Blocks for Continuous High Horsepower Applications
The little Holden seems to be able to handle much higher power levels for shorts bursts quite well, and indeed there are some very high powered blown drag race engines around. But if you run the engine for prolonged periods at horsepower levels over the high 200s you will probably find the block will be quite susceptible to cracking and splitting, and while there are some things you can do to help there are definite limits to how much power can be made reliably. Pushed hard, they will split horizontally right down the left hand side, the crack intersecting the welch plug holes. Very high rpms seem to be the main cause of the breakages, though builders of blown motors in may also have to choose between higher boost levels and block durability.
The breakages seem to be more a result of vibrations and forces transferred through the block from the crank rather than simple overstressing. It's therefore more productive to focus attention on the preparation and balancing of the rotating assembly than to try to strengthen the block itself - see the sections on the crankshaft and balancing for more detail.
Running a steel girdle on the mains might help to some degree, though there seems to be remarkably few problems with main caps walking or breaking, the main benefit of a heavy rigid girdle is as a vibration dampener. Grout filling the block will also help dampen the harmonics to a degree, and also provide a little more cylinder wall stiffness. The amount of grout fill will of course will be a compromise between stiffness and cooling, particularly for engines running petrol rather than methanol. The top 30mm - 40mm of the cylinder is the section that is under the most stress, and is therefore the bit that would most benefit from some grout support. Unfortunately this would also preclude any coolant circulation so would only work with a drag engine. You can however run some grout in the lower part of the water jackets without any overheating problems even in a street or circuit car, and it will help stiffen the cylinders a little bit. Filling to the bottom of the water pump opening will result in about 50mm of grout around the base of the cylinders, and while it's not really where the support is most needed it will help a bit.
In an attempt to strengthen the block, some guys have run long head studs that run through the deck and are anchored into holes tapped into the block at the bottom of the cylinders, and this will certainly help tie things together. It's not practical to do this on the cam side as the outer block wall prevents a straight shot from the deck to the base, but on the welch plug side where the support is needed most it's fairly easy to do. Unfortunately the block is quite thin at the base of the cylinders so drilling and tapping will probably weaken this area seriously - and remember this is also where the main webs are anchored. You may be tempted to run long studs all the way from the main caps to the head, but even if you somehow get past the cam-side wall the studs will then intersect the oil passages and you'll be removing material from an area that can ill afford it. Not only that, you'll have to somehow seal around the studs to prevent coolant leakage into the sump and also at the head end. It mightn't be impossible, but I seriously doubt it would be worth the effort and the end result could well be a block that is weaker than it was originally.
To summarise, prolonged high power/high rpm levels will be problematic for any block. Drag or street engines should have few problems but for applications such as circuit racing it's something to consider. The way to help the block survive is not by working on the block itself, but by using the lightest possible pistons and rods with a properly balanced counterweighted crankshaft. More details in the relevant section. For engines that are less than very highly tuned - and any remotely sensible street engine - block breakage is unlikely to be much of a problem, even with a non-counterweighted crank.