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Old 05-03-2012   #1 (permalink)
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Default Octane vs. Horsepower

The factory rates the horsepower of the M113 engine in my 06 SLK55 at 355 hp. My owner’s manual states that the minimum octane fuel requirement is 91 (based on the (R+M)/2 ). Now the Bosch ME consist of a digital electronic control unit (ECU) that analyzes information from various sensors on the automobile's engine and uses that information to control the engine's operation. One of these sensors is a knock sensor (actually two – one for each bank).

So question number one: is the factory horsepower rating based on 91 octane fuel (approximately 95-96 octane in Europe using the Research Octane Number (RON) rating).

Second question: since I have 93 octane in Missouri (approximately 98 octane is Europe) will my engine produce more horsepower. The key to the answer is whether the Bosch ME unit will “learn” the knock threshold for the higher octane fuel or is the engine timing mapped and the knock sensor is simply a safety device?

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Old 05-03-2012   #2 (permalink)
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Based on what other tuners are doing, I think the answer is "yes", up to a certain maximum. I forget where I read it - I think one of the Kleenman forums - the max is somewhere around 100 octane where the values fall out of what a stock ECU can adapt to.
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Old 05-03-2012   #3 (permalink)
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So...to rephrase Bob's question..does the ECU "explore" the power threshold (I would assume at full throttle and/or high rpm) til it senses the first indications of detonation?

Great question Bob
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Old 05-04-2012   #4 (permalink)
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I hope to run the car on a dyno in the next couple of weeks. I had run my 2007 350Z on this dyno and obtained 266 WHp (wheel horse power) with an engine rated at 306 Hp. That is about a 13% drive-line loss which is typical for a manual transmission rear wheel drive car. I will be interested in what I get on this same dyno with the SLK. The biggest issue will be tying the car down securely since the body work is pretty low and doesn’t allow a lot of tie-down options.

Also, interestingly the Nissan had wide-band oxygen sensors which were active through-out the load range including under full throttle acceleration. As best as I can tell the 06 SLK has narrow band oxygen sensors and is therefore using a map/look-up table to establish fueling during full throttle acceleration. Does anyone know if MB has gone to wide-band sensors on the newest version of our engines?

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