Do I rebalance for new pistons?

[Mark O'Neal]

Originally Posted by Evilsports

Would I be wrong to think that pistons, being reciprocating mass, would only come out to 40gm (50%) for eight pistons on a 90° engine? Also, I was wondering if any of the real builders (I'm by no means one) here would be open to exchanging few pm's regarding overbalance as it applies to boosted applications?

You double the rotating weight and use 100% of the reciprocating.

I have a few questions particularly wondering what some of the "proprietary" calculation formulas are used to determine the boosted weight of a piston as opposed to an NA slug when determining overbalance. Thanks.

Overbalance is determined by the weight, rpm, and opinion of the builder. I like 1.5% @ 8,000...someone else will like something different. , personally, don't exceed 2%.

[Evilsports]

Haha. Something tells me you're not new to this at all. Thanks for clearing that up. I probably did a lousy job of describing what had caught my attention re: "boosted weight" I was reading up on balancing and came across this:

"Any form of blown engine will benefit from a given amount of overbalance simply due to the weight of the piston averaging artificially heavier not only from the increase in cylinder pressure at ignition, but the increase in cylinder pressure taking place while the cylinder is also filling during the intake stroke. In this instance, the piston is averaging an overall heavier weight when running at speed. A normally aspirated engine has a given amount of pressure counterbalance in that the piston is subjected to negative pressure when the cylinder is filling but is under increased pressure during compression and ignition. If an aspirated engine is working with an extremely well designed induction system and is benefiting from a ramming effect to fill the cylinders at the upper rpm ranges, then overbalancing also helps here. And then there’s the rpm factor. Balancing is linear up to a point throughout the rpm range but depending upon the masses at work within your particular assembly, there is a point in which the crankshaft rpm starts to out run the dynamics of the existing state of balance. Overbalance allows these dynamics to stay in tune or “caught up” to the rpm’s of the crankshaft."

This just made me wonder how one would go about factoring in the boost when calculating bobweight. Thanks again.

[Mark O'Neal]

I did what I always do when confronted with a situation like this...I called someone that is a lot smarter than me.

It's appears there's is no sliding scale....he just uses 2% on a blown application. N/A will range from 1 to 2 depending on the rpm and bob weight.

[Evilsports]

Originally Posted by Mark O'Neal

I did what I always do when confronted with a situation like this...I called someone that is a lot smarter than me.

It's appears there's is no sliding scale....he just uses 2% on a blown application. N/A will range from 1 to 2 depending on the rpm and bob weight.

Cool. Good to know. Thanks Mark.

[davidray]

Why is your horsey on fire Mark?

[Mark O'Neal]

Originally Posted by davidray

Why is your horsey on fire Mark?

Because my graphics guy designed it? It's hot on top of a piston?

I don't know....but I like it.

[JUNKMAN1]

I have a question about balancing also. I guess I am dumb when it is explained cause never understood the different theory's. Main question is the part that ring friction plays on rotating. Another would be the toll of compression. And after that would be the amount of explosion during combustion. These three factors are different on every build. I guess my question is how do the guys that come up with the theories on balance percentages.

[Mark O'Neal]

Originally Posted by JUNKMAN1

I have a question about balancing also. I guess I am dumb when it is explained cause never understood the different theory's. Main question is the part that ring friction plays on rotating. Another would be the toll of compression. And after that would be the amount of explosion during combustion. These three factors are different on every build. I guess my question is how do the guys that come up with the theories on balance percentages.

Compression, friction, and cylinder pressure are essentially the same on all of the cylinders.

[JUNKMAN1]

Originally Posted by Mark O'Neal

Compression, friction, and cylinder pressure are essentially the same on all of the cylinders.

I understand that but you are dead balancing the rotating assembly but different theories are used by different machines. It is hard to explain. some balancing requires 50/50 big end to small end others might require 30/70. Is there a rule of thumb for this? Are is this just theories incorporated by the individual machine's manufacturer? Or does this have direct relation to your rod vs stroke? Like I said I am dumb when it comes to this part and would like to know a little more. I think my earlier post would have been placed more on crankshaft design then just balancing alone.