Balancing an engine

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Coyote Jack

Coyote Jack

Member #55, I'm old
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I have small blocks so I figured I might as well ask this question here.
We all know that our engines came from the factory with the rotating assembly just bolted together. No special balancing.
My question is. Under what conditions is it required to get the rotating assembly balanced? ie: over a certain rpm, over a certain HP

Jack
 
I don't believe I've never heard a benchmark for when a engine needs balancing.
I would use a horsepower/cube as a guide or whenever you change anything in the rotating assembly. When I rebuilt my engine, the +.030 Egge pistons were much lighter so I had it balanced. It would have ran good without balancing but I wanted it to be done right. tmm
 
In my opinion, every engine, regardless of use, needs to be internally balanced. Factory "balancing" consists of bolting parts that are within a wide tolerance together. It's that important in terms of power prodution, life span, and smoothness.
 
factory balancing is ball park at best,and can be way off,add in replacing pistons,rods, dampener,and its like putting a heavy wheel weight on the rim,vibrates like mad,sooner or later,most likely sooner,it will let go,so do it right,and get it done,Mrmopartech
 
Coyote Jack said:
I have small blocks so I figured I might as well ask this question here.
We all know that our engines came from the factory with the rotating assembly just bolted together. No special balancing.
My question is. Under what conditions is it required to get the rotating assembly balanced? ie: over a certain rpm, over a certain HP

Jack
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Take it from me and look at my threads. Balance the motor at all costs or you "may" regret it. Whatever it takes!
 
The big thing that needs to be considered is way back when that crank was built/balanced and kicked out the door standards ie. gram/inches tolerances and the machines used to do the balance have changed alot compared to todays standards and equipment accuracy. Maybe not considering what the factories today still kick out but the aftermarket world has improved a lot. Basically in most cases cranks today have improved with the advancements. I had a 440 crank check balanced 2 years ago and you could tell this was a stock crank, hadn't been touched since built and the balance was out to lunch by todays standards (hell probably even 60's standards).

Quick answer, getting it done is cheap insurance.
 
Just went through this with my Buick 3.8 Series II supercharged motor. The crank was out about 18 grams on one end, 22 grams out on the on the other. Would have spent the money anyways (to run the double-roller timing set you have to dump the balancer because it is run off the cam. It is in the lifter valley...), never have done a motor without doing it right.
Best core motor I ever bought? A 410 Mercury Super Marauder that had been in a flash flood. When we broke it down, Jaime and Larry found that the weights of every reciprocating part was within a gram of the others (pistons, rods) and the crank was balanced perfectly. It had obvious been gone through, but it is one torquey bastard!
 
This was not a case of am I going to get it done. I am. I was just wondering if there was any set rules as to when to have it done. I have never heard of any.

Jack
 
moper said:
in my opinion, every engine, regardless of use, needs to be internally balanced. Factory "balancing" consists of bolting parts that are within a wide tolerance together. it's that important in terms of power prodution, life span, and smoothness.
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x2
 
Coyote Jack said:
This was not a case of am I going to get it done. I am. I was just wondering if there was any set rules as to when to have it done. I have never heard of any.

Jack
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I had to have mine balanced when I went from external balanced to internal balanced bottomend.This was on a 77 360LA going to a 408ci with Eagle stroker kit.
 
I'm not sayin balancing ain't important here...so don't pile on. But yall tell all that "you gotta balance it" stuff to them Saturday night dirt track boys. LOL
 
StrokerScamp said:
I'm not sayin balancing ain't important here...so don't pile on. But yall tell all that "you gotta balance it" stuff to them Saturday night dirt track boys. LOL
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I'm not in a dirt track area up here, but I can tell you the winners on the loacl short tracks ALL have special balancing done on the primary and backup engines. The shop I use has had multiple track championship engines for years. It's almost 50% of his business. If they aren't spending on it, they are missing out on some free power, and a little jump off the corners.
 
I always balance mine, I figure for the extra couple hundred bucks, if it runs smoother, gets a few more HP, and lasts a bit longer, it is money well spent.
 
Coyote Jack said:
I have small blocks so I figured I might as well ask this question here.
We all know that our engines came from the factory with the rotating assembly just bolted together. No special balancing.
My question is. Under what conditions is it required to get the rotating assembly balanced? ie: over a certain rpm, over a certain HP

Jack
Click to expand...

For me crankshaft rotating assembly balance is NOT important, you only waste time and money for it.

Before the engine assemble, engineer has calculated how part will be made, weight, length..etc to make sure that when they put them together, they will achieve the BEST balanced. As long as you re-use the crank and rod, even you has your cylinder bore and use a new piston, this will not effect bottom end.

Think about it, the motor have 8 piston and firing order go by 1 8 4 3 6 5 7 2 , ASSUME at every 90° angles, there are explodes stroke occur, so that mean the crankshaft does not turn freely but force by combustion 4 time in 1 revolution(360°) , how do you "mechanically balance" this? This can only be done by engineer by using math and physics formula to calculate at which angle(position) the crankshaft will has much/less force, then balancing the whole thing by drill a hole in balancer . The machine shop can only balance the bottom end when crank is free, no force, its a joke.
 
SuperStock said:
For me crankshaft rotating assembly balance is NOT important, you only waste time and money for it.

Before the engine assemble, engineer has calculated how part will be made, weight, length..etc to make sure that when they put them together, they will achieve the BEST balanced. As long as you re-use the crank and rod, even you has your cylinder bore and use a new piston, this will not effect bottom end.

Think about it, the motor have 8 piston and firing order go by 1 8 4 3 6 5 7 2 , ASSUME at every 90° angles, there are explodes stroke occur, so that mean the crankshaft does not turn freely but force by combustion 4 time in 1 revolution(360°) , how do you "mechanically balance" this? This can only be done by engineer by using math and physics formula to calculate at which angle(position) the crankshaft will has much/less force, then balancing the whole thing by drill a hole in balancer . The machine shop can only balance the bottom end when crank is free, no force, its a joke.
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This is exactly why I asked the question. I have seen a movie or film of the engines being assembled at the factory. They showed workers assembling the rods and pistons and then other workers inserting the rods and pistons one bank at a time on a V8. These guys were just picking up whatever piston or rod that was closest to them. They were not matched to any specific crank or to each other for that matter. We all know that factory quality control was not the best for these cars.

Jack
 
Moper described it best, that weight/balance tolerances are pretty wide in OEM applications, but were "acceptable", especially for everyday street use and lower rpm's. But they are likely better now. Just because you can't control every tiny detail, like crank twist, doesn't mean you should just let it all go. You can drive around with unbalanced tires. Some may be off enough to feel the vibration and some may not be enough to feel it. But it's not as good as it could or should be. Imagine spinning a top with a little extra weight on one side! Then remember that engines can go 6,000, 7,000 or more rpm which magnifies any imbalance considerably! Matching piston and rod weights as well as balancing the crank counterweights to them can only help HP and reliability. Any serious race engine has it done and it's a good idea for any performance engine, especially when parts start getting changed. If you can find a dependable shop and can afford it, do it.
 
Engine balancing is about reducing harmonics to as low as possible. There are no uncontrolled explosions unless the engine is pinging or detonating. It's a controlled burn with a comparatively slow pressure rise and conversion of that energy to mechanical force. Cranks twist and "un-twist" with every firing event, compression event, and change of direction of the piston and small end of rod. The force of a tiny amount of unbalance goes up exponentially with rpm. My math may be a litle off, but looking at formulas for centrifugal force.... A .1 gram out of balance of the big end of a rod on a 4" stroke small block, becomes close to 85 grams on a 2" radius at 5K rpm. now picture some rods being equal to the engineered tolerance, some being as much as 18 grams off... Then couple that with a crank whose casting left heavy spots... It ends up being a lot of stress the crank and block don't have to deal with. Like Chief says, any energy not being spent on the tires, is being absorbed by the other parts and wearing them faster. That's why even cast crank externally balanced factory displacement engines get internally balanced when I do them.
 
It all boils down to: Do you feel lucky. I've built lots of engines. Some people don't have the money to balance an engine and they run from poor balance to good balance. Every engine I've had balanced is very smooth. I'll wait for the money to get all of my engines balanced. It easily pays off in quickness and longevity. Is it worth it? Do you want to tear down your engine again to have it balanced if it has too much vibration?
 
This is not a case of whether or not I am going to get my rotating assembly balanced. I have a stock 340 forged crank, aftermarket forged pistons and forged H-beam rods are coming. This is a high HP application and I will be getting it balanced. I have just never heard of when you should and when you don't have to, and was curious. Obviously with the responses in this thread it was a good question. hehe

Jack
 
SuperStock said:
For me crankshaft rotating assembly balance is NOT important, you only waste time and money for it.

Before the engine assemble, engineer has calculated how part will be made, weight, length..etc to make sure that when they put them together, they will achieve the BEST balanced. As long as you re-use the crank and rod, even you has your cylinder bore and use a new piston, this will not effect bottom end.

Think about it, the motor have 8 piston and firing order go by 1 8 4 3 6 5 7 2 , ASSUME at every 90° angles, there are explodes stroke occur, so that mean the crankshaft does not turn freely but force by combustion 4 time in 1 revolution(360°) , how do you "mechanically balance" this? This can only be done by engineer by using math and physics formula to calculate at which angle(position) the crankshaft will has much/less force, then balancing the whole thing by drill a hole in balancer . The machine shop can only balance the bottom end when crank is free, no force, its a joke.
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Maybe if your using all factory parts you will have no problems but if you change any part of it I would balance it. I used lighter rods and pistons so I had the whole assembly (harmonic damper, crank, flywheel, rods and pistons) balanced. Those parts bins contain rods and pistons that are within 5 grams of each other by design. So in effect, the engineers pre balanced them.
 
there is no doubt that an engine that has been balanced is going to be better.and yes it should be dun but if you are buying aftermarket parts i'm sure there within a certain degree of balancing.and could be run.I have dun it.and you could tell a deference.buy the way it revved and run.wen racing it.because that same motor was taken apart and balanced only with just new bearings and run about one tenth quicker.so if I was going to do any hard thumping on it. It will be worth the extra money spent.talk to a class racer every motor they build is perfect in every way that's why they can rev higher and last longer if and wen they blow a motor its usually a part failure.....and thats my 2cents worth if its any help..Artie
 
Heres one for you all, the engine shop I had my stroker built at: Baril Engine, had a project they did for Schnieder Trucking, (big orange tractor and trailers) and maybe some of you know how big they are, anyway, the idea was to tear down an engine and balance it and see IF it got better mileage, well the diesels were getting approx 4-5 mpg before the balance and they got 6 to 7 AFTER the balancing, that might not seem like much but when some of these trucks go over a million miles that is HUGE, so what did Schnieder do?? well they did it to 10 of their fleet then quit, don't know why but thats big business. Not really answering the op's question but I thought I would give my .02
 
I "grew up" in a machine shop, and one of my jobs was all the engine, drive line balancing. So, of course I'm a firm believer in balancing. Even the 401 I built for my Jeep is balanced, and it's built to redline at 4300 RPM's (it dyno'd at 4288 ), all bottom end power.

But what nobody has mentioned yet, and I'm just throwing it out for your own information: a dollar bill, a twenty, fifty or one hundred dollar bill weigh one gram.

Just some trivia.

Also, when balancing an engine, you weigh the oil that is in the throws of the crank rod journals in computing your rotating bob weight.

Russ.
 
The factory weight balanced the parts to be within an acceptable range of balance.
I cant say wether or not they balanced each motor individually, i wasn't there...


Most if not ALL 'stock replacement pistons',bearings etc.. are all based on factory part weight, so essentially they can be bolted in minus 'no rebalance'-clevite pistons 'example'

For stock or race, balancing is a good idea ....and we all know even the factory balance job wasnt all that, and only up to around 4800, maybe.lol
 
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