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This article was first published in 2008.
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Nearly everything to do with making good engine power revolves around having the right gas flows. How much torque the engine can make on each combustion stroke depends largely on how much air and fuel has been jammed into the chamber, while how much power is available depends on how quickly that process can be repeated. The flow of the exhaust gases will also have a dramatic affect on power - get more gases through the exhaust pipe and the engine will in turn be able to breathe in more of the good gases.
However, directly measuring gas flows is usually difficult - instead, it's easier to monitor pressure differences. (Plus the required equipment is much cheaper!) But what do pressures have to do with it anyway? There are two sorts of pressures which can be measured - above atmospheric and below atmospheric. So what are they then?
Pressures
The weight of the atmosphere lies on all of us - we're at the bottom of a blanket of air. As you go higher in the atmosphere the pressure becomes less, because there is only a thinner layer above. The fact that we don't consciously feel atmospheric pressure is because the pressures inside and outside of our body are equal. When even a slight inequality occurs in that pressure - say when descending a tall hill or in an aeroplane that's coming in to land - we feel an uncomfortable sensation in our ears. So, while we don't usually realise it, at sea level we're subjected all of the time to about 14.5 psi or 1 Bar of air pressure.
If you take some of the air out of a metal can, atmospheric pressure will crush it - don't underestimate the force that is available, just because we don't usually feel it.
The flow of air into a naturally aspirated engine is an example of a higher air pressure (normal atmospheric pressure) flowing air into a lower pressure (less than atmospheric). To get more air into the engine we can't lower the pressure in the cylinder any further, but we can increase the induction pressure above atmospheric. Add a turbo or a blower and more air flows into the engine than before because the pressure difference is greater. We don't usually think about things in such detail - but you know that if you run more boost (say a lift from 7 to 10 psi) then your engine will probably go harder.
- Gauge & Absolute Pressures
The use of a pressure gauge on a turbo or supercharged car therefore makes lots of sense - when it's connected to the plenum chamber after the throttle you can see how high the pressure is that's trying to push air into the cylinders. A boost pressure gauge of this sort shows 'zero' when the engine is off. But shouldn't it still be showing 14.5 psi or 1 Bar - atmospheric pressure?
Well, yes and no. This sort of gauge is calibrated to show zero when being subjected to normal atmospheric pressure. Most gauges are like this - any reading on the gauge is technically called "gauge pressure". In some books you'll see figures like "10 psig" - it means "10 pounds per square inch [on the] gauge", that is 10 psi + 14.5 psi (atmospheric pressure) above absolute zero.
German car specs often show turbo boost pressures in absolute values, rather than gauge values. So the new car specs might say that absolute turbo pressure is 2.1 Bar - which gets lots of dumb journalists excited. But when you take note of the 'absolute' you know that you have to subtract atmospheric pressure (ie 1 Bar) to get back to a gauge boost figure - which brings it down to a much more manageable 1.1 Bar (~16 psi) boost.
It's pretty easy to understand pressures above atmospheric - as is shown on most gauges. But what about pressures of less than atmospheric? We commonly know these as "vacuum" - but that's deceptive, because there are lots of degrees of vacuum. Unfortunately, instead of calling pressures below atmospheric "minus psi" or "minus Bar", pressures of less than atmospheric are usually given weird units like "inches of water" or "inches of mercury". The higher the number, the lower is the pressure.
This is easier to remember if you picture yourself on a stepladder, a plastic hose in your mouth. The other end of the hose goes down into a bucket of water. You suck on the hose (which really means that you lower the pressure in your mouth) and atmospheric pressure pushing on the surface of the water in the bucket forces the water up the hose. The higher the water rises up the hose, the further below atmospheric that you have made the pressure in your mouth.
For example, you might make the water rise by 10 inches up the hose - that means you've created a low pressure in your mouth that's called "10 inches of water". Sucking harder might increase the water column height to 20 inches. This is why in this form of measurement, lower pressures are given higher numbers. (And yes to prove a point you could do it with a mercury column as well... but since mercury is a dangerous poison, we wouldn't recommend it. Plus you'd need to find someone who is very good at, er, sucking...)
Most flow benches use water manometers that work in design just like the sucked-on plastic tube.
Using Positive Pressure Gauges on Cars
So what use is all of this information? First up, let's look at just pressures above atmospheric - the sort that most gauges read.
As briefly mentioned above, a turbo boost gauge is of this sort. Connect the gauge to the plenum chamber and you'll be able to measure the boost pressures that are trying to force air into the cylinders. So you plumb the gauge into place and - as an example - it shows a peak of 10 psi. Easy, huh? You have 10 psi boost.
But in what conditions did this occur? What was the gear? What was the load? What was the temperature? In reality, turbo boost will be altered to a greater or lesser degree by all of these factors. You might find, for example, that the boost controller that you are using lets boost fall as revs rise - that might be desirable as the intercooler will also be heating up during this period, or it might be undesirable as you want to keep max possible power happening at all revs. Turbo cars whiz their turbos up faster on the dyno (lots of exhaust gas can be produced as the engine is heavily loaded in a way not achievable on the road) so the boost pressure behaviour will invariably be different between road and dyno.
A manifold-connected boost gauge can be watched for:
- The revs/load when boost first starts to occur
- Variations in peak boost with revs/load
- Variations in boost with temperature
- Speed with which boost comes back up after a gear change
- Variations in peak boost with different gears
In short, when a humble boost pressure gauge is studied in lots of different driving conditions, it's surprising how much can be learnt.
But there's also a completely different way of using a boost gauge. Most people leave the gauge connected only to the plenum chamber after the throttle - but what if you move the sensing hose around? For example, what if you place it just in front of the throttle? We previously recorded a peak plenum boost of 10 psi (say at 4000 rpm in second gear on a 25 degree C day). So will the boost level recorded in front of the throttle be that as well?
The answer is - probably not. Say you make the measurement and here the peak boost is 10.5 psi. What's going on - how come the pressure is higher in front of the throttle than behind it? Where does 0.5 psi of boost pressure go? The answer is that the throttle is providing a flow restriction - not all of the air can get through it.
Next you move the boost pressure hose to the throttle side of the intercooler. Peak boost here is still 10.5 psi - so the plumbing between the intercooler and the throttle is not causing any restriction. But then you find that the max boost pressure on the turbo side of the intercooler is 12 psi - there's a 1.5 psi pressure drop across the intercooler! You thought that the turbo was pumping air at 10 psi but the pressure being developed by the compressor is actually 20 per cent higher!
In fact, these pressure drops are typical and nothing much to worry about. But if you find that the pressure drops are much larger than these figures, start looking at upgrading flow capability of the obstructive components. Remember: the greater the pressure drop across the device, the greater its flow obstruction. A boost gauge isn't being used really effectively until you move the sensing hose around and get a feel for what's happening right thorough the post turbo intake plumbing.
Another excellent use for the boost gauge is to measure exhaust backpressure. An exhaust backpressure gauge is (usually temporarily) plumbed into the exhaust as close to the engine as possible. Typically, the oxygen sensor is unscrewed to allow a threaded fitting to be inserted into the exhaust at that point.
The higher the exhaust backpressure, the poorer is the flow of the complete exhaust. That includes the mufflers, pipe work and cat converter. But as with measuring boost, there are lots of subtleties to the measurement. For example, you'll find that exhaust backpressure normally peaks in first gear at max power. This is because it is in this gear that the engine accelerates most quickly, so the exhaust gases have less time to escape down the pipe. A 'traffic jam' develops and the backpressure is at its greatest. So, despite there being no more exhaust gases produced in first gear than second gear (at the same revs), the backpressure will vary depending on the gear.
The lower the exhaust backpressure the better - simply, there's no scientific basis for the concept that an exhaust needs a minimum backpressure. However, that's not the same as saying that the 'tuned' behaviour of an exhaust isn't important, and to get the best resonant behaviour happening in an exhaust of a naturally aspirated car, the exhaust plumbing might need to be smaller than would be optimal if exhaust back-pressure alone was being examined. Certainly, in a turbo car you need no such qualification - go for a pipe size that gives the lowest backpressure possible.
When actual backpressure measurements are made, many people are surprised at how high the figures are. While an engine will put up with higher backpressures without harming power (than it will cope with the opposite happening - restriction on the intake) it's still the case that pushing exhaust gases down the tube into a wall of pressure takes power that's subtracted from the flywheel output. The presence of a high backpressure will also result in more cylinder contamination as residual exhaust gases hang around in the combustion chamber.
A standard R32 Skyline GT-R's exhaust creates a massive 6.5 psi backpressure, while a standard Commodore VL Turbo was measured at close to 7 psi. One car - a highly modified Daihatsu Turbo Mira - had a peak backpressure of 5 psi in first and second gears, but this dropped to 3.5 psi in fourth gear. Although you should aim for zero, you're unlikely to get it below about 1-2 psi for the complete exhaust system with mufflers and a cat converter.
And in just the same way as the boost gauge probe can be moved around the engine bay to detect the restriction of the intercooler, throttle body, etc, if the point at which the exhaust backpressure measurement is made is moved along the exhaust a picture can be constructed of where the major restrictions are. This is easily achieved if a small hole is drilled into the exhaust pipe and (on mild steel systems) a short section of copper tube is brazed into place. A rubber hose can then be pushed over the tube with the hose led off to the gauge. For each pressure tapping point drill the small hole and braze the tube into place.
It sounds like lots of work but it isn't - and it can save literally hundreds of dollars over replacing things at random.
Using Negative Pressure Gauges on Cars
So far we've covered measuring pressures above atmospheric - or gauge pressures. But measuring pressures that are below atmospheric can be even more useful, because it can guide you as to the restrictions occurring on the intake to the engine. Any pressure drops below atmospheric in front of the intake runners (on a naturally aspirated engine) or the turbo (obviously, on a turbo engine) will have a major and dramatic impact on power, economy and responsiveness.
Think about the turbo compressor for a moment. It's drawing air in at its eye, swirling it around and then flinging it into the pipe that heads for the intercooler. Any restriction at the mouth of the turbo will limit how much air it's capable of shoving into the engine. If there's no restriction on the airflow that can reach the turbo, the air pressure at the mouth of the turbo will be the same as atmospheric. But if there's any restriction in front of the turbo (the filter, for example), then the air pressure in front of the turbo will be below atmospheric. The further it is below atmospheric, the greater the difficulty that air is experiencing in rushing-in to fill the turbo's hunger for air.
[The pedants will point out that it's more complex than this: that placing a measuring probe through the wall of the tube will result in a reading also influenced by how fast the air is moving - and that in turn will be influenced by the diameter of the tube. And they're right. But in the real world, measuring the pressure of the air in front of the turbo (or throttle body in naturally aspirated cars) works very well at showing you how well the system flows.]
So how do you actually measure pressures below atmospheric? As discussed above, such measurements are often measured in 'inches of water', and you can literally use inches of water in a column to measure the pressure. Placing a tube upright in a plastic bottle of water (eg a narrow-necked softdrink bottle) and connecting the top to the place where you want to measure the pressure will cause the water to rise up the tube - you can see how high it rises and then measure this height above the surface of the water. It's actually a very accurate way of making the measurement.
However, many people choose instead to use sensitive gauges that are designed to measure these below-atmosphere pressures. The best are the Dwyer Magnehelic gauges but other manufacturers also make similar gauges.
The maximum pressure drop will be measured at peak power, when the demand for air is at its greatest. For example, a standard 5-cylinder turbo Audi S4 had a maximum pressure drop of 32 inches of water, measured just before the throttle body. That's pretty high, and takes into account the flow restrictions through the intake, airbox, filter and airflow meter. Simple modifications to the intake system reduced this substantially - the pressure drop of the intake snorkel alone was reduced from 9 inches of water to zero, for example.
And that latter measurement brings up another point - just as when measuring boost and exhaust backpressure, you can move the measuring hose around and so work out which parts of the system are causing the greatest restrictions. In this case, drilling a small hole in the plastic of the intake system and screwing in a miniature irrigation fitting will give you a convenient hose tapping point. When you've finished, a smear of oxygen sensor safe black silicone will completely cover the tiny hole.
Conclusion
Don't overlook how effective pressure gauges - both positive and negative - can be to performance applications in your car. They can be used to reveal a huge amount of low-cost information - and all while the car is being tested on the road.
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Cheap Gauges
Pressure gauges can easily cost lots, but if you're on a budget, look in secondhand machinery salesyards where often used gauges are sold.
For most positive pressure testing (eg turbo boost, exhaust back-pressure) a gauge that goes to about 1.5 bar / 22 psi should be suitable. Note that it doesn't have to have 'boost' or anything like that written on it, and if you're using it just for test purposes neither does it have to be an automotive-size gauge. Negative pressure gauges are harder to find, but they're also available. If the negative pressure gauge uses confusing markings, clean the fitting and then try sucking on it - just a gentle suck should move the needle a long way, and full-scale deflection should be easily able to be achieved by mouth suction alone. (But don't blame me if you suck something horrible out of the gauge!)
Gauges that look well below their best can often be effectively restored. This gauge - it measures negative pressure over a range of 0 - 60 inches of water with 1 inch of water graduations - was bought secondhand for AUD$10. Its rim was rusty and the plastic gauge face badly scratched.
However, a few minutes with wet-and-dry, followed by metal polish and then a very low-cut car polish brought the plastic back to 8/10 condition, while a buffing belt was used to remove the rust and corrosion from the metal rim. After that it looked like this - excellent value for ten bucks!
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More Info?
This article summarises techniques previously covered in other more specific AutoSpeed articles. To see more details on each of the techniques do a search based on key words such as 'intakes', 'negative boost', 'exhausts' and 'boost pressure'.
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