Tuesday, March 22, 2011

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 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 maximum power, starting with the subassembly.


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 water opening off with steel plates. One of the plates must be fitted with an air line fitting that you can connect your air compressor to. Gradually increase the pressure in the block to 40 psi. Don't increase the pressure too quickly as a loose fitting Welch plug or a weak spot in the block could blow out can cause you serious injury. If everything is still in place, gradually increase the pressure to 50 psi. Now spray the block with a mild water/detergent mixture. Carefully check the block for air bubbles. If you see bubbles, either have it repaired or test another block. If you get no bubbles, release the air pressure and remove the cylinder head. Use a plug tap to clean the head stud 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 all rust and scale in the water channels, and the caked oil in the oil galleries.


Chrome-moly forged con rods
Chrome-moly forged con rods

The stock crankshaft and con 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 con rods are much stronger and are more suitable for high load, high rev engines. In either event, you should have the crankshaft and con rods Magnafluxed to check for cracks.

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 con rods. The crankshaft, con rods, and flywheel should then be balanced statically and dynamically to reduce shock loading and vibration.


Forged pistons
High strength 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, forced induction engines.

Pistons can also be either full skirt pistons or slipper type. The full skirt pistons are heavier but stronger and less prone to wobble. Needless to say, they would be the best option for any engine modification project.

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