VW Performance Engine Basics
The principle is to get as much oxygen into the cylinder
as possible, with the correct amount of fuel (about 14:1 air: petrol).
Ignite the charge at a precise moment, and remove the spent gases as
completely as possible. There are two main principles for getting the
inlet charge into the cylinder:
Normally aspirated relies on the vacuum created in the cylinder. Inlet gases are then pushed in by atmospheric pressure. There are many ways of increasing the incoming "charge", big ports, big valves, large duration camshafts, exhaust systems that "pull through" using the much higher speed exhaust gases, to help evacuate the cylinder on camshaft overlap, Usually the larger the overlap the higher up the rev-range the power is produced. The higher the compression ratio the more the "suck" by virtue that the space at top dead centre is smaller to start with. Higher compression also releases its energy more violently. Rod length moves the piston faster or slower away from TDC. The optimum length of the conrod is in the hands of whoever builds a particular engine, ask ten engine builders you'll probably get ten different answers. Large stroke crankshafts create faster piston speeds, large bores give the availability to fit larger valves. Big strokes, short conrods, and big pistons combined with high RPM create "massive" forces, the piston has to stop at both TDC and bottom dead centre, fortunately its both accelerated and decelerated gradually, a 94mm piston at 9000 rpm could weigh as much as 3 tons, the conrod and wrist pin have to be strong enough to handle these forces 300 times per second. The valve train is the most limiting factor for high RPM, the heavy lifter, long push rod and rocker arm all combine to make a very heavy valve train, modern engines are now almost exclusively overhead camshaft the valve spring only has to return the valve. The crankcase and crankshaft have basically remained unchanged since its design, over 65 years ago. The 3 bearing crankshaft creates power robbing flex, combined with a crankcase (engine block) that was really only designed to handle 40 horsepower, these point to unreliability. Nonetheless the VW drag racer has to work within these confines, to extract the maximum amount of power, but still remain reliable.
Ways of producing more power:
Both can create a runaway chain of events, if not handled properly, the basic principle is to make an engine more efficient by introducing extra oxygen combined with the correct amount of fuel. A turbocharger uses the power of the exhaust gases (heat) that would normally be lost to atmosphere, a supercharger on the other hand, is mechanically driven. Although the boost from a supercharger is almost instantaneous it takes mechanical power from the engine, (parasitical power loss) this can be quite substantial, an engine could show 300 bhp, but for the power robbing supercharger it could be 400 bhp. The down side of turbo's are turbo lag, also turbo's have a tendency to heat the incoming air, the engine efficiency falls as the air becomes hotter. To counteract this the air can be cooled by an intercooler, making the air denser. An engines compression ratio is arrived at, by dividing the swept volume (overall cylinder area + head area + working clearance at BDC) by the volume at TDC (head area +working clearance). This gives a 'static compression ratio'. In practice a normally aspirated engine may not be able to fill the swept volume completely, conversely a forced induction engine could fill the cylinder more than 100%. This is why super/turbocharged engines run lower static compression ratio's. In practice the static compression ratio must be multiplied with boost pressure, to get an indication of the true compression ratio. If a 2 litre turbocharged engine can force a similar amount of air, to that of a 3 litre normally aspirated engine, the turbo engine will perform similar to the larger engine (providing there's boost pressure) without the extra mechanical masses of the larger engine. What an engine needs to perform is cool-thick air, a turbo produces exactly the opposite, hot-thin air, albeit lots of it. An intercoolers job is to convert hot-thin back to cool-thick, the amounts of air involved are quite substantial, the design of intercoolers are a specialised subject in its own right. Nitrous Oxide on the other hand bypasses the main problems, it doesn't heat the incoming charge, the hotter the cylinder the more likely the chances of detonation, It introduces more oxygen/nitrogen, the nitrogen content helps to keep detonation even more under control. The extra oxygen when combined with the correct amount of fuel is the area were the power derives from. Nitrous motors can also benefit in that there's no restriction on the exhaust, the engine can use the benefits of a fully tuned freeflow exhaust system. The next part of this article will deal with, how Bugster Britain has used Nitrous, detailed information on Nitrous and how we use it, also getting technical on turbo's. If anybody has any questions no matter how small, or anybody would like to add their content, please use our questions and answers message board or 'e" mail.
(This article was written in july 2000 before any conjecture on porsche heads)