Octane Question

[bluedutch]

We just got our 4R on Saturday and I had the same question. I live in NM at a relatively high altitude (~8000 ft).

The regular pumps all seem to sell 86 and midgrade is 88, which is what I've been putting in.

But as gas prices rise, I wouldn't mind lowering to the 86. But if 87 is recommended...

Is going down to 86 going to cause a problem, or is it best to stick to midgrade?

At what altitude is it better to go up to 88?

 

[43242code1]

I am a new owner of a T4R of about 2 months now.

I have been using shell 91 and occasionally 87. In my observations the 91 usually gets me more miles on a tank than the 87. Could be driving habits or weather or any other number of factors.

I remember the first time i used shell 91 in my MarkVIII i thought i was actually going to blast off in that spaceship!

city is sea-level, between 55-85 deg. on a consistent basis.

 

[Annoyed]

Info from the owners manual for an '06 pasted in below.

But, in short. 87 is fine, it may perform better on 91. The reason would be that the engine's control computer will back tuning off as soon as knock is detected, higher octane will prevent knock, so it won't have to back the tuning off as much.

OCTANE RATING
Select Octane Rating 87 (Research
Octane Number 91) or higher. For
improved vehicle performance, use
premium unleaded gasoline with an
Octane Rating of 91 (Research Octane
Number 96) or higher.
Use of unleaded gasoline with an octane
rating or research octane number lower
than stated above will cause persistent
heavy knocking. If it is severe, this will
lead to engine damage.

 

[RamThis]

All octane rating is, is the fuel's tendency to allow or resist igniting under compression. The higher the octane, the less "volitile" it is and wont ignite from compression and heat alone, it will need a spark to light it off. Ping or Knock is actually PREDETONATION where the fuel self ignites before the piston is in the proper timing BTDC. Fuel always ignites just a bit before the piston has reached TDC so that by time it's at TDC and into the combustion stroke the expansion is starting to take place and has time to make full use of the energy.

Some engines compress the fuel/air mix alot more than others, which makes the fuel ignite early, before the spark plug ignites it on proper timing. This causes the flame front and expansion to momentarily push really hard on the piston coming up still in it's compression stroke. Murder on conrods and main bearings.

Now, having said that, to get the MOST power and mileage from your engine, you run the lowest octane fuel you can without predetonation! Engines with knock sensors will re**** the timing, cooling the cylinders, which gets rid of alot of predetonation. These vehicles may see a loss in mileage in that instance. Ones without knock sensors, well, you'll hear it and know what you need to do. Move up a grade for a few tanks and listen, and if that fixes it, great, if not, move to premium. If premium doesnt fix it, you need to check deeper into the reason behind it. Be it carbon deposits raising your compression ratio, messed up vacuum advances, etc.

I run 87 in my 06 4.0 V6 and get about 19MPG out of it.

I run 93 in my 2000 Ram with some mods and an SCT tuner, and I have some mild detonation. I know the reason though, oil consumption leading to carbon deposits.

 

[p nut]

All octane rating is, is the fuel's tendency to allow or resist igniting under compression. The higher the octane, the less "volitile" it is and wont ignite from compression and heat alone, it will need a spark to light it off. Ping or Knock is actually PREDETONATION where the fuel self ignites before the piston is in the proper timing BTDC. Fuel always ignites just a bit before the piston has reached TDC so that by time it's at TDC and into the combustion stroke the expansion is starting to take place and has time to make full use of the energy.

Some engines compress the fuel/air mix alot more than others, which makes the fuel ignite early, before the spark plug ignites it on proper timing. This causes the flame front and expansion to momentarily push really hard on the piston coming up still in it's compression stroke. Murder on conrods and main bearings.

Now, having said that, to get the MOST power and mileage from your engine, you run the lowest octane fuel you can without predetonation! Engines with knock sensors will re**** the timing, cooling the cylinders, which gets rid of alot of predetonation. These vehicles may see a loss in mileage in that instance. Ones without knock sensors, well, you'll hear it and know what you need to do. Move up a grade for a few tanks and listen, and if that fixes it, great, if not, move to premium. If premium doesnt fix it, you need to check deeper into the reason behind it. Be it carbon deposits raising your compression ratio, messed up vacuum advances, etc.

I run 87 in my 06 4.0 V6 and get about 19MPG out of it.

I run 93 in my 2000 Ram with some mods and an SCT tuner, and I have some mild detonation. I know the reason though, oil consumption leading to carbon deposits.

Good post!

 

[4boer]

I havent had, or even seen (heard?), a real case of engine knock in the last 20 years. Not a mechanic, just and old-fart. Use low octane.

 

[RamThis]

I havent had, or even seen (heard?), a real case of engine knock in the last 20 years. Not a mechanic, just and old-fart. Use low octane.

Alot of the Rams with 5.2 and 5.9L engines suffered varying degrees of predetonation, mostly because we had a thin steel belly pan bolted to an aluminum "barrel" intake. The thin steel belly pan is what enclosed the intakes plenum from the crankcase. As we all know, steel and aluminum expand and contract at different rates. This makes a lovely shearing force on that rigid fiber gasket between the steel and aluminum. Once the gasket finally breaks loose, the intake vacuum can pull in oil vapor from the crankcase. Such was the case with my 2000 Ram. Now I think it may have worn down my oil control rings due to the heavy carbon deposits which can act like sandpaper. It's nearly as bad as just running the thing without an air filter really, might as well have just stretched a piece of window screen over the throttle body to keep out bugs and rocks that might hold a valve open lol. Anyway our fix is to replace the steel belly pan with a thick aluminum plate that is cnc machined, and a new gasket and some Permatex Ultra-Black on both sides. Seals the intakes up for good.

But, the ping is very definitive, it sounds like someone dropped a ball bearing down the spark plug hole of each cylinder. Mine is starting to ping again since my last SeaFoam treatment, so it's time to do it again. Eventually I'll need to pull the engine and re-ring it and have the heads freshened up to stop the oil consumption and carbon deposits, thus eliminating the ping I keep getting.