HT: Where should we start?
LW: I think a Dart block is a great foundation. They have a B20 version with a tall deck, so I can put longer rods in, and it's strong as hell.
HT: Why the Dart block? We're not going turbo (yet - wink, wink).
LW: Like I said, Honda builds an incredible engine. Only recently have high-end race engines started using machining tolerances as tight as Honda's factory production pieces. But to get 300hp, we're going to need more displacement than any stock B-series block is capable of. We'll have to buzz it higher than a stocker, too. That leaves us with 2 block options: we can either sleeve an existing Honda block, or use a purpose-built Dart.
HT: Why a sleeved stock block and not just a stock block?
LW: A sleeved open-deck block isn't as rigid as we'd like for the horsepower and RPM we're going to generate. But stock B-series cranks are incredibly strong; I've seen them on 800hp turbo motors. What actually happens is the crank keeps things together while the block flexes, so we need a rigid block and that's what the Dart provides.
HT: Still, why not sleeve a stock block? Isn't it a lot cheaper?
LW: Yes and no. The cost of a Dart block is less than most people think, and there are problems with sleeving.
LW: First is cylinder roundness. If you look at a sleeved block, you can see where the sleeves butt together; they have flat areas on them where they meet. When you have material of varying thicknesses, it expands at different rates. Consequently, as temps go up, the cylinders don't tend to stay round. Anyone who's ever built engines for serious competition will tell you ring seal is everything when it comes to making power.
The Dart block has sleeves that are the same thickness all the way around, so when the block heats up, even in machining, the cylinders stay round. Straight bores are something that Dart has specialized in for years. The Dart casting also has heavy main bearing webbing, so it supports the bearings and crank instead of the other way around. When you sleeve a Honda block and you mill out the original cylinders, it causes the whole bottom end of the Honda block to relax and the mains go out of alignment. When the sleeves are pressed in, it forces them even further out of alignment. In order to fix this you would have to align-bore, or align-hone the mains, and there aren't many shops doing that correctly, keeping the crank centerline precisely where it needs to be. This is important, because any crank offset will dramatically shorten the life of the oil pump. This process also needs to be done with a torque plate in place.
HT: Ok, we get that taking out material from the Honda block makes it less stiff on the bottom, but explain align-hone and the torque plate.
LW: An align-hone, or line-hone, is an extremely rigid [and round] bar with an abrasive coating that's slipped though the mains of the block. It's expanded and rotated simultaneously, boring all the main journals to the same size, as well as making them perfectly straight. You can imagine how a block might sag a little on the ends so everything's not in a straight line any more. When the mains aren't straight, the crank won't rotate freely. Even on a properly machined, sleeved block, at high stress levels the only thing that really keeps the mains aligned is the crankshaft's rigidity. Fortunately, Honda's factory crankshafts are more than up to the task, but in an ideal world where we're looking for both power and longevity, the block should support the crank.