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5 Things I Thought All Tuners Knew

January 05, 2018

 

 

Hi all, this is another rant video. I’m calling it 5 things I thought tuners knew, because I’m always a bit surprised when tuners ask this. You could probably make some “no true Scotsman” type argument and say that real tuners do know these things, but I don’t want to get into a definition argument of who qualifies as a real tuner, I’m just going to say the 5 things here! They aren’t really related in any way, except for that I’ve noticed them all since my last rant article.

The first one is really basic, it’s the definition of pressure and how it’s measured. Pressure is absolute; it’s relative to zero pressure which is a perfect vacuum. Atmospheric pressure at sea level is about 100 kPa, which means 100 000 newtons per square metre, which means if you had a square 1 metre on each side, the force pressing on it would be the same as the weight of about 10 tonnes. So why can is this empty plastic bottle not crushed by this pressure forcing in from all sides? Because it has the same pressure inside the bottle as well as from outside, so there’s no crushing effect. If you sucked out all the pressure from the middle of the bottle, it would be crushed, and you can see this when you close a bottle at high altitude, and then bring the bottle down to sea level, it is crushed in.

Because this pressure is all around us all the time, we often don’t notice it, and when we have positive pressure (or a partial vacuum), we often don’t care about the actual pressure, but about the pressure difference from atmospheric pressure. For example to get a wastegate actuator to deflect, we have atmosphere on one side and our pressure source on the other, so it’s the pressure relative to atmosphere that causes the actuator to move. We call this pressure “gauge pressure”, presumably because a gauge also measures the pressure above atmospheric rather than absolute pressure, so in the atmosphere it shows zero pressure (instead of about 1 bar, 101 kPa or 14.7 PSI). Many pressures such as oil pressure, tyre pressure and so on are conventionally expressed as gauge pressure.

Absolute pressure, ie relative to a perfect vacuum, is what you need to use to calculate how much gas is in a particular volume. In the ideal gas equation pV = nRT, the pressure is an absolute pressure (and temperature is also absolute). So when you’re trying to calculate the mass of a gas at a specific pressure, you care about the absolute pressure, and that’s why the “A” in “MAP” for manifold absolute pressure.

In some cases it’s actually the pressure difference we care about though. For example in the boost control example, often a wastegate actuator will have two ports, and it’s the pressure difference across those two ports that causes the diaphragm to deflect, so in that case it’s actually the differential pressure that’s relevant.

 

 

This means that you need to be careful when talking about, for example, fuel pressure. Because if you’re trying to work out how much pressure the fuel pump needs to provide, and how good your seals have to be, then you care about the gauge pressure (ie, the pressure above atmospheric). However if you’re trying to calculate how much the injectors are going to flow, and how hard they are being forced shut by fuel pressure, it’s actually the differential fuel pressure, ie the fuel pressure minus the manifold pressure, that you care about. And this is further complicated by the fact that MAP is usually an absolute pressure, whereas fuel is a gauge pressure, so to subtract the two you need an atmospheric pressure measurement.

 

 

The second thing I’m going to talk about is doing the minimum that replicates the problem. I’m going to do a whole article just on problem solving and troubleshooting, but just in this one I’ll say there’s no need to push a system with a problem harder than you need to in order to replicate the fault. For example if you have a trigger error, then that will often replicate with no load on the engine. So doing power runs, with a known trigger error (which could cause the ignition to fire at the wrong time) is asking for trouble, and not necessary when you can replicate it in the no-load condition.

Number 3 is rotary ignition timing. Rotary engines normally have 2 spark plugs per rotor; a leading plug at the bottom and a trailing plug at the top. Usually, it’s the leading plug that initiates the combustion because it fires first, and some time (or angle) afterwards, the trailing plug is fired. This difference in firing angle is referred to as the ignition split, or split.

 

 

Therefore when you tune a rotary, the value that you’re entering on the ignition map is the timing for the leading plug. A separate split table gives the split value, and this is subtracted from the main ignition table to give the trailing ignition timing.

 

 

 

For example if you have 25° in the ignition map and 10° in the split map, then the leading plugs will fire at 25° BTDC and the trailing will fire at 15° BTDC.

 

 

TDC is defined the same as with a piston engine; the crank position that gives the minimum clearance volume. Sometimes people get confused by this because there is no factory timing mark at TDC; on the FC engine there’s one mark at 5° ATDC and another at 20° ATDC, whereas on the FD there’s only one at 20° ATDC.

 

 

It is possible to run negative split; the RX8 runs negative split at idle from the factory for example, and in this case it’s actually the trailing plugs that initiate the combustion. So if you have 5° in the ignition map and -10° in the split map, then the trailing plugs will fire at 15°BTDC. But the ECU will still report the ignition timing (which means the leading ignition timing) as 5°.

 

 

Number 4, I don’t know why there’s this misconception out there that if you have a misfire you’ll get a rich reading from an oxyen sensor. I think maybe people have forgotten their basic chemistry. The chemical reaction for complete combustion of a hyrdocarbon is:

CxHy + nO2 -> xCO2 + y/2 H2O

An oxygen sensor, in the exhaust, measures the leftover oxygen. Ie the amount of oxygen that wasn’t consumed in this combustion. So if you have a lean mixture, there will be lots of oxygen left over – at stoich you get some left over because nothing’s perfect and the richer you go, the less oxygen remains in the exhaust. If you have a misfire for any reason other than a lack of air going into the engine, you’re going to end up with a LOT of excess oxygen. Even if it’s a rich misfire, and definitely if it’s an ignition misfire. So a misfire gives a lean reading, if your equipment is quick enough to pick it up. I don’t want to hear anyone saying again that a misfire gives a false rich reading unless they have some data to back this up.

The last one is somewhat related, and that is that people, many people including experienced tuners, say that backfires are an indication of a rich mixture. Let’s first describe what people mean by “backfire” – because traditionally it meant combustion that occurs out through the intake. But commonly now it’s used to mean combustion outside the engine, eg in the exhaust system. Pops and banks from the exhaust would often be described as a backfire. I guess this is possible in theory for a rich mixture to cause a backfire, but I’ve never seen it. The reason is related to the previous one, if you have an overly rich mixture then you’ll have very little excess oxygen so it simply can’t burn. It could burn when it gets to the end of the tailpipe and mixes with the fresh air from outside, but on most front-engined cars the exhaust will be so cold by this point that it’s not going to ignite in this condition.

On the other hand, if you have a mixture that’s too lean, then it can be pumped through the cylinders but be too lean to combust in the chamber (ie cause a lean misfire), so you have a mixture of fuel and air that comes out into the exhaust system. This can be ignited by very hot exhaust gas from another cylinder, or a hot spot in the manifold, exhaust valve and so on. This is what you hear when you hear a popping or banging in the exhaust, and richening the mixture usually makes it go away because it takes away the excess oxygen.

The last one is really based on my own experience, but I’ve had this discussion with tuners many times when they haven’t been able to fix their backfiring on overrun but not in fuel cut problem, they’ve tried reducing the fuel and it gets worse. Maybe there are others where being too rich has been the cause of the problem and I just haven’t seen them…. but I doubt it!


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