Questions: Octane and modern engine technology

My understanding about octane is pretty simplistic.  Higher octane prevents premature combustion.  Premature combustion causes knocking.  If your engine has higher compression you are more likely to have premature combustion when low octane fuel is used.  The explosion occurs before the spark plug ignites because regular fuel explodes  at lower compression. High compression engines need high octane fuel.  Low compression engines do not.  Higher octane fuel run in low compression engines is a complete waste of money.  It generates little to no extra power or fuel mileage.  I used to run regular in my IM with the watercooled 2.0 vw/engine.  My new cam has increased compression so I now run premium.   High octane fuel is only useful in high compression engines where it will explode with greater compression and create more power at just the right time when the spark plug fires.  Billions of money each year is wasted by putting high test fuel in cars with low compression engines.  Just my 2 (billion) cents.

I don't think your understanding is simplistic at all, Phil.

As for the Frazzled one: I'll bet your iffy injector was made whole again by means of the "miracle in a can" fuel treatment, rather than the "high-test". If the manufacturer of the engine (Mazda, in your case) recommends regular fuel, then higher octane doesn't do anything for you, for all the reasons Phil mentions.

However-- there's gas, and then there's gas. Bargain basement 87 octane fuel often gets bargain basement detergent packs. Some injectors don't seem to mind, some are more finicky. I've got 3 work-trucks-- 2 FCA "Pentastar" V6s, and one Ford 2.5L 4. The Pentastars are much more fuel particular than the Ford. If we burn enough "Hucks" 87-octane without some additive, they'll throw a code. Oddly, my wife's minivan with the same engine is not so fussy, although she exclusively uses Costco (top-tier) fuel in it.  You can throw whatever swill you desire in the Ford, and it'll run the same, no matter what.

If you put name-brand (or unbranded top-tier) 87-octane regular in the Mazda, I'd bet you'll be just fine. If not, a can of injector-be-good every now and then doesn't hurt anything. I've never been a believer either, but it's worked well on the FCA 6s, so I'm not questioning success.

Your mileage may vary.

BTW, Kelly-- hit the return key (↩) every now and again. It'll make your posts easier to read.

Kelly,

My T4, air from Raby, dual plug  2.7L actually specs the 93 oct. and that is all I have ever used in it.  Not sure if Rich D.  did the same?

My S60R and the 986 definitely call for no less than the 93, but as you said the ECM will play games with spark timing if Knock sensor identifies  the engine knock that would follow with 87. 

Only stating my experience. 

Kelly: good it fixed. I had a similar thing a couple years back with my 2.4l Nissan. My code was EGR system and it's a bear to get to, so I put in a bottle of Lucas fuel treatment. The CEL went away. Came back a few tanks later, so, another bottle. Then gone. I think it used three or four bottles eventually. The CEL has not come back for two years and I passed emissions. The truck has about 185k miles on it.

Your V6 is a Ford product and specified 87 octane. These "misfire" codes do happen occasionally without any apparent reason and without requiring any particular fix. I like to just reset the code and forget about it unless or until it gets persistent.

Octane is a rating of how much the fuel can be compressed.   The higher the octane, the more it can be compressed.

OK, so far nobody has said one way or the other if the knock sensor actually adjusts the timing, advancing it as much as possible short of knocking.  If it works that way, then seems to me regardless of compression ratio, and within certain limits, of course, if you run low octane, you get a certain spark advance, and if you run higher octane, you may get a different (larger) advance.  If the engine can now run with slightly larger advance, will that mean it is running at slightly higher power/efficiency?

Yes the Mazda says run regular gas, and that is what I have been doing since new.

I fully understand that with a set advance curve, pre-programmed, if you will, by the springs, etc. in the dizzy and where you set it at 3,000 rpm, then the octane you want will be a a strong function of your compression ratio.  and with our non-EFI, non-ECM, non-variable-valve-timing cars, with carb jetting set, and timing set, and cam set, the engine is going to run at peak or optimum really at only one speed. Or at best, over a small but useful range of speeds.

As I understand it, modern engine management systems can't compensate for more octane being used than the engine was designed for (you would have to be able to increase the compression ratio to see any benefits, but it is a fixed entity and part of the engine design) but they certainly can protect the engine from damage if fed less octane. 

The amount of ignition advance that is optimal at any rpm and throttle setting is a very small window and reducing advance slightly to compensate for bad gas doesn't mean that you can pump in significantly more advance to take advantage of higher octane gas than the engine was designed for. There's no advantage, as you've surmised, in running more octane than the engine was designed for.

OK, so it was a squirrely(?)  injector that was giving me trouble, and it got cleaned out (thankfully) and so runs better now.  Duh!

 Someone used the phrase "detergent package" above as meaning there are differences in fact in additives used at various brands and at various price points and octane ratings.  I have figured all of this Super Nitro Mumbo Jumbo as Madison Ave hype, kinda like how they sell laundry detergent, womens skin care and make-up, etc.  And while there may be differences in fact about how much of this or how much of that is actually added, the net benefit or actual difference, if any, is hard to discover.  Anybody know otherwise??

I agree, as I have witnessed tanker trucks delivering gas. I have watched one truck stop at two locations on the same trip, one premium and one bargain. Go figure.

I used to run 87 octane in my Allroad. Now I run 91 or 93, which is what it's designed for. No codes ever, but it certainly gets better economy, almost 17 instead of 16. But WAY more pep in it's step so to speak. I can't really complain about the mileage, AWD, 4700 pounds, and 300hp v-8.

As to the aircooled engines, I'm pretty close to the ragged edge there. Almost 10:1 and over 170hp. It's all in the ignition system. Crankfired is dead on balls accurate. An 009 is just a rough guess. Crankfired allows you to run right at the edge of detonation without going over the line. A distributor requires you to dial the timing back a bit because of its built-in inaccuracy.

Yes, crankfire on a Speedster/Syder, would be nice.  One of these days, perhaps.  Can't argue about the logic there.  My theory here wrt the Mazda is that the knock sensor, the CoP system and ECU accomplish the same thing: get you to but not over the ragged edge. And furthermore, that edge will move a bit as you up the octane supplied.  Just guessing tho.

As you've figured out, the various sensors on the engine do vary the advance and will run as much advance as possible (as programmed by the software engineer who set the computer map for that engine) under power requests, to the edge of detonation and then back off however much the engine designers told the SW guy to program in (1º - 2º ).  When Chris was running his Eclipse at the drags, he had one ECU map for "hot street" (his daily driver mode) and several others for the track, depending on air temp and barometric pressure and a few other things.

As much as varying the octane rating of the fuel will affect the point at which that engine will begin to knock, the ECU program will read the sensors and provide the optimal advance map that the designers wanted.  The system doesn't care much about octane rating - it's watching how the fuel is burning and trying to get the flame results "right" per the program parameters.

I can run 87 Octane in our cars (Subaru and Nissan) without issue, but they both seem to have more "pep" when I treat them to a tank of 91.  OTOH, it might be me hoping for a better ROI on my higher-grade purchase.

Regardless, I usually toss in a partial bottle of Injector Cleaner (I use CRC brand) once a month in both cars, especially in the winter, just to keep the injectors clean.  This was sorely needed when I was running diesel fuel down south.  The fuel down there wasn't especially good IMO and would clog injectors in a few months.  If I ran CRC injector Therapy I never had a problem, so started doing it in my gas cars, too.  It's cheap compared to replacing injectors.

Here's a good article from Hot rod Magazine about different Octane ratings:

http://www.hotrod.com/articles...09-fuel-octane-test/

Good info Gordo, txs.

Bump: according to AAA there is a thing called Top Tier, which proclaims to understand about additives and sets a standard for these.  Could just be an industry front, but whatever, you can see the gas brands that toe the line here at toptiergas.com.  FWIW.

I always buy gas at Costco and their gas is Top Tier.

Mostly, I've found gas to be pretty much the same everywhere EXCEPT for any gas from the service plazas on the New Jersey Turnpike, and any non-truck-stop gas along I-95 in North Carolina.  Don't know what they buy to resell to travelers, but it ain't all that good.  

The worst tankful I've had in 20 years was from the Vince Lombardi Service Plaza on the Jersey Pike - My V8 would run randomly on 5 cylinders til I finally emptied the tank and refilled it just below DC.  Got something like 8 MPG on that one tank (usually saw twice that).