balancing rotating assembly

[85_IDI_4x4]

bear with me for a second here cause my memory is a bit fuzzy on what im about to ask.
I was doing some reading up on balancing the rotating and came across a thread on another forum about it. I could not tell you what forum nor can i find it anymore.
one guy stated that he had his engine balanced and everything was within like half a gram. But he said that either it was balanced 2% or 2 grams over/out. I cant remeber exactly what was said but it was something like that and im pretty sure the number was 2
Does anyone know what he meant by this? To me it seems that either the piston assembly or bob weights were 2g or 2% heavier than the other. Can anyone shed some light on this?

 

[85_IDI_4x4]

ha ha ha now i feel a little stupid. A google search turned up the info on overbalancing the crank.
It is used to counteract vibrations at high rpms and boost among several other things. In our case it would have to do with boost causing the piston to artificially weighing more while the cylinder is filling with air.

So for the sake of discussion whats everyones opinion on this?

 

[bike-maker]

I used to balance rotating assemblies at a speed shop. When I used to balance a crank shaft, the target was to get either end of the crankshaft to be within 2 grams of being perfectly balanced; this is probably what he was referring to. Any more accurate than that, and I would end up chasing the imbalance around and not gain anything; and it was always preferable to leave the counterweight side of the crank as the heavy side. Most production machine shops do not work to these tolerances; they probably keep the counterweights within 10 or so grams of being perfectly balanced. I have never used any kind of percentages to describe balancing techniques.

And that whole shpeal is just balancing the crank. Before that happens, all the individual components of the rotating assembly have to be balanced to each other (big ends of rods, small ends of rods, pistons and pins). On these components, I always got them within .5 grams for each measurement - a typical production machine shop would probably be more like a 5 gram tolerance.

You take the weight of the big end of the rod (rotating mass) and add 1/2 the weight of the small end of the rod, piston, pins, rings (reciprocating mass) - multiply the whole mess by 2 - and you have the "bob weight". The bob weights are the weights that are bolted to each of the crank journals to replicate the weights of the rest of the rotating assembly.

Hopefully I answered you're questions somewhere in that mess.

 

[bike-maker]

As far as overbalancing; that was seldom practiced. Only place we used that was on stuff like sprint car engines or blown/alcy motors that only ran at 7000-10,000 RPM. In this case we would leave the counterweights heavy by about 10 grams. We never did any high performance diesel balance jobs, so I am unsure of the overbalancing concept on a diesel. But as far as boost numbers, some of the blown/alcy motors I overbalanced were running at as much as 40 pounds of boost. With the amount of boost that IDI's can survive at, I would think the overbalancing concept is a not worth worrying about.

 

[RLDSL]

I used to balance rotating assemblies at a speed shop. When I used to balance a crank shaft, the target was to get either end of the crankshaft to be within 2 grams of being perfectly balanced; this is probably what he was referring to. Any more accurate than that, and I would end up chasing the imbalance around and not gain anything; and it was always preferable to leave the counterweight side of the crank as the heavy side. Most production machine shops do not work to these tolerances; they probably keep the counterweights within 10 or so grams of being perfectly balanced. I have never used any kind of percentages to describe balancing techniques.

And that whole shpeal is just balancing the crank. Before that happens, all the individual components of the rotating assembly have to be balanced to each other (big ends of rods, small ends of rods, pistons and pins). On these components, I always got them within .5 grams for each measurement - a typical production machine shop would probably be more like a 5 gram tolerance.

You take the weight of the big end of the rod (rotating mass) and add 1/2 the weight of the small end of the rod, piston, pins, rings (reciprocating mass) - multiply the whole mess by 2 - and you have the "bob weight". The bob weights are the weights that are bolted to each of the crank journals to replicate the weights of the rest of the rotating assembly.

Hopefully I answered you're questions somewhere in that mess.

Finally someone speaking on engine balancing who actually knows what the heck they are talking about . I remember many years ago back when I ran an engine shop,I got sick of arguing with clowns about that stuff( they were all friggen experts since their 16 year old buddy told them so . the terms having an engine balanced and blueprinted were thrown around so much that every idiot thought they knew what they were talking about to the point where I wanted to throw up whenever the subject arose.

 

[OLDBULL8]

Geez, Robert hope you don't throwup when you read his other post about what I said about balancing.

Bill

 

[RLDSL]

Geez, Robert hope you don't throwup when you read his other post about what I said about balancing.

Bill

I used to have to deal with countless kids walking in the door every day , wanting a bazillion hp out of the small block chevy they ripped out of their clapped out camero , piggy bank and hammer in one hand and a years worth of Hot Rod magazines in the other, and they all just knew EVERYTHING about how to build an engine... except how to make their combined design last more than 30 seconds
The smart ones would sit down and listen and we'd come up with something a lot simpler and a whole lot cheaper that they could actually put on the street, the not so bright ones would take it down the street to someone who would do anything for a buck and then point to their not responsible for custom work sign when they came back crying that it blew up.

 

[85_IDI_4x4]

i hope your not saying im one of those stupid kids??? Ive got plenty of respect for those that have been there done that, but when i get told "it wont work" or "didnt work for us" i want to know why it wont work or didnt work the first time.

I actually have a problem of over reasherching something to the point of information overload. I knew the basics of what balancing was and the reason behind it. But overbalancing was new to me so i asked about it. Which it seems is more of a "black magic" compared to just neutral balancing a rotating assembly.

 

[RLDSL]

i hope your not saying im one of those stupid kids??? Ive got plenty of respect for those that have been there done that, but when i get told "it wont work" or "didnt work for us" i want to know why it wont work or didnt work the first time.

I actually have a problem of over reasherching something to the point of information overload. I knew the basics of what balancing was and the reason behind it. But overbalancing was new to me so i asked about it. Which it seems is more of a "black magic" compared to just neutral balancing a rotating assembly.

NAw, when you are out here actually spending the time to research and ask teh right questions, you are on teh right track. The kind of clowns I used to have to deal with are the total know it alls who would read the build of the month tip for a year out of the magazines without doing any background research at all and insist that they all needed to go into one engine... that was going to be driven on the street, leading to a mis matched hodge podge that would never work in anyones wildest dreams where they would wind up with an engine that is part top fueler, part roundy ronder, part drag boat part street racer, part.....total friggen disaster, and a lot of these guys could not be resoned with because , they read it in a magazine they had no concept that those articles were directed at people in a specific audience who already had built engines a certain way who could benefit from a certain modification, and they would go deer in the headlights when trying to explain that to them

I didn't mind taking 4 of 5 grand from rich guys for custom engines for their ski boats so they could take them out and blow them up the next weekend, we had a few regulars who would do that all the time and that's how they enjoyed spending their money, but I couldn't stand when some kid came in trying to hand me that kind of money to do something that he was going to regret, when we could build him something that was going to last and still make monkeys out of most that he would run into for a fraction of the cost.

 

[icanfixall]

I would like to add something I was told about balancing the factory Mahle pistons... Thats what our engines were made with and thats what I use with respect... I had a balance issue with a set of Mahle turbo pistons. The shop doing my balance shot told me they could not take enough weight off the pistons to get them the same weight.... So I called Mahle and talked for along time to the head engineer named Eric. This was about 4 years ago.... He told me any idi engine will run fine with pistons that are as much as 14 grams differance in weight..... He stated that our rpm was not going to show any problems with pistons being that much differance in weight and the engines will live a long life... I ended up sending back my set of pistons for a "closer set". About 3 weeks later I got a set of 7 pistons instead of eight... I was really pissed. I finally got another piston that weight very close to the same weight as the other 7. Then I balanced my own pistons. They are now within 1/4 to 1/3 gram differance in weight. I did not need to get them that close but why not. Then the shop did the complete rotating element. That included the balancer and the flexplate. My engine runs smooth. You mostly year the engine running but you can't actually feel it running.....

 

[RLDSL]

I would like to add something I was told about balancing the factory Mahle pistons... Thats what our engines were made with and thats what I use with respect... I had a balance issue with a set of Mahle turbo pistons. The shop doing my balance shot told me they could not take enough weight off the pistons to get them the same weight.... So I called Mahle and talked for along time to the head engineer named Eric. This was about 4 years ago.... He told me any idi engine will run fine with pistons that are as much as 14 grams differance in weight..... He stated that our rpm was not going to show any problems with pistons being that much differance in weight and the engines will live a long life... I ended up sending back my set of pistons for a "closer set". About 3 weeks later I got a set of 7 pistons instead of eight... I was really pissed. I finally got another piston that weight very close to the same weight as the other 7. Then I balanced my own pistons. They are now within 1/4 to 1/3 gram differance in weight. I did not need to get them that close but why not. Then the shop did the complete rotating element. That included the balancer and the flexplate. My engine runs smooth. You mostly year the engine running but you can't actually feel it running.....

Oh, you would have LOVED it back when I was in the machine shop and folks would come in and INSIST on me ordering them a set of TRW pistons , no matter how hard I tried to talk them out of it. Turned out that TRW was nothing more than rebox like beck arnely, and when you got a set of 8 TRW pistons, they would usually be from at least4 or 5 different manufacturers, sometimes all different. You want to talk about balance issues, the freeking skirts didn't even look the same most of teh time.
Most of these experts would insist that we just bolt them in as they came, because that's what the advertising was telling them, buy TRW.... AH advertising, aint it a wonderful thing

 

[typ4]

RLDSL, x2 on the trw stuff, for stock or mild perf SBC when I worked in the machine shop, we used silvolites, inexpensive and very tough for a cast piston. Had a 9to1 14 second camaro running them, that was 18 yrs ago, engine is still together.

My IDI is balanced weights to 1/2 gram. Standard of the shop I had do it.

 

[icanfixall]

A lot of us older guys speak from hands on experiance.... Not..."I read this and I want this"... Advertising really works on some people. I'm kind of the show me type of buyer for anything advertised... Even then I know someone is trying to sell me something.... Building an engine is not just doing one thing tothe engine. Its doing something to the complete air path thru said engine... I had a engine built for nitro and alcoho to compete in hill climbing. It was a two stroke. When you did anything to the ports like raising the exhaust you reduced the troque but increased the hp and rpm. The bottom of that port was not adjustable because the piston could only go down so far in the cylinder. Raising the intake did simaular things... Adding a third transfer port helped. Changing the design of the transfer ports did more. A trials bike engine has a very small exhust port way down low in the cylinder. Those engines made diesel kind of low end torque but nohigh rpm.... Its "what do you want" kind of work that needs to be done to aengine.... Just changing a cam or increasing the compression ratio is not going to get you what you want.... Our diesel engines benifit from adding a turbo but... Leaving it just that way will not ring out all the hp and torque available. Increasing the exhaust to 4 inch and running a straight thru pipe will work wonders. Installing a matched set of new BB codes and either a Moos or Baby Moose will add to this... A cold air intake... Even more... An intercooler...Even more... Turning up the fuel and running apyro... Even more... Then we get into the engine internals... Ceramic coating the pistons... Milling down the pistons... Line boreing the crank webs... Decking the block... Porting the heads.... Installing a typ4 cam... Is this list endles... Yes... We have only a few choices for after market proformance parts...

 

[RLDSL]

RLDSL, x2 on the trw stuff, for stock or mild perf SBC when I worked in the machine shop, we used silvolites, inexpensive and very tough for a cast piston. Had a 9to1 14 second camaro running them, that was 18 yrs ago, engine is still together.

My IDI is balanced weights to 1/2 gram. Standard of the shop I had do it.

Good choice on teh Silvolites, we used a bunch of them. Had a bunch of folks trying to argue with us because they didn't see a bunch of advertising for them, and they thought they were some kind of Gumby brand, but they were one of the OE suppliers for that engine, and a number of others.THey made some high quality stuff. We never had issues with them, granted this was 30 years ago. Aparantly they make different types of pistons now and only do performance pistons now out of that division ( and they actually advertise