Discussion and questions related to the course Practical Engine Building
I'm building an BMW M50B25 inline 6, ready for forced induction. Crank is balanced and the engine is unlikely to rev any higher than 7200 rpm. Prior to subscribing and watching these videos, I took my my conrods / pistons to a machine shop and asked for them to be balanced. I was told simply that the Molnar branded rods (that I have) are usually spot on and the pistons 'should be fine'. I'm aiming for what would be considered a relatively minor/moderate power level on this platform, so I guess my question is whether this is absolutely essential for all engine builds or, just when you're pushing the platform you have to it's limits. Ideally I'm sure the answer is everything should be balanced, but how necessary is it?
Motivating factor - there is significant investment in tools to achieve this myself, unless I find another shop willing to do it for me.
Opinions may differ, but...
The primary difference is the engine will run slightly more smoothly, but how significant it is when considering the other loads on the engine is questionable. Given the option, I would have the whole assembly balanced, as changing the pistons, and rods, will potentially alter the counterweight balance needed on the crank, as may front pulley assembly, and flywheel and clutch assembly.
It isn't a recommendation, but many years ago in the UK a drag engine with three different brands of pistons and two of the 'rods (might have been the other way round) set a national record - apparently they'd had a lot of breakages and used the best of what they had left over.
If it is within your budget i would recommend to make it balanced especially if it is planned to get high loads such as drag racing or time attack or rally. If it is not planned to withstand high loads yiu can get away without balancing as inline engines are more forgiving for not being 100 percent balanced comparing to V shape engines. I'm building 2uz engine now for 1400 hp and I'm gonna definitely balance all rotating assembly.
Thanks guys, appreciate the feedback! I've weighed the pistons and they are all already within a tenth of a gram of each other, so I guess it's just the rods left to go and I've come this far - so I might as well keep on going :)