Thread: balance the motor or not?

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Originally Posted by pro70z28

Your not the first one to mention that. If I eat right though & in moderation I never fall down, but sometimes I learn toward junk food.

sorry my down fall is soda pop

Smokey Yunick in his book Power Secrets- "Frankly, I think engine balancing is anything but an exact science, and you don't need to get upset if things are off a little.
There's no such thing as an engine that is perfectly balanced at all engine speeds. You can determine a recommended balance weight according to the formula that the crank manufactures use, and pick a crank speed-say 5000rpm- to put in the formula with a different crank speed-say 7500rpm- the recommended weight will be different. So, at best you can only gain a "true" balance within a very narrow crank-speed range.
And from what I've seen, this true balance is at best a approximation. When you look inside the oil pan as the engine is running at high rpm, there is a tremendous amount of oil caught up in the windage from the spinning crank and wrapped around the crank throws. This oil has a lot of weight, and as the engine runs, this oil will migrate all over the assembly. Sometimes it hangs around the front throws, someetimes it is in the middle. The pattern is totally random and the size of the oil cloud can get larger or smaller. All of this happens in a test engine sitting on a dyno, so who knows what it's like when the engine is in a moving car and you have all sorts of front, rear and side loads in the picture.
This oil cloud must affect the balance of the operating assembly- and there is no way to account for it- so I little reason to worry much about engine balance. If you have to replace the pistons in an otherwise intact assembly or you have to do some machining or grinding- as is occasionally the case when you change a cam and a little more piston-to-valve clearance- I don't think you have to go to the trouble to rebalance the assembly if you can get the weight on the piston end of the rod within about two grams of the original balance weight. If you can get the replacement weight slightly less then the original balance weight, I'm sure you won't have any trouble. However, if things get out of hand-more than about three grams difference- you should get the assembly rebalanced to be on the safe side.

Wheelkid,
First, though you did not mention being new here, welcome to CHR. We look forward to hearing about your projects and seeing what you're working on as time goes by.
On balancing, it sounds to me like you're focused on the balancing of the reciprocating weight - the pistons, pins & rings, but not really considering the rotating weight and the dynamic balance. From my limited knowledge on the subject, the basic balance is three pieces - the reciprocating weight; the rotating weight or "bob weight"; and the oil weight, assumed to be 4 grams. Here's a link to a Ford Muscle overview that I found quite helpful in understanding the process - Engine Balancing and Crankshaft Balancing - FordMuscle
After the engine is balanced the flywheel, clutch & pressure plate are generally balanced, too, so that the entire assembly is considered. Once that process is complete, if you have to modify things later then you are working from a balanced starting point, and it's generally not necessary to re-balance the entire assembly - you can simply work from the base numbers, and modify your replacement parts to match, like changing out a piston you can match the weight of the one removed, as close as possible. No one is going to argue with you about the distribution of oil being less than perfect, but it's the starting point that's important. If you have a few grams of oil moving around would you rather it be from a very closely balanced base, or is it OK to have it being added to an imbalance of ten, twenty, fifty or more grams that's causing the crank to flex? I'd rather start from a solid dynamically balanced base, giving a much smoother engine. Others who have more experience in this arena may shoot me down on this, which is fine. I love to increase my understanding of these processes!!

Reading the above regarding Smokey.

1. Smokey's time was in the 60's /70's, equipment has come a long way since then;

2. Smokey mentions weights in grams. How would you know the weights? Unless it was disassembled and each part of the rotating assembly weighed!

In Smokey's defense of the above, he's already done the heavy lift and is now discussing maintaining a given balance. (IMHO)

In regards to the balance variable shifting in the rpm range. This is called harmonics and is a college course study in itself! Notwithstanding this fact, it is still far better to make every attempt to cancel out any vibration(s) for added longevity / reiability.

Seems Roger types more words... but alot faster!!

Originally Posted by 34_40

Seems Roger types more words... but alot faster!!

Mike, my thought was that you said much the same thing much more efficiently!!

OIL ? this is abit of a joke with some of my freinds we know were the oil is THE OIL PAN ? just not were it.s at 8000 grand and harmonic s first post on this site i realy pist some guy who is not on hear any more about harmonics frequencys I was not going to go there in a small post and what happens in a oil pan or cylinder bores . rods big ends piston pins. push rods can flex go out of round ,aluminum heads were is the seat when the heads hot ? all things about can move from power . heat . distortion and at times harmonics . from were bolts are when tighten up some things we do things to insure stuff stays round when you bore or hone a block but we can not use the real heads . we can not bore and hone the engine under load but many do the best we can do . some way better then others smokey was one but he was off abit off on the 10 pound rule . same with the crank we do know oil ropes the crank we try to stop this . but at the end of the day you do the best you can and get the mass balanced. it will affect the engine very low speed if off alot. at the end of the day with all new parts we try to get them very close add for some oil on parts. pistons are alumiunum takes alot to throw a crank off for a fly cut for valves after going to a bigger cam .pistons they all are going to be even with same weight . as they were balanced before you cut them?? better to have a crank heavy then to lite so pistons you can go less gram piston all in match sets .but not one or 3 pistons heavy there ways to make things work if you get in to a engine that lost one or 3 pistons should buy a new set but it happens were the new pistons will not be the same weight piston pins can use a heavy or thin wall pin .pin end of the rods can be re worked . every part of a engine and action in it self is a science. the way we machine a engine is a science . the more you know the less you know about whats really going on. many know nothing

Originally Posted by pat mccarthy

OIL ? this is abit of a joke with some of my freinds we know were the oil is THE OIL PAN ? just not were it.s at 8000 grand and harmonic s first post on this site i realy pist some guy who is not on hear any more about harmonics frequencys I was not going to go there in a small post.

every part of a engine and action in it self is a science. the way we machine a engine is a science . the more you know the less you know about whats really going on. many know nothing

Charlie, you upset someone?? Nah.. don't believe it!!

And sometimes.. ignorance is bliss! HAH!!

So--------------while you guys been talking on the web about it---I've balanced 3 engines plus going out of town for 3 days--------

but-----------keep in mind that the oil going arond with the crank is ain a fluid condition,and will not affect the balance of the mass of solid materials

Originally Posted by jerry clayton

So--------------while you guys been talking on the web about it---I've balanced 3 engines plus going out of town for 3 days--------

but-----------keep in mind that the oil going arond with the crank is ain a fluid condition,and will not affect the balance of the mass of solid materials

OH and i have not ??? your not the only guy on here that like hitting his head on the wall Jerry ..oil hanging in the wrist pins .. oil can fill drill holes in the crank and gets thrown out ?. oil is still weight i know were splitting hairs

Originally Posted by 34_40

Charlie, you upset someone?? Nah.. don't believe it!!

And sometimes.. ignorance is bliss! HAH!!

charlie ? mccarthy Thanks you knew my greatgrand dad from Cork Ireland. or are you saying i am wooden dummy with someones hand up my back side ?

http://www.me.cycu.edu.tw/~merc/%E8%...Crankshaft.pdf

Crankshaft Balance Factors

http://www.me.cycu.edu.tw/~merc/%E8%...Crankshaft.pdf

Originally Posted by jerry clayton

http://www.me.cycu.edu.tw/~merc/%E8%...Crankshaft.pdf

no go joe... let me think it a post saying oil has no weight ok its working

Don't ask me any questions--I didn't read them--I was looking for something else and decided to try and copy/paste something------

Is all the snow melted over there yet????

Is this new guy from Michigan??? there is also a recent new guy posting similar Smokey stuff at Club Cobra however under different name/location (Mo) could be mis typed