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Discussion Starter #1
I recently bought a 2011 Challenger R/T and 2011 Charger R/T and upon reading the manual to see the recommended fuel type, I see that my Challenger (6 speed) calls for 91 octane, while the Charger (or automatic transmission Challenger) calls for 89. Does anyone know why the manual transmission model calls for a higher octane fuel?
 

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The 6spd cars have a more aggressive ignition timing curve. The Hemi in your Charger has the same compression ratio,10.5-1 and will benifit from the higher octaine.
 

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Discussion Starter #3
From the Charger manual:

"The use of premium gasoline is not recommended, as it will not provide any benefit over
regular gasoline in these engines."

I just thought it was odd to see a higher octane recommendation, based upon the difference in transmission...that is, unless there's a mechanical difference with the engine (like you said ignition timing curve).
 

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The automatic 5.7s have 89 recommended, but can run 87, but with lower peak output and likely slightly reduced MPG (timing being pulled if 87 isn't adequate enough).
 

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The automatic 5.7s have 89 recommended, but can run 87, but with lower peak output and likely slightly reduced MPG (timing being pulled if 87 isn't adequate enough).
I understand that. I still don't quite get why the transmission would dictate the grade of octane being used, unless something was different on the engine.
 

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Something is different on the engine with six speed cars. It's the tuning as mentioned before. That's why the automatic cars are advertised as 370 or 372 HP and the manual cars are advertised as 375 HP. The 91 octane will allow the engine to take advantage of all the timing the computer has to offer.
 

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I understand that. I still don't quite get why the transmission would dictate the grade of octane being used, unless something was different on the engine.
The engineers opted to tweak the exhaust on the 6-speed applications (it was the loudest that could pass the EPA drive-by standards) and since it breathed more efficiently, they could tweak the tuning [more timing advance and some fuel trim modifications] and get some hp and tq. that in turn required 91 octane for max output.

The MDS versions required an exhaust system that would attenuate the effect of the exhaust tone when in 4cyl mode and that put a bit more back pressure and reduced the output potential, compared to the 6-speed setup.

As another potential consideration, a lot of US buyers are a bit put off with having a premium-fuel only requirement (many European makes are premium only for example], and Chrysler probably opted for a more conservative tuning with the 'auto/5.7 and went with 89 preferred, but 87 as a minimum octane rating to keep operating costs perceived by potential buyers as being more affordable.
 

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None of this makes much sense to me. I've wondered for years why manufacturers keep pushing premium fuel when the state of modern electronics compensates for high compression ratios and a host of other factors of the fuel in the tank. A friend of mine has a new SUV with a CR of 13:1 that runs on regular 87.

I'm also one of those who simply will no longer buy a car that runs only on premium.
 

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None of this makes much sense to me. I've wondered for years why manufacturers keep pushing premium fuel when the state of modern electronics compensates for high compression ratios and a host of other factors of the fuel in the tank. A friend of mine has a new SUV with a CR of 13:1 that runs on regular 87.

I'm also one of those who simply will no longer buy a car that runs only on premium.
Compression ratio isn't a factor if the timing is later. Hard to have pinging if there's no fuel or spark when it would matter. :)

As was already mentioned, the 6-speed cars have a different tune that makes a little more power due to more advanced timing. The main reasons to run higher octane are higher CR with the same timing or more timing with the same CR.

Wife's Honda V6 has a CR of 10.5:1 but only calls for 87 (unless towing, in which case Honda recommends 91), same as my 6-speed R/T that calls for 91+. It's all in the tune.
 
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