my 2001 grand cherokee runs on 87 octane in Texas where I live....flat land driving and good power in metro driving in the Dallas area....now I took a trip to Vail, Colorado and of course buying the cheapest gas at 3.29,it being 85.... is it me or do you loose power to the lower octane and high altitude....
Nope, it's not you, engines loose power at higher altitudes, that's why piston engine planes have superchargers.
Jeff DeWitt
First, do not confuse power with octane rating, they arn't related.
The higher the octane number the harder the fuel is to ignite. And higher octane fuel has no more power than lower octane fuel.
What your experiencing is lower air pressure at altitude. The same thing that makes you tire more easily at higher elevations also means your engine is not getting the same amount of oxygen with each intake. Less oxygen means less fuel can be burned. Less fuel of course means less power unless you can push more air into the engine with a supercharger or turbocharger.
You want to use the lowest octane fuel that does not cause ping.
Actually they are a LOT more related than you think in some engine that have knock sensing timing control
No it will light just as easy, the differnece is that it has a higher auto ignite tempature and a slightly slower flame speed but it is no harder to light its fuse with a spark
Actually it has a few less BTU in it but in a higher compression engine it allows for a more favorable ignition and expansion cycle than low octane fuel does (which has to use a retarded curve to prevent to rapid of a pressure build up and auto ignition or detenation)
Mostly true. In the old days you would advance base line timing about
2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees or so. It did not restore all power but it does help some. Some modern cars can advance timing a bit more automatically as well as lean mixture out. At start up it reads MAP sensor just before engine cranks to set elevation data. The problem with this is if you start engine at 4000 feet and drive to 9000 feet the computer does not know it is at 9000 feet until you shut it out and restart it.
This is not cut and dried as it seems. This works with old fixed timing engine but not with some modern engine with knock sensor because many of them (especailly GM's ) will never knock no matter what, performance will just suffer. BTW, octane requirements do decrease generally with elevation and this is why above 4000 MSL or so you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus and 91 for premium. You do not want a tank of 85 when you are headed to lower elevations especailly in hot weather (ambent temp effects octane needs too) Generally if your engine has a CR of 9 to 1 or higher you cannot burn 87 at sea level without some spark timing trickery as 9 to 1 and above really needs 89 or more (especailly in warm weather) for optimal timing curving. At 10 to one and above you really need 91 or better unless you run a compromised timing curve which also reduces efficency and power too.
----------------- TheSnoMan.com
sooo, if I increase my octane to 87 at say 9,000 or so altitude, I will gain some power back ?? my jeep is a 2001 grand with the straight 6, 4.0 engine...
Compression ratio is not necessarily the only factor involved. Engines with aluminum heads can run higher compression ratios than engines with cast iron heads due to thermal conductivity of the metal. Also, the valve timing influences octane requirements. More overlap on the valve timing reduces cylinder pressure, which is the true determining factor for octane requirements. Other factors that affect octane requirements are the efficiency of the combustion chamber design and the quench area around the outer diameter of the combustion chamber.
I have a 350 in my S10 truck that runs on 89 octane with a true blueprinted 10.7 compression ratio and cast iron heads, but it also has a cam with quite a bit of overlap and the combustion chambers are highly polished. The quench distance between the top of the piston and the head is .038" which is the tightest recommended distance, but also the best for reducing detonation. I do not have to compromise my ignition timing with this setup. I am running 12 degrees initial timing and 24 degrees mechanical.
Chris
The best thing to do is try it. There are so many variables that it really is hard to give a "for sure" answer.
Chris
M> sooo, if I increase my octane to 87 at say 9,000 or so altitude, I will gain
The crux of what you said is that cylinder pressure is the true determining factor for octane reqs. All that stuff you talked about has to do with the air/fuel mixture getting "squeezed" as the piston comes up. (All this commentary is probably way more than Mindy ever wanted, but hey, ask a technical question...) Anyway, I might be wrong, but it was my understanding that the higher the octane, the more the a/f mixture can be compressed before it self ignites (ping). That's why high-compression racing engines use VERY high octanes. If you have higher compression, you can squeeze in more a/f and produce a bigger bang when the spark plug ignites. That's why higher octane is associated with more power. No?
I think you gave Mindy the best answer buy telling her to try it and see what happens because this crap can be complicated. But simply put, that's the main purpose of higher octane.
Ok SnoMan, tell me where I'm wrong. :)
Pete
Pete
I know, I get carried away sometimes, but I figure if someone can understand something better if I explain my reasoning, then it is worth typing.
Anyway, to answer your question, you are pretty much right. The one thing I would say is that the high compression does not directly allow more A/F to get in to the engine. It does allow the engine to be built with a bigger camshaft and still maintain the proper cylinder pressure.
The reason a big cammed engine can use higher compression ratios is that the camshaft, by design, keeps the intake valve open long after the piston has reached the bottom of the intake stroke, and is already on its way up on the compression stroke. This reduces the percentage of the compression stroke that is actually used to compress the mixture. the benefit to this is that the A/F mixture is still actually filling the cylinder, even though the piston is on its way up.
Just an FYI for you. The NHRA Pro Stock drag race engines are built with around 18:1 compression, which is the same as some diesel engines.
Here is an article explaining pretty much what I explained:
Others have chimed in but, simply put:
As you get to high altitude areas, the regular gas sold is lower octane than it is near sea level. This works out okay most of the time because the air is thinner, and your effective engine compression is less.
Even though your compression ration may still be 9 to 1, for example, the incoming air is less dense, and therefore, the air at the point of compression is also less dense.
The first time I recall coming across 85 octane regular (from big name brands) was in Utah, in 1998. I was confused at first because everywhere else I'd been (California, Oregon, Washington, British Columbia, Idaho) on that trip, regular was posted at 87. The first time I gassed up in Utah, I went for middle grade, but later ran 85 with no problems. This was a 92 XJ, 4.0, in 100 degree heat.
In an nutshell, you lose power at higher altitudes because you are dealing with less dense air, with less oxygen for combustion. Modern engine control systems MAY compensate somewhat by advancing the timing, but you are mostly stuck with lower power in the mountains as a fact of life, without major engine changes.
The only thing you need to keep track of is engine ping. As a general rule, if you hear ping, use higher octane gas. Sudden, severe pinging, is another thing, and can be caused by a number of things that require immediate investigation.
Regards,
DAve
Your body has less forced induction?
Your body has less oxygen to burn fuel in?
Well, if you hadn't already, you can certainly add physiology to that ever-growing list of topics you know absolutely nothing about.
Wow.
Aw, you first, Moby. You can be the Pace Slug.
If you ran a block in under 2 days and didn't end up in Expensive Care somewhere, you'd be on the cover of USA Today or other fine journals.
You'd have been a real hoot up in the Peruvian Andes. Gasping like a trout, twitching like a baby covered in poison ivy and crying for mommy-goat.
Yeah, supper southern fried california body, dripping grease and oozing body-fat on the carpet.
You know what they say, once a lazy f***ss greaser coward, always a lazy f***ss greaser coward.
Go play with your toy soldiers, Billie, the real ones don't want you.
In fact, 'real' anythings want no part of you.
OK, folks, gather 'round for the highly clever C&P sketch......!
Possibly, if is worth a try. It is not because fuel has more energy but because it has a better burning curve in a high compression engine. My wife has a 2000 Cherokee that we bought new and it is a stripped 4cyl with a stick and A/C that she uses to go to work and run around. It has been a good trouble free car and has over 80K now but even it runs a lot better on 89 or better octane in warmer weather at the 1000 MSL where we live. It gets better overall MPG too. (it average 19 to 22 MPG in city/urban driving (no highway) using A/C too. On highway it gets 23 to 25 with A/C depending on speed. We bought it as a cheap knock around vechile that could carry outsized cargo and such and it has served well for this. We have a bigger vehicle for really long trips that is not used in daily cycle.
----------------- TheSnoMan.com
I would run more octane and a bit more spark too. I used to race SB's years ago and 38 to 40 degress total advance is sweet spot for SB. You want to start mech advance at about 1200 to 1500 and have it fully in by about 3800 RPM. (1900 distributor RPM) You have a fairly nice build but it would do more with better fuel and more timing tweaks.
----------------- TheSnoMan.com
Well, in my case the added timing doesn't help. I've run my truck at the strip, and did lots of testing with it. Anything more than 36 degrees and it slows down in the quarter mile, even with higher octane fuel. Most of the engines I've built in the past ran best with 38 degrees, but they were much more race oriented than this engine one is.
Chris
Mindy proclaimed:
Altitude always causes power loss. It also cuts down on the octane requirement, at sea level octane 87 tends to be regular, at altitude 85. Only way to get the power back is to raise the compression ratio or add a turbo.
The only way you would get any "power" back is if your engine has variable ignition timing and has retarded the spark due to 85 octane. You don't really get any power back except when the engine is lugging...the computer will allow a slightly higher spark advance if there is no ping. Note that this does not mean your engine HAS knock sensors and spark retard. It also doesn't mean you'd gain one mousepower if you waste money on more expensive gas...particularly around Vail.
Your engine is an air pump. If it can pump more air and compress it to a higher pressure, you get more power. The fuel just provides the energy to pump the air. Used to be you could get special high altitude engines with a slightly higher compression ratio, often called Denver Heads.
Mindy proclaimed:
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