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2004 N* fuel choices...premium wins

5K views 37 replies 11 participants last post by  Ranger 
#1 ·
I had been running my 04 DHS on 87 octane without problem but after carefully reading the wording of the OM, I switched to premium. Yes, the computer will retard the timing and make all of the adjustments to use regular gas but boy did my car wake up with the use of premium gas.

I can't be too surprised that a 10:1 engine would perform more efficiently on a higher octane fuel than it would on 87 but what surprised me was how much better it performed. On 87 I had done a couple of WOT to pass another car and quite frankly was not impressed with the results although I would rate the passing efforts as "adequate" overall. Moving to higher octane gas opened for me, a whole different perspective of the N*. I tried it today on the hwy between towns (6mi, no traffic to hit) and a stomp from 80kph had me up to 140kph in a few short seconds with noticeable push back into the seat and a beautiful, previously unheard song from under the hood.

For the difference of a very few $ per tank offset by a + in the fuel economy, I'm sticking with the good stuff, not the "bar stock" for my Cad.

Opinions please ???


J.R.
 
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#2 ·
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unless you are towing - or racing -
you are wasting money buying anything higher 87 gas -

you won't get any more power - or better gas mileage -

as far as the "BUTT-DYNO" test goes - it proves nothing -

but if it makes YOU feel better - go for it -
the gas companies appreciate the extra money -
 
#8 ·
90+% of my driving is done in a company vehicle (loaded 2013 Chrysler Town & Country, soon to be replaced with a 2014) and they pay the gas, etc. so when I drive my personal vehicle I can almost consider it a hobby. My personal fuel costs per year are minimal to say the least so I feel I can afford to experiment and evaluate without breaking the bank as it were, hence I will indulge my DHS as it warrants. I don't have kids or pets so I can spend whatever I feel like on my wheels. To each his own and here's to ya'.


J.R.


J.R.
 
#10 ·
90+% of my driving is done in a company vehicle (loaded 2013 Chrysler Town & Country, soon to be replaced with a 2014) and they pay the gas, etc. so when I drive my personal vehicle I can almost consider it a hobby. My personal fuel costs per year are minimal to say the least so I feel I can afford to experiment and evaluate without breaking the bank as it were, hence I will indulge my DHS as it warrants. I don't have kids or pets so I can spend whatever I feel like on my wheels. To each his own and here's to ya'.


J.R.


J.R.[/QUOTE
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well - in THAT case -
there are LOTS and LOTS of products on the auto parts store shelves that you should try -

and EVERY ONE OF THEM will work - just as well as using premium gas -

enjoy!
 
#4 ·
I've only just started with this guys so some more shall we say, "scientific" results will come down the road. It idles somewhat smoother and pulls noticeably better when kicked running premium. I'll try it for a while to see what the real differences are since it only adds about $6~$7 per tank for the premium so no biggie. If it actually turns out that the car prefers the higher octane, then it gets it but if there is no long term appreciable gain perceived, it goes back to 87 octane.


J.R.
 
#7 ·
The engine does not idle better, it's all in your head. Placebo effect.

"Premium" fuel is not any "better" than "regular". There are no more additives, it is no cleaner, and it has no more hydrocarbons (energy content). The ONLY difference is its resistance to igniting under the heat of compression.

The engine cannot sense what fuel is being used. The only time the PCM reacts to lower octane is IF AND WHEN detonation occurs, and that will a under high load, high RPM conditions. Idle is not even remotely close to causing detonation with lower octane. This has been discussed to death in lots of threads.

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SUPER LAC DEVILLE said:
try 100 octane racing fuel... it cost more
Must be better then :sneaky:
 
#14 ·
Well Ranger, as I had stated earlier, I have no idea how the car was treated previously but since I have owned it I have subjected it to to the rigors for which the engine was designed and despite the decriers, I have found that so far the use of 92~94 octane gives allows the engine to give a noticeable/differential "push" @WOT over 87.

Perhaps my driving style is the lynchpin given that I'm not especially easy on it as compared to the O.O. who paid over $65kCdn. new for it and very likely babied it. My relative "abuse" may be waking it up???

It is what it is, I love my Cad, I went from my 85 Deville to an F150 for 7 years because of my retail business where I needed a truck but I closed the store recently and went back to Cadillac as soon as I didn't need the truck. Old habits die hard and truly, we don't have to justify them. My .02Cdn.


J.R.
 
#13 ·
I know my car quite well. Even of using premium fuel have you a whole 5 extra HP, you cannot feel or hear that difference in an engine that makes 275-300 HP. In a '60's beetle, maybe, but not in these cars. The demographic that the cars were designed for is completely irrelevant. Idle is totally and utterly unaffected, and any power difference to small to be noticed.

I'm not saying you shouldn't use a higher octane fuel, it's your car, do whatever you'd like. But unless you have dyno slips showing a power difference, or have run several tanks of fuel under IDENTICAL conditions using different octanes, and have recorded a difference in MPG, there's nothing to support the claims that "premium" is better for a Northstar.

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knotgoalie said:
Some of us younger (I'm 57) folks have to wind them out and make them sing every now and then so the premium fuel is the best slow burning insurance we have right now.
Insurance against what?
 
#16 ·
I know my car quite well. Even of using premium fuel have you a whole 5 extra HP, you cannot feel or hear that difference in an engine that makes 275-300 HP. In a '60's beetle, maybe, but not in these cars. The demographic that the cars were designed for is completely irrelevant. Idle is totally and utterly unaffected, and any power difference to small to be noticed.
Ok, first of all you could not be aware of what I do for a living. I work in tv/motion picture production and have spent decades working with picture cars...all of the traffic and any vehicle shots you see on screen are totally controlled and choreographed. We have to work with everything from rusted out shitboxes to new exotics every day depending upon what the scene calls for.

I'm currently working on a movie set in the 40s using both civilian and military period vehicles. Deal with 70+ year old vehicles for months at a time when they have to be running/ moving on cue and you realise that your long aquired "instant evaluation" skills can apply to any vehicle be it new, not really new, or old so I'm trusting my experience with my own car...no disrespect intended to you because in the first instance, I know nothing of your technical background my friend and you in turn know nothing of mine. I ask that you not denigrate my suppositions outright until one or the other of us is able to present definitive data re 87v94 octane fuel used in a 2004 Deville DHS...K?


J.R.
 
#17 ·
What does any of that have to do with anything?

You're saying your fuel mileage has increased - Fact: You cannot calculate a fuel mileage increase (especially a very small one) by running a tank or two of a different fuel and driving normally. You need to run several tanks of each fuel through the engine, and drive under identical conditions, on identical routes.

You're saying there's more power - Fact: No human being can do the job of a dynamometer, your occupation does not grant you super sensory abilities. You simply cannot feel a tiny power increase in a powerful engine, especially in a heavy vehicle. The power to weight ratio is hardly affected. That would be like noting acceleration differences in a bicycle after removing the reflectors and skipping breakfast.

You're saying the car idles better - Fact: Unless there was something very wrong with the 87 octane fuel you were using, there is absolutely no difference in how your engine idles. There is essentially no load, no extra high cylinder temperatures from high RPM operation. Everything is operating exactly as it would with 87 octane. The engine would idle just fine with even lower octane fuel.

You mentioned premium is "insurance". You should do some reading on the Northstars knock detection system. Nothing bad is happening to your engine by using 87 octane fuel, even when the timing needs to be retarded.

Like I said, use whatever fuel you'd like, but premium does not "win". There is no disadvantage to using it, there is no real notable advantage to using it. It's only better if it makes you feel warm and fuzzy inside.
 
#30 ·
I think Moist is absolutely correct here (meaning all his posts). In fact I think everyone here should realize that moistsubbass.... nevermind I can't remember, but I do know I get better gas mileage and more power when I put gas in the car. Also more importantly it's 60 degrees outside in DC.
 
#22 ·
Update...data bs on the premium, so far. To a degree the "butt dyno" has been correct but not for the right reasons. I committed the ultimate dumb of evaluation by switching fuel grades to premium after giving the previous tank of regular a mild overdose of Lucas injector cleaner/upper cyl. lube. Who knows if or when it had been done before? I very much suspect that the smoother idle and pretty song @ wot are far more attributable to the cleaner than to the higher octane gas so I retract my previous follies and fuel on 87 heretofore. Should I however decide at whatever time to do a late night superhighway/between major cities "how fast will this barge go" kind of run...I'll run it low and then fill with premium just to be sure:D


J.R.
 
#24 · (Edited)
-----------------------------
"how fast will this barge go"
it will ZOOM right up to 112 MPH - then the fuel injectors will shut down -

once your speed falls below 112 - the injectors will become activated again -

speed limiters were set according to the type of tires it received at the factory -

L75 mph
120 km/hOff-Road & Light Truck Tires
M81 mph130 km/hTemporary Spare Tires
N87 mph140km/h
P93 mph150 km/h
Q99 mph160 km/hStudless & Studdable Winter Tires
R106 mph170 km/hH.D. Light Truck Tires
S112 mph180 km/hFamily Sedans & Vans
T118 mph190 km/hFamily Sedans & Vans
U124 mph200 km/h
H130 mph210 km/hSport Sedans & Coupes
V149 mph240 km/hSport Sedans, Coupes & Sports Cars
W 168 mph
270 km/h
Exotic Sports Cars
Y 186 mph
300 km/h
Exotic Sports Cars


base Deville and DHS models got "S" rated tires -
the DTS normally got "H" tires -
a few - VERY FEW" - got "Z" rated tires -

THERE WERE A FEW EXCEPTIONS -
for instance - in 2002 - there were 33 DHS models -
that were ordered with the RPO code "QNI" - 225/60/16 "Z" rated tires -

just FYI - Seville SLS models also got "S" tires -
and MOST Seville STS models got "H" tires -
and a few got "Z" tires -
 
#23 · (Edited)
For the 2004, does your owner's manual say something like "Use regular unleaded gasoline with a posted octane of 87 or higher. However, for mountain driving, trailer towing, or maximum performance operation you may want to use a premium fuel".

That's a generic statement from the 2000 and later Northstar manuals - the 2000-2004 engines had a 10.0:1 compression ratio, slightly different spark plug location, and timing set to use 87 octane fuels. (Ignition timing will NOT advance past the preset specs in response to using premium fuels.) Maximum advance in these engines is 35° BTDC, and at 60+mph road speeds the timing hovers around 31° - 34° BTDC. Touch the gas pedal to pass or rapidly accelerate and timing instantly drops back to 18° - 25° BTDC, regardless of fuel used. At WOT, 130 mph in 3rd gear (STS) the timing is about 20° BTDC.

For someone who likes to monitor their engine functions, take a look at a ScanGauge-II. www.scangauge.com
 
#25 ·
when i first bought my 04 Deville i was using 87 octane but it would knock when going up hill, i read the owners manual & it said u could use the 92 if it does that, so i kept using the 92 octane fuel & it clogged my convertor & i had to replace it, so now i stick to 87 octane with no problems & no expense of a $1400 exhust bill.......LOL
 
#26 · (Edited)
Post #24 is correct ---- and the speed rating for your car is printed on the tire specs sticker on the driver's door edge.

maxjeepster, Using premium fuels in your engine had exactly nothing to do with "clogging my converter". The knocking when going up hills or on mild acceleration was probably caused by partially clogged EGR passages. As a matter of fact, 89 and 91/93 octane gasoline burns slower than 87 in order to control flame travel at high power demands.

There are simply too many 2000 - 2004 Seville/Deville drivers in here who use 91/93 octane fuels almost exclusively, with NO problems, to support the theory that running higher octane fuels can damage a catalytic converter.

EDIT: Please Google "top tier gasoline" and do some studying.
 
#36 ·
SUPER LAC ........... EGR ............ Exhaust Gas Recirculation - an emissions system that recirculates (!) some exhaust gas from the right (rear) manifold into the incoming intake air in order to dilute the oxygen content, thus slowing down combustion in order to control emissions of nitrous oxides. Given all the ignition, mixture, EGR, and air controls these engines have, ANY screwed up intake fuel mixture can cause a "knock" - in this case it's the old "dry peas in a can" analogy: With restricted EGR flow you get a slight increase in fuel burn rate - a "ping" - upon part throttle or passing power demands.

Years ago we could blank off the (prehistoric) EGR system, fool with timing adjustments, and get a TINY power increase way up in the power band. Not so today, with the tight electronic emissions controls.

On your engine the EGR valve is on top of the right (rear) cylinder head, just about in front of where your gas pedal toes would point. About the size of a little Minute Maid orange juice can.
 
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