Engine Balance Question

Originally Posted by woody1

...I can balance an external crank using the customers flywheel and harmonic balancer. Get it balanced within .02 grams on both planes, I can then just loosen the flywheel bolts move the flywheel .100 thousandths of an inch either direction then retighten the bolts, respin the crank and it will be off 5 grams on 1 of the planes.

So moral of the story make sure you bolt your flywheel or flexplate back on the crank in the EXACT same position that your machinist did when he balanced it originally. AINT GONNA HAPPEN......

And that's exactly the point - errors and tolerances add up, so for best results, you generally want to minimize all of them that are practical. However, by your own numbers (.02 grams to 5 grams out), a misalignment can cause an out-of-balance by a factor of 250. That would mean a stock imbalance error of 20 grams (your other example) could yield a potential dynamic OOB of over 11 pounds. Wow. On just one part. And that's okay, eh?

David

Originally Posted by PSIG

That would mean a stock imbalance error of 20 grams (your other example) could yield a potential dynamic OOB of over 11 pounds. Wow. On just one part. And that's okay, eh?

David

That far out on the stock application is not ok with me, but I am nobody, Ford builds a few 100 thousand engines per year and it seems to work ok for them.

Originally Posted by woody1

That far out on the stock application is not ok with me, but I am nobody, Ford builds a few 100 thousand engines per year and it seems to work ok for them.

That's cool - no arguments here. Your points are all valid - I was just coming froma different perspective. And no argument that Ford's minimal balancing tolerances work on most production Mom 'n Pop engines.

My assumption (right or wrong) was that most members of a performance forum - and especially a subset on strokers - would be looking for a bit more performance than stock. Even a moderately modded small block can have a reasonable redline increased from 5500 to 7000 rpm. That's only a 1500 rpm increase. However, as you know (but many on this list may not) the forces increase exponentially as rpm's increase and from 5500 to 7000 the forces go up over 60%. So, that 20 grams goes to 18 pounds of offset dynamic weight. Things can get out-of-hand pretty quickly if enough details are disregarded or overlooked.

David