Piston and rod balancing

[Chaz]

omc 3 cyl crank got 3 sets of counterweights as per what you would see in a single,

And why do think that is ... ?

Its a big jump to comprehend a v6 at 60 or 90degree to then have none less the ones on the end?

I felt you were being swayed by the V configuration , so I tried taking that out of the picture and just have you look at the 120 degree pin and counterweight arrangement.

I wonder what a flat 6 2 stroke crank would look like?

Porsche .....

[powerabout]

And why do think that is ... ?



I felt you were being swayed by the V configuration , so I tried taking that out of the picture and just have you look at the 120 degree pin and counterweight arrangement.



Porsche .....

3 cyl crank very simple plus I guess its the only way to do it

I need to re read the post

thanks Chaz

[Dave Strong]

As far as the V6 vs I3 thing goes look at the old Merc Race motors that were V6's with only one bank loaded.

Dave

[powerabout]

sst90, I guess it had a std v6 crank?

[roadkill636]

Lets see what this stirs up. Remember watching the whole tractor move back and forth a but just idling.

[Onetime]

Looks like a hit or miss engine. Super slow rpm with that huge flywheel to keep things moving. Always fun to watch them run. Can get to see them at antique tractor shows.

[Mark75H]

I wonder what a flat 6 2 stroke crank would look like?
Koenig anyone?

I doubt you'd find a Konig 6 apart to show the crank. Konig's used single crankcases for the opposing pairs, the paired cylinders fire together; the Porsche crank would look the same even though the cylinders fired alternately.

Corvair crank



Konig used pressed together segmented cranks, so it wouldn't look like this, but would have the same bearing & throw layout

[powerabout]

So the Konig didnt have counterweights on each cylinder then as above

[BarryStrawn]

Just to be helpful, V6 engines of 60 or 90 degree bank angles don't balance worth a crap without external balance shafts. Look at any auto V6. They don't spend that money unless they have to. So an outboard is going to shake, just a matter of how bad at what RPM.

Some folks that know a lot more than I say that most outboards are balanced to produce the least noise, vibration and harshness at idle. Has little to do with how it runs at WFO. And at idle, the different suction/pressure forces on top and under the pistons are significant compared to an four stroke auto V6. So traditional thinking may not explain the factory counterweighting.

The flat 6 has the pistons moving opposite so the reciprocating forces almost cancel and a very good balance should result. The forces don't perfectly offset because the crank journals and cylinder bores are offset by the space needed for the crank journals, bearings and webs.

[powerabout]

2 cyl etec skidoo has a balance shaft these days

[powerabout]

So heavy to light rods in omc looper, can you tell the difference?
where do I grind the crank to get back to 50,1,or 2%

[Chaz]

Just to be helpful, V6 engines of 60 or 90 degree bank angles don't balance worth a crap without external balance shafts. Look at any auto V6. They don't spend that money unless they have to. So an outboard is going to shake, just a matter of how bad at what RPM.

Some folks that know a lot more than I say that most outboards are balanced to produce the least noise, vibration and harshness at idle. Has little to do with how it runs at WFO. And at idle, the different suction/pressure forces on top and under the pistons are significant compared to an four stroke auto V6. So traditional thinking may not explain the factory counterweighting.

The flat 6 has the pistons moving opposite so the reciprocating forces almost cancel and a very good balance should result. The forces don't perfectly offset because the crank journals and cylinder bores are offset by the space needed for the crank journals, bearings and webs.

While it's true that 60 and 90 degree 6's do better with the addition of a balance shaft. It is far from needed. The automakers go way out of their way to make cars feel like your not riding in a car. When gas prices were working their way to $5.00 a gallon , the auto industry went with smaller more efficient powerplants. Hence the extra shaft.
I can tell you from experience , an outboard motor is smoother from idle on up after it is balanced. And as stated before , go watch them do a crank balance job from start to finish, and you will see for yourself.
On race car motors , I like to use a vacuum pump that is regulated to -15 lbs. So there are crankcase forces at work on both 2 and 4 cycle engines ...

[BarryStrawn]

No argument from me that custom work can make an outboard run smoother. Same as it has always been for any mass produced engine. But there is no solution to make a V6 have perfect or even very good balance. You can just make them suck less than the factory job. If it is worth the time and expense is up to the individual.

[Markus]

While it's true that 60 and 90 degree 6's do better with the addition of a balance shaft. It is far from needed.

Chaz, a balancing shaft is needed if you want to actually balance the engine, not just weight match the components of the rotating assembly.

Only a cross-plane V8 can be balanced with crank counterweights.

[Markus]

No argument from me that custom work can make an outboard run smoother. Same as it has always been for any mass produced engine. But there is no solution to make a V6 have perfect or even very good balance. You can just make them suck less than the factory job. If it is worth the time and expense is up to the individual.

Here is where a like button is really needed.