Practical Engine Building: Tuning Related Failure
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Tuning Related Failure
10.25
| 00:00 | - When analysing a failure, it's important to examine the components and consider whether they show signs of a tuning related issue that may have contributed to, or resulted in the failure. |
| 00:11 | When I'm talking about tuning related failures, I'm really talking about two situations that can be easily influenced by the engine tuner. |
| 00:20 | These include an engine that has been run too lean, and an engine that has been suffering from detonation. |
| 00:26 | Many times the engine may experience both of these scenarios simultaneously. |
| 00:31 | It's also possible that the engine tuner may have exceeded the mechanical strength of the engine components, but we'll cover this is a separate module. |
| 00:40 | Now that we understand the two situations we're interested in, we can consider what each of these failures may look like. |
| 00:47 | We'll start with detonation, as this is normally very evident and hence quite easy to diagnose. |
| 00:54 | We need to start with a very brief refresher on what detonation or knock as it's also known as, actually is. |
| 01:02 | Detonation occurs after the spark event has initiated normal combustion and this separates detonation from another type of abnormal combustion called preignition. |
| 01:14 | Preignition as its name implies, occurs prior to the spark occurring, and the terms detonation and preignition are often confused. |
| 01:23 | When an engine suffers from detonation, pockets of unburnt fuel and air around the outside of the combustion chamber, spontaneously combust, as the pressure and heat in the combustion chamber rise. |
| 01:36 | During normal combustion the combustion process proceeds smoothly and gradually resulting in a smooth release of energy and a gradual increase in pressure inside the cylinder. |
| 01:48 | When detonation occurs however, large amounts of energy are released rapidly, much like what we could expect with a stick of dynamite for example. |
| 01:57 | The pressure released during detonation causes damage in a number of ways. |
| 02:03 | Firstly the pressure spikes can strip away the boundary layer of gases that normally protect the aluminium piston and cylinder head from the full heat of combustion. |
| 02:13 | In mild instances this may result in a slight sand blasted appearance to the piston crown, usually around the outside edge of the crown. |
| 02:23 | While it might seem illogical, often this damage is first apparent on the intake valve side of the piston. |
| 02:30 | As the specific power level of the engine climbs, the damage that occurs due to detonation also becomes more severe, and the damage happens more quickly. |
| 02:40 | In moderate cases we can see melting and deformation of the piston crown, while in extreme cases the entire side may be melted out of the piston as if the piston has been attacked with a gas cutting torch. |
| 02:54 | Consequently this sort of damage is often confused with a lean mixture which we'll discuss shortly. |
| 03:01 | In cast pistons that are much more brittle than a forged piston it's common to see mild to moderate levels of detonation result in sections of the ring land being broken out of the piston. |
| 03:14 | You could liken the forces of detonation to someone slamming into the piston crown with a hammer and this is what results in this damage. |
| 03:22 | In some instances this sort of damage may not be apparent until the engine is actually stripped, as a broken section from the second ring land, will remain trapped between the rings. |
| 03:34 | In this case you may notice significant oil consumption to alert you to the fact that something is not right. |
| 03:41 | If you're using an aftermarket piston that includes contact reduction grooves or anti detonation grooves on the top ring land, mild detonation can result in these grooves showing pitting or sand blasting. |
| 03:54 | Often detonation will be visible here before it shows up on the piston crown, so it's very important to inspect this area during an initial tear down. |
| 04:04 | Detonation can also have the affect of crushing the top ring land down on the top compression ring, and this will be noticeable because the ring will no longer be free to move in the ring groove. |
| 04:16 | For this reason I always like to make sure that the rings are free to move smoothly in the ring grooves. |
| 04:22 | If you've got an engine that's been suffering from moderate to severe detonation, then it's also possible that the cylinder head may show signs of damage in the form of pitting around the outside of the cylinder. |
| 04:34 | This can often be seen in the squish pads or squish bands on the cylinder head. |
| 04:40 | Again this is easy to overlook and it can require some corrective machine work, so it's important to inspect the heads during disassembly along with the pistons. |
| 04:50 | In some cases severe detonation damage can also affect the heat treatment of the head, leaving it soft in places which can affect the head's ability to seal correctly against the head gasket. |
| 05:03 | If you've got severe detonation occurring, then you may also see signs of this around the top of the cylinder on the block, particularly if you have an alloy block. |
| 05:13 | Another aspect that's worth looking at when investigating detonation, is the condition of the big end bearings. |
| 05:19 | Detonation results in a severe shock load being transferred through the piston and into the conrod. |
| 05:25 | This can result in wear or damage to the big end bearing shells, where the bearing has been pushed through the oil fill and resulted in contact with the crank journal. |
| 05:35 | This will initially be obvious as damage to the upper shell of the big end bearings. |
| 05:41 | The next aspect we'll consider is a lean mixture or lean air fuel ratio in other words. |
| 05:47 | In my experience detonation related failures are actually most frequently misdiagnosed as a lean mixture because someone has seen that the piston has begun to melt, or perhaps has a hole melted out of the side of it. |
| 06:01 | To confuse matters though, often a lean mixture can promote detonation due to the resultant higher combustion temperatures so often it can be tricky to clearly define the exact failure mode. |
| 06:15 | When the engine is operating under high load we're typically going to use a richer mixture which supplies excess fuel to mix with the intake air. |
| 06:24 | We're relying on this excess fuel to help cool and control the combustion chamber, and combustion temperature and ensure engine reliability. |
| 06:34 | When the tuner reduces this excess fuel, we have what we refer to as a lean mixture and this can result in higher temperatures in the combustion chamber. |
| 06:43 | With this in mind we can consider what the likely failure may be resulting from a lean mixture. |
| 06:49 | The excess heat will be transferred into the crown of the piston and to a lesser degree, the skirt of the piston. |
| 06:56 | This results in thermal expansion and the piston may show signs of partially seizing in the bores. |
| 07:02 | Most often this will be apparent by scuffing and galling on the skirt of the piston, and there may also be signs of material transfer between the piston skirt and the cylinder wall. |
| 07:13 | This sort of failure can have similarities to what we'd expect to see from an engine that has been machined with insufficient piston to bore clearance. |
| 07:22 | However in this instance we're likely to see pronounced damage on the ring lands, whereas insufficient piston to bore clearance will begin to show up on the skirt initially. |
| 07:32 | If the engine has been operated with a lean mixture for an extended period of time, it's also likely that you'll be able to see a burnt or ash appearance on the underside of the piston crown, where the oil mist in the block has essentially begun to burn to the underside of the piston, leaving a visible deposit. |
| 07:51 | Excessive combustion temperatures can also result in melted or damaged exhaust valves, although care needs to be taken when considering this sort of failure in isolation as a burnt exhaust valve on its own may be more in line with debris becoming trapped under the valve seat or incorrect valve lash adjustment preventing the valve from properly dissipating heat from the valve head into the valve seat when the valve is closed. |
| 08:19 | When we're investigating a failure, we need to consider the entire engine, rather than a single, isolated cylinder. |
| 08:26 | For example if we have a failure on number one cylinder and it's clearly showing signs of detonation, but the other seven cylinders look perfect, it may not be the tuner's fault. |
| 08:37 | When a failure is isolated to a single cylinder, or even perhaps a pair of cylinders, we need to consider other alternatives. |
| 08:46 | Perhaps the injector on that cylinder has become partially blocked or perhaps there's a fuel supply deficiency on that particular cylinder. |
| 08:55 | Before jumping to conclusions, it's important to get a complete and well rounded view of all of the parameters that may have added up to result in the failure. |
| 09:05 | Only then can you draw a logical conclusion as to the cause of your failure. |
| 09:10 | Tuners often get a bad rap, and the finger is inevitably pointed when something goes wrong. |
| 09:16 | While this may well be justified, often there's more going on than is first apparent. |
| 09:22 | The best way of avoiding tuning related engine failures, is to ensure that you select an engine tuner that has a great reputation, and it would also be advisable to select someone who is experienced with tuning for the same application you're dealing with. |
| 09:37 | That doesn't necessarily mean the same exact engine, but rather if high boost methanol drag engines are your interest, then you'd want to select a tuner who is experienced in this area. |
| 09:49 | I'll point out that no tuner is 100% infallible, and sometimes problems will arise. |
| 09:55 | It's always worth discussing your expectations with the tuner so everyone is on the same page. |
| 10:02 | For example if you're wanting every last horsepower out of a particular engine and you're prepared to risk an engine failure to achieve that, then fine. |
| 10:11 | Others however may not be able to stomach the financial outlay required if an engine failure was to happen, and hence a more conservative approach from the tuner may be justified. |
