Fuel Economy...

Jstas

OK, I will clear up what I was trying to over simplify.

First off, we will define octane:

From Wikipedia
Octane - Octane is an alkane hydrocarbon with the chemical formula CH3(CH2)6CH3. It has many isomers.

One of the isomers, iso-octane or 2,2,4-Trimethylpentane, is of major importance, as it has been selected as the 100 point on the octane rating scale, with n-heptane as the zero point. Octane ratings are ratings used to represent the anti-knock performance of petroleum-based fuels (Octane is less likely to prematurely combust under pressure than heptane), given as the percentage of 2,2,4-trimethylpentane in an 2,2,4-trimethylpentane / n-heptane mixture that would have the same performance. It is an important constituent of gasoline.


Octane ratings are a measure of how much fuel can be compressed before it spontaneously ignites. Given that, 87 octance can handle the least amount of pressure before it ignites through compression. Since high compression engines often exceed the point at which 87 octane will ignite spontaneously, they need to have a higher octane rating in thier fuel. However, this spontaneous combustion is called knocking. The audible noise you hear in knocking is when the flame front expands and contacts the upwards moving piston that is trying to compress the now expanding fuel/air mixture. It's extremely bad because that pressure that normally pushes the car is now fighting the car and it breaks rods, pistons, cranks and valves. It also produces a considerably lower amount of power and kills fuel mileage.

Now, just because it is harder to ignite higher octane fuel through compression doesn't mean that it is harder to ignite with a spark. The higher octane fuel will ignite at a lower temperature from the spark plug so it is easier to get the combustion process started. The benefit here is that you can squeeze that high octane fuel to compress it even more and build up energy and heat. Now, spark plugs have a ceramic insert to insulate teh electrode so that the spark occurs at the tip where it is supposed to. The "heat range" of the spark plug is determined by the shape of that insulator. That heat range is important because the insulator gets hot and keeps the electrode from being corroded by deposits. If that insulator is hot enough and teh fuel is compressed enough, that insulator can ignite the air/fuel mixture long before that spark ever fires. That causes knocking and we know what that does.

Some engines require a "hot" plug. The reason it is called a hot plug is because the ceramic insert in this plug has a smaller contact area with the metal part of the plug so the plug runs hotter. A colder plug has more ceramic insulation so it runs cooler obviously. Manufacturers specify what temperature range in plugs is used in the engines because of compression ratios, ignition timing and octane levels that the car is intended to use.

One way to band-aid the pinging that a car with high carbon build up experiences is to run a colder plug. This keeps the engine from pre-igniting because of the higher compression ratio experienced from the lower chamber volume due to the carbon buiild up. The other way is to use a higher octane fuel to resist the tendency to ignite under compression only.

So yes, higher octane fuel is harder to ignite but in the environment of an internal combustion engine where high pressure and temperature levels are already present, it can be and usually is easier to set off combustion with a spark in higher octane fuel because of it's tendency to resist ignition during compression.

I hope that made sense.

More reading about octane and octane ratings:

http://auto.howstuffworks.com/gasoline3.htm
http://www.turborick.com/gsxr1127/gasoline.html
http://theserviceadvisor.com/octane.htm
http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm

Jstas

No premium. Run regular or whatever the owner's manual says to. Premium will not help you unless the car is pinging badly.

Jstas

PolkThug wrote:

Wait a sec, the higher the octane, the less septane. Septane is easier to ignite than octane.

My hypothesis is that lower octane gas can ignite easier and sometimes when you don't want it to. Under pressure, a higher octane gas will be less likely to ignite.

Octane only resists ignition through compression. The rating does not easily correlate to ignition through a spark. Gasoline will ignite from a spark at one atmosphere. There doesn't need to be pressure for a spark to ignite gasoline. The level of octane vs. heptane will also change not only the temperature at which the fuel ignites but it will also change the temperature of teh flame front, the temperature at which the end-gases ignite and the speed at which combustion happens. The only thing octane can tell you by just looking at the number is how much resistance there is to ignition through compression. To know anything more, you need to know the RON and MON ratings of the fuel.

AsSiMiLaTeD

makes perfect sense to me, good write up

Markymark

wow, nice explanation, Jstas.

aaharvel

Good post Jstas. The whole "premium gives me better gas mileage" argument REGARDLESS of car is a placebo effect from hell- but when it comes to Carbon buildup (like i alluded to with miles on an engine), you're right on!

The reason I mentioned the master cylinder was because my older Acura averaged about 21mpg. When the Cylinder discorporated, my car's mileage went down to around 15. That's the only reason I mentioned the possiblility of the Master Cylinder.

CrBoy

Thanks fellas...

Jstas

aaharvel wrote:

Good post Jstas. The whole "premium gives me better gas mileage" argument REGARDLESS of car is a placebo effect from hell- but when it comes to Carbon buildup (like i alluded to with miles on an engine), you're right on!

The reason I mentioned the master cylinder was because my older Acura averaged about 21mpg. When the Cylinder discorporated, my car's mileage went down to around 15. That's the only reason I mentioned the possiblility of the Master Cylinder.

The gas mileage probably went in the dumper because it not only started dragging pads/shoes but if it was a power brake setup it probably developed a huge vacuum leak. That would affect the fuel/air mixture and could cause the engine to run rich and kill gas mileage.

The reason running rich is bad is because any cabon/resin/varnish deposits in the engine will soak up the unburned fuel where it will smolder. It causes hot spots that will pre-maturely ignite not only the incoming air/fuel mixture but it will also ignite the unburned gases exiting the exhaust. Now that is bad on a whole 'nother level. Unburned gases getting ignited in the exhaust will not all burn because of the nature of the exhaust gases to expand rapidly with the help of the heat of combustion. It's how exhaust systems work to move gases down the line. However, not all of that unburned fuel gets burned. It settles in the catalytic converter and starts to smolder at temperature. This increases the heat in the catalytic converter drastically. It ends up melting the catalyst in the converter which then starts falling off in molten globs and exiting out the tailpipe. That, for very obvious reasons is extremely bad. What is worse is that the catalytic converter is now hot enough to set the car on fire all by itself. The reason it doesn't while moving in most cases is because teh air flow of the moving car helps to keep the converter cool. This phenomenon is called slagging and very, very bad. It's downright dangerous and exactly what can happen if a rich fuel mixture condition is not fixed promptly.

MacLeod

Makes sense to me. In fact my owners manual actually recommends AGAINST using anything higher than 87 claiming it will hurt performance.

John, whadya think of these fuel injector cleaners then?

I run Valvoline's Super Concentrated Fuel Injector Cleaner at every oil change (3000) miles to clean up any carbon buildup and gunk in the fuel system. These things actually work or are they just snake oil?

AsSiMiLaTeD

I also have used the Valvoline stuff for quite a while on several vehicles. While I can't see any direct impact in our current vehicle (2004 4Runner), I was able to tell noticeable difference when I put it in that POS Jeep and that POS Cavalier we had...but those were older higher mileage cars anyway...

ND13

I highly recommend Lucas products for your vehicles. Very good products. They were recommended to me by several techs, truckers, and gear heads. :cool:

PolkThug

I think the "mechanic in a can" fuel injector cleaner that you dump in the gas tank is bunk.

Jstas

I use the fuel system cleaners that go in the tanks. They don't necessarily hurt and they can help alot. What they do help to do is keep deposits from forming on stuff where it can really louse things up. However, like what was mentioned, some vehicles have parts in the fuel system that can be damaged by such products so be sure to consult your owner's manual or at least a dealer service department if the manual isn't available.

These cleaners will do several things. They will remove the varnish that you usually get in the fuel. They will also help to break up carbon deposits. They will remove any moisture from the system too. They also usually have conditioners in them to help prolong the life of your injectors and other soft parts of the fuel systems. Using it every 3K might be excessive. You should check the bottle to see what they recommend. I usually only run it twice a year. Once after the winter is up to clean out the crap from the oxygenated gas and then once again at the end of the summer to make sure that there isn't any water in the lines and there is nothing gumming anything up that might hinder cold weather starting.

Also, it should be mentioned that these cleaners, by nature, are an octane booster. They barely raise the octane by a 1/4 of a point in most cases so it's not a huge deal. However, if you are like me and have a 25 gallon tank, these bottles usually only treat 20 gallons so I have to use two. That can actually make a noticable difference. It is not going to harm your vehicle in any way. Unless of course the owner's manual says it might do harm.

However, stay away from oil additives and stop leaks and such. They do nothing for you and only serve to prolong a needed repair in most cases. The oil companies do a fine job of providing a high quality oil with all the detergents and solvents that your engine needs. Even with the snake oil they try to sell you, those added detergents and such only last so long anyway and you have to replace them with the regular oil change. It has also been shown that several of these oil additives have shown no benefit at all and are usually gone from the system or rendered inert within the first 100 miles after an oil change. Totally not worth it.

Change your oil regularly and you won't have to worry about it. BTW, sludge from oil is not necessarily dirt in the engine. Sludge is actually the parrafin and other bitumen that is suspended in the oil. It becomes sludge when the oil is so old that it is broken down and can no longer suspend the parrafin and such in the suspension. It falls out and collects in places in the engine clogging things up and causing problems. Regular oil changes ensure that it will not build up and if it starts to, there are enough detergents in the fresh oil to break it up and send it down to the filter where it gets removed from the system.

The only additives I use are the fuel system cleaners and this stuff called Water Wetter. It is an oil that lowers the surface tension of the water so that it gets better contact with the surfaces of the cooling system. It basically makes the water more efficient at conducting and dissapating heat just like soap makes water more efficient at cleaning your hands. This stuff will not necessarily drop the temperature that your engine is running at but it will make it more tolerent to outside temperature changes and in my case, it stabilized the temperature of the engine on hot days. I use it both in the radiator and the intercooler. It also has some oils in it that help lubricate the pumps which will help them last longer. Unlike alot of other snake oils, this Water Wetter has been shown to actually work. It's made by a company called Redline and will last about 30,000 miles which is when you should be changing your coolant anyway.

One thing about the fuel system cleaners. Using them in a car that has many miles (we're talking almost 100K or more) and never had the fuel system serviced can cause problems. This is because the cleaner may be able to dislodge a large chunk of funky and not be able to dissolve it all the way. This undissolved chunk of funky can get lodged somewhere else along the line and cause fuel flow problems. Most fuels nowadays are very clean and highly refined so it is not much of an issue but the issue is still possible. So if you have a car that is running really rough and you think this might help, you might want to have it checked out by a mechanic. There may be a problem too big for a silly little bottle to help.

I tell many people that the answer is never in a bottle. The contents of the bottle only serve to help you deal with a situation. The answer to a problem won't be found there. If you have a problem, get it checked out before it ends up getting worse and costing you a fortune.

RSTERN

Jstas
Good read, I thought I was back in automotive school again.
Noel
I also like BG products. Not sure where around here. I thing I paid around 15 for a bottle of fuel injector cleaner last time I bought some.
Rob

Jstas

One more thing I forgot to add.

DO NOT, under any cicumstances, use a botth labled Carb Cleaner in any form in a fuel injected vehicle. It is a different cleaner and much more powerful. It will destroy a fuel injected fuel system.

That said, if you gotta clean something good and don't mind destroying it in the process, Carb Cleaner will clean the brown off of dog poo if you want. It's nasty, caustic stuff and it will do damage to alot of things so if you have it and use it, be careful.

Speaking of dangerous cleaners, brake cleaner. It comes in two bottles, red and green, for a reason. Green is usually non-chlorinate and safe to use on braking systems where the cleaner might come into contact with the brake fluid. The red stuff is chlorinated and red for danger! The red stuff works better than the green stuff but if you get the red stuff in contact with brake fluid, it will cause a chemical fire and possibly an explosion. Bad news, don't mess with it. This was mentioned here before a few people tried it and had no problems but they were just lucky.

george daniel

Excellent post, sir

Markymark

Jstas wrote:

DO NOT, under any cicumstances, use a botth labled Carb Cleaner in any form in a fuel injected vehicle. It is a different cleaner and much more powerful. It will destroy a fuel injected fuel system.

I use carb cleaner to find vacuum leaks on my intake manifold, but that's it.

Jstas

I've done that too but ether, otherwise known as starting fluid, is a better option. It doesn't harm plastics or rubbers in the engine bay, won't damage sensors either. It also gets a more pronounced response in engine RPM than carb cleaner does.

It's a hell of alot cheaper too!

TroyD

Good posts, John, thanks!

BDT

nadams

Jstas- what do you think of SeaFoam? Ever heard of it/use it? Basically, you let the engine suck it in via a vacuum line and it's supposed to clean out the injectors and deposits on the pistons and valves and such. I think it's basically a "steam cleaner" for your engine. When I ran a half can through my engine it smoked like a chimney until it got cleared out (mostly white smoke).

They also say you can use it as a fuel additive, and in your crankcase, though I haven't tried it in either of those two places...

website- http://www.seafoamsales.com/motorTuneUpTechGas.htm

2000th post

Jstas

Never heard of it. Then again, I wouldn't force anything through my fuel injection system. Too much stuff to go wrong. If it is really in dire need of that level of cleaning, I'd take it to a shop somewhere and have them clean the injectors. When they do that, they remove basically the entire fuel injection system, disassemble it and clean it with a solvent that you can't buy on a retail shelf. Then bad seals are replaced, it is all reassembled and any injectors that need to be replaced are replaced. They then flush the system with fuel and hook it back up. They will usually change the fuel pump and fuel filter too. No sense in feeding a fresh, clean system with crap. It's labor intensive and can cost almost two grand depending on what parts are needed. I pulled parts for a job like that several times when I worked teh parts counter at Pep Boys. For your typical V6 engine, the injectors can cost you close to $200 a pop. Add another $200 for the in-tank fuel pump and $35-$65 for the fuel filter. If nothing else goes wrong, you are out about $1200-1600 in parts and it is usually 8-10 hours worth of labor. At $65 an hour for labor as the going rate, you can see the prices mounting. Unfortunatly, if it is going to happen, there is no way to prevent it. One thing you can do is not to buy cheap gas and the periodic fuel system treatments will also help to warn off such a dreadful repair. Keep the car in clean air and fuel filters too and you will most likely not need new injectors for at least 120,000-150,000 miles.

beardog03

CrBoy wrote:

Any brand in particular?

go to a chevron station and buy a bottle of techron...

I swear by the stuff...I worked at chevron during a shutdown , and while I had time on my hands, cruised the intranet and researched this product...outstanding product IMO...

oh ya...my wife has been at Chevron for 33 years also..

Change your plugs and wires and when your down to 1/4 tank , before you fill up, and a bottle of techron, and fill up withat least 89...

should make a big difference

CrBoy

Geo Metro 95 Owner's Manual
"...Use regular unleaded gasoline rated at 87 octane or higher..."
"...Be sure the posted octane is at least 87, If the octane is less than 87, you may get a heavy knocking noise when you drive..."

Regular Fuel: 88 Octane
Premium Fuel: 9? Octane
I called the oil/fuel company and asked them about the Octance ratings...

So regular it is...

wodom1

Any tips to get more than 10.6 mpg in the city in an '02 Dodge Durango with the 4.7L V8? I don't pay for my gas, but I would still like to get more milage!

nadams

haha.... nope, no suggestions. My brother has a 01 durango w/ the 5.9l v8 and he averages 12mpg.

Markymark

wodom1 wrote:

Any tips to get more than 10.6 mpg in the city in an '02 Dodge Durango with the 4.7L V8? I don't pay for my gas, but I would still like to get more milage!

hahaha
Trade it in for something that has a small I4

ND13

wodom1 wrote:

Any tips to get more than 10.6 mpg in the city in an '02 Dodge Durango with the 4.7L V8? I don't pay for my gas, but I would still like to get more milage!

Yep, but you'll have to spend about $400 or so to do it.

The first $50-60 goes to either a Green or K&N performance air filter. Good thing is that you'll also never have to buy another air filter again. Just clean, reoil, and reinstall. Good for about 10 HP and 2 mpg. YMMV.

Second, head on over to the muffler shop and get a three chamber Flow-Master muffler put on. It's a little louder, but only under hard accelleration. It sound real nice, imo. Again, good for up to 20 hp and 2-3 mpg. YMMV.

Combined, you should see about 4-5 mpg increase and 20-30 hp increase.
More air in/more air out= more hp and better mpg

Markymark

to go along with the more air comment (good idea) would be a cold air intake. It increases mpg if you can keep your foot out of it.

nadams

That's the thing... I've done exhaust and intake to my truck (94 ranger w/ the 4.0 v6), and my mileage suffered for a while because i couldn't keep from putting my foot deep in there and listening to the intake growl

I've since gotten a little more resposible, but theres still some days where I hurt my gas mileage by a bit because I feel agressive...

ND13

That's what the YMMVs were for