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4.0 sohc octane rating?

Ohhnulix

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My 4.0 sohc has about 180k miles on it, it’s always felt like a sluggish engine but never anything too bad, especially compared to my last 3.0. But I’ve noticed my truck seems to run considerably quieter and feel stronger if I run a higher octane fuel, usually midgrade which is like 89 octane around here. I know the 4.0s are suppose to run on 87 octane but I’ve seen a few people say the noticed the same affects while others say it’s all a placebo effect. if that is the case and my truck magically wants the expensive shit over stock, is there a reasoning behind it? Is it just something with the factory tunes or a mechanical thing? I’m not even sure where to begin looking
 


rusty ol ranger

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Its probably in your head.

Ive heard on newer engines with sophisticated timing controls higher octane could in theory make a difference because the ECM can advance the timing more without it pinging...but ive never noticed anything. Even on my newer shit
 

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I have never felt my 4.0 SOHC engine was sluggish and am very happy with the performance. However, there may be something that could explain what you have experienced when using higher octane fuel. The 4.0 SOHC has a knock sensor that can retard timing if knocking is detected. I understand the 4.0 SOHC engine is very near needing higher octane than 87 and may ride on the edge of having the timing retarded. So, it may be when you use a higher octane it could allow the timing to return to normal and your performance could improve. You may see improved performance with 89 but I doubt going to 91 would give you even more improvement. Many vehicles have this feature which allow them to run on 87 octane but will have optimum performance with higher octane.

I have tunes I can use on my Ranger that do give improved performance with 91 octane. However, they are custom tunes specifically for that higher octane. I normally run on an 87-octane tune because it is more economical.
 

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I understand the compression ratio on a stock 4.0 sohc is high enough to warrant the 89. Knock sensor was put it, like previously said, to allow the truck to run 87.

I wouldn't think the difference would be super noticeable, but its there. May also increase MPG by .5 or something.
 

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The only way to know would be to datalog and see if the knock sensor is pulling out timing when you run the 87. My bet is that its not, because the knock sensor on that engine (at least in the tunes I have played with - 03 manual and 09 auto pcm) will pull out timing crazy fast if it ever thinks it hears knock, so you generally feel the power drop when the knock sensor kicks in. One of the first things I did in the tune was to change the rate that it pulls out timing to something halfway reasonable rather than what they put in the factory tune.
 

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4.0l SOHC runs 9.7:1 compression ratio
9.4:1 is the limit for 87 octane

So yes, engine spark needs to be adjusted(knock sensor) to use 87 octane and EGR is also used more in the 4.0l SOHC

Running higher octane will increase power, for sure, its just science :)

If engine is under 9.3:1 CR it wouldn't matter, a gollon of 87 to 93 octane contains the same "energy", octane is just a heat rating, i.e. the temp rating when it will ping/knock/self ignite when compressed
 

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My 4.0 Ranger runs happier and gets slightly better fuel economy using 93–octane instead of 87. It's noticeable and not my imagination. Especially if I plan a trip, it gets premium.

I look at it this way: if the slight extra cost of premium keeps this engine trouble-free longer, it's worth it.
 

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Mine does something that may be tied in here also.

When under heavier acceleration ( leaving a light, getting on a highway, passing, going up a steeper incline),.......... When I give it the gas it just bogs down, more gas will not help, after fighting my way to get up to speed and have at long last ( and with great difficulty ) done so, with it seemingly objecting every bit of the way.

Now I ease into a cruise level, and as the pedal comes up off the gas a bit, the engine goes into rocket mode !! ??
 

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With my 2001 V6~4.0L~SOHC, normally run the cheap gas,
but in hot weather with the AC compressor load, I do fillup with premium several times thru the summer,
as I do notice slightly more pep, but no real change in MPGs.
My suspiction is that its more due to the additives than due to the Octane rating.
Gets ~17 around town, 18+ on longer trips, no matter which gas or how heavy my foot is.
 
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rusty ol ranger

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My credo
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4.0l SOHC runs 9.7:1 compression ratio
9.4:1 is the limit for 87 octane
Not to argue with you but if thats true then why does my carburated Sportsman 500 with 10:1 compression or so run just fine on 87?
 

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The sohc has aluminum heads, and it is well documented that aluminum heads can run one point higher compression ratio with the same octane fuel due to the better cooling effected by the aluminum v. iron heads. So 9.7:1 on regular 87 gas is no problem at all in the sohc.

Higher octane gas does not have more energy in it - that is an old wives tale that just keeps coming back. The only way to get more power out of an engine when using higher octane gas is to put more timing advance in the system. The pcms that have octane sensors (and E85 sensors for that matter) have a different spark advance table for the different fuels, and they provide more spark advance with the higher octane fuels - that is the reason for more power from higher octane fuel.
 

Roert42

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Mine does something that may be tied in here also.

When under heavier acceleration ( leaving a light, getting on a highway, passing, going up a steeper incline),.......... When I give it the gas it just bogs down, more gas will not help, after fighting my way to get up to speed and have at long last ( and with great difficulty ) done so, with it seemingly objecting every bit of the way.

Now I ease into a cruise level, and as the pedal comes up off the gas a bit, the engine goes into rocket mode !! ??
I’ve had that same issue on my one truck for a while. Manual transmission 4.0 sohc.

If you lean on the throttle real light it run fine, but once you give it too much throttle it start bogging down like there is low fuel pressure.
 

ekrampitzjr

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I’ve had that same issue on my one truck for a while. Manual transmission 4.0 sohc.

If you lean on the throttle real light it run fine, but once you give it too much throttle it start bogging down like there is low fuel pressure.
Time for a new fuel filter?
 

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Not to argue with you but if thats true then why does my carburated Sportsman 500 with 10:1 compression or so run just fine on 87?
Yes, you can adjusted spark timing for lower octane, same as Knock Sensors do on EFI higher compression engines
Dynamic compression(running compression) might be lower than spec
As said its just science and 9.4:1 is a "rule of thumb" with all things equal, loads of variables with "rule of thumb"
 

Roert42

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Time for a new fuel filter?
Since that's fairly easy, that was the first thing I did. I had also recently installed a cheap Chicom fuel sender, and I suspected that the pump may have been week. So I changed that out as well.

I was thinking maybe a vacuum issue.

Truck has at least 250K miles on it.
 

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