in-line 6 Cylinder Engines, 'straight-six'

[beiland]

For years now I was aware that the in-line 6 cylinder engine was likely the BEST configuration for a 4-cycle internal combustion engine (the most naturally balanced configuration). I had been told this long ago by some car racing buddies, but I don't know that I ever looked it up in the library or whatever,...at least I may have, but I had forgotten the specific’s.

Today I decide to see what I might find via google, and this site was the first to pop up:
http://en.wikipedia.org/wiki/Straight-6

Wow, this is pretty inclusive. I'm going to excerpt a few of the passages from that site for posting here on this subject thread that might ask a few more questions of more specificity as related to 6-cylinder Marine engines.

The straight-six engine or inline-six engine is an internal combustion engine with the cylinders- mounted in a straight line along the crankcase with all the pistons driving a common crankshaft. The bank of cylinders may be oriented at any angle, and where the bank is inclined to the vertical, the engine is sometimes called a slant-six. The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders.

Balance and Smoothness
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. 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.

An inline four cylinder or V6 engine without a balance shaft will experience secondary dynamic imbalance, resulting in engine vibration. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed — for example, if the speed doubles, vibration will increase by a factor of four. In contrast, inline six engines have no primary or secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb torsional vibration, will run more smoothly at the same crankshaft speed (rpm). This characteristic has made the straight-six popular in some European sports-luxury cars, where smooth high-speed performance is very desirable. As engine reciprocating forces increase with the cube of piston bore, straight-six is a preferred configuration for large truck engines.

Straight-Six Diesel Engines
The straight-six in diesel engine form with a much larger displacement is commonly used for industrial applications. These include various types of heavy equipment, power generation, as well as transit buses or coaches. Virtually every heavy duty over-the-road truck employs an inline-six diesel engine, as well as most medium duty and many light duty diesel trucks. Its virtues are superior low-end torque, very long service life, smooth operation and dependability. On-highway vehicle operators look for straight-six diesels, which are smooth-operating and quiet. Likewise, off-highway applications such as tractors, marine engines, and electric generators need a motor that is rugged and powerful. In these applications, compactness is not as big a factor as in passenger cars. Reliability and maintainability are much more important concerns.

Most of the engine components and accessories can be located along both sides, rather than on top of or underneath the cylinder banks, meaning that access and maintenance is easier than on a V engine in a truck or industrial configuration. In addition, a straight-six engine is mechanically simpler than a V6 or V8 since it has only one cylinder head and the overhead camshaft configuration has half as many camshafts

[cat man do]

One of my favourites The Gardner 6LXB

Gardner 6LXB Marine - YouTube

[beiland]

...on another forum...

6 cylinder straights have more harmonic vibration than any other engine.

That appears to be in direct conflict with this quote from that weblink??...and I'm sure I could find many other praises for the natural balance of an inline 6.
"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. 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."

They are also not the best at breathing. Using exhaust pulses and intake pulses the straight 5 cylinder is better.

I would ask you to find some verifiable references to that statement.

I do remember that Chrysler did some experimenting on their slant-6 and achieved some success in racing when engineers utilized the slant of the engine for very long intake ports to boost horsepower by tuning the intake system.

Any breathing problems for a marine 6 would be overcome by turbo boosting which is a natural fit for a good strong 6 engine not worried about turbo-lag as in quick acceleration in a vehicle.

A slant 6 or any slant is to use gravity to combat side wall wear on the cylinder opposing thrust side

I'm not quite sure what you are referring to here?....more wear or less as a result of the slant?

I seem to recall that the Plymouth Valiants and Dodge Darts had quite a good record for durability....exceptional I believe?

[bobconnie]

Just ask 40 or 50,000 fish boat owners anywhere what works, Gardners, Lehmans,671 detroits. some of these motors go so many hours with simple maintaince, and good oil, then most folks would ever believe!! Ive personaly had 85 and 120 hp Lehmans with over 15000 hours with a upper end job in those hours somewhere !! and I wont even coment on Gardners or 671s cus ya would never believe me anyway !! LOL Lets just say with normal maintaince, some 6 cyl and even 4 cyls can run well a lot longer then most yacht owners would ever believe if they could keep there engine rooms cooler at least thats my 2 cents

[David M]

The Cummins B-Series is also an excellent straight 6.

[gjordan]

Unfortunatly in the under 100 HP size that most sailboats use, the 6 cylinder is not a very practical engine. The scale of size makes for too many small parts which means more coast to build or rebuild. It would also be longer which would cut into accomadations. In the size range that most cruisers own, the 2, 3, and 4 cylinder engines are the most common and , with proper maintainance will outlast most owners. There is nothing wrong with 6 cylinders, but they belong in bigger boats than most cruisers own. Another 2 cents worth.____Grant.