I got into a similar debate about floating vs. non-floating pistons. At what point does it make a difference?

At 8,000 RPM, the engine is revolving 133 times every second. 1.33 times every 100th of a second.

Take a stopwatch and try to stop it after 1/100th of a second. I challenge you. I used to do it with my friends as a kid. I think my record was 0.03 seconds as the fastest I could physically hit the buttons. The engine would have spun around 4 times by then.

Fast.

I cannot tell you for 100% absolute certain that an engine WOULD detonate if 24# out of balance at 8,000 RPM.

I CAN tell you for 100% certain that if it is MY $6,000 engine, I would spend money to make sure it is AS IN BALANCE AS POSSIBLE before re-assembly. I would and I did. $425.

But I'm also the fool that invested in full-floating rods, so what do I know.

 

What's with fools investing in full floating rods? I've apparently not heard this debate.

 

All I know is that my engine is the sweetest running, fastest reving and quickest of my mates/clubs engines and i'm the only one who took balancing seriously.

I had a race with my mates c5 z and he was shocked how quick my car was for a mild 355.

Balancing seems to be thought of as something that only race cars need, not so. i think people assume new parts are balanced because it says 'internally balanced' etc, that just tells the builder HOW to balance it.

People probably spend more money getting their wheels balanced, than their engines

Therefore I think it's impossible to build an engine at home (unless you're building a stocker), give the parts to a shop and get them to lighten/balance and assemble the short block, get it back and do the rest.

It's not much $$ on top of all the parts and the end result is a sweeter engine that you can feel where your $$ went

 

 

Apologies. Different thread.

I was involved in a heated debate about the extremely small frictional differences betweed full-floating and press-fit rods. I argued that while the frictional difference are, indeed, very small, when they are multiplied by speed and time, the cumulative impact is very significant.

Not all believed my engineering/fluid dynamics based argument.

This feels like that. A very small weight issue can get exponentially amplified by the speed and the distance from the center of rotation. A small problem is a big problem.

Balancing an engine is a wise investment. There just isn't any getting away from that fact.

 

Flinging #24 around 133 times a second will cause some stresses, twist and harmonic vibration your calculation is totally not taking into account. The forces compound as RPM increase. Put a pair of coveralls in your washer. Wash them. Everything is fine until the spin cycle starts. All of a sudden you here a weird noise coming out of your laundry room. It is increasing in intensity and volume. You get up and run over the the laundry and open the door. Your washer is jumping around and banging like crazy. It is unbalanced to the point your drum is bouncing off the side of the washer case. You lift the lid, Redistribute the coveralls and close the lid. It spins up smooth as silk. you just seen what happens when the unbalanced forces compound as RPM increase. Harmonic vibration intensifies it further and how the closer the balance is the smoother it runs. Now spin that washer up to 8000 RPM and see what happens, Even though the load is redistributed it will still start a harmonic vibration and start banging again at a certain RPM. Ask Gkull about breaking a crank. He snapped a forged 4340 crank in half in the middle a couple years back. Harmonics do weird things like cause microscopic stress fractures over time that turn into catastrophic failure prematurely. Here is a high speed video showing what compounding harmonic vibration will do to a spring. 1000-6000 RPM(Spring Surge)http://high_speed_video.colostate.ed...pm_1000fps.wmv

 

I do it makos way when i'm looking for serious hp.its a very time consuming and tedious job.
however i think its a waste of time and money to do it on a cruiser or a DD,same with balancing, the way it was built 30 or 40 years ago was more than adequate for a long and trouble free life with the ocasional blast to redline.

 

full floating rods are your best friend when you are blueprinting an engine,at least in my experience the pistons tend to get juggled from hole to hole and from rod to rod pretty often.

 

I didn't see anything odd here either.

 

Most of the engine stress is due to moving the entire mass of the piston/pin/rod at that high rpm state. But, the amount of out-of-balance results in vibrations/pulsations that result in further dynamic complications. If you are going to run an engine up to 7000-8000 rpm and have it FEEL like it won't throw craps on you....smoother is better.

If you're building an engine that will turn 6000-6500 and using quality parts, using the parts "as is" in the build might be fine. But stresses increase logrithmicly as rpm's increase. It become "iffy" at 7000 and up if the rotating assembly isn't balanced in some reasonable manner.

 

Good Info guys!! I definately learned something new..
Thanks

 

Thanks for the info! I was 90% sure that I need to go ahead and have the assembly rebalanced with the flywheel, etc, but wanted to be 100% sure. But I also learned something new, so I declare this thread a win!

 

Those engines built 30 or 40 years ago were all internally balanced from the factory as well. As I said there is a tolerance. The tolerance on externally balanced 400 SB and 454 BB were felt to be not as critical because they were originally designed to be used in Trucks, Motorhomes, Ect, Low RPM, high torque, For hauling loads and towing (LS6 was an exception not the rule.). The solid lifter engines were better balanced, forged cranks, bigger dampers, ect.

 

First, I am a big believer in balancing my engines. Once, when I built a 406 many years ago (my first build), I didn't have it balanced. After driving it with an annoying vibration for several months, I tore it apart; had it balanced and was very pleased with the smoothness.

On all my later builds, I weigh all the rotating parts (including the big end of the rods), and weigh all the reciprocating parts (including the small end of the rods), and adjust each component to the lightest of the group. I then calculate the bobweight and furnish that result to my machinist to use that bob weight when balancing the crank. If it's external balanced, I furnish the HD and the flywheel/flexplate. If it's neutral balanced, I don't include the HD or the flywheel since I want the crank itself balanced rather than the end pieces because these pieces may be replaced in the future and I don't want to lose the balance if I replace a flywheel - after all, a neutral balance flywheel should be neutral - if it's not, it's not the brand for me.

Above, we saw the calculation for a 2 gm rotating force, but when looking at the response of the engine, one must consider the dynamics of the entire engine as a unit. Further, a rotating forcing function will have the greatest response at the resonant frequency of the engine system in its mountings - at frequencies higher than this, the amplitude actually decreases. [As an example, that is why an unbalanced tire will have a peak vibration at say 70 mph, and then decrease somewhat at higher speeds.]

Anyhow, if one looks at the aforementioned 2 gram imbalance, and with the knowledge that at rpm higher than resonance, the center of rotation will be around the cg of the rotating system. So, in the 2 gram case, let's assume that the imbalance is at the #1 crank journal... Since a SBC crank weighs about 56 lb with an additional ~2 lb of rod big end weight at each journal, we have an approximate weight at the #1 journal of 64/4 or 16 lb. Further, since the 2gm is 2 inches away from the centerline (with a 4" stroke), we are looking at (2/454) lb x 2" or .009 in-lb. To find the cg offset, divide .009 in-lb by 16 lb, which equals .00056". This means that the instant center of rotation will be offset by barely more than half of a thousandth of an inch, which is well inside the main bearing clearance. Bottom line, at high rpm, a 2 gm imbalance will be insignificant.

 

Intersting stuff

Well I took the vette out yesterday after a couple of months of not driving her and WOW she's fast and the engine is beautiful, flawless acceleration, smoked the tyres in 2nd for a fair distance before she gripped (with 285 tyres) and into 3rd flat out and 4th flat out, then 5th.
And the EBC Yellow Stuff brakes are light years ahead of the crap that was in there, I can brake sooo much later and harder, haven't locked them up once yet.
The TKO 600RR ( close ratio fifth 0.82) is also a great box now that it's worn in, never miss 3rd now, and I can change a lot faster and more accurately than with the Muncie.

I can now say I have a Vette that keeps up with most modern fast cars, it steers, handles and brakes like a Vette never did.

Just gotta change the shocks and i'm done with the mechanicals and can finally get the thing painted

I don't know much of the tech stuff associated with lightening/balancing but all I know is my engine is a beauty for it

 

Just been researching engine balancing, and found this thread. And from what I can see here, and other places, it's well worth pulling the trigger and going for it. I have a stock crank and rods with new 0.20 cast flat top pistons from Northern Automotive. The last thing I need is this.......

 

Thanks for the visual backup for post #26

 

That's impressive.

 

Thanks to that harlem shake video, my life is now complete.

 

And here it is again with a cartoon face