Crank weight vs harmonic vibrations.

[Thad]

OK, so it not a Ford six but this is where intelligent motor head hang and it is a big Ford four. (Hey flattery works)

Planning for the Merc 4 banger that I mentioned before.
Seems big cube four cylinder engines have an inherent harmonic vibration getting strong at about 3600 rpm. This vib is not a balance issue. Some big cube 4's used a balance shaft to counter this. This fix is not practical for what is planned.

My question is what effect would crank weight, less or more, have on the magnitude of these harmonic. The crank would be balanced as close to zero as possible.

Would reducing or pendelum cutting the counter weights increase or decrease the magnitude of the vibrations? Or would increasing counterweight be the answer?

Ball is rolling, somebody kick it?

 

[PaulS]

The harmonic vibrations are induced by the firing of four pistons in two revolutions at 180 degrees. The only way to get rid of the problem is to modify the crank/firing sequence (not firing order but the number of degrees between the firing of each cylinder).
That can't be done well without adding more cylinders. If you use a very heavy flywheel and a 90 gegree crank and an appropriate cam and distributor you can fire all four in one revolution and then let the engine coast through a full revolution but that leads to a shudder between coast and firing revolutions.

 

[80broncoman]

Many fours don't use a balence shaft or a harmonic balencer at all.
Just look at the old pinto 2.3.
I'd try to look into what roush used on his 3.0 and larger fours back in the 80s and hope that you can afford the same thing. I've heard fours have less problem with crank twist from imbalence be cause the cranks are shorter, stiffer, and have less points of stress.

 

[Lazy JW]

More weight will help smooth it out but hinders acceleration.

All inline fours have an inherent vibration problem as mentioned above. Many millions of these have been produced and run smoothly enough but they accomplish this by simply adding weight and limiting rpm; just look at tractor engines, Model T's, Model A's, etc.

Those old Pintos could sure shake at certain speeds
Joe

 

[Stubby]

Weight helps. Mercury's solution was extra heavy crank and hang a prop on it.

ATI makes the best damper on the market. They might be able to help tame the beast.

When you think about harmonics and balancing a crank, start by thinking about a single cylinder. The crank is balanced with 100% rotating and X% reciprocating. The X is a variable based on intended usage, rpms, and smoothness. You can somewhat balance an engine for rotation. However the inertial loading and cylinder firing create problems.
Most engines are overbalanced. 100% rotating plus x% of reciprocating. If the bob weight were based solely on rotating mass it would be balanced.
We have to add part of that reciprocating stuff and it gets crazy. Part of the recip is rotating in a circle, this part is from at or below the crank center line then it swings way out beyond the center line. This causes an oscillation of the crank. Add in the inertial loading from the piston and pin, and fire the cylinder every other stroke and it is a wonder it all holds together.

If the crank is heavy it will dampen the osculations. Heavy pistons at low RPMs will also dampen things.

Now lets play Hot Rod. Lighten the pistons and we help inertial loading. However we also add compression and fire it harder. If you lighten the crank it will also affect the osculations.

Remember the one cylinder engine? If we take four of these mounted inline and oppose two of the cylinders we will have a crank that will spin by itself and be balanced. That does not mean it is balanced to any particular bob weight. This is where most engines get away from the engineered path.
If you look at one throw of the crank and consider the factory bob weight for that counterweight. If we offset grind the crank for a longer stroke, install lighter pistons and rods, or any number of other things, we change the balance of the engine. Not only did we change it, but we have no way of balancing it properly. Our inline engine has opposing mods that cancel out each other.

The good news is, most engines will tolerate overbalance. This means they have more counterweight than they need. This is desirable in several forms of racing. Mostly where acceleration is needed. How much is good? I don't know.

I spent many years balancing race engines to a theory. I did this on a Hines digital balancer that came with a manual that explained this theory.
So remember, when someone talks about balancing engines, they are talking about a theory.

 

[XPC66]

Isn't the imbalance from the rapid unloading of the crankpin when the piston goes on blowdown? This causes the pin to flex and rebound, setting up an oscillation that can get out of hand. If you knock metal out of the counterweight, expect the thrower to deform more easily.

The problem I had on taking the balance shaft out and having the crank balanced and bullnosed/tapered was that there was an rpm band where it just wouldn't behave. I learned from that and just leave the shaft in place. There is still generally a need for a damper.

 

[Stubby]

This one doesn't have a balance shaft. It is a Mercruiser 224Cu IN 4cylinder.

With a little cam, it will shake the paint off the fenders. 8)

I agree with the comment about taking weight out of the counterweights, if you go under the proper amount for the application.

 

[Stubby]

These links will give an idea of what you are dealing with. I personal feel that most of the racing four cylinders with vibration problems could be better, if they would consider all of the dynamics.

http://www.prestwich.ndirect.co.uk/tech ... ancing.htm

http://www.modelenginenews.org/etw/etw_bal/p2.html

http://en.wikipedia.org/wiki/Straight-4

I don't believe all of the WIKI stuff. But it has some relative info.

After reading about how critical a single cylinder balance is, imagine how bad four of them at the same time would be.
Then picture one journal as a single cylinder engine. After changing pistons and rods and anything else that affects the mass of the throw, the counterweight is no longer the correct size. You can get by with doing a lot. There are people who are racing four cylinders all over the world.

I am just saying that if you want to get down to the real nitty gritty, you would need to cut up an old crank and make a one cylinder version of your crank. Then, stroke it, grind it, balance it and see exactly how much counterweight the engine needs. Then profile all of the counterweights to match. Then after a normal balance you would have a better crank. Technically this would require all of the surfaces to be consistently machined. Over balanced engines are better in most cases. I just don't think it is better to have a waaaaaay over balanced engine.

That being said.

Thad, I have a bad crank in my possession that I will ship you, or I can put it in the band saw and send you one cylinder. It has been outside so it has surface rust and would have to be fully prepped. If you could spark some interest in your favorite machinist he might be willing to absorb some cost of the education.

 

[StrangeRanger]

This one is a real can of worms.

The secondary imbalance of an inline 4 has nothing to do with crank weight or counterweight. It is due to the differences in piston acceleration away from BDC vs. away from TDC. Because of the geometry involved the acc. from TDC is much greater. The max values also occur at different distances from end of stroke. This sets up a vibration which occurs at twice the frequency of the crank rotation and cannot be eliminated except by setting up a counter vibration via a balance shaft. The only ways to mitigate this secondary vibration are longer conn. rods which reduce the difference (nothing whatsoever to do with our other ad nauseam discussion of the effects of conn. rod length) or lighter pistons and rods which reduce the forces imparted to the block by the diferences in acceleration.

And before you ask, no I6 engines do not see this same problem. They have a minor third order vibration which is generally imperceptible and can for the purposes of this discussion be ignored.

If you want the details on this pick up a copy of The Design and Tuning of Competition Engines by Philip H. Smith ISBN 0-8376-0140-1

 

[Broncitis]

The old chopped in half 460, I have an infinity for the unusual and looked at this engine many times for my bronco, I wish you well with this and hope to hear more, I think its a great idea and alot of torque for a 4 cylinder, it was very reliable for mercury.

 

[Stubby]

I did not mean to imply that balance is the cause of the four cylinder problem.

I just think more attention should be paid to the use of proper counterweights. I have never had a vibration in a four cylinder that I couldn't live with. This leads me to believe that some people are compounding a problem with imbalance problems. When I am faced with an obsticle that I can't get around, I tend to hunt all of the other things that I can use to my advantage. Basically, eliminate the obvious and see whats left.

I would think lighter pistons and rods, longer rods, heavy crank and flywheel should all help mask it to some extent.

 

[Thad]

Thanks all, Stubby I've got two cranks one is scored, so that's covered. The crank is hell for stout and heavy too. Every bit the same design as the 460 Ford. Wondered why and the feed back here explains.Have set of Engel BBC forged "H" section rods 6.8" vs the stock 6.605", planned on KB 132 piston which are lighter than stock.So accidently got two things right, longer rods and lighter pistons.When grinding mains to BBC journal size will destroke slightly to get compression height right, 0.010 below zero deck. So that should help some more.Actual crank balancing, friend allow me use of all his shop equipment and a Sun Super balancer is included. 1grm is easy and can take the time  to  get near zero.Now to calculate how much counter weight and how to measure.Thanks again.Regretable is all planning and calculating, got a lot to do after Ike.

 

[StrangeRanger]

I agree completely about not lightening the crank. The flywheel is a different matter. On the much smaller (1.6 litre Kent) Formula Ford engine, reducing the weight of the flywheel not only increases performance but also improves life by reducing stresses in the crank. I don't know how good or bad the Mercruiser crank is by comparison (I suspect much better, the Kent crank is old technology) but it is food for thought.

 

[Lazy JW]

The late Gene Berg struggled with vibration problems in the old air-cooled Volkwagens. He experienced problems when power output went up; one solution was running lighter flywheels and actually ADDING weight to the crank pulley. The VW's didn't use an actual damper, it was just a pulley and the added weight helped calm down the vibration nodes traveling through the crank.
Joe

 

[StrangeRanger]

Flat 4 engines don't have the same dynamics as I4s. There is no secondary vibration. Because of the left to right offset in the cylinders they do have a couple that sets up a slight rocking motion which IIRC occurs at crank frequency. A heavier pulley combined with a lighter flywheel would tend to make the resistance to motion equivalent on both ends of the engine.

 

[Lazy JW]

Yup. The rocking couple would wipe out the center main bearing on the VW's, those little crankcases just couldn't stand up to the stress. Gene also added counterweights (the stockers have none).

I think counterweights and a good, big damper up front will be about as good as it gets on the inline four.
Joe