Fuel Grades

J

jase386

5.7L LT1 Member
Joined
Feb 23, 2005
Messages
455
Reaction score
5
Points
16
I am looking into buying an Ultra. 2001 model. and i read where they require premium gas.

The older Caddys required premium as well, but i dont know of anyone who ran it, they all ran regular.


Who all here runs regular in their Ultra?
And why would the S/C version of the same engine require different fuel?
 
jase386 said:
I am looking into buying an Ultra. 2001 model. and i read where they require premium gas.

The older Caddys required premium as well, but i dont know of anyone who ran it, they all ran regular.


Who all here runs regular in their Ultra?
And why would the S/C version of the same engine require different fuel?
Click to expand...

the Ultra uses a supercharged version of the 3800 engine. Boost from the supercharger increases cylinder filling, and creates more cylinder pressure for more power. That requires higher octane fuel to prevent detonation/knock. Higher compression engines need higher octane for the same reason. The engine computer listens for detonation by monitoring the knock sensors, and the computer will pull ignition timing in an attempt to protect the engine. That is called knock retard (KR), and it reduces power and efficiency (gas mileage). You can use regular gas in your Ultra if you must, and if you do, I'd stay out of boost and drive it like your grandmother. If you drive it harder, you should have premium fuel in there. The computer can only do so much to protect the engine, and detonation can be very injurious to any engine.

Having said that, I'm sure there will be someone who posts that they use regular in their Ultra with no problems at all. GM wouldn't recommend premium fuel for no reason at all. I understand that times are tough now, but IMHO, you aren't going to save that much by burning regular, and it may end up costing you more in the long run.
 
GM wouldn't recommend premium fuel for no reason at all.

Actually, they would, just to avoid any pinging issues in hot climates. In the early 90's Devilles, GM recommended premium fuel for a 9:1 compression 4.9 engine. Most people ran regular with no problems whatsoever; and 9;1 is not a high compression ratio. They probably had some issues in hot climates and just recommended premium for everybody. I would assume that a supercharged engine would definitely need premium fuel but ya never know if you can believe "The General" or not, since they were overly cautious in the past. I'm curious if there are Ultra drivers that use regular.
 
over the years, ive never known any ultra or cadillac drivers who used premium. i have never had a car that called for it, and would most likely see how it does with regular.

Was just curious for the reasoning behind it, and the compression information helps.
 
______________________________

Help support this site so it can continue supporting you!
I use premium. I have an app called Torque on my phone that connects to the OBD2 port via a bluetooth dongle. I can watch the knock retard in real time. With regular gas, the timing gets retarded by 8 - 12 degrees any time the manifold pressure goes positive. Premium keeps retard down in the 0 - 2 degree range.
So the engine runs more efficiently with premium, plus it makes more power. I find that the car requires less throttle with premium, so it doesn't down shift as often when I hit a hill.
It really doesn't cost that much more to run premium, and the car gives some of that back in better mileage.
 
Cadillac_AL said:
GM wouldn't recommend premium fuel for no reason at all.

Actually, they would, just to avoid any pinging issues in hot climates. In the early 90's Devilles, GM recommended premium fuel for a 9:1 compression 4.9 engine. Most people ran regular with no problems whatsoever; and 9;1 is not a high compression ratio. They probably had some issues in hot climates and just recommended premium for everybody. I would assume that a supercharged engine would definitely need premium fuel but ya never know if you can believe "The General" or not, since they were overly cautious in the past. I'm curious if there are Ultra drivers that use regular.
Click to expand...

AL,
There is a common misconception about the correlation of static compression ratio (SCR) and octane requirement. The higher the static compression, the more octane an engine requires. That is not entirely true. The valve timing has a direct effect on the actual cylinder pressure created. There can be no compression until the intake valve closes on the compression stroke. The intake valve always closes when the piston is on it's way up on the compression stroke. Different cams close the intake valve later, or earlier than others. Some of the stroke is used up while the intake valve is still open. Once it closes, the remaining stroke creates compression. That compression is called the Dynamic Compression Ratio (DCR), and the DCR is what determines octane requirement. Ever wonder why an aftermarket camshaft description lists a minimum static compression? That is why. You get best performance when the DCR is in the 7.5 - 8.4:1 range. Some cams close the intake valve late enough to require a higher SCR, to get the DCR in the required range. It is possible to have a 9:1 engine with a camshaft the closes the intake early, require more octane than a 10:1 engine with a late closing intake. This applies to normally aspirated engines mainly.


Supercharged and Turbo charged engines operate above 100% volumetric efficiency, and their cylinder pressures are like an engine running much higher static compression. Turbo and SC engines commonly have a static compression ratio in the 7.0-8.5 range, because boost brings that up substantially. I've read dozens of stories about people disregarding engine knock, and paying a big price when they crack a ring land, or if they are lucky, simply blowing a head gasket. That includes people who think they can run a higher boost with a smaller pulley, without supporting mods. You might find this article on DCR interesting.

http://www.empirenet.com/pkelley2/DynamicCR.html
 
Plus, premium is only 20 cents more a gallon. On fill up your only paying $2 or $3 dollars more for your engine to run more efficient. Might as well go for the ethanol free while your at it. Ethanol is crap, and hurts your mpg.
 
Faith_G said:
I use premium. I have an app called Torque on my phone that connects to the OBD2 port via a bluetooth dongle. I can watch the knock retard in real time. With regular gas, the timing gets retarded by 8 - 12 degrees any time the manifold pressure goes positive. Premium keeps retard down in the 0 - 2 degree range.
So the engine runs more efficiently with premium, plus it makes more power. I find that the car requires less throttle with premium, so it doesn't down shift as often when I hit a hill.
It really doesn't cost that much more to run premium, and the car gives some of that back in better mileage.
Click to expand...

That is pretty cool, you can use a phone for anything. I guess it proves that the Ultras like their premium fuel. That's not the case with the Cadillac 4.9's though. They don't have a knock sensor which is one of the reasons GM covered their ass by requiring premium on such a low performance engine although it does not offer any more power and they run just as well on regular. Hot AZ climates is a much different situation I assume. The Northstars have a knock sensor like the Ultra, they retard the timing for regular fuel. I suppose a lot of Caddy drivers wouldn't miss a few hp and probably don't notice it.
 
______________________________

Help support this site so it can continue supporting you!
That there is an interesting question. All Caddy models that required premium said Premium Fuel Only on the instrument cluster(even the Northstar engines with the knock sensor) However the Park Ave Ultra with the supercharger simply says Unleaded Fuel Only. Now GM knows 99.9% of the car buying public does not read the owners manual. If it was required then why would GM not put the Premium fuel only on the PAU? My guess is that it is recommended and not required. I think it will simply cause decreased performance and worse gas mpg. But I don't think it would hurt the engine. I would think if it really needed it would be mentioned on the car. Remember this is the country that you can sue and win a lawsuit for spilling hot coffee that you put between your legs knowing full well it is hot. i would think GM would want to cover their collective asses on this
 
journeyforce said:
That there is an interesting question. All Caddy models that required premium said Premium Fuel Only on the instrument cluster(even the Northstar engines with the knock sensor) However the Park Ave Ultra with the supercharger simply says Unleaded Fuel Only. Now GM knows 99.9% of the car buying public does not read the owners manual. If it was required then why would GM not put the Premium fuel only on the PAU? My guess is that it is recommended and not required. I think it will simply cause decreased performance and worse gas mpg. But I don't think it would hurt the engine. I would think if it really needed it would be mentioned on the car. Remember this is the country that you can sue and win a lawsuit for spilling hot coffee that you put between your legs knowing full well it is hot. i would think GM would want to cover their collective asses on this
Click to expand...

I think if you ran the SC3800 on 87 grade, and beat on the engine, you would eventually frag a piston. That is what happens when you put a small pulley on the SC and don't do any supporting mods, and you don't scan for KR. My 98 Riviera has a black sticker on the inside of the fuel door that says Premium Fuel Only. I think running the engine on lower octane fuel is entirely doable if you don't go to full throttle full boost for extended periods of time. Under those conditions, the computer can protect the engine. Detonation at full throttle will destroy any engine.
 
Im a low rpm guy, Love to hear that engine growl in OD. my 2000 lesabre rarely goes over 2300 unless im merging or need to pass, even then only into the 3000's. Very seldom do i WOT. Only do that to blow it out from the low rpm build up.
 
jase386 said:
Im a low rpm guy, Love to hear that engine growl in OD. my 2000 lesabre rarely goes over 2300 unless im merging or need to pass, even then only into the 3000's. Very seldom do i WOT. Only do that to blow it out from the low rpm build up.
Click to expand...
Well if this the case, you could remove the S/C belt and run 87 octane without any problem. As mentioned, the boost is what increases cylinder pressure. BTW, the engine will still perform well enough for 95% of the drivers without the S/C belt connected.
 
______________________________

Help support this site so it can continue supporting you!
My Park Avenue Ultra has just over 150000 miles on it, and it probably has seen maybe two or three tanks of premium in its lifetime, at most. At least I know for the last two years that I've owned it I've only run it on 87 and never have had a problem, knocking or otherwise.

Then again, I'm not a lead footed driver. In an effort to save some fuel and wear-and-tear, I try to keep accelerating rpms under 2200. It must work, because I've yet to replace anything on the engine besides a S/C pulley, and my fuel economy indicator says I'm averaging 19.2 mpg. I agree with the above posters that if you run the engine gently, then regular should be no problem; otherwise premium is probably a good idea.
 
HotZ28 said:
Well if this the case, you could remove the S/C belt and run 87 octane without any problem. As mentioned, the boost is what increases cylinder pressure. BTW, the engine will still perform well enough for 95% of the drivers without the S/C belt connected.
Click to expand...

Ive heard that you can just take the sc belt off and the sc will just act as an upper intake. If this is the case, i dont see why 87 would be an issue. I have seen scans on 87, 91 and 93 gasoline in my 03 ultra and never again will I use anything less than 93. Boost and KR is a huge no no.
 
______________________________

Help support this site so it can continue supporting you!
G. Man said:
19.2 mpg! Ouch!
Click to expand...

Yeah, it's not awesome compared to other's numbers. I just filled up with some 93 from Kwik Fill, so we shall see if it improves. I already noticed I am now getting instantaneous mileage of 20-21 on a hill in my area that I used to only get 13-14.

Funny thing is there is a slight surging now when shes going up this hill in low rpm. Seems slightly worse now then when i was on 87 but my cousin (who had it before me) told me that the car has a non-supercharged PA tranny installed, so it could just the gear ratios are off.
 
Last edited:
Both my 99 and 2000 ultras have premium only stickers inside the gas doors. I've tried to run regular in my 2000 which btw is my work car I drive 100 miles a day to work and back with cruise set at 85. Its has 230k on it right now and when I did run regular in it my gas mileage dropped from 26.5ish to 20-21ish on the highway. And although it seemed to run fine, it was definatly noticeable in power as well. And yes I've had 2 Auroras with northstars that barely wanted to run on regular, they pinged and kncked so bad it sounded like torture, and I had a 94 Deville with a 4.9, same thing, ping and knock. I say for the couple extra bucks is it really worth the risk of what you can do to it? If you dont wanna spend the extra $$ on gas find a regulafr non sc park avenue then it wouldnt matter
 
The surging went away, so it may have been an odd quirk or some bad gas. My mileage on premium is now reading 18.7, but I did reset my indicator when I put premium in, and I'm due for an 02 Sensor replacement, so we'll see if it gets better.

Haven't noticed a sheer horsepower improvement, but there definitely is some improvement.
 
You must log in or register to reply here.

Cars Connected in the Apple App Store Cars Connected in the Android App Store
Great products:
- VIN Check
- Windshield repair kit
- Clean your leather!
- Safe Emergency Charging
- Chemical Guys HydroSpeed Ceramic Quick Detailer
- Chemical Guys HydroCharge High-Gloss Hydrophobic SI02 Ceramic Spray Coating
- Get a dash cam!
- Auto Buyers Market
- Foreign invaders vs USA gangs
Protect your vehicle from code scanners #ad

Share this page

Back
Top