Thread: Premium Fuel?
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Old 03-28-2013, 02:12 PM   #9
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Join Date: Jul 2011
Location: Tucson, AZ
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The REAL difference between the 5.3L LS4 V8 and the 1.4LT is their respective 'sensitivities' to knock (the CAUSE) and its attendent amount of 'knock retard' (the EFFECT).

The V8 is a constant compression engine but the turbocharger makes the 1.4 a "variable compression" engine. Hence, the V8 ECM typically (depending upon fuel quality, ie: octane value) isn't constantly 'fighting' between optimum spark-advance and too much advance, which is exactly what happens when the turbo spools up on the 1.4LT and starts pumping air into the engine--effectively pushing the static CR of 9.5:1 up to as much as 22:1. And, that increased CR needs a higher octane level or the engine begins knocking (mild) or dentonating (extreme) instead of smoothly "burning" its A/F-mixture.

This is why using higher octane fuel in a turbocharged engine almost always produces both more POWER and better fuel ECONOMY, because the engine ECM can advance the timing to extract MORE power from the fuel under load without damaging the engine. Conversely, using a lower octane fuel won't *hurt* the engine because the engine ECM will constantly "pull" timing to keep knocking from occuring, but in doing so, both the power (significantly) and fuel economy (slightly) are reduced.
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Last edited by 70AARCUDA; 03-28-2013 at 02:14 PM.
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