitely an oval bore of about 500CC or larger displacement.

It competed for several years and then faded away, for some type technical problem.


ADD: Try Googling "Honda GP Bike History" and then use the search function for NS500. Two connecting rods for each piston, 8 valves per cylinder, 155HP @15,000RPM Four Stoke Year 1987. This will give you info on both 2 and 4 stroke bikes that Honda built to compete in GP 500CC which was the ultimate in engine and bike development for all the manufacturers that competed.

Each "piston" in the NS500 was oval in shape and filled what would be two round cylinders put together to make one cylinder oval in shape. A 750CC version was also made.

[Mark75H]

It was a 4 stroke

[Bill Van Steenwyk]

Its a shame all the late 500cc jap engine tech has been kept out of the public eye so we have never seen the top end of 2 stroke technology.
I heard Honda worked out the bores should be oval anyone know about that one?
( I dont)

I think you see it now and have for several years (latest Japanese technology in 2 stokes) with the GRM and VRP PRO engines built in Italy at this time. Carlo Verona built and may be still building cylinders and other component parts for the Japanese for bikes used in F-1 racing.

I would venture to say those engines are probably the pinnacle now in 2 stroke design, as what used to be F-1 Motorcycles is now "Moto-GP" and is all four stokes in the largest classes. I think they still run 2 stokes in 125 and 250, but not sure.

[Powerabout]

the 2 stokes left the Gp bikes at the end of last year due to some lobbying by Honda