Smoothest Engine

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?

manyboats said:

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?

Click to expand...

Eric, are you forgetting the Buick OHV straight eight. Smooooooooth!

manyboats said:

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

Why do you think that?

Click to expand...

I've had both over the years and I think the inline 6 is the smoothest !

Forgive me if I misstate something in the following, I am reciting this from memory as a result of many years of reading Road and Track (too many years).

The straight 6 and 8, the 90 degree V8 are all inherently smooth and the 60 degree V6 almost so due to the balance of reciprocating mass and power pulses.

A straight 4 will never be as smooth because of the imbalance, but modern auto based engines (and the JD I think) use a balance shaft which gets close in smoothness. That is why the Cummins 4BT is often panned as being rough. It is big engine that is unbalanced. The 4 cylinder Yanmars are pretty smooth due to some very well engineered (and expensive) engine mounts.

The 90 degree V6 was used early on as it could be bored on the same line as the American V8s but it took a complex offset crank throw to smooth it out decently.

The only takeaway in all of the above is that a straight 4 without a balance shaft will be rougher than the others.

David

Don wrote;

"Eric, are you forgetting the Buick OHV straight eight. Smooooooooth!"

Indeed and I've known no 6cyl engine that comes close. But the inline 8 Buick is smooth for numerous other reasons. I think the combustion chamber was even designed for smoothness. "Fireball Valve in Head" was printed on the valve cover. I've owned several Buick 8s and I remember specifically w a 3 spd stick I could go around a 90 degree corner fully engaged in top gear at FIVE mph. And in total smoothness. But the Buick is/was not your average straight 8. Packard, Chrysler, Pontiac, Oldsmobile and Hudson and I believe Studebaker offered straight 8s and ALL were smoother than the flat head 6 cyl engines of the day that are directly comparable to the eights other than Buick. The F head RR Silver Cloud engine I have not experienced and often wondered why RR didn't build an 8. I'm guessing they built the RR 6cyl like GM built the Buick ... to be exceptionally smooth. For example if one were to employ a heavy enough crankshaft and flywheel combination by that alone one could build a smoother engine.

I think I know what FF is talk'in about as I've heard of it. "IT" is some aspect of mechanical smoothness of reciprocating and/or rotating parts that measure up to be smoother than it's 8 cylinder counterpart. But in the real world that aspect I suspect is fly stuff in the pepper and rarely if ever noticeable.

In the real world of working engines that are comparable except for some having 6 cylinders and some having eight the 8 will be very noticeably smoother. I don't think this balance feature FF is talking about is going to be more effective re smoothness than the more frequently firing 8 cylinder engine. I don't think FF should allude to it as it's false and misleading.

David,
In a boat it's likely that the only time an 8 will be noticeably (as in seat of the pants) smoother than a 6 is at slow idle. So I agree w your "takeaway" comment.

Are we forgetting the boxer design?

A good read:

engine smothness

manyboats said:

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?

Click to expand...

Eric, that wasn't FF's statement above. He quoted Brian E's statement. Remember Fred's keyboard is broken and he's using<> instead of "" now.

Spy,
Quite basic .. actually too basic.
He dosn't even consider torsional vibration. That created from combustion and compression. Watch any engine idling and if running on soft mounts one can actually see the engine's movement. Most of the movement is radial. There are very strong forces that accelerate the the rotation of the crankshaft in one direction. And there are forces that decelerate the crankshaft of much less magnitude (compression) in the other direction. Recall hand cranking. As the crankshaft rotates very fast these forces are kinda like a jack hammer. But these forces are rotational. Not vertical, horizontal or sideways. Those movements are caused by moving parts and I think (but don't know for a fact) that the torsional vibration is considerably greater. However a lot of why I think that is based on observations of engines at idle or not far from it. At higher speeds it's harder to see the engine move.

All engine mounts that allow movement in one direction more so than another seem to universally favor allowing for more radial movement that any other. However engine mounts need to control not only vibration but thrust in both directions (most boats take full thrust on their engine mounts) and other control considerations.

But I'm quite sure torsional vibrations are of the highest magnitude and the number of cylinders controls the amount of vibration.

OK Craig,

But I'm responding to the fact that he has said over and over that the the 6 cyl engine is the smoothest.

OK so Brian said it .... I'll argue w him on the issue too.

It may be that the 6 is more "inherently" balanced but I'm saying that the engine (6) dosn't run as smooth .... as an 8 cyl. So I'm think'in it's silly to imply that the 6 is smoother by say'in it's more "inherently" balanced. I don't think it should be said because it's misleading.

Actually I'm getting a bit sorry I started this because I can't prove I'm right. But in my opinion nobody else has shown that a 6 is smoother. As soon as someone does this thread will be worthwhile. That is a little harsh ... it already has been some fun.

After all it's no fun to kick something around that you already know. It's fun when you know something about it but the reality is somewhat elusive.

Interesting conversation. I've also had two Buick Fireball 8s. Silky. Also two Pierce Arrow V-12's used in Arens Fox Fire Engines, but I think a friends Cad V-16 took the cake from both. In Diesels, I also found that my restored 1946 GMC with its 471 Detroit shook the mirrors less than any truck I ever had, including at idle (probably because it was firing with each stroke). Another smoothie truck application was a 67 GMC with a 702 V-12 gasser, which was actually two 351 V-6's mated together with a common crank. When the carbs were tuned, it ran well with good power, but terrible fuel consumption. I had plenty of stovebolt Chevys. They were pretty smooth too. Smoothest V8 I ever had was again, the old Buick nailheads, especially the 322. They whirred.

healhustler said:

Interesting conversation. I've also had two Buick Fireball 8s. Silky. Also two Pierce Arrow V-12's used in Arens Fox Fire Engines, but I think a friends Cad V-16 took the cake from both. In Diesels, I also found that my restored 1946 GMC with its 471 Detroit shook the mirrors less than any truck I ever had, including at idle (probably because it was firing with each stroke). Another smoothie truck application was a 67 GMC with a 702 V-12 gasser, which was actually two 351 V-6's mated together with a common crank. When the carbs were tuned, it ran well with good power, but terrible fuel consumption. I had plenty of stovebolt Chevys. They were pretty smooth too. Smoothest V8 I ever had was again, the old Buick nailheads, especially the 322. They whirred.

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Walter L. Marr-Buick's Amazing Engineer
Walter L Marr: Buick's Amazing Engineer: Beverly Rae and Cox, James H. Kimes: 9780976668343: Amazon.com: Books

Walter Marr was the grandfather of a good friend of mine. He lived for many years on Signal Mountain, TN and did a lot of his design work for Buick from there. He was largely responsible for Buick's valve in head engine. There first straight 8 OHV was I believe in 1932. It was amazingly smooth for it's time.

SaltyDog said:

Are we forgetting the boxer design?

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Quite right, we shouldn't. I'm always amazed how smooth and vibration free our Sub Outback is, which is a 3L flat six, horizontally opposed, so-called, Boxer engine.

Don very good stuff about the B8. Only recently has Buick taken it's eye off the smooth ball. Even in the 90s Buick cars had a wallowing pillow ride. There were Bar (I think) marine manifolds for the B8. It would be a dream engine for an old Chris Craft but the flat head sixes were very smooth too. And the flatheads were lower. I suspect the flat head combustion chamber helped w the smoothness. Flathead engines don't ping even w high compression and advanced timing. I think it's because the combustion takes more time ... is longer in duration. However if you were to lug a six on a hill the drive shaft would make noises from vibration. The 8 cyl cars never did that.

Peter I've had mixed experiences w the boxers. The old air cooled VW boxers were very smooth. I had a 57. I lusted after several cars w boxer engines thinking they'd be as smooth but were not. The Subaru 4 cyl never did seem smoother than the many in-line 4s I compared them to. Seemed about the same to me. I'm now lusting after a Scion sports car that is powered by a Subaru 4. Haven't driven it and probably won't.
I think the boxers have a "rocking couple" issue. That's caused by the cylinders on each side aren't across from each other but staggered. I'll go back and read Spy's link as I think there was a blurb on boxers ... that I didn't read.

I looked at the link and think it's absolutely false. The boxer engine would need to have yoked connecting rods to eliminate the rocking couple. Here's an example of the rocking couple. A boxer twin (like a BMW air cooled twin) will have each con rod connected to it's own crank journal one ahead of the other. So the engine will "rock" sideways in a yawing motion. Kinda like a vertical twin w a 180 degree crankshaft where the pistons rise and fall alternately. The engine in this case will "rock" in the pitch axis.
Re 6 cyl boxers I only know the Corvair and it was very smooth. Not as smooth as an 8 though.

healhustler wrote;
"I also found that my restored 1946 GMC with its 471 Detroit shook the mirrors less than any truck I ever had, including at idle (probably because it was firing with each stroke)" That's almost a given. The 4-71 DD has exactly the same number of power strokes and in the same sequence as the Buick straight eight. I think there's more to it too.

Amazing what happens to ANY engine when the rods, pistons and crank are 'balanced and blueprinted'...the difference can be startling...

Porsche six is the smoothest fast revving I have ever messed with. By a long shot.

BMW then Jaguar in-line 6s.

SaltyDog said:

Porsche six is the smoothest fast revving I have ever messed with. By a long shot.

BMW then Jaguar in-line 6s.

Click to expand...

Now that I think about it, I have to agree that the Jag twin OHC 3.4, 3.8 and 4.2 sixes really were silky smooth. And by the way, those Pierce Arrow V-12's were often found in American LaFrance and Seagrave Fire Engines, and not the Arens Fox (which I never had). It reminds me that this old memory is about as smooth as a 3208 Cat with two bad injectors.

Gee, I thought all of my engines were "smooth," from my one cylinder Seagull, my four-cylinder VW bug, to my six-cylinder Jeep and Explorer, as well as the Coot's four-cylinder JD. Honest, no shaking!

For most cruisers the 6 cylinder engine , marinized from a car (yanmar) or truck sourced will be the best easiest to live with choice.

Which engine to select , thats almost a religion.

MY choice is always an engine that the Mfg has the confidence to produce the usual 4 power ratings for day boats to 24/7/365 .

What about the Jag V12? They are touted as THE smoothest! Also Honda's flat 6 used in Goldwings and Valkyries. I had a Valk and I did the trick of standing a nickel on the valvecover and cranked the engine without it falling! The power and smoothness of that engine made the bike feel almost electrically powered! It'd be neat to have a gen-set powered by one of those if they could pull the Tq curve down to those RPMs.

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.

manyboats said:

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?

Click to expand...

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

Two Volvos in the "Holy Place."

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

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...

psneeld said:

but I'd be a lot more worried about reliability, source of parts and ease of repair before I worried about "smoothness".

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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!

twiisted71 said:

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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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....

SaltyDog said:

Porsche six is the smoothest fast revving I have ever messed with. By a long shot.

BMW then Jaguar in-line 6s.

Click to expand...

BMW in line 6 gets my vote

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.

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