Anyone switched to 87 octane?

nekkidhillbilly

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had 87 ran in it the whole time. run these chargers at work on it as well and they get beat to death. same engine iirc.
 

HEMIMANN

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I'd like to take credit for that very valuable post from @Hemi395 but it wasn't mine :). I am learning this as he posts it as well.

I read through it all and pondered.

It seems Chrysler (it was Chrysler when Ram was designed) elected to have the PCM ignition timing settings ST & LT for a learning algorithm, much like transmission shift programs do.

The quick setting ST to react to preventing knock damage (how quickly I don't know, which is what I was asking for knock damage risk assessment), then the LT 'learns' to look for that conditional input the next time and holds the retar*ded timing for some pre-estimated longer time period, to prevent ST 'toggling' ret***** and advance so much.

This all seems like a normal adjustment algorithm design to me. So I'm back to the real question - 87 octane should do no harm to the engine - ASSUMING the PCM engineers worked with the mechanical engineers to make the timing ret*ard fast enough when knocking is detected.

You and other reports of being able to hear it does not give me great confidence they hit that parameter. Still too many design teams working in functional silos instead of cross-functional project teams. Ask me how I know.
 

Hemi395

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I read through it all and pondered.

It seems Chrysler (it was Chrysler when Ram was designed) elected to have the PCM ignition timing settings ST & LT for a learning algorithm, much like transmission shift programs do.

The quick setting ST to react to preventing knock damage (how quickly I don't know, which is what I was asking for knock damage risk assessment), then the LT 'learns' to look for that conditional input the next time and holds the retar*ded timing for some pre-estimated longer time period, to prevent ST 'toggling' ret***** and advance so much.

This all seems like a normal adjustment algorithm design to me. So I'm back to the real question - 87 octane should do no harm to the engine - ASSUMING the PCM engineers worked with the mechanical engineers to make the timing ret*ard fast enough when knocking is detected.

You and other reports of being able to hear it does not give me great confidence they hit that parameter. Still too many design teams working in functional silos instead of cross-functional project teams. Ask me how I know.
Correct, in theory 87 is safe to run because the pcm will compensate by pulling timing. I just don't like how it has to see Knock first before it adjusts.
 

Hemi395

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Throwing this out there even tho it's not a Hemi, I've been driving my wife's 3.6 Jeep GC from our home to Boston and back several times recently which is about 95 miles one way. I hooked my scantool up and just watched it for a trip there and it 0 ST or LT Knock on 87. Like nothing. I've looked at the tune file for it and even tho it's a V6 vs a V8 the parameters are similar to my trucks.

Off the top of my head I don't know the compression ratio for the 3.6 but I just found interesting that it was that solid on 87. So if you have a Pentastar don't waste your money on anything but 87:cool:
 

ramffml

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Correct, in theory 87 is safe to run because the pcm will compensate by pulling timing. I just don't like how it has to see Knock first before it adjusts.

Have you ever hooked up your scan tool and then compared "ice cold" vs "heat soaked" or as close to those extremes as you can? My ears tell me that I never hear knock when it's cold, but when I do hear it it's when the engine is hot, the hotter it is the more knock I hear. I would love to see that backed up by a tool as "hearing things" is not always reliable in my case.
 

HEMIMANN

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Correct, in theory 87 is safe to run because the pcm will compensate by pulling timing. I just don't like how it has to see Knock first before it adjusts.

Thanks, @Hemi395 - got it. As I thought, and also concur, FWIW. I always fill with 89 octane prior to pulling anything, but haven't pulled as much this summer with extreme heat plus high gas prices. Rode my Harley more.

Now I'd like to understand the 89 octane vs. 91 octane ignition timing more. That experiment needs to use identical base fuel source - % ethanol blend must be identical, in my view, as ethanol and gasoline have different burn rates regardless of octane rating.

Some stations around here offer 91 octane with 10% ethanol, others 91 nonoxy (no ethanol), but it is illegal to use for automotive vehicles - meant only for motorcycles, small engines, 2-stroke engines. Of course, there's no enforcement, so...

I think @Burla was going to experiment with intuitive feel, but not datalog.
 

Hemi395

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Have you ever hooked up your scan tool and then compared "ice cold" vs "heat soaked" or as close to those extremes as you can? My ears tell me that I never hear knock when it's cold, but when I do hear it it's when the engine is hot, the hotter it is the more knock I hear. I would love to see that backed up by a tool as "hearing things" is not always reliable in my case.
Your ears are correct, the colder the motor is the less knock you'll get. In the winter I never see any timing pulled or hear any knock until its completely warmed up
 

HEMIMANN

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Yup - detonation (knock) is a function of in-cylinder temperature, which is a function of pressure and ambient temperature.

Which is why high compression spark-ignited engines need high octane, because high pressure = high temperature. PV=nRT
Which is how a diesel works without a spark plug.

Diesels were great, until EPA turned 'em into science fair engines. I'm not pro-pollution either, but their is a point of diminishing returns on investments. Just today today in the NYT was a big article about how all the U.S. wildfires have completely eliminated any pollution reductions from all the engine modifications. If all vehicles had been diesels, there'd be a helluva lot less fuel consumed, CO2 discharged, and vehicles sold (diesel lasts much longer).

Ah, well. *end rant*
 

ramffml

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So I wonder then, how "long term" are the "lt" tables. For those who are getting knock, it might be better then to pick an octane and run with it permanently, instead of switching back and forth with towing etc. If it takes longer than a tank for the LT table to update then the silly thing is permanetly confused as we mix and match octanes.
 
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