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Discussion Starter · #1 · (Edited)
It seems there may be some confusion about gasoline octane and the new 5.7L Hemi truck engine. The Hemi has 346 cubic inches or 5.7 liters of displacement and makes 345 horsepower and 375 pound feet of torque (SAE corrected power.) At over 60 HP per liter (normally aspirated pushrod engine) the 5.7L Hemi is and should be considered a high performance truck engine. To achieve this kind of HP and torque a 9.6:1 compression ratio is used, which is roughly 1 point higher than some previous truck engines. The Hemi makes use of this higher compression ratio by being calibrated on 89 pump octane fuel.

The Hemi’s speed density engine control unit (ECU) utilizes dual knock sensors to protect the engine from harmful spark knock when fuel with less than 89 pump octane is used. Is it OK to run 87 octane? Absolutely. Will you get less HP and torque? Yes defiantly. Will you feel the difference in the butt dyno? No, probably not. With the use of knock sensors the calibration doesn’t need to be anywhere near as conservative, in terms of spark advance. The dyno and vehicle calibrators can calibrate the spark advance right to the edge of spark knock without fear of damaging the engine because the knock sensors will pick up the spark knock (usually before it’s even audible) and tell the ECU to retard the spark advance. When the spark advance is retarded HP and torque are reduced but the biggest reduction is going to be from the torque converter stall speed up to peak torque.

Will your fuel economy be less with 87 octane? Yes. Will you notice the difference in fuel economy under normal driving conditions? Probably not, but when the engine is under medium to heavy loads with the reduced spark advance the ECU does add addition fuel (which also reduces power output) to keep the exhaust manifolds and catalysts from getting too hot. A stock Hemi will make slightly more HP and torque on 91 octane fuel because it insures that there is no knock retard at any RPM. You should always run a fuel slightly higher than what you’re calibrated for on a knock sensor equipped engine. When a cylinder starts to spark knock the knock circuit doesn’t just remove 1 degree of spark advance, it removes about 4-6 degrees to stop knocking immediately then it slowly adds the advance back in unless spark knock returns or continues. When modifications are made to increase HP and torque the increase comes from additional cylinder pressure. This cylinder pressure increase can cause spark knock at a speed/throttle where there was no spark knock previously.

Most of the popular MODS for the Hemi increase airflow. The O2 sensors will recognize and adapt for the additional airflow and increase the fuel delivery at all part throttle or stoichometric air-fuel-ratios (closed loop) only. This addition fuel is carried into medium and wide-open-throttle (WOT) speeds/loads but does not guaranty that that air-fuel-ratio (AFR) is correct for the best HP and that the AFR is rich enough to keep things from getting too hot. When the AFR is too lean the engine is a lot more prone to spark knock as well which goes back to the reduced spark and reduced HP/torque.

If you have engine MODS (headers, CAI, exhaust, power wire) or tow with your Hemi I strongly recommend using the highest octane fuel you can get short of race gas. The power wire especially should be used with 93 octane because it tricks the ECU into not reducing the spark advance automatically when the inlet air temps get above 100°F. The knock sensor then reduces spark when knock occurs as mentioned above by removing 4-6 degrees of advance when the correct inlet air temp would have only reduced the spark advance by 2 degrees to stay out of spark knock. On a dealer scan tool (DRBIII) there are parameters called ST and LT knock retard. If your truck has extensive MODS and if you have access to a scan tool you can check to see if your getting any knock retard with the fuel you’re using by driving around a bit and doing some WOT runs with the scan tool connected while having someone view these parameters.

If you have knock retard, try better fuel. If you’re using the highest octane possible you’re probably running too lean or you need to reduce the spark advance. If you’re too lean you can check the exhaust gas temps (EGT’s) with thermocouples. Around 1650°F measured 2” down from the exhaust manifolds in the exhaust pipe is staring to get too hot. I use a digital two channel gauge called a Pyro 2000-Y. http://www.hoytmeter.com/pyro2000/pyro2000.html You can also use a wide range O2 sensor to get the exact AFR but at higher engine speeds your mainly fueling to control temps.

All Hemi durability/endurance testing is done on BP/Amoco 93 octane fuel. The 3.5 years that I was at GM they also used only BP/Amoco 93 octane fuel for all durability testing. Where I live BP/Amoco 93 octane is only $0.10 per gallon more than 89 octane. I try to use only BP/Amoco 93 octane in my Hemi Ram when ever possible as it insures that I’m not losing any performance.
 

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DAMN! Paragraphs....that is long as hell.
 

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Anyone got Cliff's Notes for it?
 

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That was a pretty good read. Some of it is pretty standard stuff you can get from allpar some of it is kinda technical. Now can you tell us your creds. and where you got the information?
 

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Discussion Starter · #6 ·
Lew360 said:
That was a pretty good read. Some of it is pretty standard stuff you can get from allpar some of it is kinda technical. Now can you tell us your creds. and where you got the information?
Almost twelve years engine dyno experience, a small race shop for 1.5 years, GM for 3.5 years and DCX for the past 7 years.
 

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Lew360 said:
That was a pretty good read. Some of it is pretty standard stuff you can get from allpar some of it is kinda technical. Now can you tell us your creds. and where you got the information?
Yes, known buy some but not all. Good read, i'm sure it will enlighten a few if they take the 2.5 min to read it
 

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Discussion Starter · #10 ·
one short of said:
When does the movie come out by the time i got done reading i forgot the first half.
Thanks for the negative comments! I'm glad I took the time to try to help clear up any confusion as to how the engine control system really works! I wish I could include a link to the pictures of the torn down Hemi engine that had 5 damaged pistons that was posted here a few weeks ago. If someone could post a link to them I think they would be a great addition to this thread. You could see in the pictures the short block with the cylinder heads on the bench next to it with the shorty headers still attached to the heads. I think the dealer replaced the the whole engine under warranty. Absolutely unbelievable. The engine clearly ran lean and spark knocked itself to death. As for the owners manual I didn't see anything in there about any of the MODS we do to our trucks either.
 

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Discussion Starter · #11 ·
KIP311 said:
Yes, known buy some but not all. Good read, i'm sure it will enlighten a few if they take the 2.5 min to read it
Thank you for the positive feed back. If it only helps one person it was worth the time it took to compile and write.
 

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dodge3471 said:
Thank you for the positive feed back. If it only helps one person it was worth the time it took to compile and write.
No problem, I accept and appriciate all info that can potentially save my truck and/ or money. Keep the knowlege/experience coming... :rck:
 

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dodge3471 said:
Thanks for the negative comments! I'm glad I took the time to try to help clear up any confusion as to how the engine control system really works! I wish I could include a link to the pictures of the torn down Hemi engine that had 5 damaged pistons that was posted here a few weeks ago. If someone could post a link to them I think they would be a great addition to this thread. You could see in the pictures the short block with the cylinder heads on the bench next to it with the shorty headers still attached to the heads. I think the dealer replaced the the whole engine under warranty. Absolutely unbelievable. The engine clearly ran lean and spark knocked itself to death. As for the owners manual I didn't see anything in there about any of the MODS we do to our trucks either.
All comments are in fun and much like my self I am sure that every body is grateful somebody is doing research for us/the guys who would rather sleep or drink and watch the family guy all day.

ALL INFORMATION IS GOOD INFORMATION WHEN USED.
 

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one short of said:
All comments are in fun and much like my self I am sure that every body is grateful somebody is doing research for us/the guys who would rather sleep or drink and watch the family guy all day.

ALL INFORMATION IS GOOD INFORMATION WHEN USED.
Yep, I'm that guy :rck:
 

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dodge3471 said:
Almost twelve years engine dyno experience, a small race shop for 1.5 years, GM for 3.5 years and DCX for the past 7 years.
All that info makes perfect sense, now you got me wondering about my 4.7, I only use 87 octane, what is the stock compression ratio on the 4.7? If it helps your answer, I also have some mods that increase airflow such as a fastman tb and h.o. cams, what octane would you recommend I run? Please note, unlike the hemi, the 4.7 manual calls for 87octane for stock engines. Thanks for your input.
 

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awsome info, thanks :rck:
 

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Dodge3471,
Do you recomend any type of octane booster? And if so what brand. I can only get 91 octane where I live. I have mods that require 93+ octane. I tired a few but don't really know wich is best. Any help would be great. By the way that was an awesome read on how the engine control works Tahnks!!!
Mitch
 

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dodge3471 said:
Almost twelve years engine dyno experience, a small race shop for 1.5 years, GM for 3.5 years and DCX for the past 7 years.

Just wanted you to know I wasn't knocking you. :D I was just trying to find out how reliable the info. was. You obviously know about what you wrote.

It was worth every second it took to read since I have no experience with the hemi and plan on getting one very soon.

Thanks. :gr_patrio
 

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Dodge3471, that was a GOOD read. Solid, factual and useful information provided by someone who knows what he's talking about is always appreciated. Nice work.

Now...will you turn your attention to the 2004 Torque Management problem? You'll be a real hero if you can tell us how to override it. I absolutely HATE this feature of our trucks. It destroys driveablility in some situations and severely limits the performance we SHOULD get out of our Hemi's!

 
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