All combustion engines require the flow of gas. In a petrol or diesel engine, the flow of intake gas (air) starts at the intake to the airbox. The air flows into the airbox, passes through the filter, exits the airbox and then often passes through an airflow meter. In some cars it then moves straight onto the intake manifold; in others it passes through a throttle butterfly. There might also be one or more turbo compressors ahead of the engine.
If at any point in this process there are restrictions, the pressure of the air will decrease. Therefore, by measuring pressure drops through the intake system, you can quickly identify where the restrictions are. Clearly, the process will work best if the airflow is at the same levels that occur in the engine when it is operating; therefore, the technique is most accurate if it is undertaken when the car is being driven on the road.
On the exhaust side, the exhaust gases are forced out of the engine by the rising pistons. The gas then passes through a catalytic converter, perhaps a particulate trap or NOx adsorber, before then moving through one of more resonators and mufflers.
If the exhaust flowed perfectly, there’d be no back-pressure. But we know that’s not the case, and so by measuring how high the pressures actually are, we can see how well the exhaust is flowing. Once again, the process is most accurate if it is undertaken when the car is being driven on the road.
You can see that by measuring pressures in both the intake and exhaust, you can quickly and easily get a really accurate picture of what is occurring.
Straight away, you can know for sure whether it’s worthwhile changing the factory air-filter for an aftermarket unit, whether the intake snorkel needs an upgrade, or whether the tubing between (say) the air box and the turbo is restrictive.
On the exhaust side, you can immediately see whether the exhaust system is restrictive – and also in what driving conditions it is most restrictive. Guesswork is minimised.
So how do you make these measurements?
On the intake side, restrictions will show up as pressure readings that are below atmospheric. That is, if there’s a restriction to flow, not all the air available from the atmosphere will ‘get through’ – resulting in a pressure drop. Measuring pressures below atmospheric can be done with a manometer or a Dwyer Magnehelic meter.
The easiest way is to use the Dwyer Magnehelic gauge (other manufacturers also make similar gauges but they're not nearly as common). These gauges are incredibly sensitive to very small pressure variations and have superb, clear markings and a large size.
If you want an even cheaper approach, make your own manometer. This'll take you about 5 minutes and cost very little.
You need: a plastic soft drink bottle (1.25 - 2 litres), a metre of so of dowel (any small bit of wood will do), sticky tape, 4 metres of small-bore flexible clear plastic tube.
Attach wood to bottle, so when bottle is sitting upright, wood protrudes vertically above it.
Attach plastic tube along length of wood, one end of the tube going to the bottom of the bottle. There will be lots left over at the top.
Three-quarters fill bottle with water (put some food colouring in it if you want).
Wherever the top surface of the water ends up, mark a line on the bottle. Every inch above that line (working your way up the stick) place another line. Write some numbers on the wood to mark off 5 inches, 10 inches etc, counting upwards from the surface of the water.
The way this manometer works is similar to sucking water up a straw. Except in the case of car testing, the pressure drop in the intake does the sucking for you.
In a naturally aspirated car, measuring the total pressure drop (ie total restriction) is as simple as using a quarter-inch rubber hose to connect the vacuum port of the Magnehelic gauge to the intake system just in front of the throttle butterfly, and then pulling peak power from the engine on the road. (With the water-and-tube manometer, the long length of free tube goes to the intake. Make sure that you don’t suck the water out of the manometer – this might happen if the intake system is incredibly restrictive!)
In a turbo car, the maximum intake restriction is measured in front of the turbo. In a naturally aspirated car without a throttle butterfly, you can plumb the manometer or Magnehelic gauge to the intake manifold.
Connection to the intake system can normally be made by temporarily pulling a breather hose and using that point to make the connection.
As the airflow rises and so the intake system starts to become restrictive, the measured pressure drop will increase. To put it simply, the water level will rise higher in the manometer, or the Magnehelic gauge will show a greater negative pressure (sometimes also called a ‘vacuum’).