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1999 Cadillac Seville STS (November 2012, 183k)
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Discussion Starter · #1 ·
It's a 99 so it needs Premium (in my case 91) gas right? I only fill up with Chevron or Shell, mainly Shell. With the way gas prices are I was just wondering.
 

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2001 Seville STS, 1990 Seville (RIP), 1972 Sedan Deville
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26,323 Posts
Nope. Your engine requires 91 octane regardless of where you buy your gas. 2000 and up were designed to run 87.
 

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2010 DTS
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88,145 Posts
Sure you can. It has a very effective knock sensor. I ran my '97 on 87 octane for years and never noticed any difference (performance or mileage wise) . Give it a try. You won't hurt a thing.
 

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1992 eldorado. 2007 dodge magnum hemi r/t
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I've had my 97 sts for 5 years and have always used 97 chevron, shell up here (or at least guys I know) would never use shell.
 

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1992 eldorado. 2007 dodge magnum hemi r/t
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I've had my 97 sts for 5 years and have always used 97 chevron, shell up here (or at least guys I know) would never use shell.
Duh!, my mistake , I have always used 87 in my 97 sts, it runs fine.
 

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2001 Seville STS, 1990 Seville (RIP), 1972 Sedan Deville
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26,323 Posts
Sure you can. It has a very effective knock sensor. I ran my '97 on 87 octane for years and never noticed any difference (performance or mileage wise) . Give it a try. You won't hurt a thing.
When knock is detected timing is retarded, no? How could that not effect performance? Why would GM tell their customers to use premium if it isn't required? Wouldn't make sense marketing wise to increase the cost of ownership for no reason. Even my '01 owners manual states that using lower octane may decrease performance.
 

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2010 DTS
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Yes it will retard the timing and in theory it will affect performance, but I never noticed it. Might need to take it to the track and time it to tell. It's not a big difference. If jcresciSTS wants to race it or be ready for a race at any time and wants peak performance, run 92. At today's prices he might be more concerned about price. Bottom line is he CAN run 87 in the pre 2000 engine with very little to no noticeable effect.
 

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1962 coupe (selling), 1995 Seville SLS, 1966 Sedan DeVille
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244 Posts
Would any of you try that on a '95? Since newer N* engines are not interchangeable with the older models, is there anything unique on the 2000 and up models that you can run with 87 octane? In GA, there's a minimum 30 cent per gallon difference between regular and premium.
 

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2010 DTS
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What I said is pertinent to ALL pre 2000 Northstars. GM lowered the compression ratio in 2000 from 10.3 down to 10.0 to allow for regular gas. That's it. I think even the 2000+ manuals say "for optimum performance" use 92 octane. Like I said, try it and see for yourself. Won't cost you anything nor will it harm anything. You can always switch back.
 

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1962 coupe (selling), 1995 Seville SLS, 1966 Sedan DeVille
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244 Posts
Will do. In the Atlanta area, any time there's a gas shortage (real or induced), mid and super grades become unavailable. I was afraid of breaking the 11th Commandment: "Thou shalt not use regular gas in THIS vehicle...Or I will smite your car with 10 plagues....starting with your head gaskets!"
 

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99 STS, White Diamond
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792 Posts
'99 DEFINITELY allows regular (87 octane) unleaded. Specifically stated in the owners' manual. Premium is suggested only for "best performance".

The car senses the actual octane of the fuel being burned and adjusts timing as necessary to reduce knock with the lower octane. Supposedly this was quite a technical achievement and I recall reading it was one of the first (perhaps the first) high-performance engines that did NOT require premium fuel.

Every car I've seen that requires premium has the words "Premium Fuel Required" written right by the gas gauge.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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My 2002 exists on 87 octane Shell. It runs fine, pulls hard, and will easily go to 130++ in a hurry without a hiccup. IF - IF knock is sensed, timing is retarded slightly, but only for the duration of the load event. Hook your OBD data port up to a real-time scanner and watch timing as you do all sorts of crazy maneuvers. You might be really surprised: timing in these engines, under normal and abusive driving conditions, varies from 3 degrees BTDC to about 35 degrees BTDC, and it is constantly changing. The "93 octane runs better" is a complete butt-dyno myth for engines that are designed to run on 87...........and running 93 will NOT advance timing.

The owner's manual does not say anything about 87 octane causing "reduced performance". It says to use 87 octane and, if towing a trailer or for maximum performance, you may want to use 89 or 91 octane fuels. "Maximum performance" does NOT mean the engine will run better or have any more "horsepower" at city and highway driving speeds. "Maximum performance" is operation under heavy load or WOT.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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When all else fails, read the correct year owner's manual - there's about 3 pages devoted to fuels and requirements. I do know that the 2000 redesign lowered the compression ratio to 10:1, down from ??? 10.4:1 ???, so the later engines would always be happy on 87. Regardless, your engine has a knock sensor which operates nearly instantly when needed, so the chances of doing engine damage by running 87 - or losing measurable performance - are slim and none.

BTW - Since Post #14 we're now in Norfolk, VA after a 220 mile run down the eastern Shore - on 87 octane we averaged 24.7 mpg - 2 adults and a load of birthday presents and luggage. 4.4 hours including a stop at a Burger Doodle for a gutslammer.
 

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I run 87 for both of my cars, never noticed any knock. Performance decrease? I think the wind speed/direction/resistance will affect the performance more than the octane difference can do.
 

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2010 DTS
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OK guys, here is what the Guru had to say about it in his own words.


"First, spark knock or detonation, is very dependent on the ambient conditions that the vehicle is operating in. The humidity in the air is probably the single biggest factor in whether the engine detonates on a given fuel octane rating or not. Dry air promotes detonation and humid conditions tend to prevent detonation. Air pressure or the barometer reading can make a difference. The higher the baro day the more cylinder pressure and the greater propensity to knock. Same here with altitude. An engine that requires premium at sea level can run comfortably on regular at Denver and on even lower grades of fuel higher up....the baro keeps dropping the higher in altitude you go. Ambient temp has some effect as it will drive the intake temp up and the coolant temp up...but it doesn't have nearly the effect humidity (or the lack there-of...) does on detonation. Cold temps can cause detonation as the humidity drops with low temps and the air is denser..more charge in the chamber. VERY cold air is the worst condition for detonation many times as there is zero humidity and very dense air. All these things make it hard to make a solid statement as to fuel reguirements. Generally, the fuel that will perform correctly over the widest variety of conditions is specified.

The 4.9 engine does not have a knock sensor so it generally needs premium fuel to avoid spark knock or detonation. You will audibly hear the spark knock in that engine so if you try lower grades of fuel and they do not knock then it is fine to use them. If you do hear knock then use a better grade of fuel. The 4.9 is a premium fuel required engine....although, as mentioned, under many conditions it will operate fine on lower grades of fuel.

The Northstar engine for 1993 was designed and optimized for premium fuel operation. Period. The spark timing, compression ratio, power level, etc...were all optimized for premium fuel. The Northstar does have a knock sensor that is very sensitive to picking up detonation and very effective knock control software in the PCM. It also has a combustion chamber that is very fast burn and detonation resistant. The chamber design is such that the engine does not need much spark advance to run properly, so, when the knock control system retards the spark to squelch detonation the driveability is barely affected if at all. The knock control system is sensitive enough and the spark retard "smooth" enough that you willl likely never hear the engine spark knock even on the worst fuels. The engine will not be damaged running on lower grades of fuel as the protection methods at WOT of spark retard and extra richness will squelch detonation with all but "bug spray" gas. The spark retard is transparent to everyday driveability but will cause some power loss at peak power. Not a seat of the pants thing...but...it will not make rated power on lower grades of fuel.

The Northstar engine has the same basic design for compression ratio and such from 1993 thru 1999. This would mean, on the surface, that the 93-99 Northstar engines are all premium fuel required. And they are.....but there is more to the story.

Two things became evident as the years progressed.... One, a lot of customers did not like having to use premium fuel...enough that it became a complaint item for Cadillac with the Northstar engine. Second, the engine runs so well on regular fuel that most people would never know the difference. It will not make rated power, but most people would never ever know.

So.....the twist is that even though the engine did not physically change the fuel requirement changed.....from Premium REQUIRED to premium RECOMMENDED and later to no real mention of fuel requirement other than to say regular was OK.... So that is why there is a difference in fuel "specifications" for the 93-99 Northstar engines. If you want the absolute best power under all conditions then use premium. If the cost bothers you, use regular. Honestly, despite claims otherwise, I have rarely if ever seen of felt any driveability issues with regular fuel in any of the earlier engines. I barely shows on the dyno, either.

Note, since the Eldorado was being dropped from the lineup shortly after 2000 model year and was being discontinued the decision was made to NOT change the dash legends saying "Premium" fuel....so...the 2000 Eldo does say Premium Fuel on the dash but it really isn't required. Definitely a source of confusion. sorry.

The 2000 and later Norhtstar engines are optimized in the same way for REGULAR fuel. The combustion chamber was significantly redesigned for 2000 and the compression ratio dropped .3 to accomodate the regular fuel. So, regular is the recommended fuel for 2000 and later and that is all that is required for rated power, fuel economy, etc.

If you read the first paragraph about the things that affect octane requirements and detonation maybe this explaination will make more sense...

I usually tell people that the 93-99 engine was "happy" about 25 percent of the time on regular fuel (no knock control activity/no knock control spark retard) and that about 75% of the time the knock control would be active on regular. Premium was required about 75 percent of the time if the owner/driver wanted to be absolutely sure to have max power.

Similarily, the 2000 and later Northstar is "happy" about 75 percent of the time on regular and that about 25 percent of the time the knock control is going to be pulling some spark out using regular. In other words, about 25 percent of the time even the 2000 and later engines will need mid-grade or poremium to avoid a slight power loss and/or knock control activity.

So....there is really no perfectly clean answer here. No black and white one in anycase...LOL

If you are towing a trailer thru Death Valley or racing your buddy or running wide open across Northern Ontario in the winter (very low humidity and cold, dense air) use premium in either engine to get max power.

If you are on the interstate on vacation on cruise....the engine is probably going to be perfectly happy on regular...any of them.

In any case, any concerns over future engine damage due to running poor fuel (octane "poor" I mean) is unfounded. It will not hurt any of the Northstars.

Understand that there is MUCH more regarding fuel than just octane rating when it comes to driveability. If someone senses a loss of driveability due to a certain grade of fuel I would suspect that it is due to the volatility , the blending or the amount of "other" added to the fuel..i.e...methanol, ethanol, MTBE, etc. rather than the octane rating."
 

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1995 Eldorado
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Thanks for this thread. I mean to only use premium in my new used 95 Eldorado but I am noticing a lot of gas stations only advertising one grade of gas and if that's all there is, it's nice to know I may get away with it if I am stuck somewhere without options.
 
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