I was amazed at what I saw at the garage this weekend. An $80,000 MK1 TT that was sold by it owner for… $11,000. Reason in two words? Nitrous Backfire.
Yes, this 500HP+ 4 cylinder TT lost its life in just a few seconds, as a nitrous oxyde backfire took the engine out.
Nitrous oxide (often referred to as just “nitrous” or “NOS” as the latter refers to the NOS brand, derived from the initials of the company name Nitrous Oxide Systems) allows the engine to burn more fuel by providing more oxygen than air alone, resulting in a more powerful combustion. (By weight, Nitrous contains 36% oxygen while air has only 23%.) The gas itself is not flammable at a low pressure/temperature, but it delivers more oxygen than atmospheric air by breaking down at elevated temperatures. Therefore, it is often mixed with another fuel that is easier to deflagrate. Nitrous oxide is stored as a compressed liquid; the evaporation and expansion of liquid nitrous oxide in the intake manifold causes a large drop in intake charge temperature, resulting in a denser charge, further allowing more air/fuel mixture to enter the cylinder. Nitrous oxide is one of the simplest ways to provide a significant horsepower boost to any gasoline engine.
As with all modifications to increase power, the use of nitrous oxide carries with it concerns about the reliability and longevity of an engine. Due to the greatly increased cylinder pressures, the engine as a whole is placed under greater stress, especially the parts involved with the combustion chamber. An engine with components not able to cope with the increased stress imposed by the use of nitrous systems can experience major engine damage, such as cracked or destroyed pistons, connecting rods, or crankshafts.
Even if the engine is up to the task, severe damage can occur if a problem occurs in the fuel system; an engine running with nitrous oxide depends heavily on the proper air to fuel ratio to prevent detonation from occurring. For example, if the engine’s fuel supply were to be reduced, this would cause the engine to run lean by whatever degree the fuel delivery was reduced, which can lead to engine knock, detonation or backfire. Depending on the engine, this may only need to occur for a matter of seconds before major damage occurs.
It is essential not to reach a fuel cut rev limit as this will also momentarily restrict the fuel flow to the engine and as nitrous is still being injected into the engine without the additional fuel the engine will again run lean and cause detonation. Good optimisation of enrichment fuel is essential otherwise the fuel can ‘drop out’ and puddle in the intake tract, potentially causing a backfire.
Audi TT Nitrous Backfire
So what happened exactly to this TT? Not sure. My theory: a) The driver launched the nitrous at a too low RPM or; b) Ran the TT too lean.
At low RPM, think about what’s going on: you’re spraying nitrous into the intake at a constant flow. That is, the nitrous bottle and solenoids have no idea what RPM you’re at, and they’re just pushing it into the intake at a constant volume. Inside the engine, though, the nitrous and fuel combination is being sucked into the cylinders during every stroke. The net result is that at low RPM, you’re getting far more of the mixture into the cylinders. At 3000 RPM, for example, you’re getting twice the amount as at 6000 RPM. So, you can imagine that running nitrous at, say 1000 RPM, is far more stressful on the motor as at 3000 RPM, and probably caused the TT’s nitrous backfire (the nitrous/fuel combination exploded in the intake manifold rather than in the cylinders.
Likewise, running the car too lean was most likely causing preignition and ignited the mixture at the wrong time, such as above. Because nitrous is more oxygen-rich than air, the recommended air fuel ratio becomes 9.5 parts of nitrous to 1 part of fuel (9.5:1). That means when oxygen-rich nitrous is introduced additional fuel must also be supplied in order to maintain the optimum ratio. Without the additional fuel the mixture would become dangerously lean – circumstances that always lead to severe and expensive damage… and in this case $69,000 loss…
Watch a nitrous backfire on a Mustang