Early Hemi Crank Balancing

Something like thrust and coast !!! >>>>.

 

Model T engines had no counterweights. Engine ran fine. Car just had that up and down hop to it.

Balancing was ins***uted in later engines to achieve a comfort zone for the operators.

Usually power output is not affected by balance. Reliability can be depending on the balance factor and harmonics involved. Harleys have the hop due to the firing order as do odd fire V6's. All those engines are balanced at the factory to what the factory feels is correct for operator pleasure.

Also on the power stroke a piston now has a very imbalance in the form of high lbs of force load introduced into the rotating m*** that is way out of balance, like hundreds of lbs of force.

Harmonics rears its ugly head at various points in a hp application. A bit of NASCAR technology is that the counterweigts are as small as possible to cut down inertia drag for accelleration purposes off the corners, and balancing is done to control destructive harmonics in the power range, which can be quite narrow.

Harold Elliot, who was an engine builder years ago for Rusty Wallace used to cut the counterweights almost totally off and tuned the rotating m*** to achieve what he wanted in reliability.
I would balance the rotating m*** to whatever the balance guy recommends. Street is one comfort zone and race is another.

At one race, I broke a con rod in a VW engine. Engine still ran on 3, but the car hopped sideways almost. Bad balance situation for sure.

Hope this helps.

 

The points I'm making:
1. the original text on how it's done etc. are all well said and accurate
2. but it's actually more complex than it appears
3. even the best job is a compromise (except L6 and V12, which can be almost perfect)
4. the forces should properly include compression and combustion - and never do
5. do the best job you can afford

 

Did you happen to be on a debate team in high school ??? >>>>.

 

I have always wondered about this myself. I'm not familiar enough with the process to know what exact voodoo they use in a good balance job, but is there anymore info on this specific subject? Does anyone use a sort of "universe" figure to add into the equation to represent the combustion process?

 

if we balance a crank and rod,s and pistons what about ... timeing chain,s and there gears??

 

I like all this theory and explanation, but truth be told, not many of us would realize it if someone strung together as much sophisticated ********.

For my part, I'll tell you my balancing story.
Motor was purchased off a local racer, fresh from a known machine shop. Knowing there work, I was comfortable that it was as advertised, a STD/STD rebuild on a motor of some miles. Clean crank, deglazed bores, new pistons and rings. I couldn't ID the pistons for a while. I let it sit a year before looking it over and found the deck clearance of every piston varied. Stock deck. Tore it down.
The stock rods were "resized" but not matched. OK for a daily pickup, but not for what I had in mind. I bought a set of **** rods for what labor would cost and threw away the stock rods. Remember those pistons? Found an exact match on the shelf at the machine shop. They presented two problems. The compression height was way short for my stock deck and they were some 30 grams heavier. The new rods were some 70 grams heavier. ****! Now I need to mill the block and balance everything. I weight matched all the rod ***emblies and put it together. Test fit of rod & main bearings was slightly loose, but in spec. I hauled it to the shop for balancing and the guy pulling it down mistook my parts for worn and tossed all my bearings. The shop replaced them with better and it tightened the clearance just right. Now they make up bob weights for this mess. Something crazy heavy. No biggie. M*** is good in drag racing. Spin my crank a few times and tell me I need 230 or so grams of mallory metal to net 104 grams on the crank. ****! That's about $250 in name brand tungsten alloy. I couldn't swing it. Nice guys at the shop marked the location and weight on each counter weight for me and I picked it up. I didn't find a crank that I could afford, but I did find the alloy recipe for the Mallory metal. A few google searches later and I had order 2 slugs of appropriate weight and dimension for $25, incl. shipping! I bought 2 drills of the right size so I could put the metal in coaxially rather than radial as the shop planned. I hung my crank in my drill press and drilled all the way through both counterweights as marked. The metal had a slight interference when I used a c-clamp to press it in. I welded around both ends of the slugs with silicon bronze and took it back to the shop. It took 4 spins and about 8 holes to get total imbalance under 1 gram. Total cost $205. Then we decked the block and ****oned it all up. The lessons here are too many to count. I would call out the main one that a rookie with a drill press and buddy with a balancer can do this stuff.

It was fun. Enjoy yours.

 

That stuff should all be radially symmetrical, and doesn't need anything other than good quality control.
The cam has actually been argued as an unbalanced force (both the eccentric lobe placement, and the spring forces), but the very small "mean radius of gyration" and the low (1/2) speed makes any imbalance very small in terms of force.

 

Scotty you just explained a chapter from a normal day in the life of an automotive machine shop !!! >>>>.

 

I'm not surprised. You may also recall answering a few PMs for me about that time. I still owe you one. I still need a set of pushrods, too!

Thanks again!

 

This sounds reasonable. Some reading I did way back actually placed more concern on harmonics induced by spring pressures as you got more serious.

 

I always thought they come up with 50% for a V8 pretty much by trial and error... it was the std when I started balancing.
A lot of guys dont use 50%... and some guys add 4 grams for oil in their bob weights... some add more some add less... no real formula that I ever found.... just a lot of opinions.
I always snuck up to speed with a crank cause some would actually jump out of the machine the first spin up.
And yes when your done balancing the crank it is smooth on the readings... but... as was said its only balanced to the bob weight you put on the crank.
I know a guy who built a lot of super speed way Nascar motors (for the lower cl***es) and he kept changing the percentage until he liked how the bearings looked after a long race.... and it wasnt 50%.... his stuff seemed to run just fine.

 

why i asked the question was i read that harmonic,s can eat a engine and make it self destruct they have been doing testing on the harmonics at high speed so could we asume that over 5000 rpm could create problems ??

 

#### This by far is the best quote yet !!!! >>>>.

 

Boy, go on the road for a couple of days and I miss all kinds of good stuff. All of the theoretical math is entertaining but as is the case in alot of 'everyday' applications, hands-on, practical experience is what we really rely on for day to day operations. Ottoman's observation are good in that observing the condition of the bearings can be an excellent 'interpretation' of the quality of the work done. Unfortunately, most street guys do not have the luxury of pulling down the bottom end to check bearings after 100hrs of run time to see if they have what they paid for (unless something breaks) so they/we need to be comfortable with the 'experts' that engage in a 'black art' like balancing. I'm all for including a good dose of science in a rebuild but hard earned experience is hard to duplicate.



.

 

Agree.

 

50% is a good number for street use, for higher reving engines I use 52% as a rule. These are percentages for 90* V8 engines the numbers change when you get into differant V6s (90*, 60*, 15*) Odd fire Buicks use a differant percentage than even fire, then inlines use no bobweight at all but reciprocating weight is still seperate from rotating weight.
When you start hanging things off either end that starts the job all over, but I find it better to do flywheels and clutch covers on the crankshaft after the crank has been balanced.

 

Ted explains it very well here.

http://www.eatonbalancing.com/blog/2007/11/21/engine-balancing-part-1/