What octane level?

[bobbyo]

I've looked through some threads on this but they all seem to degenerate into discussions about ethanol.

Leaving ethanol out of the discussion, please, what is the preferred octane level people are using for their 4.0L engines? I'm used to running and paying for premium (93 octane) on my other vehicles since they require it and its relatively cheap here in Missouri compared to other states I've been to. I know the age old argument about not using premium if you don't need it, just throwing money down the gas tank. I've also discussed this with a respected mechanic who only works on high end cars, and he claims to have noticed a difference in wear over time on engines using premium vs. regular, so there could well be two sides to that argument.

Any how, I know the manual says use 87 octane and don't use premium. I've never used 87 octand except on my lawnmower, which seems to like it, but not on my cars. On the other hand, it does seem using premium would be overkill. I was thinking of just using 89 octane and calling it a day. Just curious what others think.

 

[Apparition]

87 is all you need

 

[D's Jeep Wrld]

Long story short: 87 octane is all you'll ever need

Longer story: Even with the bigger cam I run and other mods I've done to my engine and the extra timing I've added to my tune,thinner headgasket etc. I run nothing but 87, SoI can definitely say that 87 octane is all you needAnything more than 87 is just a waste of money

 

[suicideking]

Agreed, don't bother with higher than 87. TJ's can barely go 65 with their 190HP motor. Higher octane isn't going to help.

 

[KennethS]

The 4.0 engine is specified as 8.8:1 compression ratio, which is why the manufacturer recommends 87 octane fuel. There’s really need for higher octane with that compression ratio, which is confirmed by so many of us running 87 for years with no issues.

 

[lBasket]

Wait you're supposed to use 87?? I always use 85

 

[KennethS]

Elevation matters. Most areas of the country have 87 as the lowest.

 

[NashvilleTJ]

I've looked through some threads on this but they all seem to degenerate into discussions about ethanol.

Leaving ethanol out of the discussion, please, what is the preferred octane level people are using for their 4.0L engines? I'm used to running and paying for premium (93 octane) on my other vehicles since they require it and its relatively cheap here in Missouri compared to other states I've been to. I know the age old argument about not using premium if you don't need it, just throwing money down the gas tank. I've also discussed this with a respected mechanic who only works on high end cars, and he claims to have noticed a difference in wear over time on engines using premium vs. regular, so there could well be two sides to that argument.

Any how, I know the manual says use 87 octane and don't use premium. I've never used 87 octand except on my lawnmower, which seems to like it, but not on my cars. On the other hand, it does seem using premium would be overkill. I was thinking of just using 89 octane and calling it a day. Just curious what others think.

The entire purpose of higher octane is to prevent pre-detonation (pinging). If the engine is not experiencing pre-detonation, the higher octane makes zero difference.

Now, what primarily drives pre-detonation is higher compression and aggressive timing. That is why performance engines, and particularly boosted engines, require higher octane. Engines which require higher octane fuel are also tuned to take advantage if it.

You can avoid pre-detonation by backing off the timing (which is what modern, computer controller engines do automatically), but then you are impacting performance.

Your stock 4.0 does not require higher octane for the above reasons. And yes, you are wasting money on anything above 87.

I speak from experience. For years I ran a supercharged 4.0 in my rig, which did require 93 octane or it would ping like hell. The 4.0 does not have knock sensors, so it has no way to deal with pre-detonation.

I run 93 in my Hemi, again because of high compression and aggressive timing (my Hemi is not a stock Hemi configuration).

With both the Hemi and the SC 4.0, I carry a bottle Octane boost in case I am somewhere where I can’t get premium. It makes that big of a (negative) difference.

 

[Geok]

"Generally, engines with compression ratios of 9.3 : 1 or less will safely operate with unleaded 87 octane fuel.
Most fuel refiners blend fuels for geographic areas and adjust their blends seasonally. These blending techniques compensate for the decrease in oxygen content with an increase in altitude and compensate for volatility during the warmer or cooler seasons."

It's about volatility of fuel. Lower octane gasoline is more volatile than higher octane. If you have a lower compression engine, you NEED lower octane to get full combustion in the combustion chamber. If you don't get full combustion, you get unspent fuel running down the exhaust ports into the next combustible area, like the catalytic converter or exhaust manifold. This will prematurely destroy the catalytic material or potentially crack an exhaust manifold with enough heat or pressure from detonation.

Conversely, if you have a high compression engine, or an external source of potential increased compression (ie turbo charger), you need a more stable fuel that will not detonate as early. If the compression stroke ratio is higher than the volatile fuel mix being compressed would allow, you get predetonation. If a cylinder detonates out of sync with the other cylinders for an extended period, the stressors on the crank shaft and other moving engine components can lead to catastrophic failure.

 

[NashvilleTJ]

…

It's about volatility of fuel. Lower octane gasoline is more volatile than higher octane. If you have a lower compression engine, you NEED lower octane to get full combustion in the combustion chamber. If you don't get full combustion, you get unspent fuel running down the exhaust ports into the next combustible area, like the catalytic converter or exhaust manifold. This will prematurely destroy the catalytic material or potentially crack an exhaust manifold with enough heat or pressure from detonation.

…

Running high octane fuel in an engine which does not require it will cause no problems - except to your wallet. Lower compression engines do not require lower octane fuel.

 

[D's Jeep Wrld]

Agreed, don't bother with higher than 87. TJ's can barely go 65 with their 190HP motor. Higher octane isn't going to help.

Cruises just fine all day everyday at this speed even back when my lj was stock and right now in its current form,However i get what you mean thougn! In this pic I was trying to get the shot of it at 244,444 miles.....but it changed right to 244,445 right as I took the picture

 

[jjvw]

Why aren't we just telling him to run regular, instead of mid or premium? That eliminates the variances in octane adjusted for elevation.

 

[NashvilleTJ]

Why aren't we just telling him to run regular, instead of mid or premium? That eliminates the variances in octane adjusted for elevation.

Had to think about this for a sec - your regular is what, 85? What's your premium? I believe in UT it's 91.

 

[jjvw]

Had to think about this for a sec - your regular is what, 85? What's your premium? I believe in UT it's 91.

No idea. The boring Jeep gets regular and the fun Jeep gets premium now that it has a supercharger.

 

[lBasket]

Had to think about this for a sec - your regular is what, 85? What's your premium? I believe in UT it's 91.

85/89/91 I think is what we have. Or is the middle 87?

 

[jjvw]

85/89/91 I think is what we have. Or is the middle 87?

Depends on where in the state you need fuel

 

[tworley]

85/89/91 I think is what we have. Or is the middle 87?

In CO I typically see 85, 87, 91.

I run 85 in my vehicles

 

[lBasket]

Depends on where in the state you need fuel

Where is it different? West Kansas area?

 

[jjvw]

Where is it different? West Kansas area?

Don't know because I don't care what the specific octane number is for regular, mid, and premium.

 

[Apparition]

Don't know because I don't care what the specific octane number is for regular, mid, and premium.

My Jeep has never liked 85 out west. I've always needed to use 87 or it will run poorly.