93 Octane Benefits?

[Zebvance]

I have been meaning to start this discussion for a while now. What are the benefits of running 93 octane in our 4.0s? Are there any? I have heard many different things from people and I'm trying to figure out what is fact and fiction. To start off I use to have a 97 Firebird Formula (RIP...) with a bolt on modded LT1 in high school. It would run like crap on regular gas and needed 93 octane to run fine. What are the reasons for this?

 

Here are some of the things I have heard about running 93 octane:

 

Better Gas Millage

Burns Hotter, therefore helps to burn off excess crud out of the engine

Burns Cleaner

Gives more Horse power. If so how much?

 

I have filled up my tank in my 88 4.0 mj with 93 octane a couple times now and I couldn't tell much of a difference. Is it doing good things for the engine that can't be felt?

 

 

I have one last question, as I'm sure most of you have seen, there is octane booster available in a bottle at most if not all local auto parts store. I have even seen some that claim they boost the octane to 108! But why would I need that? What is it doing?

 

Just trying to shed a little knowledge on a subject I don't know much about. All inputs are welcome!

[ftpiercecracker1]

Higher compression engines (9.5:1 and up) require a higher octane to stave off detonation caused by the higher pressures in the combustion chamber. When i first thought about the ICE (internal combustion engine) i originally thought that the gasoline literally exploded when the spark plug fired. As i have since learned this is not the case, instead it is a very precise and controlled burning of the fuel(s) that causes the expansion of gases to push the piston back down, keep the engine turning and you moving. Almost all High/Mild performance engines have higher compression ratios compared to their stock counter parts. Most of the time this is done intentionally. What is the benefit of running higher compression ratios? I don't exactly know, but i do know you must run higher octane fuels to compensate or you will experience detonation aka: the 'ping' (fuel is acutally exploding from the pressure) and premature and sometimes catastrophic failure of the engine. I think methanol/alcohol injection systems can help prevent this as well, but just running 93 all the time is far simpler.

 

Our engines (4.0l) run somewhere around 8.5:1 to 8.8:1 compression ratio depending on the condition of the engine. Dragsters and other extreme performance gas burning engines can run as high as the mid teens (15:1) That is why they have specialized 'race' gas that is 108 octane or more.

 

 

On a side not, diesels actually do run on detonation or a least a form of it. Since there are no spark plugs to ignite the fuel at the top of the pistons travel it is left up to the shear power of compression to ignite the fuel. That is why diesels have extremely high compression ratios and are able to run insane amounts of boost. They are built extremely robustly and can handle massive amounts of pressure and torque.

[Timeless]

This thread will likely end up a mess.  Just saying.

 

Higher octane fuels resist ignition due to compression better than lower octane fuels.  Thereby, an engine can be designed to run at a higher compression ratio when it is fed higher octane fuels.  The more compression an engine runs, the more efficient it runs too.  Therefore, the engine can produce more power from the fuel and/or achieve better fuel economy.

 

To answer your first question, there is no benefit to running high octane fuel in a low compression engine, like the 4.0L.

[87Warrior]

The only benefit of running higher octane fuel in our 4.0 is the lack of ethanol.

[Keyav8r]

This thread will likely end up a mess.  Just saying.

 

Higher octane fuels resist ignition due to compression better than lower octane fuels.  Thereby, an engine can be designed to run at a higher compression ratio when it is fed higher octane fuels.  The more compression an engine runs, the more efficient it runs too.  Therefore, the engine can produce more power from the fuel and/or achieve better fuel economy.

 

To answer your first question, there is no benefit to running high octane fuel in a low compression engine, like the 4.0L.

On some later model vehicles with higher tech engine controls, you can operate on either the lower octane "regular" unleaded or higher octane "premium" unleaded. The engine management system will adjust timing, etc to run on the regular fuel without knock. However, with the premium fuel, there is a noticeable performance improvement. I experienced this in my better half's 99 Lexus and my buddy says his wife's later model Lexus did the same. I would run regular most of the time but put in a full tank of premium occasionally. However, the BMW Z4 she drove for a couple of years refused to even consider lowering itself to burn regular. It was so much fun to drive I didn't mind the extra fuel cost.

[Keyav8r]

I meant to add that our trucks don't have high enough compression ratios to need higher octane fuels and the ECUs don't have the capability of "modern" engine management systems, so higher octane fuel is a waste of money.

[johnj92131]

The engine is designed to run on 87 octane pump gas from the factory.  There is no provision in the engine management system to use higher octane fuel.  In the "old days", you could change the ignition advance curve to get more power by advancing the timing or change the timing curve to use lower grade fuel without detonation.  But today, you have to be a programmer to change the ignition curve.

 

Stop throwing your money away.

[91Pioneer]

But, what if you do cruiser's "CPS timing advance mod". Then there should be benefits from higher octane, no?

[HOrnbrod]

The stroker I have has a 9:1 compression ratio due to a slightly decked head and mild cam. It will ping under load on 87 octane fuel, like climbing a long grade under power, so I use at least 91 octane always. But for a N/A 4.0, anything higher than 87 octane is wasting $$. 

[Incommando]

This thread will likely end up a mess.  Just saying.

 

Higher octane fuels resist ignition due to compression better than lower octane fuels.  Thereby, an engine can be designed to run at a higher compression ratio when it is fed higher octane fuels.  The more compression an engine runs, the more efficient it runs too.  Therefore, the engine can produce more power from the fuel and/or achieve better fuel economy.

 

To answer your first question, there is no benefit to running high octane fuel in a low compression engine, like the 4.0L.

So true. Some of the latest engines that can adjust timing and other things more efficiently on the fly based on multiple sensor readings may show a slight increase with an increase in octane. In such engines you will see a decrease in performance from running a lower than specified octane.This certainly does not apply to a renix or OBDI MJ. If your manufacturer didn't recommend a higher octane you are pissing your money away unless your engine is modified in terms of compression or such. There is no better mileage or performance gains in using higher octane in a stock tune & compression engine that did not require such fuel in the first place. 

 

Octane is a numeric representation of knock/pre-detonation resistance. In tunable high power engines with a lot of compression you can access more of the available power by advancing the timing without suffering knocking/pre-detentaion. Running a lower octane in such engines means you have to retard the timing and are leaving some power on the table. Again none of this is applicable to an MJ.

[gogmorgo]

As was said, higher octane ratio fuels resist compression ignition (pinging, detonation) better than lower octane fuels. This is unnecessary in most engines as they are tuned to run on regular, our Jeeps being among those. In a stock engine, it's a waste of money.

Some higher-perfomance cars are tuned for higher octane fuels, and the manufacturer will indicate this. Mostly it's in higher compression engines or those with forced induction (super/turbo chargers). The compression ratio is the ratio between the volume in the cylinders at bottom dead center vs at top dead center. A higher compression ration indicates that you're squeezing the air/fuel mixture into a tighter space, so you have a relatively larger "bang" and the expanding gasses from the ignition expand further, thus you make more power out of the same fuel, which can lead to better fuel economy, but typically doesn't due to the heavy foot effect. A stroker engine, like the common 4.0 w/ 4.2 crank is an example of an increased compression ratio. Vs stock, the stroker has higher displacement, but the same volume in the head (compression chamber), which is why, as Hornbrod points out and as anyone with a stroker will attest, they require higher octane fuel.

Higher compression requires more compression-ignition resistance because temperature goes up with increased compression. Higher octane fuels don't burn hotter than lower octane fuels, they auto-ignite at hotter temperatures. This may actually take away a small amount of performance/economy in an engine not tuned for higher octane fuels due to incomplete burning, although it wouldn't be all that noticeable to a butt-dyno. Incomplete burning would also lead to carbon buildup in the engine which will reduce performance and would produce "dirtier" exhaust.

Most "name brand" fuels will also have much higher levels of detergents and other useful additives in their higher octane fuels as well, leading to improved fuel economy in the form of a better running/cleaner engine, but for the most part those improvements are minimal and probably not noticeable in a properly maintained engine.

 

As a bit of anecdotal evidence that running premium is a waste of money, my dad had an '85 Chevy van with the 305 V8. He had issues with run-on (dieseling... the engine would still compression ignite after it was shut off) for as much as a minute afterwards. He "solved" the run-on issues by switching to premium fuels. I wound up borrowing the van for a summer a few years back, and sick of poor running and paying for premium in a vehicle that made no power, I replaced the spark plugs. The original ones were coated in carbon. He has the opposite of a heavy foot, basically drives like a grandpa, so no surprise there. The new plugs sorted out all run-on issues even on 87 octane, and I noticed about a 20% gain in fuel economy over my dad, despite the fact that I was pretty well running a 20 mile rallye stage every morning on the way to work. After he got the van back, he was still insisting on running premium, but was back down to only a 5% improvement vs before the new spark plugs.

As more anecdotal evidence about octane ratings vs compression ignition, with the same van. I was nine years old at the time, so I've only heard this second hand, but apparently somehow the distributor cap and rotor melted, resulting in the van running like crap. There was a noticeable improvement in running once switched to regular from premium because regular fuel was mostly compression igniting on the cylinders getting crappy spark, whereas they weren't really igniting on premium fuel because they weren't compression igniting. Don't really know how one would go about melting a distributor cap without serious heat damage to the engine, but again, I was only nine years old when it happened.

 

A piece of trivia on octane ratings: there are a couple different systems of measuring it. In Europe and other parts of the world, they don't use the same one as we do in North America, so the fuel carries higher octane numbers despite similar anti-knock characteristics for their regular/mid/premium grades as the fuel grades we use.

 

Another piece of trivia on compression ratios, the Atkinson cycle engine is similar to the typical (Otto cycle) engine, but through some mechanical means has a longer ignition stroke than compression stroke. This makes for a comparable improvement in efficiency as in a higher-compression engine, but without as much boost in power because you aren't sucking in more air (and fuel to maintain proper mixtures). In the original system, this was done with a complex crank that competed all four strokes in one revolution, but modern "Atkinson" engines such as the one in the Prius accomplish the same simply by leaving the intake valves open during the first part of the compression stroke, so some of the air is pushed back out into the intake and a similar effect is achieved: hot combustion gasses can expand further than their original, uncompressed volume, so less energy is wasted out the exhaust. Not so great for increasing power output though because you aren't dumping in more fuel.

 

Due to oversight on my part, I once wound up running my MJ on race fuel from the shop, which is somewhere between 94-104 octane, depending on what's available. I didn't notice a difference either.

Also worth saying, some of the octane boosters do so by adding in methanol or other alcohols, and can be as corrosive to an engine as ethanol. Alcohols are a cheap way of increasing octane levels, but definitely not always the healthiest.

[benjy_26]

I actually noticed a DROP in economy when running premium in my 2.5 MJ. My 4.7 stroker ZJ pings BAD on regular. My CR is somewhere in the high 9's.

[Eagle]

I meant to add that our trucks don't have high enough compression ratios to need higher octane fuels and the ECUs don't have the capability of "modern" engine management systems, so higher octane fuel is a waste of money.

Not so fast.

 

What you wrote is true for the HO versions with the Chrysler ECUs and injection, but the Renix engines have a knock sensor, and higher octane reduces knock. Which means you can work the engine harder without having the knock sensor shut things down. Also, at cruise throttle it allows more spark advance, and that means better fuel economy.

 

I ran my '88 XJ on high test most of the time when I first got it. I was getting in the mid- to high 20s for gas mileage on it. Best ever was 28 MPG for a 250 mile trip.

[ftpiercecracker1]

 

I meant to add that our trucks don't have high enough compression ratios to need higher octane fuels and the ECUs don't have the capability of "modern" engine management systems, so higher octane fuel is a waste of money.

Not so fast.

 

What you wrote is true for the HO versions with the Chrysler ECUs and injection, but the Renix engines have a knock sensor, and higher octane reduces knock. Which means you can work the engine harder without having the knock sensor shut things down. Also, at cruise throttle it allows more spark advance, and that means better fuel economy.

 

I ran my '88 XJ on high test most of the time when I first got it. I was getting in the mid- to high 20s for gas mileage on it. Best ever was 28 MPG for a 250 mile trip.

 

 

 

 

 

 

Best ever for me was 21mpg on regular and thats mixed driving, so maybe it is possible. :dunno:

[Akula69]

I too did some research on the octane additives due to the knock sensor.

 

I hooked up the SnapOn MT2500  "red brick' to our 89/99 (when it was Renix), ran the cord into the truck and drove it under varying conditions. The engine did have a problem with the 87 octane with ethanol and knocking, ultimately affecting fuel economy (through the short and ultimately the long term fuel management). We were getting about 12-13 MPG at the time. I started adding an octane booster to the 87 fuel (not so much so for MPG but to partially negate the ethanol) and the MPG rose (over about 6 months time) to 15-17 MPG.

 

Put the brick back on and noted the knock sensor activity was greatly diminished, if not completely gone from the equation.

 

Looked at the price per bottle for the octane additive and realized: at the price of 87 octane per gallon (vs. the 91 octane) we were saving money over the long run. Looking at the prices now it is no longer the case....

[johnj92131]

 

I meant to add that our trucks don't have high enough compression ratios to need higher octane fuels and the ECUs don't have the capability of "modern" engine management systems, so higher octane fuel is a waste of money.

Not so fast.

 

What you wrote is true for the HO versions with the Chrysler ECUs and injection, but the Renix engines have a knock sensor, and higher octane reduces knock. Which means you can work the engine harder without having the knock sensor shut things down. Also, at cruise throttle it allows more spark advance, and that means better fuel economy.

 

I ran my '88 XJ on high test most of the time when I first got it. I was getting in the mid- to high 20s for gas mileage on it. Best ever was 28 MPG for a 250 mile trip.

 

 

It is true that the Renix engines have a knock sensor.  But the question is what does the computer do with the knock sensor data?   Answer:  I "think" it retards the spark to stop the knock.  Don't think the Renix ecu uses the knock sensor data to advance the timing curve to where you would gain from 93 octane fuel.   .

[Eagle]

It is true that the Renix engines have a knock sensor.  But the question is what does the computer do with the knock sensor data?   Answer:  I "think" it retards the spark to stop the knock.  Don't think the Renix ecu uses the knock sensor data to advance the timing curve to where you would gain from 93 octane fuel.   .

You are correct -- the knock sensor (and ECU) retards the timing when it "hears" ping. If higher octane gas reduces or prevents ping, the timing can stay more advanced under a wider range of operating conditions, and that leads to both more power and better fuel economy.

[Zebvance]

So have we come to the conclusion that there is Benifit to 93 octane?

[Zebvance]

So have we come to the conclusion that there is Benifit to 93 octane?

[Incommando]

Most research shows no benefit in any vehicle not designed for it or modified. Anecdotal evidence has been presented in this thread that an octane boost will not improve economy or performance over factory specifications but may prevent them from worsening in a specific 4.0 renix MJ. If you want to consider that as conclusive evidence feel free to do so. A more controlled and scientific test that rules out things such as driver input caused by being more aware of MPG while actively engaged in testing as well as many other variables will be needed before I give up regular.

[HOrnbrod]

^^  Agree. Engine tuning, drivetrain condition, driving habits, and tire pressure, in that order, all have more to do with optimizing performance vs. economy than the fuel octane rating. Correct all the above before wasting money on fuel.

 

Case in point: I once had a 1963 Studebaker Avanti R2. The engine was a 289 V8 with a Paxton supercharger and 9.75:1 compression ratio. It would only run w/o pinging on 103 octane rated Sunoco 260 gas. Mileage was 9MPG on a good day. After a complete tuneup w. dyno testing and new tires mileage went up to a consistent 12-13MPG and I was able to use a lower rated fuel. Just one example; there were lots more over the years. Unfortunately I never changed my horrendous driving habits though.......

[Incommando]

 

Case in point: I once had a 1963 Studebaker Avanti R2. The engine was a 289 V8 with a Paxton supercharger and 9.75:1 compression ratio.

Drool.

 

The single greatest factor in mileage is the loose nut behind the steering wheel. :) . But no one wants to adjust it. It is kind of like those who say "I would do anything to lose weight.." without adding "...but eating right and excercising."

[Keyav8r]

OK - based on Eagle's point about the Renix knock sensor, I amend my statement to say that most of our trucks don't have high enough compression ratios to require higher octane fuels for normal operation and the ECUs don't have the capability of "modern" engine management systems, other than retarding timing in the event of knock, to make full use of the higher octane rating. Therefore higher octane fuel is, for the most part, a waste of money.

 

There now, that should be sufficiently watered down to be generally acceptable.