Technical Balancing an engine!

I was just wondering about engine balancing, thought maybe some folks here could spread some enlightenment on it.
I am confused why in-line engines don’t need the same balancing as a V-8. Why does the angle change the balance? Also, to me, the weight of the piston has a different action on the crank at different spots on it’s rotation.
That is the weigh has a different effect at 0 and 180 degrees than 90 and 270 degrees on the crank, in my mind, anyway.
Are there any balancing wizards , here , that can enlighten me?






Bones

 

It is not an easy subject to tackle. The engine layout, crank layout, and firing order all have an effect on the balance requirements. In the end it all comes down to the math.

Wikipedia has a good article on the subject.

https://en.wikipedia.org/wiki/Engine_balance

 

All the forces on an inline are vertical. A V engine has them at 90 degrees to each other.

But the main thing is that an inline can always have the pistons up/ down, or 120 degrees apart, in matched pairs. A V engine can't do that, so it takes more effort to balance it, the crank counterweights have to match piston/rod weight.

 

I’m not a genius at this but I know side loads (v8) act differently then straight up and down loads (inline engine )

I know when I built my 261 I got it balanced and when I farted around with the flat 6 in my Chrysler it also has to do with how an inline engine fires , there is more overlap on combustion strike etc so it’s more balanced then even a 2 or 4 cylinder inline engine .

the 261 when I fired it up it did run ridiculously smooth ! I’m guessing an inherently well balanced engine now actually has a balanced rotating assembly is the best of both worlds .

I used new pistons amd he needed to shave them a bit , but I also went through about 3-4 sets of connecting rods until he came up with a set that he could match balance , a couple of the rods where off by huge numbers in grams !!

 

Thanks , Frozen Merc, that was quite a read! I learned a few things, but I will have to read it again after some of it soaks in! Lol





Bones

 

Does the crank shaft really know it’s side loaded or top loaded? Is a slant six MoPar, balanced differently that a Ford six cylinder?






Bones

 

Just as squirrel said.
In the inline 6 engines, for every motion, there is an exact opposite and perfectly canceling counteracting motion.
For some reason, the inline 4 cylinders don't enjoy this perfect balance. I don't quite understand why.

 

I think the straight eight engines are also inherently perfectly balanced. Yes?

 

Odd about that. My sister had a Vega in HS, was like a vibrating chair at idle. My dad took it to the Chevy garage in town and they pretty told him it is what it is.
Both of my forklifts have 4 cyl Continental engines, idle smooth like a sewing machine.

 

It's all about harmonics - ever notice how when a wheel is slightly out of balance it will badly vibrate at a certain MPH? The same thing happens with an engine that's out of balance... Somewhere along the RPM range, the harmonics will cause vibration and vibration leads to fatigue which leads to failure.

 

As I stated earlier, it comes down to the math. There is a Cosine function in the equation that describes an engines balance (see Wikipedia article for full explanation). A 4 cylinder inline is 180 deg apart, and due to that cosine function, it effectively behaves like two (2) cylinder engines and doubles the imbalance force. A 6 cylinder inline that is 120 deg. apart, through the same cosine function causes the imbalance forces to zero out. It is one of the mathematic wonders of trigonometry and trigonomic reflections. The same rules apply to single vs. 3 phrase AC electrical currents.

 

Bah, looks like someone has been messing with the Wiki articles, there was a good article on primary/secondary imbalance that was separate from engine balance, really good at explaining. Now it looks like some of the links lead to GDP discussions . Wikipedia Keyboard Warrior Muppetry.

Read up on primary and secondary order imbalance a few times to understand what is going on.
Piston count, cylinder count, crank pin count(shared or not), crank pin angle, and how many banks there are all affect engine balance.

Read the secondary order imbalance paragraph.
Piston speed in the cylinder is different if it is the upper(TDC) rotation vs the bottom half(BDC) rotation. Trigonometry and ish is involved.

Inline 4 cylinder, 180° crank pin engine, remove the head. Rotate the crank 90° from #1 TDC. All the pistons will be at the same depth in the cylinder bores.
But they are NOT halfway down the bore. This shows(visual trigonometry and ish) how one cannot fully balance this type of engine without outside components(balance shaft(s))
This is not a weight/balance issue, it's a difference in piston speed and location relative to crank angle.
Forces are different. Opposite but unequal.
This is why flat crank engines vibrate. *Exception to the rule is the Flat/Horizontal V engines, which is NOT the same as a Boxer engine(con rod pin layout).*
Advantage is the opposed cylinders statically balance each other(which makes for a lighter rotating assembly) but dynamically the forces in a spinning rotating assembly do not.
It's also why, normally, inline four cylinders were kept to ~2 litres or less. Creep past that size you will need to install balance shafts to maintain an acceptable level of vibration from the engine.
There are crossplane 4 cylinders, they are not as quick to rev, but the advantage is a smoother power output. I think they do this in some motorcycles for traction reasons.

Inline 6 cylinders cheat as they are really two triples in line.
I3 engine with 120° crank angle is inherently balanced. Each cylinder is 'chasing' the other, which makes for a smooth running engine. However this chasing causes the engine, as a whole, to rock quite a bit. Take another I3, place that crank 180° out of phase of the first I3 and the rocking motion is canceled out which makes for a suhmoooth running engine.
Want even more power and smoothness, add a second bank to compliment the crank angle, we get the coin balancing capabilities of a an idling and revving V12.

 

Scrounged up this good read on Piston Motion.
Fig 3.


Note that the conrod is 2" from centerline, indicating a 4" stroke crank.
It is at 90° from TDC, note the piston height is 2.337" down the bore. Not 2".
Upper 180° sweep(90°BTDC - 90°ATDC) has the piston traveling 4.672" (58.4%)
Lower 180° sweep(90°BBDC - 90°ABDC) has the piston traveling 3.328" (41.6%)

In a flat plane crank, two neighboring pistons traveling in opposite directions will have a significant difference in piston speed.

 

Not inherently. Here is a table of the 10 most likely straight-eight crank layouts from a 1927 SAE discussion of straight-eight engines:

This is the author's comment on balance:

Most volume production straight-eights were type IX or type X, which allowed for a five main bearing engine. Duesenberg has been criticized for sticking with the type III, which is two four cylinders phased 90 degrees apart, with the issues inherent in a four remaining.

 

Last motor I balanced flipped off the work bench. Shoulda used a 4x4 instead of stacking 1bys. A couple of tie down straps might have helped too

 

LOLOLOL

 

Interesting thread and links. Thanks to all you contributors!

 

I really don't think a crank, rods and pistons care whether they are straight up and down or in a V configuration. Balance is balance. Hang a heavier rod on there and see how smooth it is. Lippy

 

I think-and probably wrong- objects going up and down cause less vibration than opposing each other.

 

One thing that is easy to miss is that the piston motion relative to crank position isn't proportional. While the piston is at TDC at 0 degrees and BDC at 180 degrees, the piston is quite a bit below the half way point at 90/270 degrees because of the angle of the rod. Probably a minor detail, but things aren't always as simple as they appear.

 

The 90 degr of a V-engine has the two pistons of the same crankpin at completely different speeds, when one is at TDC, the other is near or already over max speed - this alone helps a lot to avoid vibes. High reving engines will need a higher balance factor than a slow turning motor. I balanced a lot of V2 motorcycle engines with only 50 degr cylinder angle and bore and stroke of 84 x 90mm. The tourers at 2500 to 3000 revs were really nice to ride at 35 to 38% whereas the race engines that were run up to 6500 revs needed at least 47% to be bearable. On parallel twins its completely different, you will need 80% at 6500 to make it tolerable for the rider.

 

@Boneyard51 what are you building? Inline 6 or V8? Are you swapping aftermarket parts in

Machine shop balancing is the best way to go, especially with lightweight piston or aftermarket rods or any major changes from stock
https://www.google.com/amp/s/www.motortrend.com/how-to/ccrp-0803-engine-balancing/amp/

If it's a stock replacement type parts build, you can do some static balancing of the parts. Weigh the pistons that you're removing. Make sure the replacement pistons are weight matched across the set. Weight match the rods - Needs a stand to compare big and small ends weights and overall. Only taking off enough to get a matched set.

None of this replaces a true spin balance, but it's sometimes surprising how unmatched the weights are on older factory engines.

 

I like this guys channel even though he covers mostly OT stuff, the same principles apply. He has an entire series just on engine balance and he explains it incredibly well with accompanying animations.

 

I thought the OP was asking about balancing an engine, not engine design and construction. Lippy

 

At present, not building an engine, just talking about engine balancing in general to promote discussion.
I do have an engine build in my future, a 462 FE, but it’s going to be a low speed torque engine. I think the rotating assembly will be sufficiently balanced for my needs. That will be determined later.
Back when I was building backyard racing engines , I used your method of balancing engine parts. I used a balance beam scale from my school. Back then I didn’t balance both ends of the rod, just the total weight. Figured it was better than nothing.







Bones

 

Yep, Better than nothing is correct. Stock replacement parts / driver engines.

462 FE, now that's interesting.

 

462 FE = 428 bored .030 with a 4.25 Scat stroker crank. I’ve got the 428, the three twos, the 65 long tubes, but no stroker kit….yet. Going to try to wake up my avatar car!




Bones

 

Well thanks to all the replies I now have a headache! The only thing I know about balancing is my first and last day working at a bank. I customer asked me to check her balance so I pushed her over...

 

Maybe Dana Barlow will join in I recall he used to Balance engines.