4LH OR 6LYA which vibrates more

[SILENTKNIGHT]

is there more vibration on twins 240's in a 2006 Mainship 34T?
I was told the 6LYA-STP is smoother ? less vibration.

 

[Dougcole]

Not sure about the 6LYA, but my 4LHA-STPs vibrate quite a bit at idle, then smooth out tremendously at about 1,000 RPM. They run silky smooth above idle, they just shake at idle. I recently had the valves adjusted and that helped a lot, but my mechanic describes them as "shakers."



That said, in the 8 years that I have owned them they have been the best marine engines I've ever run, inboards, outboards, gas, diesel, whatever. I love them. The same mechanic says that they are among the most trouble free engines he works on, including all the Cats, Cummins etc. He thinks it is one of the two or three best engines Yanmar has ever built.


I'm not in any way knocking the 6LYA, just passing on my experiences.

 

[Ski in NC]

Any four-banger will be rougher than a straight six. Mid range the four is ok, at high rpm the second order vibe comes into play if there are no balance shats (which the 4lh does not have). At idle the four is rough. With good mounts, the four can be tolerable.

 

[SILENTKNIGHT]

Yes, your info is very credible. I just did a survey on a 4LH. Rough at idle, and at backdown....but smooth underway....although motormounta were needing replacement and cutlass bearings dry rot. Going to see a 6LYA today to compare.
I had a 6LP on a prior mainship 30 pilot and was very smooth at idle...

 

[bayview]

SKI I have always heard that 4 cyl diesels are shakers but 4 cyl gassers are common in cars and dont seem shaky or vibration prone. Any idea why?

 

[Duetto]

most modern 4 cyl car engines have balance shafts to eliminate the shakes.

 

[rslifkin]

The vibration issue also gets worse as you make the engine bigger. Most modern 4 cylinder car engines are fairly small displacement.

 

[keith c]

I have twin yanmar 4s. They do vibrate at idle but stop at 1000 rpms or when engines are at running temp. I had motor mounts replaced/ no change.

 

[Nomad Willy]

With twin engines there is another vibration that often is very annoying. The harmonic vibration caused by the difference of frequencies of vibration that the two engines create.

It has a “reverberating” sound much lower in frequency. Kind of a whum whum whum ... Any twin engined vehicle will generate this harmonic vibration. It’s like the two engines are harmonizing w each other. They are. Hence the name harmonic I’m sure. The only way to fix or vastly decrease the harmonic vibration is to run both engines at EXACTLY the same rpm. And that’s the job of the engine synchronizer.

 

[High Wire]

... The only way to fix or vastly decrease the harmonic vibration is to run both engines at EXACTLY the same rpm. And that’s the job of the engine synchronizer.

...or the human ear.

 

[Ski in NC]

SKI I have always heard that 4 cyl diesels are shakers but 4 cyl gassers are common in cars and dont seem shaky or vibration prone. Any idea why?

There are two vibe scenarios on four bangers: At idle the firing impulses are far enough apart (180 deg) that at low idle engine shakes side to side. On cars, they really refine the motor mounts to make it as smooth as possible. Idle rpm is also usually higher than the marine engines, so that makes it smoother. And smaller engines make less shake.

The second vibe is a second order at higher rpm. This is because the piston motion does not really cancel the inertial forces out exactly. One would think two pistons going up would cancel the inertia of two going down. They do... mostly. But due to the geometry of the crank/rod, when the crank is at 90deg from tdc, the piston is not half way down. It's a little more than that. This vibe is 2x crank rotation frequency and goes straight up and down. Worse on larger engines, and worse at high rpm. Some larger 4-bangers use twin counter rotating balance shafts running at 2x crank speed to get rid of that vibe. But the balance shafts do nothing for the idle shake.

Take a Honda Civic and have someone hold the pedal to keep steady 4000rpm. Extend your index finger straight down and touch the top of the cam cover. You will then understand the second order vibe of a 4-banger!

The 4LH is the nicest running larger 4-banger diesel that I have been around. They use very soft mounts to handle the idle shake, and the second order vibe really is not offensive even without balance shafts. Not sure how they accomplished that.

 

[DavidM]

Thisi is my experience with different engine configurations/cylinders:

1 cylinder- Pretty rough at all rpms
2 cylinder- About the same
3 cylinder- Fairly smooth. My 3 cylinder turbo Mini is really smooth
4 cylinder- Rough at low rpms, smooths out at higher, better with balance shaft
5 cylinder- In between a 4 and a 6
6 cylinder inline- One of the smoothest engines made
6 cylinder 90 deg V6- More like the 5cylinder, a little rough
6 cylinder 60 deg V6- Smooth like the straight 6
8 cylinder inline- Don't know, never drove one
8 cylinder 90 deg V8- Very smooth, like the straight 6
8 cylinder 60 deg V8- Don't think they exist

David

 

[Nomad Willy]

David,
Re the eight cyl in-line I have owned many Buick straight eights.
Smoothest engine I ever had. One can go around a 90 degree corner in 3rd gear (top gear) and all the way around the corner .. in the city at a customary speed (15mph?) ..... perfectly smooth.

 

[Shrew]

I have a 6LPA-STP2. No vibration issues.