Last month we looked at how crankshafts are manufactured, what options are available and what these options mean in plain English. This month we will dig a bit deeper and look at the starting point of any performance engine: the cylinder block.
There are lots of options available to consider and a confusing array of technical jargon that comes along for the ride. We will cut through the jargon and find out what really matters when it comes to choosing a suitable engine block for your next build.
Iron versus Alloy
For many years the world of production V8 engines has been dominated by the good old cast iron block. For better or worse this was all that was available, and cast iron is still the material associated with most of the traditional performance-oriented V8 engines that are popular for modification.
Cast iron isn’t a bad material for an engine block, and while the resulting engine is inevitably pretty heavy, it is also quite strong and rigid — and both are important traits when trying to produce a lot of power.
Aluminium engine blocks have become more common in production cars over the last couple of decades as manufacturers looked for improvements in engine efficiency. The aluminium block offers significant weight savings over cast iron, thus its use lowers the overall weight of the car and so improves fuel economy and performance.
Since aluminium is a soft material, it is not possible to run the pistons directly against it as we do in a cast iron block. There are a couple of ways around this. The most common is to fit each bore with a thin iron sleeve during the block casting. An alternative is to apply a hard coating directly to the aluminium, something like Nikasil, an avenue that is common in motorbike engines. Both techniques need some thought when modifying an aluminium block, as your options may be limited when it comes to boring the block to a larger diameter.
Aluminium is not without its disadvantages, and when it comes to ultimate strength in an all-out performance engine, an alloy block is not usually as strong as an iron equivalent. Since the aluminium is not as rigid as cast iron, head gasket integrity can be a problem with high combustion pressures. In extreme cases flexing or cracking of the thin iron sleeves can occur, rendering the block suitable only for scrap.
AfterMarket Sleeve Kits
For the more popular alloy blocks, aftermarket companies have come to the rescue with sleeve kits available off the shelf. These kits require extensive machining to the block to allow the weak alloy bores to be replaced by a thick ductile iron sleeve. The sleeves normally form an integrated deck surface to improve the head gasket integrity at the same time, thereby dealing with most of the downsides of the alloy block.
If you are searching for the strongest blocks available, then billet aluminium is undoubtedly the best option. These are the blocks used for Top Fuel drag engines capable of producing in excess of 700kW per cylinder. Typically a billet block will be fitted with replaceable sleeves made from ductile iron for the pistons to run in. While these blocks are only really suitable for drag racing, it is still interesting to look at the technology.
The strength advantage of the billet block comes from three areas.
A billet block starts with a forged billet of aluminium, which has a much higher density and a superior grain structure than a cast item. This improves rigidity as well as ultimate strength.
The quality of the billet material can be controlled much more accurately than a casting, and unwanted material inclusions and porosity can be completely eliminated.
Billet blocks for drag racing use are machined without water jackets. This means the cylinder sleeves are better supported by the surrounding material, and flexing and cracking under extreme pressure is eliminated.
As a side note, billet blocks can also provide other special features unavailable in a production unit, such as extra head studs to help improve head gasket sealing.
Solid filled blocks
As we have already seen, the thickness of the cylinder wall has a big impact on the overall strength of the block. A thicker cylinder wall will also provide more power, as the cylinder will be more dimensionally stable under very high combustion pressure. This means the cylinder will not deform in shape, and the rings will retain a good seal on the bore walls.
For drag-only use it is common to fill the water jackets of a production block with a special compound to improve strength and eliminate bore flex and distortion. The block filling compound is usually a special type of concrete, which is mixed with water and then poured carefully into the water jacket before being allowed to set. An important aspect of the compound is that once set, it provides the same thermal expansion properties as the parent material of the block. This ensures that no stress or distortion is introduced as the block reaches operating temperature.
Supporting the Crankshaft
In high power and high rpm engines, retaining the crankshaft securely becomes critical to ensuring reliability. Under high loads, the caps that retain the main crankshaft journals in the block can flex and fret against the block, or in extreme situations they may even fail completely.
Older generation engines will typically hold each main cap into the block with a pair of bolts (that’s known as a two-bolt main). While this is sufficient for a low-power application, raise the stakes and some alternative measures are required.
Most late-model performance engines will provide four-bolt mains, which means that each main cap is now secured to the block by two bolts on each side of the crankshaft.
The four-bolt caps may also be fitted with larger diameter or higher grade bolts to provide a better clamp against the block.
In extreme applications, the ultimate solution is the six-bolt main. This takes the four-bolt idea one step further and adds a pair of lateral bolts that normally bolt into the cap through the side of the block, providing superior strength and rigidity.
If you are stuck with a two-bolt block all may not be lost. Discuss your intentions with your engine builder and you may find that the strength of the two-bolt block will be sufficient for your needs. If you will be exceeding the limit of the two-bolt block then there may be options available to convert the main cap to four-bolt, although the cost of the machining required can often come close to the price of a better block, so make sure you do your homework on what is available.
Unfortunately, in the world of performance engine building there is often more than one option that will suit your requirements. It would be great if everything was black and white, but life just isn’t that easy. This article is aimed at helping you understand some of the technical aspects of engine blocks so that you can make more informed decisions. The best advice is always to deal with a reputable engine builder who is familiar with your specific engine. This way you can guarantee that the block you choose will handle your intended power level.
Words: Andre Simon
This article is from NZV8 issue 65. Click here to check it out.