If you're planning to do some serious modifications to a four-stroke engine, you'd better do it right if you don't want your engine to end up with an expensive pile of scrap metal. It's easy to slap on a turbo and run mild boost on a stock engine or even fitting a bigger turbo to an OEM turbo engine, but if you're looking for serious power, you have to rebuild the subassembly to ensure that it can handle the additional power without disintegrating. Obviously you need to ensure that your drive train can handle the extra engine power as well, but in this section we'll discuss engine building for more power, starting with the subassembly and the cylinder block.
The Cylinder Block
You've got to start by ensuring that your cylinder block is race grade. Even if you're just building a street race car, engine tuning would be senseless if the block is not up to the job.
Start by pressure testing the block. If you have an air compressor you can do this yourself. Strip down the engine but leave the Welch plugs and oil gallery plugs in place. Fit the bare cylinder head to the cylinder block using new head gasket or one that's not too worn. Close all the water opening in the cylinder head off by bolting steel plates over the water openings. One of the plates must be fitted with an air line coupling that you can connect your air compressor to. Gradually increase the pressure in the block to 40 psi. Be careful not to increase the pressure too quickly as a loose fitting Welch plug or a weak spot in the block could blow out and can cause you serious injury. If nothing pops or gives and the pressure remains constant, gradually increase the pressure to 50 psi. Now spray the block with a mild water/detergent mixture and carefully check the block for air bubbles. If you see any bubbles, either mark the area and have it repaired at an engineering shop or reject the block and test another one.
Preparation and Cleaning
If you get no bubbles, release the air pressure and remove the cylinder head. Use a plug tap to clean the head studs and main bearing cap threads and chamfer any stud hole that is not already chamfered. This will prevent the thread from pulling up. Grind away any casting sag, especially around the main bearing webs, the sump pan deck, and the valley area of a Vee engine. This will prevent cracks from developing. Now remove all the Welch plugs and oil gallery plugs and have the block boiled and cleaned in a chemical bath. This will remove any rust and scale that might be building up in the water channels, and any caked oil in the oil channels and oil galleries.
The Crankshaft and Conrods
The stock crankshaft and conrods (connecting rods) are usually cast iron items that can be retained if the engine is not required to handle high boost pressures, high horse power, and high revs. Forged crankshafts and conrods are much stronger and are more suitable for high load, high rev engines. In either event, you should take the crankshafts and conrods to an engineering shop and have them magnafluxed to check for cracks.
If your stock conrods are acceptable for the level of power or RPMs you will be running, you should have the stock rods polished, magnafluxed again and shot peened, and then have the big (bearing) end re-sized. Smoothing and polishing the conrods is the first step. This removes any stress risers or surface imperfections that could lead to a crack. Once the rod is polished you should magnaflux them to detect any existing hair-line cracks. If magnafluxing does not show any cracks, the next step is have them shot peened and then have the big end re-sized using aftermarket conrod bolts, such as SPS bolts.
Balancing the Conrods
Finally, have the rods balanced, which basically means ensuring that each rod's small end is the same weight, that each rod's big end is the same weight, and that each rod is the same weight. So how do you weigh the big end or the small end? Simply support the end opposite to the one you're balancing so that only the end you're weighing rests on the scale and then ensure that the ends of each rod is weighed in exactly the same position. Also, when weighing the big end, you should weigh them with the bolts installed and mark each bolt so that you can fit them to the same side of the same rod that they were in when you weighed them.
Checking the Crankshaft
Next is the crankshaft. If the crankshaft has no cracks, check it for straightness. A crankshaft that is even 0.002in out of straight will increase bearing load and will be the cause of bearing failure. If your crankshaft is out of straight, you have two options – either have the crankshaft straightened or machine the crankshaft's main journals so that crankshaft rotation is true. However, straightening a crankshaft that is to be used for a high boost, high horse power, and high rev engine is a waste of time and money as the combustion pressure and inertia loads will reverse the straightening process. Machining the crankshaft journals will also weaken the crankshaft. Ultimately, replacing a bent crankshaft is your best option.
It goes without saying that all the crankshaft journals should be checked for roundness and size. The same goes for the big end on the conrods. The crankshaft, conrods, and flywheel should then be balanced statically and dynamically to reduce shock loading and vibration.
Cast, Hypereutectic Cast and Forged Pistons
The next thing you need to consider is the pistons.
Most OEM engines are fitted with cast aluminum pistons with a slotted oil groove. High performance OEM engines may be fitted with hypereutectic cast aluminum pistons that have a higher silicon content. The higher silicon content makes the cast material much harder and more wear resistant, which allows these pistons to withstand greater temperature and pressure loads. This makes these pistons ideal for street racers. However, the higher silicon content also makes the pistons more brittle and prone to breaking under detonation. Thus, these pistons are not a good choice for forced induction applications where the possibility of detonation in greatly increased.
Forged pistons, on the other hand, have much denser and even harder than hypereutectic cast aluminum pistons but are not as prone to breaking under detonation. Forged pistons also have drilled oil holes round the oil groove rather than a slot in the oil groove. This makes them the best option for high horse power and forced induction engines.
Full Skirt and Slipper Pistons
Pistons can also be either full skirt pistons or slipper pistons. The full skirt pistons have a round skirt, which makes the piston resemble a pot. Slipper pistons also have a skirt but the skirt is not round but squared off at the gudgeon pin holes. Full skirt pistons are heavier but are generally stronger that slipper pistons. They are also less prone to wobble as the skirt extends right round the circumference of the piston. Needless to say, a full skirt piston of the same grade as a slipper piston would be the better option for most engine modification projects.
Dish, Flat-top and Dome Pistons
Pistons can also be dish pistons, flat-top pistons or domed pistons. Dome pistons can be used to increase the compression ratio as the dome extends into the combustion chamber of the cylinder head. This requires the dome to match the shape of the combustion chamber and often requires valve pockets to prevent the often catastrophic results of the piston and valves making contact. Dome pistons are usually used to increase compression on naturally aspirated engines, and can be used with Nitrous injection (NOS). They are not advised for turbocharged or supercharged engines where the intake charge (the air/fuel mixture) is already compressed outside the engine. Flat-top pistons can also be used to increase the compression ratio if the stock piston is dish piston. Flat-top pistons might also require valve pockets. Finally, dish pistons can be used to reduce a too high compression ratio or for applications such as turbocharging and supercharging.
Once you've got your conrods, crank shaft and pistons sorted, the next step is to balance them. We've already balanced the conrods. We must now balance the pistons and then balance the piston and conrod combinations and mark each combination as we did with the conrods and rod bolts. This part is static balancing and should be followed by dynamic balancing, which is done by an engineering company and entails balancing the harmonics of the crankshaft's rotational frequency. This basically means, balancing the centrifugal rotation of the crankshaft, conrods and piston assembly. This balancing reduces vibrations makes the engine more efficient and reliable. It will also allow it to operate more reliably at higher engine speeds.
Cylinder Head Sealing
Another important consideration when putting your engine back together is the cylinder head sealing. There are two things to consider here: aftermarket high-tensile strength cylinder head studs and an aftermarket cylinder head gasket.
Cylinder Head Fasteners
Some engines have cylinder heads that are fastened with head bolts rather than studs. These bolts should, however, be replaced with studs if you're looking for serious power as a stud carries a more evenly distributed torque load. The reason for this is because a bolt is subject to both a twisting force and a clamping force as the cylinder head is torque into place. A stud on the other hand is not subject to any twisting force and is only subject to a vertical clamping force that it shares with the nut.
Cylinder Head Gasket
A stock cylinder head gasket might maintain proper sealing on a stock engine, but when you increase the compression ratio and combustion pressure that gasket will in all probability not be sufficient. Replacing the stock head gasket with an aftermarket multi-layer steel gasket would be your best option. However, not all multi-layer steel gaskets are the same. Some are laser cut while others are stamped from a die. The later tends to be of better quality as stamping provides better dimensional consistency but the ultimate would be what is called a "stopper" head gasket. These are stamped from dies but also have a folded "stopper" layer around the openings for the cylinders that increases the gasket thickness around the cylinders by about 0.15mm. This helps increase the clamping load and the seal around the cylinders when the cylinder head is torque in place.