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Old 04-16-2005, 8:33 PM   #1
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The RR Journals: Mixing octane

Most of you know that the TL calls for 91 octane, which of course is not available anywhere that I know of. Most fuel is delivered to stations in 87 octane and premium octane of 93 in most places. They are blended to produce the 89 mid-grade octane fuel.

We have been assuming that that would be a 50:50 mix approximately: 87 + 93 = 180/2 = 90 (89).

I contacted one of my SAE colleagues who is a fuel engineer, and he said that actually, 89 octane is achieved by mixing at about 65% 87 to 35% 93 octane! As this is not my area of expertise, I am just passing this along.

Naturally, I asked whether that also applied to mixing 89 with 93 to obtain 91 octane, and he indicated that it did. Therefore, there is some savings to be had in mixing at 65:35 rather than the 50:50 we have been working with. With fuel prices at the level they are at, for some people this could add up over time.

And since using higher octane than needed has absolutely no benefit, those of us who use 93 octane in our cars have the opportunity to "mix" in a manner that does not let the 2 extra points just blow out the tail pipe. Personally, I have a "thing" about waste, and in handing over a sheckel more to Big Oil than i need to.

Some will no doubt say "why bother?" - for those people, this thread is not intended.
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Old 04-16-2005, 8:55 PM   #2
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Good to know. Thanks. But for my area of the country where the top octanes are 89 and 92, it may not be worth the trouble of the mental gymnastics and mixing to get 91.
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Old 04-16-2005, 9:26 PM   #3
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Interesting information to tuck in some corner of the brain - for those of us who live in full service states (NJ). I can only imagine the look on the gas station attendants face if I were to say "gimme 15 bucks regular and 9 bucks premium"
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Old 04-16-2005, 10:01 PM   #4
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Your formula still only gets you to 89 octane..

Most of the fuel stations in my area already sell 89 octane.


To get to 91 it is still closer to 50:50 actually 50/50 is only 90 octane

To hit the 91 octane a 9 to 4 ratio should do it. Or for simplicity..

It needs to be about 75:25 Which will give you 91.5 octane
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Old 04-16-2005, 10:04 PM   #5
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Quote:
Originally Posted by wakattack
Your formula still only gets you to 89 octane..

Most of the fuel stations in my area already sell 89 octane.


To get to 91 it is still closer to 50:50 actually 50/50 is only 90 octane

To hit the 91 octane a 9 to 4 ratio should do it. Or for simplicity..

It needs to be about 75:25 Which will give you 91.5 octane

I think you missed the point.
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Old 04-17-2005, 2:01 AM   #6
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Quote:
Originally Posted by Road Rage
Most of you know that the TL calls for 91 octane, which of course is not available anywhere that I know of. Most fuel is delivered to stations in 87 octane and premium octane of 93 in most places. They are blended to produce the 89 mid-grade octane fuel.
You should come out West. Here in Phoenix our choices are 87, 89, & 91 octane. It's possible they mix it themselves upstream somewhere, but they do sell it here directly. I think I paid about $1.60/gal the other day, lately it seems to go up about a nickel a gallon each time I get gas .

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Old 04-17-2005, 10:03 AM   #7
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Old 04-17-2005, 10:11 AM   #8
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Quote:
Originally Posted by rsc123az
You should come out West. Here in Phoenix our choices are 87, 89, & 91 octane. It's possible they mix it themselves upstream somewhere, but they do sell it here directly. I think I paid about $1.60/gal the other day, lately it seems to go up about a nickel a gallon each time I get gas .

Lance
$1.60 and you are whining? A lot of other places in the country haven't even seen 87 that low for months and months. You have it nice.

I don't worry too much about specific octanes that I have in my tank, I just put in 89 or 93, whichever I feel like at the time
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Old 04-18-2005, 5:36 PM   #9
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Quote:
Originally Posted by UnsanePyro
$1.60 and you are whining? A lot of other places in the country haven't even seen 87 that low for months and months. You have it nice.

I don't worry too much about specific octanes that I have in my tank, I just put in 89 or 93, whichever I feel like at the time
Oops, I meant $2.60 a gallon. Man, I even proofread the thing first...

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Old 04-18-2005, 10:09 PM   #10
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Interesting. I was taught years ago that the relationship between octane ratings was linear. I remember being told that in the past gas stations used to have pumps that allowed consumers to 'dial' the octane that they wished, and the correct parts of high-test and low-test were added to the tank. It seems hard to imagine that pumps back then could have operated on any assumption more complex than a linear relationship.

Sorry for babbling about 50's gas pump technology, I'll get to the point. If the octane calculation is based on (R+M)/2 for all fuels at the pump, then a linear relationship when mixing octanes would seem to exist...
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Old 04-18-2005, 11:38 PM   #11
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91 octane seems readily available around here (NC). There are a few places that have 93 octane (Amoco) but most have 91 or 92 octane.
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Old 04-19-2005, 1:52 AM   #12
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Quote:
Originally Posted by ndabunka
91 octane seems readily available around here (NC). There are a few places that have 93 octane (Amoco) but most have 91 or 92 octane.
hmmm, yeah, 91 is available all over. i dunno why RR says "...91 octane, which of course is not available anywhere that I know of."

i'm certainly not gonna waste my time at any non-91 station to mix-pump to get 91 just to save a few cents, my time is much more valuable than that. if anyone got a TL and finds the need to mix-pump, return your TL asap and buy a ford focus or something.

and yes, i read the last line in RR's post.
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Old 04-19-2005, 8:38 AM   #13
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Quote:
Originally Posted by RacialSlurs
hmmm, yeah, 91 is available all over. i dunno why RR says "...91 octane, which of course is not available anywhere that I know of."

i'm certainly not gonna waste my time at any non-91 station to mix-pump to get 91 just to save a few cents, my time is much more valuable than that. if anyone got a TL and finds the need to mix-pump, return your TL asap and buy a ford focus or something.

and yes, i read the last line in RR's post.
In certain parts of the country, mostly in the east, 91 octane is very rare indeed. Can't find it at all where I live in VA. RR is correct, you are simply ignorant in this case.

Waste your time? I doubt your time is that valuable when you spend much of it droning out worthless crap and berating the valuable posts of others. In fact, I can't say I've read one post of yours that has any value whatsoever.

Also, if you know anything about RoadRage, you'd know that his information is not shared for pinching pennies. Trust me when I tell you the man can afford any octane he chooses...and then some. It is a fact, however, that any octane ABOVE what your car uses is simply wasted, not usable. This is not only a waste of money but more importantly, a waste of natural resources.

I think I speak for many here when I say, I certainly wish YOU had bought a Ford Focus.
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Old 04-19-2005, 9:08 AM   #14
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Now, back to our regularly scheduled program. I've long been a self-schooled student on the octane issue starting when I bought a car that actually recommend 89 octane! Here, our octane choices are always 87, 89, and 93. Most gas stations run a pricing ladder of about 2.10, 2.20, and 2.30. This puts 10 cents between each grade, but let's do the math. If I buy a 13 gallon tank of 89 straight from the tap, I pay $28.60. I could also (using the standard 4:9 ratio) just put in 4 gallons of super for $9.20 and 9 gallons of regular for $18.90. That's the same tank of gas for $28.10, a savings of 50 cents. Would I spend my time doing this for 50 cents? Of course not, but this adds up for the gas stations as they can simply sell the same stuff out of a different spout for a higher margin! Genious!

Now, do I mix gas for my TL to get 91? Nope. But when my big hog Montero Limited is empty, (also requires 91, runs like shit without it) you bet I pump 93 and then throw a few dollars of regular in, just to save on the crude. It's only responsible.
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Old 04-19-2005, 9:38 AM   #15
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he writes
Quote:
Originally Posted by JackSprat01
This is not only a waste of money but more importantly, a waste of natural resources.
and then says
Quote:
Originally Posted by JackSprat01
.... But when my big hog Montero Limited is empty, (also requires 91, runs like shit without it) you bet I pump 93 and then throw a few dollars of regular in, just to save on the crude. It's only responsible.
thats retarded, but i guess if you enjoy doing that then you should buy more gas guzzling hogs, its better for the environment
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Old 04-19-2005, 9:54 AM   #16
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Quote:
Originally Posted by ndabunka
91 octane seems readily available around here (NC). There are a few places that have 93 octane (Amoco) but most have 91 or 92 octane.
It is? Everywhere I go, they have 93...can't remember when the last time I saw 91 was. 92????
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Old 04-19-2005, 9:57 AM   #17
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I finally found 91 octane, in the Buffalo area. Could only find 87 - 89 - 93 in Columbus. Lo and behold, the 91 octane in Buffalo was HIGHER priced ($.02 per gallon) than the 93 octane. Needless to say, I got the 93 (as I always do).

Anyone know why that may happen, unless it's supply and demand...
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Old 04-19-2005, 10:25 AM   #18
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I have found 89 isn't always available; besides, since its just a intermediary between 87 and 91 then why not just mix 87 and 93 to get 91... which if you do the math (assuming its a linear relationship) comes out to one third 87 and two thirds 93.

87 * X + 93 * ( 1 - X ) = 91, X = 1/3
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Old 04-19-2005, 9:22 PM   #19
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Interestingly enough, I read an article recently (I think in one of the car mags) where someone mentioned that all gas stations built within the last 20 years or so mix the fuel at the pump and only store two grades (high and low). They don't store the middle grade (or 2 middle grades in the case of some gas stations like Sunoco.
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Old 04-19-2005, 11:40 PM   #20
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There's a huge gasoline faq here: http://www.faqs.org/faqs/autos/gasoline-faq/

One section says:
6.15 Can I mix different octane fuel grades?

Yes, however attempts to blend in your fuel tank should be carefully
planned. You should not allow the tank to become empty, and then add 50% of
lower octane, followed by 50% of higher octane. The fuels may not completely
mix immediately, especially if there is a density difference. You may get a
slug of low octane that causes severe knock. You should refill when your
tank is half full. In general the octane response will be linear for most
hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89.
--------------
This seems to imply that the relationship isn't ALWAYS linear. It may depend on what the octane-enhancing ingredients are. Sometimes linear, sometimes not. However, for the purposes of what we put into our cars, if you assume linear, it won't be too far off.

Also, the octane available in specific parts of the country varies. Altitude has a lot to do with it. (Lower octane is needed at higher altitudes for the same engine. You probably won't find 93 octane as premium in Denver). So there's no need for people to start arguments about what octane gas is available at gas stations, saying other people are wrong.....
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Old 04-20-2005, 3:51 PM   #21
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It is my understanding that octane requirements are dynamic. High load conditions, like driving uphill in a higher gear, and high ambient heat conditions increase an engine's octane requirement.

I just use the 93 available here and I don't fill up until miles to empty reads zero. That way I minimize the octane impact of the half gallon or so 87 octane still in the fuel line from person that just filled up.

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Old 04-20-2005, 3:58 PM   #22
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So -- why doesn't everyone buy 3 of those plastic 2.5 gallon gas cans you use for your lawnmower gas? Put 87 in one and 93 in the other. Mix it yourself to get 91, if you like...
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Old 04-21-2005, 7:24 AM   #23
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Quote:
Originally Posted by KilroyR1
It is my understanding that octane requirements are dynamic.

Don
Of course it's dynamic. This is why Fords are continuously spark knocking only when you STEP on the gas. :dog:
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Old 06-06-2005, 11:46 AM   #24
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R&T says higher octane not wasted (!)

The most recent Road & Track tech column stated that if you put 93 octane premium gas in a Honda Accord which calls for 89 (or 87) regular gas, it will in fact produce more horsepower (about 10 was what the reader had heard). New car engines' computers can compensate apparently. This sounds questionable to me, but I wonder (Road Rage, are you home?). My new (used) Honda Civic Si calls for regular gas...

BTW, I have often used mid-grade gas on long highway trips where I don't anticipate VTEC usage, and was under the impression that the engine would "throttle down" to compensate for the lower octane. Harmless, no???
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Old 06-06-2005, 12:05 PM   #25
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Quote:
Originally Posted by ndabunka
91 octane seems readily available around here (NC). There are a few places that have 93 octane (Amoco) but most have 91 or 92 octane.

Huh??? I haven't seen 91 octane anywhere around here since... ever.

If there is a station you know about in Raleigh, please let me know!
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Old 06-06-2005, 12:18 PM   #26
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Read this too...

Quote:
The most recent Road & Track tech column stated that if you put 93 octane premium gas in a Honda Accord which calls for 89 (or 87) regular gas, it will in fact produce more horsepower (about 10 was what the reader had heard).
Basically what the item said was that Honda has designed their engine management system to allow as much spark advance as the knock sensors will allow. So higher octane will result in reduced knock and more advance translating into more horsepower. Although it didn't specifically say so, I'm assuming the TL will behave the same way under the same conditions.

If this is true, then running higher octane than recommended isn't wasted on a Honda engine.
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Old 06-06-2005, 12:27 PM   #27
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Maybe................ But it is highly dependant upon what the knock sensors will allow and we don't have a clue about that. I hope more spark advance is possible from our engine management systems so that some of us can benefit from higher octane, but only you guys having access to 92 and 93 octane can reap the rewards.
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Old 06-06-2005, 4:49 PM   #28
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Computers are pretty smart when it comes down to fuel management in our cars. Most of GM cars that set to run premium car now run on 87 octane car at reduced power. I suspect that Honda engines will do the same. Even those our car can run on 91 octane performance and mileage would improve with 93 octane gas. There were some stations that used to have 92 octane gas in my area, but my gas mileage went down when I used the 92 octane. I didnt save anything by using the 92 octane even though it was a little cheaper.
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Old 06-07-2005, 9:48 AM   #29
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Quote:
Originally Posted by JackSprat01
In certain parts of the country, mostly in the east, 91 octane is very rare indeed. Can't find it at all where I live in VA. RR is correct, you are simply ignorant in this case.
Try going to a Sunoco. I live up the road from you near the Springfield Interchange and I had never seen 91 octane until last fall, when I found that the Sunoco stations now carry it instead of the 94 octane they used to have (i.e., they used to carry 87, 89, 93, 94; now they carry 87, 89, 91, 93).

I've used Sunoco's 91 with no problems at all and my father reports the same on his TSX. I don't use it very often, though, just because the nearest Sunocos are out of the way as to any driving I normally do and I'd waste more money on gas driving there than I'd save by using the 91 octane, which is usually about 3 a gallon cheaper than the 93 octane.
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Old 08-18-2005, 1:38 PM   #30
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I can't believe I just wasted ten minutes of my life that I'll never get back, reading a thread by people who (have way too much time on their hands) payed 30k+ for a car and have actually wasted brain cells thinking about mixing 89 and 93 octane to get to 91 and save what? 40 cents.

Jeez people, isn't there something better going on in the world to talk about.
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Old 08-18-2005, 6:50 PM   #31
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Quote:
Originally Posted by bwilder10h
I can't believe I just wasted ten minutes of my life that I'll never get back, reading a thread by people who (have way too much time on their hands) payed 30k+ for a car and have actually wasted brain cells thinking about mixing 89 and 93 octane to get to 91 and save what? 40 cents.

Jeez people, isn't there something better going on in the world to talk about.

And you just wasted another minute belittling our converstion. Certainly you have something better to do than THAT? This is an internet forum; SKIP the threads 'unworthy' of your time.
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Old 08-18-2005, 7:23 PM   #32
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The only way the ecu can know anything about the octane of the gas
you put in it is through the knock sensors.
I doubt any manufacturer would build an engine that knocked on the recomended
octane rating under normal conditions.
If anything, I think they would give a safety margin.

Knock sensors are a sort of last ditch engine protection system, NOT a fine
tuning type system.
All knock is bad, and the sensor has to detect knock before the ecu retards
the timing to prevent more knocks.
Knocking is like hitting motor parts with a hammer, its not good.

If the ecu updates often, you can have a lot of knocking, it will just be short
duration instead of constant.



Keep in mind, knocking is not steady state stuff, air intake temps have a BIG
input, as does load on the engine, rpm's, carbon buildup on piston tops
and valves, etc.
Its quite likely you could drive around normaly on cool days running octane
well below what is recomended, and its also possable you could get some knocking even running 93 octane on a real hot day running up hill at full throttle
at lower rpm's.

Various factory manuals say what knock sensors do, how they work
(they are sort of like microphones), but I have never seen any info on just
how the ecu is set up to react to them.
The ecu retards the timing, but for how long, how often they dont say...

Brett
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Old 08-19-2005, 9:25 PM   #33
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Originally Posted by bwilder10h
I can't believe I just wasted ten minutes of my life that I'll never get back, reading a thread by people who (have way too much time on their hands) payed 30k+ for a car and have actually wasted brain cells thinking about mixing 89 and 93 octane to get to 91 and save what? 40 cents.

Jeez people, isn't there something better going on in the world to talk about.
Unbelievable. Not worthy of a reply, but since you are so passionate I will apologize for wasting your time.

Sorry if we interrupted your thoughts on the Heisenberg Uncertainty Principle.
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Old 08-19-2005, 10:34 PM   #34
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Originally Posted by kosh2258
Basically what the item said was that Honda has designed their engine management system to allow as much spark advance as the knock sensors will allow. So higher octane will result in reduced knock and more advance translating into more horsepower. Although it didn't specifically say so, I'm assuming the TL will behave the same way under the same conditions.

If this is true, then running higher octane than recommended isn't wasted on a Honda engine.

If this were true then it follows that the ecu continuously advances the spark until it detects a knock. Then since the advance is continously "tested", it wouldn't matter what octane is used, (engine damage wise), since at any octane level the ecu would force some knock. Just at a lower octane the spark advance range would just be lower. Less power but same "damage"
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Old 08-21-2005, 2:53 PM   #35
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If this were true then it follows that the ecu continuously advances the spark until it detects a knock. Then since the advance is continously "tested", it wouldn't matter what octane is used, (engine damage wise), since at any octane level the ecu would force some knock. Just at a lower octane the spark advance range would just be lower. Less power but same "damage"
No, it doesn't work that way, in the systems I have examined, and worked on. (If you have specific info that demonstrates that Honda uses a "continuous spark advance" model, and has no upper limit, then that is something altogether different).

The spark and fuel maps are just that - maps - they have set limits (example: the max number of degrees of advance BTDC), because things like emissions figure into the fuel/spark issue. The mfrs leave a fair amount of spark advance "on the shelf" because as a car ages, its octane demands almost always rise, too much spark can cause damage, even faster than the EKS can respond occasionally. In some cars, the EKS (engine knock sensor) has some software behind it to double check at a few intervals, since there can be mechanical noises that may "fool" it. So if they pushed the envelope too hard, they could put the engine at risk. But the major dirver is ensuring that the emissions does not get compromised. That would be very costly to correct.

I developed software years ago allowing the first TPI Corvettes to have their spark maps examined and adjusted "in the field". Worked at first with a major aftermarket chip supplier.

Now, I watch with interest, as my buddies pull up much more sophisticated tools and adjust those maps. Lately, a lot of work on SVT Cobras (Diablo Chips). But there is a fine line between ever more spark advance, and the grenading of the engine. And absolutely no guarantees about passing emissions tests.

The Diablo tine we did on an SVT Cobra with lots of mods had a bunch of spark map programs, but not one where the limit was "unlimited":

1) Stock
2) Aggressive street (extra spark at lower RPM, leaner A/F)
3) Track on Pump gas with (more of #2, with risk of engine damage)
4) Track on Race Gas (more of #3, with even greater risk)

By the way, the Supercharged 4.6L DOHC Cobra engine does not have aknock sensor at all. My understanding is that it has to do with the issue of other mechanical sounds making it impossible - could it be the scream of the supercharger in full song? (stopck boost = 8#'s. I was running about 12#'s, and have seen in excess of 20#'s and huge power from some of the crazies at SVTperformance.com
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Old 08-21-2005, 4:50 PM   #36
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Post Not exactly...

Road Rage,

The 2003-2004 Mustang Cobra and 1999-2003 Lightning have Knock Sensors, pigtails and wiring. They are turned off inside the EEC-V computer. Yes, the Eaton M112 blower makes a distinctive sound that triggers the KS and pulls timing. So Ford disabled them electronically.

Please remember that the 2003-2004 Cobra utilizes FORGED PISTONS, FORGED CONNECTING RODS (MANLEY), FORGED STEEL CRANKSHAFT, ARP HEAD STUDS, A CAST IRON BLOCK WITH 4-BOLT MAINS AND DOWEL PINS. It's strong, plus the 4.6L always used steel laminated head gaskets. You can break these engines, but they'll take a lot more abuse than any 4.6L 2V, 3V or 4V naturally aspirated engine that has a supercharger.

I've seen NATURALLY ASPIRATED 4.6L 2V engines with broken rings lands on the hypereutectic pistons. Knock is very bad! I myself have lost a piston ring land due to knock. It was on my supercharged 4.6L 2V T-Bird engine. I never heard anything, it just laid over and stopped making power. Compression test verified the horror, 7 cylinders at 180 psi and the #2 at 88 psi. Wow! BTW, my engine does not have a KS, just me listening (and it would be to late by then).

Forged parts are NOT indestructible either. I've seen plenty of professionally built forged 4.6L engines with wiped oil pump gears from detonation. Wiped oil pump means no oil pressure and well you get the idea. I know of one case where the detonation was so server it lifted the cylinder heads off the block and puked coolant all over the engine bay. All this with ARP head studs. We're talking 15-20 psi of boost though.

I could go on forever on this subject. My passion for the last 10 years was Ford...specifically making my T-Bird run 12's all day long.

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Old 08-21-2005, 4:54 PM   #37
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The best stuff out there...

Road Rage,

My car was dyno tuned by an ex-Ford engineer...tell your pals to seek SCT tuning for the best stuff. He and Chris Johnson own SCT and they know the know...not just hackers reverse engineering the PCM program.

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Old 08-21-2005, 6:05 PM   #38
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Originally Posted by Atrain
Road Rage,

The 2003-2004 Mustang Cobra and 1999-2003 Lightning have Knock Sensors, pigtails and wiring. They are turned off inside the EEC-V computer. Yes, the Eaton M112 blower makes a distinctive sound that triggers the KS and pulls timing. So Ford disabled them electronically.

....

A-Train
A-train. I owned one - see sig.

But I stand corrected. And how!

I left off the word "working" ...

"By the way, the Supercharged 4.6L DOHC Cobra engine does not have a WORKING knock sensor at all. "


.......and look what it got me?

The net is the same, whether the pigtails are there or not, right?
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Old 08-21-2005, 6:44 PM   #39
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All premiums are 91 here in western Canada.

If i had choice of 93, I would go with 89 and add just a bit of 93. so what if I'm running at 90 not a big deal.
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Old 08-21-2005, 9:15 PM   #40
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Absolutely...

Road Rage,

Yes, I agree with you.

Somebody with the proper software could flip the I/O switch and turn them back on. They are there and can function if allowed to. However they are off, so it's a moot point.

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Old 08-21-2005, 9:15 PM
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