Less Than Ideal Engine Balancing - Limits?

[burbfixer]

I have run into a snag with component weights for my 454 project.
I am just freshening this engine, but am using new dome pistons to raise compression. I do not want to have to re-balance the stock crank, but the new piston/pin/ring assembly weighs a whopping 68 grams more than the stock components, almost all of which is the piston.
I doubt I can safely remove that much, no matter what I do, but I know I can get most of the way there if I include lightening the small end of the rod.
This is a daily driver, low rpm application - 5000-5200 max (but rarely).
Question:
In your experience, what weight difference can I safely allow here without having this thing shake too much? 0 difference? 10grams? 20? More??

 

[JayMaechtlen]

So a quick Google tells me the stock pistons weigh about 880 grams?
- sounds heavy.
Anyway, that would say your new pistons are 7-8% heavier than stock?

Seems like the percentage would be more important than just weight.


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[SwinnyGG]

68 grams is WAAAYYY too much to do nothing.

The general consensus is that you can go lighter as much as you want (this will just put the assembly into an overbalance condition, which some people want anyway) but if going heavier, anything in excess of 10 grams requires rebalancing.

This balance job will involve adding weight to the crank, which means ballast metal, which means money.

Sounds like your rotating parts are already purchased. Is it too late to change pistons? use a longer rod instead of higher crown to get your compression?

 

[Tmoose]

How much do the piston pins weigh in the new pistons?
How much do the piston pins weigh in the old (stock ?) pistons?
Are you running the pins with the factory interference fit, or going "full float" ?

Were you satisfied with how smooth the engine was before ?

 

[burbfixer]

Thanks for the replies!
I will just re-use the stock pistons if I can't trim enough weight from these new ones to get them into an acceptable range.
Stock press fit pins are 151g
New press fit pins are 152
There is 8g difference in the old vs new rings - the originals are shallow groove, so are lighter.
I experimented last night with an identical old rod and was able to lighten it by 30g just by removing the big balance pad on the small end.

 

[burbfixer]

Of course, the right answer is to properly balance the crank to the new piston/rod assemblies. I have done that with builds in the past, and will likely do it in the future, but this time I can't spend the extra $ for balancing.
My understanding is that there are very complex things going on inside a reciprocating AND rotating assembly, and it gets even more complicated when it's a V-style engine. So much so that some have said even using proper, accepted balancing practices still doesn't perfectly 'balance' an engine.
I understand I'm basically asking 'how to do it wrong', and wouldn't fault anyone for not wanting to get involved. Still, it seems there might be some margin for error here. I just don't know what that margin might be.
Funny, when I don't have lots of money to spend, I end up learning more about things like this. Lack of finances forces me to test some limits that I wouldn't normally. It's interesting!