Jake, I'm just offering a pragmatic counterpoint for those who don't have access to such good equipment. We have to trust that the guy doing our balance work did it as well as you do - which can take a big leap of faith. A tiny imbalance in the flywheel assembly is going to cause much more trouble than a gram or so difference in piston weight, and I don't know of anyone who tears their whole engine down for a complete rebalance if they have to change a clutch cover. Nor do most folks obsess about the balance of their clutch disk, trans mainshaft, gears....
Off the balancing machine and in operation, a piston that's being pushed down on the power stroke is going to "weigh" a lot more than its brother that's sucking down against vacuum on the intake stroke, and these dynamic differences are going to vary by a lot more than the static weight differences - getting reciprocating weights within one-half of what Porsche uses should be sufficiently anal for most of us.
"Perfect" is always best of course (by definition nothing can be better), but in the real world for the average guy building his own motor on the kitchen table there are more important issues to worry about IMO, like rocker geometry and bearing clearances.
Balancing pistons
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Tom Simon
Balancing pistons
I'll go along with that, Jake. I know you already know this, but for the benefit of some of the younger guys builder their first high performance engine on a tight budget:
A lot of people wonder why quality workmanship costs what it does. Over the years, I seems every time I've asked myself, "why can't I just do this myself, I have a scale?" (for instance, I was going to balance my own con rods for a SB Chevy in 1980) After a little investigation, I found a pattern developing. A lot of jobs require expeience, knowledge, usually special tooling and expensive equipment to get it right. Doing it yourslf is always possible, but in many cases, just not cost effective unless you build engines for a living. In other cases, inexperience and lack of knowlege fool the builder into thinking after, say, an hour or two with a dremel and carbide burr tool, he now has a set of "ported race heads" for instance.
The automotive machining market is so competetive, that price gouging almost never happens. If you pay $120-$200 for a complete dynamic balence job, you can pretty well be assured the machinist is spending 2-3 hours on your job.
Balancing your own pistons is great, but it is only a small part of a balance job. Why worry about the pistons if you are not going to do the rods? I guess it's better than not checking at all. Why the rods if you don't do the crank, flywheel and front pulley? You probably save yourself 15 minutes of shop time by doing your own pistons, and the guy who balences the rest of you assembly will want to re-weigh your pistons, anyway.
A lot of people wonder why quality workmanship costs what it does. Over the years, I seems every time I've asked myself, "why can't I just do this myself, I have a scale?" (for instance, I was going to balance my own con rods for a SB Chevy in 1980) After a little investigation, I found a pattern developing. A lot of jobs require expeience, knowledge, usually special tooling and expensive equipment to get it right. Doing it yourslf is always possible, but in many cases, just not cost effective unless you build engines for a living. In other cases, inexperience and lack of knowlege fool the builder into thinking after, say, an hour or two with a dremel and carbide burr tool, he now has a set of "ported race heads" for instance.
The automotive machining market is so competetive, that price gouging almost never happens. If you pay $120-$200 for a complete dynamic balence job, you can pretty well be assured the machinist is spending 2-3 hours on your job.
Balancing your own pistons is great, but it is only a small part of a balance job. Why worry about the pistons if you are not going to do the rods? I guess it's better than not checking at all. Why the rods if you don't do the crank, flywheel and front pulley? You probably save yourself 15 minutes of shop time by doing your own pistons, and the guy who balences the rest of you assembly will want to re-weigh your pistons, anyway.
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MASSIVE TYPE IV
- Posts: 20132
- Joined: Fri Jul 07, 2000 12:01 am
Balancing pistons
Thanks for the posts guys...I knew you would understand my point.
The reason for a bobweight card is for future reference and to see the entire rotating/reciprocating mass at a glance.
I balance parts to .02 ounce inch for high revvers and no higher than .035 for a low revving street engine. The "ounce inch" is a sum of the averages of the front and rear imbalance readings added and averaged by the balancer computer. This is much more accurate than the older strobe methods of weight imbalance. The specs I use on my lowest revving engine are better than Porsche used on the 911 series engines.The tightest tolerance is the same as used by NHRA and NASCAR for full race, 9,000RPM plus engines.I have balanced a few gas generator assemblies for turbine engines that spin 18K RPM, the specs on this were balanced to .3 oz inch, but there is no reciprocation there...
There is nothing like a correctly balanced engine, I had to buy my balancer because I could not find anyone who would spend the needed time of the job to get it as tight as I desired. It takes me 6 hours on average to balance all parts individually, then progressively assemble and re verify balance, and fill out the bobweight card.
For the normal guy a gram scale that measure in .5 gram increments is great, or just go to a body shop, they use the same scale I have with my balancer for mixing paint.
I believe that balancing is just as important as setting compression ratio..thats why I had to spend alot of money..
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Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com
The reason for a bobweight card is for future reference and to see the entire rotating/reciprocating mass at a glance.
I balance parts to .02 ounce inch for high revvers and no higher than .035 for a low revving street engine. The "ounce inch" is a sum of the averages of the front and rear imbalance readings added and averaged by the balancer computer. This is much more accurate than the older strobe methods of weight imbalance. The specs I use on my lowest revving engine are better than Porsche used on the 911 series engines.The tightest tolerance is the same as used by NHRA and NASCAR for full race, 9,000RPM plus engines.I have balanced a few gas generator assemblies for turbine engines that spin 18K RPM, the specs on this were balanced to .3 oz inch, but there is no reciprocation there...
There is nothing like a correctly balanced engine, I had to buy my balancer because I could not find anyone who would spend the needed time of the job to get it as tight as I desired. It takes me 6 hours on average to balance all parts individually, then progressively assemble and re verify balance, and fill out the bobweight card.
For the normal guy a gram scale that measure in .5 gram increments is great, or just go to a body shop, they use the same scale I have with my balancer for mixing paint.
I believe that balancing is just as important as setting compression ratio..thats why I had to spend alot of money..
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Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com
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Pillow
- Posts: 2940
- Joined: Mon Oct 15, 2001 1:01 am
Balancing pistons
I can vouch for Jake's work on the balance machine... Seen it and built engines with his balanced parts.
I cannot put into words how smooth the motor was/is right from initial startup. Well worth the money and extra time!
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Adrian Pillow
'66 VW Westfalia - "Biscuit"
'67 VW Deluxe Westfakia - "Dino" - FS
'79 Porsche 911 SC - "Cochese" - FS
I cannot put into words how smooth the motor was/is right from initial startup. Well worth the money and extra time!
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Adrian Pillow
'66 VW Westfalia - "Biscuit"
'67 VW Deluxe Westfakia - "Dino" - FS
'79 Porsche 911 SC - "Cochese" - FS