Originally Posted by bruce.augenstein@comcast.
Forgetting about the personality characteristics of the bragging gentleman who prompted your note, let me ask another question:
Why would any manufacturer hand build (to whatever extent) an engine. For marketing bragadoccio?
It's about two things, the first of which is money.
It simply costs more to build/assemble each individual engine by hand compared to an automated process - but, if you're talking about very low production numbers, it'll cost more per engine if you amortize the costs of designing and building the automated tooling necessary to keep Dieter or Guido (or whomever's) hands off the build process.
So, some Lambos, Ferraris, etc. get their engines hand built because it's more cost effective to do it that way - but so do some Chevrolets.
Why on earth would that be?
Because of the second reason you do this, which is that you can build a better engine through hand assemby.
When you're machining various engine parts prior to final assembly, there will inevitably be variances in the final product. Take a cylinder block, for example. Boring each cylinder heats the block in that area, and depending on cylinder placement and the volume of adjacent material, the amount of heating will vary. Then, boring another cylinder starts with a hotter block, and so on.
In addition, once you've bored your hundredth block with the same cutter(s), they have worn a bit.
And so on.
Let's say you call for five thousandths of an inch piston clearance, plus or minus a thousandth, in the build process.
Hand assembly can result in an engine that has five thousandths of an inch clearance on every cylinder, plus or minus nothing, because pistons have tolerances as well, so "oversize" pistons can be mated to oversize cylinders, and so on.
Such an engine will tend to make a bit more power, run more smoothly, and perhaps even get slightly better mileage.
50 years ago, an engine like this would've been termed "a freak" in the parlance of the day, resulting from the chance assembly of exactly the right piston for each cylinder, etc., creating more power, running more smoothy, and so on.
Nowadays, overall tolerances are tighter, so there is less engine-to-engine variation. But there still are tolerances in the process, so if you take ten automatic M3s selected at random off the assembly line, you will still get ten different ETs and speeds over a quarter mile, even if each is using launch control. I'd venture to guess two to three tenths variation in ET, and perhaps as much as a two mph difference in trap speed. (Note that only some of this variation comes from differences in engine power. The rest can be attributed to overall build tolerances in the drivetrain with it's associated bearing clearances, etc.)
If those M3s had hand-assembled engines, there would tend to be less variation, and the results would be clustered closer to the quick and fast end of the spectrum.
So to recap, hand assembly will tend to result in an overall "better" engine.
Edit: PS - To head off anyone fixating on something beside the point, my example of ten M3s wasn't designed to denigrate the car, but merely to illustrate that hand assembly of engines can result in better overall results.