What Octane Grade Fuel for 3800 SC?

Premium only!

One of the first things you can do to make more power is to use the right gas. The L67 engine was designed to run on premium fuel only. This means 91 or better octane. If you can find 94, go for it.

Higher octane fuels help prohibit engine knock, which is caused by excess heat inside the engine, a lean air/fuel mixture, and/or using a fuel that is too easily combusted. A higher octane fuel helps to resist cylinder combustion until it is ignited by the spark, so the burn is more precise and controlled. Knock exists even in stock engines, and can result in decreased power or possible engine damage if left unchecked. If you aren't using the highest possible octane, you are missing out on some of your engine's potential, and you could even be compromising its longevity.

I only use 94, and around here you can only get it at sunoco, so that means if I run out of gas and there isn't a sunoco around, I'm out of luck.

I used 93 since I got it and a couple weeks afterwards, but then I pulled up to sunoco and decided to give 94 a try.

Man I noticed a difference as soon as I turned the car on! I mean instantly. The gas pedal was stiffer and harder to put wot. It was a heck of a lot faster too! Big difference in only one percent more octane!

I use the best I can find... unfortunately all there is to be found this winter is 91. I'm hoping the 92 comes back this spring. Don't recall the last time I saw 93.

What's anyone think of 100 octane unleaded in the Riv? Theoretically the supercharger should love it...or at least a mix with 93. Granted...at $5.99 a gallon it certainly is not cost effective for a daily driver. But once in a while...oooo...that sweet exhaust smell!!

The higher the better!

so i usually put 92 octane in my car when i fill the tank but when my dad fils it he puts in 87 octane how much of a power loss and am i going to kill the engine or whats the worst that will happen??

I noticed great power loss before I started putting 94 in my riv. Mine was only a little faster than my 92 bonneville series I. You won't kill the engine most likely. Just don't really lay the throttle down unless you have mainly all 92 left. Let the 87 burn off and tell your dad to put in the good stuff or nothing at all.

I hate the fact no one understands why our cars need premuim only. I try to explain and I just boggle their minds with talks of compression, octane, knocking, and predetination. I just show them documentation that says premium only.

when i first got the car my dad said..every couple months put in 93 to keep the supercharger clean...so my first few tanks were 87...then about a month ago or so..(only had da car 2 months so far)..i put in 91...noticed a difference really quick...so my next tank i was like..screw this..im goin to 93..so i put in 93...i noticed a slight increase in mileage per gallon...and i def felt a bit more power..ill be stickin with 93 from now on...

If you fill the tank with 92, and then your Dad puts some 87 in when it's half empty, it's not the end of the world. The octanes will mix, and you'll have a blend of about 89/90. This is okay for normal driving (not racing, though). BUT... if the tank is empty and you put in 87, drive the car like Grandpa until you can get some premium. And don't use octane boosters, use real premium gas.

The reason 87 is such a bad thing is briefly explained at the top of the thread. Basically, fuel needs air & heat in order to explode. Air is provided from the intake, and heat is provided by one or more of these factors: 1) spark, 2) compression, and 3) temperature of intake air/combustion chamber. So as you can guess, the air/fuel can explode from reasons other than spark, and herein lies the problem. Ideally, we want fuel to burn only when the spark lights it. In a supercharged engine, this requires higher octane fuel. Here's why:

The amount of heat it takes to set off the air/fuel mix is determined by the octane level. Octane resists combustion, so higher octane gas will not burn until heat levels are very high. The spark from a plug is a very hot source, so it overrides the octane's resistance to combustion, and boom... you have an explosion. A controlled burn caused by true spark is a good thing for the simple reason that it's timed. Timing is everything.

Now, if you used low octane such as 87, the air/fuel mix can explode with a much lower level of heat. This causes problems in a supercharged engine because of the high compression that occurs in the cylinders. If you remember from chemistry, compressing anything makes it hotter. Superchargers compress the intake air, so it gets hotter inside the manifold. The pistons then compress the air/fuel mix, so the air gets hotter yet, and the fuel itself also warms up. The end result is the fuel exploding before the spark occurs, and this is bad. Timing is gone, so the piston isn't pushed optimally on the downstroke. Also, the air/fuel mix may explode violently rather than in a controlled manner. This is often referred to as knock, detonation, or ping.

But knock isn't the end of the world. Sometimes it's very minimal, and in normally aspirated engines it's almost tolerable at times. But supercharged engines are much more prone to knocking because of their increased compression. Although technically our compression ratio is only 8.5:1, once you squeeze the air through the blower and the piston reaches TDC, you're looking at a much higher overall ratio, probably somewhere above 12:1. It's important to realize that there are different levels and types of knock, and the causes of it can also vary from a lean mixture to lower octane, but heat is almost always a key factor.

If you read up on how KR works to fight knock, it would seem our PCM could just retard the spark timing to delay the burn until the proper time, right? That's how it usually works, but if the fuel itself is igniting before the spark even occurs, how does KR effectively help? Easy answer: sometimes it doesn't. KR is effective if the knock is a result of incorrect spark timing or improper air/fuel ratio, but if the air/fuel mix explodes on its own due to lack of octane, your engine is going to knock as long as you keep the gas pedal depressed. You may even hear a "ping-p-p-ping-ping" when it happens. When you do, it's time to let up on the accelarator... and get some premium fuel.

As stated, occational knock won't kill your engine. Your PCM can even learn when your engine has the wrong gas and switch over to a different set of tables to help band-aid the problem, but this really isn't the answer, and it doesn't always work. If you drive a car with the wrong gas for long enough under the right conditions (especially of you race the car on low octane), you could see a special case of extreme knock where the combustion happens so early (up to 180º) that it fights the upstroke of the piston. This is called preignition, and you better pray it never happens in your engine. Usually the result is near instantatious destruction to your piston... without a sound or hint that anything is wrong until it's too late. Instant engine rebuild from using the wrong gas. It can happen.

Read the owner's manual, the fuel filler door, and the little label on your dash. USE PREMIUM FUEL ONLY. Our cars aren't the same as some with N/A engines that will run okay on 87 and receive an extra power boost with the use of premium. Supercharged Rivieras need PREMIUM ONLY. If you don't use it, you're not getting the most out of your car, and you may be taking a risk.

18 gal x $.20 premium for premium=$3.60/fill up (assuming near empty tank)

Estimates:
20 mpg @ $3.00 regular=6.66 miles per dollar
22 mpg @ $3.20 premium=6.87 miles per dollar
or
21 mpg @ $3.20 premium=6.56 miles per dollar
or
20 mpg @ $3.20 premium=6.25 miles per dollar

More power, less detonation=priceless

Really there is no reason to cheap out with a lower octane. Even if you manage the same mileage with premium as you would with regular the difference in miles per dollar is nominal, only .41 miles. If you manage to get 2mpg better with premium vs. regular you're actually saving money and getting .21 miles more for your dollar. I would not want to risk engine damage to "save" $3.60 per fill up.

Agree, it's worth paying more for the fact that the engine runs better under heavy throttle. But I am confused when I hear some claiming better economy with premium fuel. Maybe you'd get slightly better economy in stop & go driving, but I can think of no reason why anyone would get better (hwy) mpg just by switching to premium.

i only drive city/suburb roads with my Riv. lots of hills and stop signs..so therefore slightly better gas mileage..and i acutally did the math too the other day with my exact numbers..i threw them out..but i myself ended up savin about 2.75 with premium..NOT including the trip to the gas station..granted u are makin a special trip to it...plus like you all said...new engine....4,000 bucks...in the long run...premium saves money.

AA wrote:

Agree, it's worth paying more for the fact that the engine runs better under heavy throttle. But I am confused when I hear some claiming better economy with premium fuel. Maybe you'd get slightly better economy in stop & go driving, but I can think of no reason why anyone would get better (hwy) mpg just by switching to premium.

My roommate says he gets better econ with 93 or 94. Don't know if it is for real tho. I don't see how higher octane would give better mileage???

My coworker had a Grand Prix with 3100, and said the same thing. He thought the higher octane made his engine run better, and said he had done a test to prove fuel economy had improved. I told him he was wasting his money, and if he let me borrow his car for a week, I'd do 3 tests to prove him wrong.

After explaining to him what octane is, he agreed there must've been some other variable in his test he wasn't considering... It's not easy to explain how premium gas improves economy, but it's very easy to explain how it doesn't.

The public assumes premium fuel is better for engines and will give higher economy because it costs more. The truth is, it'll only do good if your engine is designed to utilize its properties in a way that yields more power, and the benefit is almost always in the form of power, not economy.

To make matters worse, Shell has marketed it's V-Power premium gas with the idea that it is a performance fuel that will give more power and increased performance. The truth is, V-Power contains lots of detergents that keep engines running clean, and can even clean up a dirty engine, much like a fuel cleaning additive. In this case, lost power and economy can be restored, but additional power is never gained. Because of these detergents, Shell even recommends V-Power for cars that don't need the higher octane. Very tricky if you ask me. A clever way to sell unneeded octane to the masses... I wonder why they don't have V-power reg and plus grades? Hmmm.

No doubt V-Power is good gas. I use it sometimes, but my car can use the octane. I'm guessing lots of drivers are confused at why V-Power high-octane can be recommended for their car, while other premium brands offer no benefit. It's a hard case to prove to most, but without the cleaning agents in V-Power, it would be pointless to use it in engines that don't require premium. It's a brilliant tactic, imo. Someone at Shell got a fat raise when they came up with the idea, I'm sure.

My last car was a 00 mitsu eclipse gt 5spd and it said to use premium gas (91 oct?) and for the 156k miles i or my father owned it, it had premium in it twice - when we got it from the dealer, and when i used a F.I. cleaner and felt i should balance the difference from the additive i put in. Other than that, ive never used it in the car and never had a problem with it. for that reason i kind of got the idea in my head that the oct. wouldnt make any difference, and i would think the fuel tolerences in that car are more strict.
But you have convinced me enough to use premium in the riv now i think
My only question would be should i use 91/89 or 93
I belive the owners manual says premium (89-91). Would it then be reasonable to probly use 93 when running with higher boost?

Use the highest octane you can find in your area. In my state, there's 94 at every Sunoco, so I use that, but I run a 3.4" SC pulley. 91-93 should work fine for a stock car.

Also, I don't always use the same octane. If I'm going to the track, it's pure 94. If it's daily driving, I'll mix 94/93/92 throughout the week. If it's a 400 mile trip, I can get away with 89 (forces me to drive conservatively, too!). If it's through the mountains, I'll use 91.

I'm not just basing this on what I think. I've scanned with different grades, and KR levels do increase/decrease with your octane levels. Once I was running 89 on a trip through TN, cruising in OD up a steep grade, when I heard "clank-clank-clank". Scanned to find KR levels way up... stopped at a nearby station and topped it off with 92, which eliminated the problem. lesson learned... No more 89 through the mountains!

But unless you scan and really know your car, it's best to use 92-94 all the time. Another thing to consider is the source. If you use the same filling station all the time, and trust them, go ahead and fill up; but for me, if I'm at a station I've never been to, I only get enough to make it to the next station. You never know if you're really getting the correct octane, and one way to lower the risk is to mix your fills at different stations until you find one that you know has the goods.

89 (plus or mid-grade) is a strange grade, which probably came about for use in older engines in order to stop pinging. Some older vehicles with dirty engines can develop carbon build-up that will make the engine knock. Using plus can help this condition. 89 can also be used in an engine requiring premium if the extent of driving is cruising at 1800 rpm over flat ground (long trips only). But for normal driving, it shouldn't be used.

Premium being divided into 91/92/93/94 is really just a way to meet each state's laws regarding octane limit. In states like CA, it doesn't get any higher than 91, so they call that premium, but when you can buy 93 or 94, you can't really call 91 premium because it's closer to being 89.

Guess i'll be buying my own gas from now on

i only use 93 and ill put seafoam and drygas in every once in a whille. i only get a/b 18 mpg mostly city. even on trips i only get 20-21 mpg. i change the fuel filter with oil changes too. anyone no how i might get so better mpg?

Quite a bit of aero drag from those hoodscoops perhaps.

What is your definition of a trip? How many rpm's do you turn at 70? What tire pressures do you run? How often are you on the boost? Are you an "aggressive" driver? Did you have the tornado thing in the intake? How old are the plugs/wires?

it has been the same since b/f the scoops. the trips i take are a/b 200 round trip. at 70 mph im at a/b 1900rpms.i run 34 to 38 psi. i normaly just set it to 65-70 w/o using the s/c much. no im not a agresive driver. no the tornado is not in. plugs and wires less tha 10,000mi.

I'm Surprised you guys didn't mention that the Riviera, at least 97+ has a dual-stage fuel pump. When you use 87-90 octane, the fuel pumps burns the fuel up tp twice as fast to compensate for the lower octane. I use to be good friends with a buddy in Atlanta and he has done EXTENSIVE research on 3800 Series engines. You see, you are not only getting less power and may expereince some knocking, but you are also not saving any money since you gas milage will be worse with the lower octane. Sometimes around here the price difference bewteen 89 and 93 is alot. So I get away with 89 sometimes and never experience any knocking ( but my supercharger belt has been off for over year so maybe thats why) Well, even with the belt on, 89 was fine but 93 is probabaly the best recomened for price/performance. Sunoco here has 110 oactane race fuel but its leaded. What is your guys take on this?

-Chris

ps: I'm New, Hello

This also raises another question...how high of an octane rating is too high for our cars? Say youre at the drag strip and you want that extra kick in pants. But the only grades available are 110 and 112. Whats a safe mixture for our engines?

98riv65lark wrote:

This also raises another question...how high of an octane rating is too high for our cars? Say youre at the drag strip and you want that extra kick in pants. But the only grades available are 110 and 112. Whats a safe mixture for our engines?

Make sure you do not use leaded if you still have a cat, you will wreck it. If you can get unleaded use this chart. Click on the excel spread sheet.

http://www.gnttype.org/techarea/misc/Octanemix.html

I tried 89 octane for 4 weeks and I lost about .8 to 1 mpg on my driving to work which is a 66 mile trip each way on a 2 lane 55 mph road and 17 miles of highway driving. I swithed back to 93 octane and my milage can back to normal, which is about 26 mpg.

Okay, hold up here... Let's examine a few things:

quote: "I'm Surprised you guys didn't mention that the Riviera, at least 97+ has a dual-stage fuel pump."

I'd assume it has the two stage pump to get the higher fuel pressures (60 psi) required under boost, just like when the PCM switches to a hotter spark table when you are in boost.

quote: "When you use 87-90 octane, the fuel pumps burns the fuel up tp twice as fast to compensate for the lower octane."

If you are saying the PCM commands more fuel to combat knock caused by low octane, I could believe it, but typically the first step is KR, or pulling timing. In my experience, any fuel that is added is based on LTFT (long term fuel trim), which is the PCM's best guess as to whether you're running too lean or rich. And there are limits as to how much fuel can be added. When you run very lean, the PCM will not always keep up. However, a lean mix and low octane fuel are two different things, so that's why I question this idea.

Even if your theory is true, the only way the PCM "knows" you're running low octane is by listening for knock. So your engine is already knocking and then the PCM takes steps to add the fuel. This is an unhealthy way for an engine to operate, imo.

quote: "but my supercharger belt has been off for over year so maybe thats why"

That has everything to do with why! Without the belt, you can run on 84 octane!

quote: "Sunoco here has 110 oactane race fuel but its leaded"

As mentioned, the lead is not good, not only for the cat but also for the O2 sensor. But just upping the octane to 110 would be great. Besides the lead, there are no drawbacks to using ultra high octane. The only problem is, you need to tune for it to see a benefit. Just as many engines designed to run on 87 won't see a power boost with 93, your supercharged engine won't see extra power unless you change the PCM and up the boost to take advantage of it. However, once you're tuned for 110 octane, you cannot run the regular premium 93 for the same reason we normally can't run 87. If your engine runs knock-free with a higher octane and higher boost, anything lower will cause KR.