Engine Management: High-Performance Tuning: Carburetor, Fuel Injection, Ignition Systems

KNOCK, KNOCK. WHO'S THERE?

In order to gain a more complete understanding of the engine calibration, we need to look in some depth at the type of fuel we are using and how it affects the way we set up the engine.

The main criteria for mapping, or carburetor jetting, is that the engine delivers the best possible power without destroying itself! The ability of the engine to live a useful life will depend to a large extent on the fuel that we use. It isn't uncommon for people to turn up at the rolling road with a tan full of low-octane unleaded.

You can set up the engine on this fuel, but it isn't going to give anything like the power you'll get from a high-octane petrol. Let's first look at how the octane rating of the fuel affects the engine.

As we have already stated, the fuel and air mixture burns in a controlled manner inside the combustion chamber, starting at the spark plug and radiating from there.

If that combustion isn't controlled, we get 'knocking' noises coming from the engine. There is a lot of confusion about engine knocking noises, so let's look at the three most common sources of unwanted noise.


PINKING

This is the most common noise of all, and it produces a light metallic clinking sound. Pinking is actually caused by the ignition being too far advanced. What happens is that the pressure rise in the cylinder occurs too early, and the piston rattles against the bore as it goes over TDC.

This is a mechanical rattle and it has nothing to do with combustion. Pinking at light load and low speed isn't an engine breaker, in fact it is quite harmless as long as it is not allowed to continue for too long. Since you will gain both power and peace-of-mind by retarding the ignition at this point, there is no excuse for living with it.


KNOCK

Inside the combustion chamber, as the flame front spreads out from the spark plug, a series of reactions take place in the gases ahead of the flame front. We call the gasses that are furthest from the plug 'end gasses', and under certain conditions these gasses can auto ignite. in place of the smooth burn, we now have two flame fronts, which collide and produce a knocking sound.

The noise is much heavier than the light pinking previously described.

THROTTLE BODIES

Unlike carburetors, injection throttle bodies are relatively simple devices. All you need is a flap to control the airflow, and a potentiometer on the flap to register movement over the operating range. I have made throttle bodies by cutting up carburetors, machining then, and adding adapters for bell-mouths so that they no longer looked like carburetors.

If you aren't concerned about appearances, you can adapt old carburetors very quickly. The catch is that you still need somewhere to put the injector. On my own engine, I turned up aluminum bosses and welded these into the inlet manifold.

Life is a lot easier if you buy purpose-made throttle bodies, which contain the injector and come complete with fuel rail and fitting clips. Most professional throttle bodies use butterfly throttle-plates, but not all. Slide throttles were popular at one time, but they have several disadvantages (the least one not being the price).

With a slide throttle you can get sticking under high engine vacuum conditions, such as on overrun when the vacuum pulls the slide against it's runners. Ball-bearing systems do help, but you still have the slide opening the throttle from one side of the induction port. On four valve engines this is a distinct disadvantage, when ideally you want to expose both inlet valves at the same time.

• Foreword
• Acknowledgements
• Introduction
• Before You Start
• Engine Checks
• Spark Plugs
• Cam Timing
• The Knock
• What is Power?
• Carburetors
• Carburetor Theory
• Carburetor Selection
• Component Installation
• Engine Mapping
• Using the Emerald M3DK Mapping Software
• Index