Why do Cadillac's "require" premium gas?

[97SLS]

Just curious. Would 89 octane hurt my 1997 SLS? Thanks.

[beefy]

It has to do with the motor's compression ratio. Cadillac's have a fairly high 9.5:1 or so. That means the combustion chamber gets hotter before ignition. Low Octane gas ignites too quickly before the piston is in position. That causes a "knock" when the little explosion(not ignition) hits the piston. With a higher octane gas the gas is less prone too ignite too quickly, and is able to be ingited by the sparkplug when its supposed to. As for hurting your car... Using the wrong octane gas once or twice shouldnt hurt it, but i believe consistently using the wrong octane gas can result in damage or premature wear.

[adallak]

It has to do with the motor's compression ratio. Cadillac's have a fairly high 9.5:1 or so. That means the combustion chamber gets hotter before ignition. Low Octane gas ignites too quickly before the piston is in position. That causes a "knock" when the little explosion(not ignition) hits the piston. With a higher octane gas the gas is less prone too ignite too quickly, and is able to be ingited by the sparkplug when its supposed to. As for hurting your car... Using the wrong octane gas once or twice shouldnt hurt it, but i believe consistently using the wrong octane gas can result in damage or premature wear.

I am pretty sure 1997 Seville is equiped with knock sensor allowing the PCM to adjust ignition timing to lower octane. If so, 89 and even 87 won't hurt anything. You probably won't even notice any change in performance or fuel economy if you do not race.

[JimD]

I frequently run 87 octane in my '98 with no ill affects. There is a knock sensor that is effective at warning the PCM to retard the timing just a tad for a second or two if and when required.

If there is a penalty in performance or fuel economy, I can't measure it.

[BodybyFisher]

I have a 96 Deville that requires Premium. I can use 89 with no problem but 87 will make the car feel like a pig. I have heard some say that 87 performs ok for them. I would NEVER use 87 in hot weather

Honestly however, since the PCM uses a knock sensor to retard the timing using 87 can sufficiently degrade performance so that you are getting poor gas mileage and its actually costing your more per mile using cheaper gas, watch out for that... Check your miles per gallon closely.

[OynxSTS]

Using the wrong octane gas once or twice shouldn't hurt it, but i believe consistently using the wrong octane gas can result in damage or premature wear.

No, these cars all have knock sensors and will work just fine on regular without damage... though the above posters are right this will reduce performance... in 2000 the North* was redesigned to run even better on 87 regular... but premium is still recommended for high performance use.

[Ranger]

I may as well add my $0.02. I ran 87 in my '97 Deville for over 3 years. Never saw any difference in milage or performance.

Beefy,

The fuel does not "explode". It burns in a steady, even fashion across the chamber. What you are discribing is pre-ignition not detonation. Detonation occurs after normal combustion. Pre-ignition is deadly, detonation is generaly harmless. The "knock" you hear is the resonance of the high pressure spike from the spontaneous combustion of the end gasses in the chamber.

Compression ratio is 10.3:1 on the pre 2000 and was lowered to 10.0:1 on 2000+ to allow for regular gas.

[JasonA]

As others have noted, the hit in performance and/or drivability is variable. My '97 definitely runs best on premium fuel. The throttle response is much softer when using lower grades of gasoline. For the nominal increase in cost, I prefer to use the fuel that lets the car run best (and I only drive mine about 5000 miles per year, so the difference in cost really is small for me).

[GreenMachine]

Have a look around the gas stations in your area, most have "special" days where they mark down the higher grades.

Here in Buffalo the Sunoco down the road has "SUper Tuesday" and "Ultra Wednesday", 91 and 93 octane respectively (they actually have 4 grades, 87/89/91/93 or 94 and some locations in the country have street racing fuel. There's several Mobils that offer Super+ (93 Octane) 5 cents off on certain days.

I think octane more or less is more important to location and driving habits than anything, plus if you've been using 87 and it does retard the timing, how long does it take before it bumps the timing back up? How will it know that you just put in better gas since its backed t he timing for 87, you might be able to get better fuel economy and performance and not know, but then again I'm not 100% sure how that works.

What I am sure of is if you're running in hot temperatures and stop and go traffic having the engine getting hot will cause more 'pings' than just driving around town casually, I suppose highway driving where the engine is at operating temp it would also be more noticeable.

heh, maybe I should kick this dead horse somemore.

I guess try it and see what happens, 89 is closer to recommendation than 87. I usually run the sunoco 91, or mobil 93, or sunoco 93, depends on whats cheaper at the time.

[Texas Jim]

As others have noted, the hit in performance and/or drivability is variable. My '97 definitely runs best on premium fuel. The throttle response is much softer when using lower grades of gasoline. For the nominal increase in cost, I prefer to use the fuel that lets the car run best (and I only drive mine about 5000 miles per year, so the difference in cost really is small for me).

My '06 does the same thing. There is a noticeable difference in performance if you use 87 instead of 93. I haven't checked mileage using 87 as I only did it once but I would "ASSUME" that since the performance is down the mileage is also. I want it to run the best that it can, so I spend the extra 20 cents per gallon for Premium. My Owners Manual says you can use 87 but there will be a decrease in performance, therefore Premium is recommended. Therefore that is what it gets.

[growe3]

I may as well add my $0.02. I ran 87 in my '97 Deville for over 3 years. Never saw any difference in milage or performance.

Beefy,

The fuel does not "explode". It burns in a steady, even fashion across the chamber. What you are discribing is pre-ignition not detonation. Detonation occurs after normal combustion. Pre-ignition is deadly, detonation is generaly harmless. The "knock" you hear is the resonance of the high pressure spike from the spontaneous combustion of the end gasses in the chamber.

Compression ratio is 10.3:1 on the pre 2000 and was lowered to 10.0:1 on 2000+ to allow for regular gas.

I would disagree that detonation is harmless, it is very destructive to an engine.

Detonation (also called "spark knock") is an erratic form of combustion that can cause head gasket failure as well as other engine damage.

Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to auto-ignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug, and rod bearings to severe overloading.

If you do not notice a performance drop, due to a well performing spark sensor, than regular gas should not hurt the Northstar engine. In hot weather I only use premium, as I definitely have a performance drop due to the type of terrain that I drive through daily.

I have experienced both problems firsthand, although not in my STS.

Pre-ignition:

My supercharged Bayliner with a 305 cu engine, leaned out and burned a hole the size of your thumb through one of the pistons (9.5:1 compression).

Detonation:

One of the 426 wedge engines, that I built long ago, suffered cracked and crushed ring lands in all eight cylinders (13:1 compression).

Please review a brief explanation on pre-ignition and detonation from a Hastings Manufacturing web page, that has more on this subject.

-George

Link to Hastings Manufacturing

http://www.hastingsmfg.com/Service%20Tips/..._detonation.htm

Link for more reading/research

http://www.hastingsmfg.com/site_index.htm

[JimD]

....

I think octane more or less is more important to location and driving habits than anything, plus if you've been using 87 and it does retard the timing, how long does it take before it bumps the timing back up? How will it know that you just put in better gas since its backed t he timing for 87, you might be able to get better fuel economy and performance and not know, but then again I'm not 100% sure how that works....

Timing is continuously being 'corrected' by the PCM according to a long list of operating parameters. The PCM has no idea what grade of fuel is in the tank (and really, you don't either; but that is another discussion), but based on what the powertrain sensors are saying, the timing will be what is needed at that moment.

Under heavy acceleration you might get 0 degrees advance (or full retard). When coasting at no load, you could see as much as 40 degrees advance, or more. Whatever is needed.

[Ranger]

I would disagree that detonation is harmless, it is very destructive to an engine.

Perhaps "harmless" was not the proper word. Yes, it can do damage, but it takes thousands and thousands of miles as opposed to pre-ignition, which is instantaneous. That was my point, meaning that you will do no damage using it.

[adallak]

I would disagree that detonation is harmless, it is very destructive to an engine.

Perhaps "harmless" was not the proper word. Yes, it can do damage, but it takes thousands and thousands of miles as opposed to pre-ignition, which is instantaneous. That was my point, meaning that you will do no damage using it.

The Guru told to me to not worry about spark knock at all. I had it for many years as many other owners of 4.9 for that matter, and the engine still runs strong. Maybe it is just this bulletproof 4.9?

[JasonA]

Spark knock is very common in modern lean-burning engines. Granted, not a heavy spark knock, but light knock under certain situations is even considered normal. It really is pretty benign. A 13:1 426 is probably a far cry from most modern consumer engines, and may be an apples-to-oranges comparison. I noticed no additional spark knock on mine when using lower grades, but the performance was flatter.

[clarkz71]

I may as well add my $0.02. I ran 87 in my '97 Deville for over 3 years. Never saw any difference in milage or performance.

I have verified a 2.5 mpg increase between 87 and 93. Went from 22 mpg average to 24.5 mpg average with my 95 Eldo.

[thu]

97SLS,

What is the altitude where you live?

Which grade of gasoline you use really depends on where you drive your car. If you live at a higher altitude (e.g. 6000 feet and higher), you should be able to use 85 octane no problem.

My 93 Deville (owner's manual says use Premium fuel) lived its whole life above 6000 feet. I always used 85 octane in it. Using premium fuel did not affect the MPG one bit, nor did it give me any noticeable increase in power. The car now has 298,000 miles on it and there's not a thing wrong with the engine.

My 2003 STS owners manual also says that the best performance is obtained with premium fuel. However, it doesn't take into account altitude. I use mid-grade (87 octane) and I get 24-26 MPG at 80+ MPH. Changing to premium (91) doesn't affect economy. I'm considering switching to regular (85 octane).

Octane requirements drop as the altitude increases.

However, if the engine is pressurized, then we may have a different story.

[growe3]

I would disagree that detonation is harmless, it is very destructive to an engine.

Perhaps "harmless" was not the proper word. Yes, it can do damage, but it takes thousands and thousands of miles as opposed to pre-ignition, which is instantaneous. That was my point, meaning that you will do no damage using it.

Hi Ranger,

Yes, I understand a little knock is not a big issue, but some engines knock just under the drivers notice (for long periods of time) and this problem will destroy a number of related parts.

If the Northstar, in any particular year, runs on regular without knocking/pinging, paying for the higher unnecessary octane fuel is a waste.

I drive from 2,000 ft to near 0 ft and back, every day. In the cool months regular is fine, in the summer there is a definite power loss when climbing back up the hill. Moderately steep grades at 70+ MPH is the norm. Often I will go over Carson Pass to Lake Tahoe and if I have regular in the tank it is readily apparent, taking a much longer distance to pass other cars; although no noticeable pinging.

As for the Northstar it has a very good system to counteract knocking; if all of the components are working as designed.

The reference to one of my custom engines was that I can verify the actual damage caused by detonation, not just from hearsay or a picture; not to compare it to the Northstar. Aside from the piston damage on my 426, there was also damage to the rod bearings (mostly the upper shell). Approximately 10k miles on the engine when I tore it back down to fix it.

-George

[97SLS]

97SLS,

What is the altitude where you live?

I live in Ohio so it is pretty flat here. I inherited the car from my uncle who lived in Indianapolis. I notice a little better mileage with 93 versus 89 but I don't drive the car hard. I drive it mostly on the highway also and the computer says I am getting around 23.5 but I have not checked it myself.

[RichieR]

The funny thing is it's not just the pre-2000 Northstars where Cadillac mandates premium. I had a '90 Eldorado with the 4.5L SPFI engine and the gas gauge said "Premium Unleaded Fuel Only". The 4.5 was a very strong engine, but it wasn't a high performance engine such as the Northstar. The 4.5L with SPFI generated 180 HP and the 4.9 generated 200 HP. I'm not sure if the introduction of sequential port fuel injection in 1990 is why Cadillac mandated premium in that year.

Anyway, I wonder why Cadillac mandated premium on the 4.5 and 4.9 liter engines.

[WarrenJ]

This is like the synthetic oil question which pops up periodically. Fortunately, we've just been through that so it isn't due again for a while.

Here's my story and I'm stickin' to it: my '95 lost just short of 1 mpg when I ran it on regular. Additionally, on regular, it wouldn't break the tires loose when it shifted second gear. The performance loss went otherwise unnoticed. What I saved on price, I largely gave back on mileage. The difference didn't amount to a Starbucks coffee. I went back to premium.

The current steed, apparently, doesn't have the tranny tuned to break the tires loose on a one-two shift in any event. Discussion moot. Is my mileage less on regular with the newer steed? Dunno, never touch the stuff; nor will I.

Now it's nitty-gritty time. The knock system is a feedback system. It corrects knock by first detecting its presence and developng a proportional feedback signal that tells the PCM a correction of a certain direction and magnitude is needed. The important point to note is that knock MUST BE PRESENT for the knock sensor circuit to make a correction.

Now I don't know what algorithm Cadillac uses, and I'm inclined to believe the Guru, but to be on the safe side I'd prefer to be on the end of the algorithm that calls for the least possible and least frequent amount of correction.

It won't even cost me a Starbucks every ten days. The car deserves it.

It also deserves a coat of wax. Guess what I'm doing tomorrow?

Regards,

Warren

[thu]

I live in Ohio so it is pretty flat here. I inherited the car from my uncle who lived in Indianapolis. I notice a little better mileage with 93 versus 89 but I don't drive the car hard. I drive it mostly on the highway also and the computer says I am getting around 23.5 but I have not checked it myself.

I understand that it's flat, but what's the altitude? 0? 1000? 2000? 3000? higher?

[adallak]

I live in Ohio so it is pretty flat here. I inherited the car from my uncle who lived in Indianapolis. I notice a little better mileage with 93 versus 89 but I don't drive the car hard. I drive it mostly on the highway also and the computer says I am getting around 23.5 but I have not checked it myself.

I understand that it's flat, but what's the altitude? 0? 1000? 2000? 3000? higher?

It depends on what he's smoking that day. Just kidding.

[Ranger]

I understand that it's flat, but what's the altitude? 0? 1000? 2000? 3000? higher?

1000' AMSL (Above Mean Sea Level) is a good average.

[WarrenJ]

1000' AMSL (Above Mean Sea Level) is a good average.

Sorry, I thought that had something to do with Amsoil.

Regards,

Warren

NEVER MIND