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Old 08-20-2013, 10:31 AM   #21
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I had a BMW K-75 3 cyl motorcycle that not only had balancers but the flywheel was gear connected and revolved in the opposite direction. Blip the throttle and no rotational forces noted. Torque effect seemed to be gone.

This one was so smooth while riding in w 1st wife on the back in Wyoming we rode for miles one morning in 4th gear. Didn't even notice.
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Old 08-20-2013, 11:28 AM   #22
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Quote:
Originally Posted by manyboats View Post
FF wrote;
"<The in-line 6 is the most inherently balanced configuration for a 4cycle engine,.>

You've stated that the 6 cyl engine is the smoothest many many times.

Why do you think that?

Are you referring to the balance of reciprocating and rotating parts only?

Of what significance is that when the torsional vibrations from power strokes (and perhaps other factors) make the 8 cyl engine much smoother?
Hi Eric,

That was me that made that statement. Long ago when I was involved with a little car racing I had a mechanic tell me that. I said WHAT?, surely its got to be a 4 cylinder that's the smoothest for 4 cycle operation.??

No,
An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs (but of course, 360 out of phase and on different strokes of the 4-stroke cycle). That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero.

So the in-line 6 requires the least counterbalances on the crank, and combined with the proper firing sequence it is the best of all
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Old 08-20-2013, 11:49 AM   #23
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Two Volvos in the "Holy Place."
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Old 08-20-2013, 11:59 AM   #24
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Long Subject Thread

Lots more discussions over here for anyone interested,...some pretty good references as well.

in-line 6 Cylinder Engines, 'straight-six' - YachtForums.Com
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Old 08-20-2013, 12:07 PM   #25
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This is a great discussion...
I sit at the dock all day...even week after week maybe month after month with my engine out of gear listening to how smooth it is...

By far in my experience...smooth cruising boats are all about everything BUT what engine they have.

It's everything from drivetrain to prop cleanliness to engine alignment to construction and boat stowage....ad infinitum...

Then we throw in mother nature and what she hands us...sometime I really have to strain to hear/feel my engine when the wind is rattling my sliding helm side door.

Nice if you get your whole boat into perfect harmony so you can concentrate on your engine...but I'd be a lot more worried about reliability, source of parts and ease of repair before I worried about "smoothness"...unless you are going to have it blueprinted or whatever motorheads are doing these days...
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Old 08-20-2013, 02:22 PM   #26
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but I'd be a lot more worried about reliability, source of parts and ease of repair before I worried about "smoothness".
OBVIOUSLY you don't have a 1 lunged Volvo MD5 powered antique gen-set that can best be described as an angry monkey with a hammer inside an oil drum!
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Old 08-20-2013, 03:26 PM   #27
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OBVIOUSLY you don't have a 1 lunged Volvo MD5 powered antique gen-set that can best be described as an angry monkey with a hammer inside an oil drum!
My old ketch had a 12hp one lung Yanmar in it...thumper it was!!

But still...on a boat ..the engine smoothness while nice is only part of the equation....
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Old 08-20-2013, 07:20 PM   #28
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Porsche six is the smoothest fast revving I have ever messed with. By a long shot.

BMW then Jaguar in-line 6s.
BMW in line 6 gets my vote
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Old 08-20-2013, 10:38 PM   #29
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Well I see there are people here that actually believe a 6 cyl engine is smoother than an 8.

Never would'a thunk.

But we're stuck w opinions as nobody can show decisively that one or the other is indeed smoother.

I'm open to look'in at proof though.
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Old 08-21-2013, 06:42 AM   #30
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I guess a marinized Wankel rotary engine hasn't happened yet, but one can hardly debate the smoothness of that design. Ever wonder what a 6 rotor Wankel would sound like? Here it is.

This is what a six-rotor Wankel looks and sounds like - Autoblog
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Old 08-21-2013, 11:19 AM   #31
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Quote:
Originally Posted by brian eiland
The in-line 6 is the most inherently balanced configuration for a 4cycle engine
Note that I did NOT say the inline 6 was the best engine, just the most inherently balanced engine
I repeat:
Quote:
Originally Posted by brian eiland View Post
An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs (but of course, 360 out of phase and on different strokes of the 4-stroke cycle). That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero.

So the in-line 6 requires the least counterbalances on the crankshaft,

Quote:
Originally Posted by manyboats View Post
Are you referring to the balance of reciprocating and rotating parts only?
Primarily, and because there are less counterweights involved the wear and tear on the crankshaft bearings ultimately is less.




Quote:
Originally Posted by mannyboats
Of what significance is that when the torsional vibrations from power strokes (and perhaps other factors) make the 8 cyl engine much smoother?
I would challenge you to prove that statement....that a straight 8 is a better balanced configuration than a straight 6.
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Old 08-21-2013, 11:21 AM   #32
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How about this !
Mercedes-Benz to Replace V6s With Inline Engines
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Old 08-21-2013, 11:36 AM   #33
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There was a Wankel that OMC built as a OB prototype as I recall. I don't think the Mazda Wankel overcame the fuel efficiency issue as the new RX8 shows an MPG rating of 18. My 1995 3.8L Buick 4dr sedan does better than that. And in the 2013 Consumer Reports magazine the RX8 has a terrible mechanical maintenance record.

According to CR the engine is "exceptionally smooth providing you keep the engine at mid-revs, ..". At an idle my 03 Camry w variable valve timing is smoother than older sixes. It probably has balancers too.

I read about the Wankel motorcycle (Suzuki) and it was very complicated. Anti-backfire valves and lots of other stuff that was not on other engines. The induction and exhaust system had lots of gimmicks and sub systems to make the Wankel behave.

Looks like the Wankel has had it's chance but it's not totally dead yet as the RX8 is on the showroom floors and some people are buying.
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Old 08-21-2013, 11:45 AM   #34
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I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient.
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Old 08-21-2013, 12:00 PM   #35
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Very interesting Brian. (the MB link)

The big problem w inline 6s is weight and length. The crankshaft is much longer. It flexes more and thus needs to be heavier yet. And then there is the automotive obsession w FWD and inline 6s just don't "fit in". So the V6 engine will probably prevail as long as FWD.

Fours are becoming even more popular and fewer cylinders usually means better fuel efficiency. A 2.5L four will almost always get better mileage than a 2.5L six. Mileage is increasingly important so I think fours will continue to outshine sixes in the marketplace and technology seems to be reducing the smoothness advantage.

Look at the JD fours that are about the same size as six cyl Luggers and Cummins and others. JD has the balancers and it seems to make them equal in smoothness but I'd guess the JD has better fuel efficiency just because it's a four. I suspect the 4 has less surface area cyl + combustion chamber and hence better thermo efficiency through less heat loss.

Brian wrote "I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient"

I don't think that in itself is a problem and I can think of many advantages.
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Old 08-21-2013, 12:40 PM   #36
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I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient.
Why is that?
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Old 08-21-2013, 05:56 PM   #37
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Define smooth

V24 Detroit Diesel - YouTube
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Old 08-21-2013, 07:14 PM   #38
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Is it just me or does the 6 "cyl" rotary sound like one of the last big 2 stroke jet skis?
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Old 08-21-2013, 07:38 PM   #39
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Is it just me or does the 6 "cyl" rotary sound like one of the last big 2 stroke jet skis?
Yeah, that's it. Mix in the sound of a WWII buzz bomb and it would be perfect.
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Old 08-21-2013, 10:42 PM   #40
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here is a smoothyBIGGEST ENGINE
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