Gang,
Break-in has always been a controversial topic. Personally, I believe the keep-it-slow method is more for manufacturer liability than for breaking in the engine. For the most part, every one of these engines has run on a dyno to full RPM before they leave the factory. If they can do it there why can't the owner do it on the road? On the other hand, if you tell your customer to keep the RPMs down during break-in they have a chance to get to the know the machine at a reduced performance level before blasting thru the gears at full performance.
FYI, the most important goal of break-in is to allow the rings mounted in the grooves around the top circumference of the pistons to wear the inside of the cylinder (and themselves) to make a good seal. The inside of the cylinders on new engines are honed in a cross-hatch pattern to promote the process. As the rings move up and down inside the cylinder with the piston the "peaks" on the cross-hatching break off (in microscopic pieces). Theoretically, as the cylinder walls wear away they begin to exactly match the outside of the ring. The better these two pieces of metal match up, the better seal you get. A better seal improves cylinder compression, prevents blow by and reduces oil consumption.
A bad ring-to-cylinder seal allows the fuel/air mixture in the cylinder to "leak" out around the rings during the compression stroke as the piston moves toward the top of the cylinder following the intake stroke. When the mixture leaks out, there is less to burn during combustion, thus less power.
A bad seal also promotes blow-by. This occurs when hot gases flow around the piston during the combustion event adding caustic combustion products to your oil. Imagine pumping acid around the inside of your engine. That's what the oil pump is doing when it pumps acidic oil around your engine!
Finally, a bad seal increases oil consumption. The rings are supposed to "scrape" oil (sprayed on the bottom of the piston and the cylinder wall below the piston) off the cylinder wall as they move downward with the piston. A poor ring-to-cylinder seal allows more oil to remain on the cylinder wall which eventually burns off during combustion. This is what happens with engines that are said to "use" oil.
To best promote break-in and a good ring-to-cylinder seal you must force the rings against the inside of the cylinder with as much pressure as possible WITHOUT OVERHEATING THE CYLINDER! This is critical... if the cylinder overheats you could generate a "glaze" in the cross-hatching which will NOT wear away. That means your engine will NOT break-in; you will always suffer low compression, excess blow-by and excess oil consumption. Hence, BRP's admonition to not sit in traffic with the cooling fan running during break-in -- KEEP THE THING MOVING!
The rings are shaped such that the combustion event (which increases pressure inside the cylinder) pushes against the the inside of the ring forcing it outward against the cylinder wall. So it stands to reason whatever you can do to put more pressure on the INSIDE of the ring will improve the break-in process (keeping in mind the overheating issue). Riding the thing easy WILL NOT ACCOMPLISH THE TASK! You have to use frequent, hard acceleration runs to get the in-cylinder pressure high enough to do the job. At the same time, you MUST KEEP MOVING to prevent overheating.
One final warning: don't open the throttle too much at low RPM -- called "lugging" the engine. Lugging promotes overheating and puts a great deal of pressure on bearings that haven't broken in yet either.
Instead, briefly open the throttle at the mid-range RPMs, say from 4,000 to 7,000 on a regular basis. The farther into the break-in period the higher you can allow the revs to go -- right up to redline.
Following this regimen your engine will be largely broken in by 100 miles. At that point it's a good idea to change the oil and get rid of all those little pieces you broke off the inside of the cylinder, as well as the stuff that got by the rings (remember blow-by?) before they broke in.
Follow the links to the Motoman site posted earlier for a great description of a good, viable break-in procedure. It accounts for both increasing the in-cylinder pressure while preventing overheating.
Regards,
Mark Sletten