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Discussion Starter · #1 · (Edited)
If I'm going on a long trip and I set the power level to 500hp I don't see any reason why I couldn't run 89 octane fuel. Compression ratio is 9.5 to 1 and from what I understand there is no boost at the 500hp level as it keeps the bypass valve in the supercharger open. Am I missing something? Please don't quote the owners manual as I'm able to read. I just can't see why I couldn't do this while cruising down the interstate and save a few bucks on fuel as the added octane is really not doing anything for me in the scenario. I wouldn't think at 9.5 to 1 I would want to use 87. Any technical thoughts?
 

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It would more then likely be fine. Stock tune will pull enough timing to keep things safe.

That being said, keep in mind that even in 500HP mode, the supercharger is still used to torque fill and keep fuel efficiency up.
 

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It would more then likely be fine. Stock tune will pull enough timing to keep things safe.

That being said, keep in mind that even in 500HP mode, the supercharger is still used to torque fill and keep fuel efficiency up.
Exactly! The computer is smart enough to adjust for that. Even if you hammer it, you'll just be down on power but won't damage anything.
 

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Discussion Starter · #4 ·
So what do you mean by torque fill? Are you saying that it will give a little boost if it needs the extra oomph given the power profile programmed into the ecu? I'm no tuning expert, but did some tuning several years ago when I had a mustang I put a vortech on when I lived in England. So I guess what I'm saying is I'm no expert, but I'm no dumby either. Just don't ask my wife as I'm sure she'd disagree! LOL
 

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If I'm going on a long trip and I set the power level to 500hp I don't see any reason why I couldn't run 89 octane fuel. Compression ratio is 9.5 to 1 and from what I understand there is no boost at the 500hp level as it keeps the bypass valve in the supercharger open. Am I missing something? Please don't quote the owners manual as I able to read. I just can't see why I couldn't do this while cruising down the interstate and save a few bucks on fuel as the added octane is really not doing anything for me in the scenario. I wouldn't think at 9.5 to 1 inwod want to use 87. Any technical thoughts?
You are missing something. At part throttle at lower engine RPMs and with some load cylinder filling is pretty darn good. Among other things the butterfly valve is opened wider and pumping losses are reduced.

Because of this better cylinder filing detonation is very likely. The engine controller will of course dial back the timing but in doing so undoes any "benefit" you think you might get by driving in 500hp mode.

Have to mention too the timing retard amount is probably based on the engine being fueled with the correct octane of gasoline. Run a lower octane than called for and the engine controller may not have the adjustment range. Or the adjustment range is restricted because with too much retard other issues may arise. With delayed timing exhaust gas temperatures go up. You burn 89 octane gasoline and maybe burn exhaust valves. Not good.

Besides my experience with my Hellcat is while it may not make enough boost to get the pressure gauge to switch from in/Hg to PSI the supercharger being over driven is making boost from a pretty low RPM which is why the engine has so much torque at lower RPMs.

The Hellcat makes 411 kb-ft at 1,200RPM. The 5.7L Hemi makes its 410 lb-ft. peak torque at 4,800RPM.

Run the grade of octane the manual calls for. With my Hellcat on the open road (freeway) when driven with reasonable restraint it delivered around 22mpg to 23mpg fuel economy with a red key. Sure not to Prius mpg levels but not too shabby.

Might add in viewing the HP "gauge" the Hellcat engine is making no where near 707hp on the highway. More like 60 to 80hp.
 

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Yes you can get away with this idea when cruising. Some of us have had to use lower octane fuel over the years in winter time when 93 octane doesn’t get used much here in the north and you get a bad tank full. Making this a habit to save some money is not a wise choice though. Best of luck
 

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I'm going to try VALET mode next weekend when I go out of town; for mileage. It shifts a tad later than STREET mode, and maybe does a 2nd gear takeoff? If it reduces power, maybe increases mileage? I REALLY miss the 4-cylinder mode from my 392!
 

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Why you would consider using a sub grade fuel in such an expensive performance car just to save a few dollars stupefies me.
Literally tens of dollars we're talking about here. I always think that when I see someone selling their stock brembo pads on here used lol, "$75 less than 5k miles" when you can spend 90 and get the powerstops and reduce that dust issue.

LITERALLY, TENS. OF. DOLLARS. OP.
 

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Yes you can get away with this idea when cruising. Some of us have had to use lower octane fuel over the years in winter time when 93 octane doesn’t get used much here in the north and you get a bad tank full. Making this a habit to save some money is not a wise choice though. Best of luck
Do not know about 93 having found it only once and in Wyoming but I have been up to the upper part of Michigan in the winter and other areas west of the Mississippi River in the dead of winter and have not had any issues finding 91 or with the 91 being stale or in some locales 90 as the elevation is considered high enough that 90 suffices.

I don't know what I'd do in such a case but I know barring an emergency I'd never use any octane grade of fuel less than what the factory called for.
 

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Discussion Starter · #11 ·
So what do you mean by torque fill? Are you saying that it will give a little boost if it needs the extra oomph given the power profile programmed into the ecu? I'm no tuning expert, but did some tuning several years ago when I had a mustang I put a vortech on when I lived in England. So I guess what I'm saying is I'm no expert, but I'm no dumby either. Just don't ask my wife as I'm sure she'd disagree! LOL
 

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Discussion Starter · #12 ·
Thanks for all that added technical information to the discussion, much appreciated. It was just a brain fart I had the other day. As mentioned, for a 700hp car to get 25mpg is pretty amazing, at least that's what I got driving 200 miles one day on the interstate. What's not to love about that!
One other interesting tidbit regarding octane. I owned an RT 6m prior to my Hellcat. The manual specified 91 for the manual and 89 for the automatic trans? Only logic I could see in this is you load the engine more when taking off from a dead stop with the manual
 

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Imo cheaping out on the fuel is just that lol. Driving around in 500hp mode to literally save a cpl of dollars makes frugal sense but not logical sense to me personally with cars like this. Interesting question though even if it never occurred to me before lol.
 
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If I'm going on a long trip and I set the power level to 500hp I don't see any reason why I couldn't run 89 octane fuel. Compression ratio is 9.5 to 1 and from what I understand there is no boost at the 500hp level as it keeps the bypass valve in the supercharger open. Am I missing something? Please don't quote the owners manual as I'm able to read. I just can't see why I couldn't do this while cruising down the interstate and save a few bucks on fuel as the added octane is really not doing anything for me in the scenario. I wouldn't think at 9.5 to 1 I would want to use 87. Any technical thoughts?
Hell, I'm on HC #4. Currently have a 2020 RedEye Challenger. Have always run 87 Octane in them. I don't race, have never gotten a code and cars always run great. Save your money and just put in regular gas.
 

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Hell, I'm on HC #4. Currently have a 2020 RedEye Challenger. Have always run 87 Octane in them. I don't race, have never gotten a code and cars always run great. Save your money and just put in regular gas.
So you're not going to follow the manufacturers suggested octane to save $6.00 a fill up on a $70,000 car? Racing really has nothing to do with it.
 
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