What Octane Grade Fuel for 3800 SC?

xxsupergman25xx wrote:

sooo..ive run 93 in my car for about 2.5 years straight...noticed for a while i got AWFULLLL gas mileage...switched to 87 when my dad filled my tank up with it...got about 4mpg's better. about 12 mpgs better on the highway...again..with 87....i tried putting in 93 the other day...in 5 gallons...i got 63 miles. yes...93...im talkin crusing at 55 mph for 30miles no accel. with 87 im getting aruond 25 mpg at these conditions. WEIIIRDDDDD...also..no knock. the only thing i noticed difference is the power with 93 is much better...

Do you measure consumption by gauge level or fill-and-refill; and do you refill at the same pump when testing mileage?

Albertj

Wolfmaster579 wrote:

I will check out a Lower heat range plug. Kinda sucks though cause the current plugs i've only had a few months.

I know, I hate to waste anything too. But, sometimes you gotta', and plugs are fairly cheap. When we built a new motor for our boat, we had to change the plugs and the carburetor jets and metering rods...

(The installation of our plugs is the real pain!)

albertj wrote:

xxsupergman25xx wrote:

sooo..ive run 93 in my car for about 2.5 years straight...noticed for a while i got AWFULLLL gas mileage...switched to 87 when my dad filled my tank up with it...got about 4mpg's better. about 12 mpgs better on the highway...again..with 87....i tried putting in 93 the other day...in 5 gallons...i got 63 miles. yes...93...im talkin crusing at 55 mph for 30miles no accel. with 87 im getting aruond 25 mpg at these conditions. WEIIIRDDDDD...also..no knock. the only thing i noticed difference is the power with 93 is much better...

Do you measure consumption by gauge level or fill-and-refill; and do you refill at the same pump when testing mileage?

Albertj

i measure by fill and refill. my driving style doesnt change from 87 to 93. i made sure of it. i always keep track of my mileage...thats why ive noticed all this.

Here's a thread (motorcycle forum) about the same theory Scotty posted, which basically came to the same conclusion we did. One guy says he thinks 87 has more BTUs, but another poster claims the difference is too small to be relevant. I tend to agree.

http://www.advrider.com/forums/showthread.php?t=493834

hey im not disagreeing with any of u. lol im just calling it like i see it. maybe theres no valid explanation. maybe there is. lol i mean..im not worried about it lol i doubt ill have this car for its lifetime...i think the riv will be gone within the next 1-2 years. or by some slliiiiiimmmmmmm chance could put it away for storage.

But maybe there is a good reason for what you have reported. Maybe it's just that no one else has figured it out yet. I wouldn't want us to overlook it. I like to discuss this stuff, because if by chance you are on to something, I'd be very glad to see this bunch of Riviera owners figure it out. If you should find any new info that supports your theory, please let us know!

i will track my data as best i can. things ill track...
miles
gallons of gas used
my route - i only go to the same 2 places so that should stay constant with hills and straights. i drive very conservative alwayysssss cuz i drive so much now that every mpg is worth it lol. i drive 60-75 miles a day.

xxsupergman25xx wrote:

albertj wrote:

xxsupergman25xx wrote:

sooo..ive run 93 in my car for about 2.5 years straight...noticed for a while i got AWFULLLL gas mileage...switched to 87 when my dad filled my tank up with it...got about 4mpg's better. about 12 mpgs better on the highway...again..with 87....i tried putting in 93 the other day...in 5 gallons...i got 63 miles. yes...93...im talkin crusing at 55 mph for 30miles no accel. with 87 im getting aruond 25 mpg at these conditions. WEIIIRDDDDD...also..no knock. the only thing i noticed difference is the power with 93 is much better...

Do you measure consumption by gauge level or fill-and-refill; and do you refill at the same pump when testing mileage?

Albertj

i measure by fill and refill. my driving style doesnt change from 87 to 93. i made sure of it. i always keep track of my mileage...thats why ive noticed all this.

Is it possible that the gas pump might be wrong (selling you 5 gallons and only dispensing 4)? In some locales fraud is a problem.

Albertj

AA wrote:

Here's a thread (motorcycle forum) about the same theory Scotty posted, which basically came to the same conclusion we did. One guy says he thinks 87 has more BTUs, but another poster claims the difference is too small to be relevant. I tend to agree.

http://www.advrider.com/forums/showthread.php?t=493834

IDK about BTUs - BUT - 93 octane burns slower so that the mixture does not explode under engine compression. 87 burns faster and with the supercharger's air charge in the cylinders would be much more likely to pre-ignite (causing knock). Get it? 87 is more volatile than 93.

Thinking about it, may be that this is one reason why the NA riv engine has higher compression than the SC engine.

Albertj

The NA 3800's higher compression has to do with making power without supercharging. The SC version makes higher effective compression with boost, but the NA version uses higher static compression to make decent power without the blower. Even though the NA's static compression is higher then the SC's, the SC's effective compression is higher than the NA's, so this is why the NA can run 87 without knock. It also explains why the SC version makes more power.

I've heard people say that higher octane fuel burns "slower" than low octane fuel, but I never believed it. I think the higher octane inhibits flashing until a certain temperature inside the cylinder, then burns at the same rate as lower octane fuel. I don't think it needs to burn slower to accomplish this. If premium fuel really burned slower than 87, it would require different timing curves to burn different octanes in the same engine. But an engine designed to run 87 will run 93 just fine with no changes to spark advance, so I don't believe about the different burn rates. Also, some higher compression engines run up to 9000 RPM on premium.

you're right about the 93 octane burn not actually being slower. Rate of combustion is not the same as degree of volatility but "burns slower" is easy (albeit inaccurate) short-speak for the difference in volatility. Specifically 87 is more volatile than 93.

Albertj

Absolutely.

The simple and correct definition of octane rating is "resistance to detonation", not to burning.

The way I understood the octanes was 'harder to ignite'. So it takes a little more energy to start combustion of 93 vs 87, whether that energy is in the form of hot air or spark etc, that's why 93 is more resistant to detonation. It's also why the advise using the lowest octane possible in lawnmowers/chainsaws and other hand-start engines - it's easier to pull start the little bastards with 87 than with 93.

From The Washington Times (Washington, DC) | October 9, 1998 | Sikorsky, Bob:

Dear Bob: In reference to your recent article about premium gasoline, our family recently purchased a Buick Riviera and an Oldsmobile Aurora, both of which say to use "Premium Gasoline Only."

What's your opinion about using lower-octane fuel in these cars?

- H.W., Manhasset, N.Y.

Dear H.W.: You have just spent premium dollars to purchase two premium General Motors vehicles, and now you want to ply these expensive vehicles with lower-octane, nonpremium gasoline, no doubt to try to save a few pennies at the pump. Talk about penny-wise and pound-foolish!

General Motors doesn't require premium fuel for these vehicles simply because some engineer thought it would be cool. These cars need premium to operate properly and at full potential. Otherwise, the label would advise you that lower-octane gasoline is safe to use.

You have just laid out more than 60 grand for those two cars. Does it make sense - or cents - even to think about using a non-recommended fuel, simply to save a few pennies each time you fill up?

My 97 SC ran with low 87 octane for the last 13 years with no problem...But my father was the driver.. From now on it will be premium only.

fieromanie wrote:

My 97 SC ran with low 87 octane for the last 13 years with no problem...But my father was the driver.. From now on it will be premium only.

After the first few hours of driving it probably switched to the Bad Fuel Spark Table and never looked back

Or Dad never went past 1/3-throttle...

But hell, that's what the knock sensors are for.

Remember though, knock sensors only hear the detonation after it has occurred. Kind of like a smoke detector beeps only after the fire has started. The PCM programmers obviously felt KR was adequate for dealing with occasional knock on proper grade fuel - for extreme knock resulting incorrect fuel, they created a special table with retarded ignition tables, to ensure Dad will not blow up the engine when he decides to go WOT that one time.

WOOHOO! Found 94 octane 20 minutes from town. This lousy place only goes as high as 91.

My Pontiac G5 GT is 10.4 to 1 factory, she gets premium. There was never any question in my mind that a supercharged engine should get anything but premium, your just insane or stubborn not to! I can get 93 all day long at Mobil and thats all my cars have ever gotten (except my Century with the 3300, that thing would run on lemonade)

//General warning about pistons chipping and head gaskets here//

Hometown Hero wrote:

WOOHOO! Found 94 octane 20 minutes from town. This lousy place only goes as high as 91.

You lucky bastage! I'd like to cut the bells off of the iceholes at CARB that leave us with 91 in the whole fargin' state...

DecoyGSX wrote:

My Pontiac G5 GT is 10.4 to 1 factory, she gets premium. There was never any question in my mind that a supercharged engine should get anything but premium, your just insane or stubborn not to! I can get 93 all day long at Mobil and thats all my cars have ever gotten (except my Century with the 3300, that thing would run on lemonade)

One question: Does the fuel-door or owner's manual specify Premium?
I don't know your motor, but in the last decade, a lot of cars have gone to 10+ compression on regular because of ever-faster computers, knock-sensors, direct-injection, variable-cam-timing, etc...

Yeah, the G5 (GT package only) and Riviera (Supercharged) both require premium from the factory.

Hometown Hero wrote:

WOOHOO! Found 94 octane 20 minutes from town. This lousy place only goes as high as 91.

LOL you guys are funny. EVERY gas station here has 93 as their premium. Sunoco, BP, and Valero all have 94 here. That's all the Riv gets!

And since when does a little EcoTec G5 get premium? I used to work at a Pontiac dealer and I never remember that.....

FUFUFUFU!!!

The guy who puts 93 or 94 into an Ecotec,

is a helluva lot luckier than the guy who has to feed his L67 91-octane Pelosi-piss!

Eldo wrote:

FUFUFUFU!!!

The guy who puts 93 or 94 into an Ecotec,

is a helluva lot luckier than the guy who has to feed his L67 91-octane Pelosi-piss!

right?!