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100 Octane Mods for 997 M97 Engine


Old 11-23-2016, 09:32 PM
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Question 100 Octane Mods for 997 M97 Engine

I was thinking about performance mods for a 997.1S 3.8L M97 engine, and wondered how difficult it is to take advantage of a higher octane fuel.

Here in California, we only have 91 octane at most stations, even though the engine is designed for 93. Fortunately, 100 octane is readily available for a reasonable price (I don't commute with the car, so $9/gallon is fine).

To some extent, the ECU can increase the compression and produce more power with higher octane fuel by advancing the timing. However, there is a limit to this. With a custom dyno tune, how much could the timing be advanced to take advantage of a given octane? In other words, could you program an ECU map to extract power from 95 octane?

Secondly, let's assume you wanted to go all of the way and build an engine for 98 octane. To do this, I assume you need some way to mechanically increase the peak engine compression ratio. What is the lowest cost and/or least destructive way to do this? Can you just insert a thinner head gasket, or do you need to actually mill down the head? Or what?

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Old 11-23-2016, 10:48 PM
Join Date: Jun 2015
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I have experience building performance non-Porsche engines so my observations are speculative to your specific question.

First understand that there will be some level of octane that increasing above stops adding power. In other words the engine may make peak power using 93 octane and using higher octane gas will not change the power. As you mention, Porsche specifies 93 octane and relies on knock sensors (and possibly other means) to dynamically detune the engine when low octane gas is used. This would be true whether using a stock ECU tune or not. So you should be able to blend your racing gas with 91 octane to get an ideal fuel for much less than $9.

There was a time when some manufacturers would use an ECU tune that did not make optimum power because they were trying to meet other goals - usually emissions related. So it is possible that a remapped ECU could bump your power. My suspicion is that for a non turbo car this remapped ECU tune would not need to be tailored to the gas being used. In other words it would act like the stock ECU and just downgrade based on engine knock. Since your engine is stock (hardware wise) any shop offering the remapped ECU should be able to provide dyno charts showing the power changes.

As for increasing compression, both methods you suggest would work. My gut is that for the change in octane you are contemplating the thinner head gasket would not bump compression enough to take advantage of the better gas. Milling the heads raises a host of issues including possible interference of the valves with the pistons and other geometry issues. You are possibly changing the required length of timing chains/belts, etc. Also the heads could be weaker and given the cost of Porsche parts I doubt there are many experienced shops who know what is a safe amount to remove.

The safest way would be new pistons. And if you were to really go this route most builders would look at the entire engine and consider different cams and other performance parts.
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