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Fast Friday #8: Octane And The Turbocharged Engine

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Volvo Repair Database Fast Friday #8: Octane And The Turbocharged Engine

Post by matthew1 » Fri Aug 31, 2012 3:01 pm


Fast Friday #8 is by... Lucky! We're lucky to have Lucky writing all this surprising stuff. Thanks, Lucky.
Missed last week? Nooo! Unmiss it now: 3... 2... 1... Ignition.

Octane And The Turbocharged Engine

Octane on the Street
Octane in the turbocharged performance world is a fairly broad and wide subject matter and conversations abound about when and where to use it. This week on MVS Fast Friday I thought we’d take a look at octane ratings and how/when they are useful to the performance enthusiast. To keep from getting too deep in the subject and in turn get bogged down in semantics let’s discuss the matter as it relates to your typical street driven turbo model.

Higher Octane = Higher Burn Resistance... Wait, What?
Firstly Octane is both a chemical and a rating but for this conversation we’ll focus on the definition of octane as a rating of fuels and how it impacts the engine and subsequently performance. The higher the octane rating of a fuel the more resistant to burning/ignition it is. This is not just a threshold increase of its initial ignition temperature or energy requirement but extends across the fuel as a whole, meaning the higher the octane of fuel the slower it will burn. So right off the bat a higher octane fuel will require more advanced ignition timing just to account for the extended burning time to get the peak cylinder pressure to occur at ~12 degrees after top dead center, which for most situations is the preferred peak cylinder pressure point.

But Lucky, if higher octane fuels take more energy to ignite and take longer to burn then how can they be considered advantageous when measure against lower octane fuels? Good question, the answer lies in the constant battle we have with turbocharged cars: heat! Anytime you compress air it’s going to increase in temperature, with a turbo charger it’s no different. The more boost you run the more heat is imparted to the air going into the engine. This heat along with latent heat from the previous combustion cycle can increase enough that the fuel can actually ignite before the spark plug lights it off, which is called pre-ignition or ping. Pre-ignition can be very damaging to an engine if it is severe enough or occurs over a long enough period of time. So higher octane fuels help avoid this.

Enter the Knock Sensor...
Knock sensors are intended to detect pre-ignition and report back to the ECU so it can reduce timing. By reducing/retarding ignition timing the total amount of heat for that combustion cycle is reduced and the peak pressure point doesn’t occur too far before top dead center which can be damaging to the piston and rings. ECU’s try to constantly adapt to changing ignition needs and optimize timing for maximum power and efficiency. This is how an ECU can adapt to higher octane fuels and provide increased performance when they are used.

Help Your Tuner Help You
When it comes to tuning an ECU you should let your tuner know what octane is used most commonly so they can adjust the base timing map. This provides the quickest and best scenario for the ECU to advance timing to the maximum safe limit which provides the best power and performance for your engine.

Robert Lucky Arnold

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1997 850 T5, MSD ignition coil, Hallman manual boost controller, injectors, R bumper, OMP strut brace [gone]
2004 V70 R [gone]


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