Hi Jerry,
Have you ever read this book?
Fundamentals of Automotive Engine Balance
W. Thomson
ISBN 13: 9780852984093
It has a lot of interesting information regarding engine balance. It is very difficult to locate for purchase (at least at a reasonable price) , but most libraries that have exchanges with local universities can usually get a copy on loan. (I have been looking to purchase it for awhile, but never stumbled across an affordable copy).
I read it several years ago, so I don't remember all of it in detail, but what I do remember prompts me to a ask a couple of questions:
I can see how flywheel mass could be used to help dampen torsional vibrations, but isn't there a second order vibration in four cylinder engines that is inherent to the design? As I recall it was caused by the difference in acceleration between the two pistons moving in the area near TDC vs. the two moving near BDC. Since the pistons travel a longer distance from TDC to 90 degrees than they do from 90 degrees to BDC, the acceleration rate is higher, generating unequal forces between the two pairs. In modern engines, this is compensated for by using balance shafts.
I can't speak for MerCruiser applications, but early on when a variant of that engine was used as the "Iron Duke", I seem to recall it got stroked quite a bit to keep the RPMs down. I think they also had pretty soft motor mounts in those days, and I also wonder if customer expectations with regards to NVH back then were not particularly high? Later in it life, I believe that it finally got balance shafts as well when they tried to spin it faster in order to get more HP out of it.
Does this sound right to all you engine gurus? Or, did someone slip crazy juice in my diet Dr. Pepper again?
Best Regards...
