Octane requirements

What grade gas do I ened to run?

  • 87 octane

    Votes: 12 40.0%

  • 89 octane

    Votes: 5 16.7%

  • 92 octane

    Votes: 13 43.3%
  • Total voters
    30
  • Poll closed .
-
Jarlaxle

Jarlaxle

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I just bought a Duster. (Insert happy dance here.) It seems to be a stock Magnum long-block with a Holley QF carb on an RPM Air Gap intake, headers, Hughes cam kit with 1418ALN-10 cam and roller rockers, about 9:1 compression, iron Magnum heads. Considering this, will this run on 89 octane? Or even 87?
 
Should be fine on 87.

I switch between 87 and what they call Rec 90 in Georgia. Non ethanol 90 octane. I can't tell a difference in my 5.9 Magnum stock bottom end, .218/.224 @.050, .517 lift, Edelbrock 2.02 heads, Air Gap and AVS 650, headers.
 
The magnums (5.2, 5.9) came in big heavy trucks and SUVs and ran just fine on 87 octane. But they had long slow advance curves and knock sensors that were very active. If you’ve played with the timing at all and added a performance style curve you may find that it rattles at 9:1 with iron heads and 87 octane. But you won’t know until you test. The answer to test each fuel and use the lowest octane that the engine doesn’t rattle on is the correct answer.
 
TT5.9mag said:
The magnums (5.2, 5.9) came in big heavy trucks and SUVs and ran just fine on 87 octane. But they had long slow advance curves and knock sensors that were very active. If you’ve played with the timing at all and added a performance style curve you may find that it rattles at 9:1 with iron heads and 87 octane. But you won’t know until you test. The answer to test each fuel and use the lowest octane that the engine doesn’t rattle on is the correct answer.
Click to expand...
Or retard the timing till Ya have a Slug! Lol not Acceptable
 
TT5.9mag said:
The magnums (5.2, 5.9) came in big heavy trucks and SUVs and ran just fine on 87 octane. But they had long slow advance curves and knock sensors that were very active. If you’ve played with the timing at all and added a performance style curve you may find that it rattles at 9:1 with iron heads and 87 octane. But you won’t know until you test. The answer to test each fuel and use the lowest octane that the engine doesn’t rattle on is the correct answer.
Click to expand...
I ran the Hell outta my 2nd owner 02 Ram, last year of the 5.9. When my Bro bought it, they had to call the Owner of Jerry Ulm Dodge in Tampa. Cause he was rolling it! It was an 85k truck new... death by Advance Auto hub bearing (China).... could killed some people! Anyways, I Ran the Crap out of that 5.9, in the 4 Dr, For YEARS, good ol cheap *** 87 pump gas
 
No one ever said to a P 38 driver or for that matter any WWll fighter, ' I bet that thing uses a lot of gas'. That answer is , if I was worried about that, I wouldn't have bought it.
 
Probably should have thought about that before you bought it. You'll be alright, you got your answers.
Have fun with it.
 
Engine isn't stock so look back at #4. Use whatever doesn't ping. Not hard to figure out. Put 1/2 tank of 87 and put it in second from a stop. See what happens, pings bad ? Put 1/4 tank of 87 test again. Same result ? fill the rest of the way with 91. Looks like 91 will be the stuff then.
 
Nothing to do with the question asked, but my 2011 1500 Sport 5.7 does not like lower octane gas. I've used 93 octane since new and it has never had an issue with knock. After gas went sky high I tried running 87 octane (lot of posters on Ram forum said it won't bother it), that thing ran like doggie poop. Missing, and knock. Back to 93 octane for me, you just have to go with what the engine likes best.
 
thats sucks since prenuim is over a dollar more a gallon now
 
MoparLeo said:
Your owners manual tells you that the 5.7 only uses premium. The V-6 3.6 uses regular
Click to expand...
I haven't checked the owner's manual in a while, I am on a Ram Forum and this has been a topic of discussion several times lately. Some say theirs run fine on 87 octane, some say they use 89 octane, and some, like me, run 93 octane. I just know my truck doesn't appreciate 87 octane. I'll check my owner's manual and see what it recommends and try to remember to let you know what I find.
 
If you have an EFI engine with a knock sensor, then you can run whatever gas you want, the computer will dial back the timing to protect the engine. Yes, sometimes it runs like crap, until the computer learns the new gas, or the operator does.
But with a carb, you are the computer.
Octane requirement is highly Load-sensitive, or actually sensitive to combustion chamber heat, which increases with increased load.
An engine that might require hi-octane gas at WOT, pretty much doesn't care what it gets for steady-state cruising.
You may be able to design a timing curve to satisfy both. Or just do what I did; install a dash-mounted dial-back retard module, and adjust the timing on the fly, right from the driver's seat. Mine has a range of 15 degrees which is plenty.
IMO, 9/1 in a Magnum is just about right.......... for now, lol.
There are several tricks that you can do, to save an engine, that has a compression ratio that is considered to be too high for the gas you want to run. The major players are combustion heat, and cylinder pressure; which are often intimately related.. Therefore, the first place to start, is a compression test to see what the actual CCP(Cranking Cylinder Pressure) is. This will point you to a minimum WOT fuel requirement. Throttled, it will often run Part Throttle on something less than at WOT. Sometimes the low rpm timing has issues. There are tricks to get around that.
I didn't vote because the answer is, IMO, impossible to give, with the provided meager information.
>Cranking cylinder pressure is a more accurate indicator of the octane requirement than is SCR(Static Compression Ratio) especially in the lower rpms, say below 3200, or perhaps a lil higher.
>With iron open-chamber heads the empiracle evidence seems to point to the following;
91 supports 160psi
89 supports 155
87 supports 150
tight-squish chambers usually support about 5 psi more.
Some guys have learned to use a lil more pressure.
Some guys have lost engines on less.
>When you run on the edge, your tune has to be sharper, and the chamber temperature carefully controlled.
You can detonate engines even down under 140psi
>The goal of street-timing is NOT to force the engine to take as much timing as possible, rather, the goal is give it just enough to satisfy her. You will not, by seat of the pants testing, on the street, feel, 3degrees short of optimum.
>If the tires are spinning, the engine no longer cares about timing at that throttle/load setting, lol.
>Generally, it is more energy efficient to run the timing up, on better gas, then to retard the timing to suit a lower octane gas;
your cost per mile may be slightly higher, but your engine will be more fun and will probably last longer.
>There are two times that timing should be a concern;1) WOT; at 3500 and up, and at stall. 2)Part Throttle at Stall . Below stall, is basically a throw away, you do what you gotta do to achieve the stall to 3500, detonation-free.
>Without a computer, it is IMPOSSIBLE to give your engine the Idle timing it wants so; IMO, Idle timing is of LEAST concern, within the parameters of; 1) not being so high that it causes the transmission to jack-hammer the driveshaft and ring-gear when you put it into gear, and 2) not being so high that the Transfer slots dry up and this causes a tip-in sag, and 3) there is no such thing as running too retarded at idle. Idle-timing controls your idle-power. The more you give it, the more power she will have at idle. This is NOT necessarily a good thing, especially with a poor starter gear, of say under 8.60, but this is somewhat camshaft dependent.

AS for me, I would rather have to deal with a lil too much pressure, than a lot not enough.
IMO, 9/1 in a Magnum is just about right.......... for now, lol.
 
AJ/FormS said:
If you have an EFI engine with a knock sensor, then you can run whatever gas you want, the computer will dial back the timing to protect the engine. Yes, sometimes it runs like crap, until the computer learns the new gas, or the operator does.
But with a carb, you are the computer.
Octane requirement is highly Load-sensitive, or actually sensitive to combustion chamber heat, which increases with increased load.
An engine that might require hi-octane gas at WOT, pretty much doesn't care what it gets for steady-state cruising.
You may be able to design a timing curve to satisfy both. Or just do what I did; install a dash-mounted dial-back retard module, and adjust the timing on the fly, right from the driver's seat. Mine has a range of 15 degrees which is plenty.
IMO, 9/1 in a Magnum is just about right.......... for now, lol.
There are several tricks that you can do, to save an engine, that has a compression ratio that is considered to be too high for the gas you want to run. The major players are combustion heat, and cylinder pressure; which are often intimately related.. Therefore, the first place to start, is a compression test to see what the actual CCP(Cranking Cylinder Pressure) is. This will point you to a minimum WOT fuel requirement. Throttled, it will often run Part Throttle on something less than at WOT. Sometimes the low rpm timing has issues. There are tricks to get around that.
I didn't vote because the answer is, IMO, impossible to give, with the provided meager information.
>Cranking cylinder pressure is a more accurate indicator of the octane requirement than is SCR(Static Compression Ratio) especially in the lower rpms, say below 3200, or perhaps a lil higher.
>With iron open-chamber heads the empiracle evidence seems to point to the following;
91 supports 160psi
89 supports 155
87 supports 150
tight-squish chambers usually support about 5 psi more.
Some guys have learned to use a lil more pressure.
Some guys have lost engines on less.
>When you run on the edge, your tune has to be sharper, and the chamber temperature carefully controlled.
You can detonate engines even down under 140psi
>The goal of street-timing is NOT to force the engine to take as much timing as possible, rather, the goal is give it just enough to satisfy her. You will not, by seat of the pants testing, on the street, feel, 3degrees short of optimum.
>If the tires are spinning, the engine no longer cares about timing at that throttle/load setting, lol.
>Generally, it is more energy efficient to run the timing up, on better gas, then to retard the timing to suit a lower octane gas;
your cost per mile may be slightly higher, but your engine will be more fun and will probably last longer.
>There are two times that timing should be a concern;1) WOT; at 3500 and up, and at stall. 2)Part Throttle at Stall . Below stall, is basically a throw away, you do what you gotta do to achieve the stall to 3500, detonation-free.
>Without a computer, it is IMPOSSIBLE to give your engine the Idle timing it wants so; IMO, Idle timing is of LEAST concern, within the parameters of; 1) not being so high that it causes the transmission to jack-hammer the driveshaft and ring-gear when you put it into gear, and 2) not being so high that the Transfer slots dry up and this causes a tip-in sag, and 3) there is no such thing as running too retarded at idle. Idle-timing controls your idle-power. The more you give it, the more power she will have at idle. This is NOT necessarily a good thing, especially with a poor starter gear, of say under 8.60, but this is somewhat camshaft dependent.

AS for me, I would rather have to deal with a lil too much pressure, than a lot not enough.
IMO, 9/1 in a Magnum is just about right.......... for now, lol.
Click to expand...
I guess I should have been more specific; it's a 2011 Ram Sport, single cab, short bed, with the 5.7 Hemi, so it has the knock sensor. It just does not like low octane gas, even at normal cruising speeds, it would hesitate, miss, lose power going uphill. I'll just pay for the premium grade and be happy with how it runs on it. Thanks for all that information by the way.
 
Show me....It should say 89 recommended depending on the year. I have never seen " Do not use" for fuel octane. I have seen use only 5w20 for MDS equipped engines.
Premium s a dollar more than regular ?? In the same gas station ?? All gas is a dollar more in California than the other lower 47 states.
 
demonjim said:
I guess I should have been more specific;
Click to expand...
Sorry man, this was not directed to any one in particular, I just saw the way the thread was going, lol. and put my 2cents in .......
=============================================================
by the way, as to your truck; This is how I learned EFI, your truck may not be exactly like this, but you get the idea;
the ECU pulls timing in steps, the first time it senses a knock event. It will continue to pull timing until the knock quits. Then, as time goes by, it puts it back. Meanwhile, you sense the crappy running.
The Ecu keeps track of how much timing it pulled. This is a short term compensation.The next time you start it up, the ECU trys to nurse the timing back up. But if you are still running the lower-octane, and it gets back into detonating, it pulls the timing out again. Now it has a long-term history.
IIRC, it takes three, short term events to send the ECU into a long-term correction.
After that, each time you start it, she will run on the long-term correction, for the next three consecutive cold-starts. On the Fourth cold-start, it will start bringing the timing back. If you have, in the mean time, gone back to adequate-octane gas, she will start building a new timing curve. If you are still running too low an octane, it will go back to square one.

You can short circuit this learning process by disconnecting the battery, for IDK how long, which will restore the ECU to factory defaults. From there, the ECU jumps past all the tom-foolery. However, when you do this, the Fuel corrections are also erased ...... so she will be hard on gas until the ECU has dialed it back.

You gotta remember, that the ECU is trying to protect your engine from killing itself thru detonation. Your engine has a different octane requirement for every load-setting and rpm. At cruising in steady state, with the throttle relatively fixed, it's EFFECTIVE compression ratio might be 5/1 or less and she might be happy with the crappiest gas money can buy. But that poor engine still has to get the 5 or more thousand pound rig up to speed, and that might take best gas, depending on in how much of a hurry you are. Once the timing has been pulled, it only comes back in small steps over a longer time period. And that is likely what you sensed.
The octane of the fuel is fixed, so the fuel you use has to be able to keep calm under the highest load. Which means, for most of us, 95% of the time, we are wasting our gas-dollars on hi-test. That's just the way it has to be.
If you want to see your ECU in action, get yourself a scanner with graphing ability. Then call up timing and MAP on the graph. Now you can see the correlation of timing to load, that the computer is running on. Maybe you can pinpoint under what circumstance the ECU pulls the timing, and avoid that situation. You can also see how much timing it restores and how long it takes to recover. If you can figure it out, maybe you can run a lower-grade fuel all the time and save some coin at the pumps. Well,
with a hemi,
maybe not, lol.
 
AJ/FormS said:
Sorry man, this was not directed to any one in particular, I just saw the way the thread was going, lol. and put my 2cents in .......
=============================================================
by the way, as to your truck; This is how I learned EFI, your truck may not be exactly like this, but you get the idea;
the ECU pulls timing in steps, the first time it senses a knock event. It will continue to pull timing until the knock quits. Then, as time goes by, it puts it back. Meanwhile, you sense the crappy running.
The Ecu keeps track of how much timing it pulled. This is a short term compensation.The next time you start it up, the ECU trys to nurse the timing back up. But if you are still running the lower-octane, and it gets back into detonating, it pulls the timing out again. Now it has a long-term history.
IIRC, it takes three, short term events to send the ECU into a long-term correction.
After that, each time you start it, she will run on the long-term correction, for the next three consecutive cold-starts. On the Fourth cold-start, it will start bringing the timing back. If you have, in the mean time, gone back to adequate-octane gas, she will start building a new timing curve. If you are still running too low an octane, it will go back to square one.

You can short circuit this learning process by disconnecting the battery, for IDK how long, which will restore the ECU to factory defaults. From there, the ECU jumps past all the tom-foolery. However, when you do this, the Fuel corrections are also erased ...... so she will be hard on gas until the ECU has dialed it back.

You gotta remember, that the ECU is trying to protect your engine from killing itself thru detonation. Your engine has a different octane requirement for every load-setting and rpm. At cruising in steady state, with the throttle relatively fixed, it's EFFECTIVE compression ratio might be 5/1 or less and she might be happy with the crappiest gas money can buy. But that poor engine still has to get the 5 or more thousand pound rig up to speed, and that might take best gas, depending on in how much of a hurry you are. Once the timing has been pulled, it only comes back in small steps over a longer time period. And that is likely what you sensed.
The octane of the fuel is fixed, so the fuel you use has to be able to keep calm under the highest load. Which means, for most of us, 95% of the time, we are wasting our gas-dollars on hi-test. That's just the way it has to be.
If you want to see your ECU in action, get yourself a scanner with graphing ability. Then call up timing and MAP on the graph. Now you can see the correlation of timing to load, that the computer is running on. Maybe you can pinpoint under what circumstance the ECU pulls the timing, and avoid that situation. You can also see how much timing it restores and how long it takes to recover. If you can figure it out, maybe you can run a lower-grade fuel all the time and save some coin at the pumps. Well,
with a hemi,
maybe not, lol.
Click to expand...
No problem. I'm a fairly easy person to get along with, unless you attack me directly, my wife, dog, or cat. It's all good.
 
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