First I'm NOT talking about a race engine. What I'm thinking is 5000-5500 rpm absolute max and that would be a fairly uncommon rpm to reach. Ok... So I've always wondered how close is close enough for a purely street driven engine when it comes to balence. Is a dynamic balence really needed? Is a static balence to really tight tolerances (say +/- 2 grams) close enough?
A buddy of mine works on some Stock Eliminator car that goes well past 9000RPM. In a pinch they pulled out some forged pistons, put in some cast pistons they had in the shop to make eliminations on Sunday. I have no idea what it felt like, but they did it. In the 70's another guy put open chamber pistons on one side of his 454 and closed chambers on the other. Swap meet junk and no budget forced his hand. Ran the equivelant of 4 Nitrous units (he called it the motor eater) and ran a few weekends like that. I also can say, run of the mill engine rebuilders grab 8 rods out of a pile, check the big end and ***emble YOUR everyday motor without balancing. For $175, i balance EVERYTHING! PS-if you look at all pieces after balancing, hardly much has been done. So, it's not big amounts we're talking about
I'm all for balancing...that said.... The engine in my truck wasn't balanced when I rebuilt it.....I tried to keep it on the cheap....new pistons, bearings, turned crank, etc..... And I'm quite pleased with how it runs......smooth as silk and revs up like a street bike, great throttle response......but I may have just gotten lucky too.
If I'm rebuilding a virgin engine, putting the same crank and rods back in, I don't bother balancing for normal street use (up to 5500 rpm). New pistons come in weight matched sets, even if they don't match the old ones exactly they work fine. If you're building a hot rod motor with forged pistons, good size cam, etc. you are doing it to turn some rpm, so you need to balance it.
I always balance when I build an engine. My machine shop does it for $100 and do verything I bring in for that price. I usually take the flywheel, clutch and pressure plate as well as the front pieces for the crank. I worked as a machinist on aircraft parts for years and watched the mechanics ***emble cooling turbines. They even weighed how much cotton they put in the oil tank. I know that turbine we worked with turned a lot faster (80-100K RPM) but I could tell a difference when machining the turbine cases on ones that weren't balanced as well. The main reason I balance is to extend life and reduce wear on the engine. And it is nice to have one that runs smooth also.
Balanceing is a good thing but it doesn't happen that often. More often than not a mill is just bolted together and run, most guys don't even check the bearing clearance not that it would make much difference I don't know that many guiys that are going to buy an extra set of inserts or two to make it dead nutz anyway. All that said if you are going through the trouble to blueprint one and make it right on the cost of balanceing is minimal. It costs me 125 here but groucho is closer to normal cost and a little below what a lot are chargeing. Anyway you look at it its a worthwhile investment if for no other reason piece of mind. That's just my .02
Like Squirrel said, if your rebuilding an engine that was a factory ***embly and your building a daily driver kind of ride it's not necessary to balance, the factory job is fine. Now, if your running a different set of pistons (forged vs cast) a different set of rods, and/or a different crank that the engine came with a balance job is worth the money for a smoother running, worry free rotating ***embly. If, on the other hand the thing is for all out compe***ion then balancing is not an option unless your a born gambler. Frank
[QUOTE A buddy of mine works on some Stock Eliminator car that goes well past 9000RPM. Not wanting top start a big deal here but this is just not possible. Maybe if it was a ****el rotory but not a stock eliminator motor. A STOCK intake and carb is not going to flow enough ,Stock heads are not going to flow enough.ECT. Of course I may be wrong ,just post the car picture and specs and show me!!
good points everybody. i have a question though. how much more does a static over a dynamic balance gain me? also is it possible to achieve a static balance so close that theres no need to dynamically balance an engine?thanks.
I did some static balancing on my Citroen gangster car Motor a few years ago (4banger) up 0.5 grams and ported the intake/exhaust. Was worth the work, runs smooth like a babys ***......
I'd like to add a question. I had my engine balanced when I built it. Pistons, rods, crank, flywheel, pressure plate and balancer. Now 6 years later I have it apart and need to replace the pressure plate. I saw where they drilled the crank and flywheel to balance them. On the pressure plate there are pieces of metal welded on in a couple of places. What are the implications of putting on a new pressure plate? Do they balance each piece seperatly then ***emble and balance again? And if I replace the pressure plate does it throw everything off significantly? I always felt the extra $100 to balance an engine was worth it after spending several thousand to build it. A machinist told me some factory balancing can be as much as 10 grams difference between the heaviest and lightest piston rod ***emblys and they get then to within .5 grams.
"Static" balance will work fine on an inline engine or flat opposed 4/6, but will be no more a roll of the dice with a V-8.
I have run engines with different pistons also without any trouble. Money no object---Balance it / otherwise I would probably just run it.
I've seen stock engines run anywhere from 3-9 grams off. Within 3 grams and it'll run really smooth. Had a VW once that was 27 grams and ran that way for 60K miles. Got it within .03 grams on the rebuild just for the hell of it, and it was sweet.
There is more than 2 grams of oil clinging to parts at any given time +-2 grams is more than good enough for that application. Older oem parts are way further out than that.
I can't remember for sure, but a while back I had a physics instructor that did the math with me and we found that one oz. out of ballance at one inch radius will create a force equal to about 70 lbs at 6000 RPMs. That's a hell of a force that the main journals and bearings have to put up with, not to mention the motor mounts and the buzz that you have to put up with the whole time you're driving on the freeway. My current engine machine shop instructor (with 30+ years machine shop experence) will not build an engine that is not ballanced. I have built more then my fair share of engines for myself and friends and I would have to agree, it's worth every cent.
yeah, one ounce is a heck of a lot of imbalance! you'd probably feel that....but most (unbalanced) stock rebuilds aren't that far off, fortunately. Just for fun, calculate the load on the mains from the pistons pushing down on the crank at that rpm, WOT....
Mr. Creosote, i will try to answer your question. All of the machinist I know balance by adding or subtracting weight from the crankshaft. The flywheel ,pressure plate and so on should be nuetral balanced so they can be replaced if required. That is the reason for weight added to them . A cast crankshaft is different though and may be balanced with the harmonic balancer or more likely the flywheel /flexplate. Hope this helps.
FWIW, it happens every weekend. There's very little that's truely "stock" about a stock eliminator car. Grab a rule book and take a look. The cam has to be within "x" of stock lift and duration, but they don't really specify WHERE those valve events happen. Very complicated and takes severe dedication to pull it off and remain compe***ive with the whole combination. I once had a 69 BOSS 302 with a "cheater" cam that was stock L/D. It pulled hard well past 7500RPM with a stock intake and carb and tube headers. Anything's possible.
First of all 7500 rpm is a long ways away from 9000 plus A legal stock eliminator engine will not run 9000 plus with cast pistons and stock rods with good bolts. If it would the stock head (with only a legal three angle valve job) would not support that kind of airflow. Lets not even talk about valve train. I have read the rulebook for 40 years and tech cl***ification sheets for almost as long. A race engine doubles the stress and strain every 1000 rpm. At 4000 rpm the stress would increase something like 40 times by 9000 plus.( with stock parts?) It is a lot of fun to speculate about this stuff . Most legal stock eliminator engines probably make the most power under 6000 rpm. The easy way to to shut me up is just post the legsl stock eliminator car with the motor trans and rear specs and the times and speeds it turns. Or you can call up one of the championship engine builders and price a legal engine that runs 9000 plus.Be prepared to have them laugh at you.
You can ride in a car with an unbalanced engine and not notice it until you ride in a balanced one, then its like Holy *hit is this thing sweet! That said Ive run a 289 when I was broke with flat top pistons & 289 hipo heads on one side and dished pistons with huge chamber van heads on the other side and ir ran like a top
I don't know anyone that hears one down and rebalances because they needed a new pressure plate. When they balanced it the first time they probably didn't balance the pressure plate they just bolted it to the rest of the rotateing ***emble before they spun it. Anyway I wouldn't bother I'd just replace the pressuer plate with a good quality one (not an O' rielly rebuilt) and go on with life. Quote: Yea but some ( most?) are heavier on one side than they are on the other at least those on an external balance motor. A good balancer ( person not harmonic thingy) won't even look at your stuff unless you bring him ( or her?) all the stuff that attaches to the rotateing essembly. Oh yea I used ride a hodaka that was pretty close to stock that would turn real close to 10,000 RPM. it wasn't mine but the guy let me ride it once in awhile. OH two strokes don't count do they.
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