Fuel and Mileage

Hello i own a 2007 Grand Prix Supercharged. 38k miles. Everything is stock.

i have two catagories of questions.

I have been filling it up with 89 octane. Gas prices are bad, and im thinking about putting in 87.
Do i run any risks?
Is 89 ok to begin with?

Second question, aside from driving 55-65 to increase mileage. What else can i do? i get about 21-25mpg on the freeway now going in the 70's mph.
Would 91 octane improve mileage?
Would getting my fuel injectors serviced do anything? (the guys at my oil change place swears my car needs a fuel injector service, cant find anything about it in my car manual.)

Thanks in advance.

Most Likely with 38K miles you deff. do not need a Fuel Injector service. Run a bottle of Cheveron Fuel System cleaner through your gas tank. It might clean some components up.

GM and we recommend that you run premium unleaded gasoline only but lower grades can be run in emergency situations only. From my experiences the mileage is better with premium versus running mid-grade or regular. Since this is a supercharged SIII 3800 engine you will run the risk of getting detonation which will cause knock retard (KR) and a loss of horsepower and performance.

But it is your decision. Your car and money.

Running 87 octane on a regular basis is bad news in a supercharged 3800. The savings really isn't that much (even at $0.20 per gallon difference it's only about $3 per tank and you'll most likely lose a couple mpg's) but the possible engine damage that can be caused by knock is huge money!

Running 87 octane on a regular basis is bad news in a supercharged 3800. The savings really isn't that much (even at $0.20 per gallon difference it's only about $3 per tank and you'll most likely lose a couple mpg's) but the possible engine damage that can be caused by knock is huge money!

Ok so is 89 ok to run in my tank?
I get my work mileage reimbursed. So it's usual to for me to rack up 1k-2k buisness miles. So i drive alot. Sure, i like the money i make off of my mileage but i also dont want to hurt my engine in any way. Driving this much..........

would 89 ocatane hurt my engine? Am i safer with 91?

:shakehead

GM and we recommend that you run premium unleaded gasoline only but lower grades can be run in emergency situations only. From my experiences the mileage is better with premium versus running mid-grade or regular. Since this is a supercharged SIII 3800 engine you will run the risk of getting detonation which will cause knock retard (KR) and a loss of horsepower and performance.

But it is your decision. Your car and money.


I completely agree with this. I bet running less than 91 might save you a few bucks up front, but in the long run I'd say you'd save more running premium.

Check this out:

http://ask.cars.com/2007/07/premium-gasolin.html

I think this person goes a little more into detail and explains it very nicely.

Running 87 octane on a regular basis is bad news in a supercharged 3800. The savings really isn't that much (even at $0.20 per gallon difference it's only about $3 per tank and you'll most likely lose a couple mpg's) but the possible engine damage that can be caused by knock is huge money!

Damn! So i've put about 8,000 miles on my car with 89 octane. i dont really hear knocks and pings. I've got to put 91 for a tank or two and then check to see or hear a difference.

Have i dont serious damage to my car??? :banghead:




P.S. i had a non supercharged GM3800 series I in a 94 buick le sabre. GREAT ENGINE. 200K only needed to change the gaskets!

We as human beings aren't smart or sensitive enough to feel knock retard (KR). You need a full function odb-ii scan tool with KR readout capability. That is why KR is called the silent killer of supercharged engines.

With mid-grade gas I've seen up to 4 degrees of KR with my scan tool running mid-grade versus 91 octane or better premium. That is not acceptable.

From what I've read up on, you lose MPG for running lower than premium unleaded, due to the knock retard.

If you do the math, it's not really worth it.

Let's say for EXAMPLE (I don't know what the car actually gets for mileage on regular) you get 22mpg vs 26mpg w/ premium unleaded. The gas tank is roughly 17 gallons. Let's say you fill from dead empty. I'm going to go with California's gas prices...4.433 per gallon for regular, 4.638 for premium. It will cost you 75.36 to fill with regular and 78.85 for premium. You can go 374 miles @ 22mpg and 442 miles @ 26mpg (regular vs. unleaded). In total, you pay 0.20c per mile for regular and 0.18c per mile for premium. Now this is all in basic theory, and the fuel economy loss may not be that steep, but you see what I mean. Do yourself and your car a favor, just fill it with premium. Trust me, the adverse affects are costly, so everyone will be happier with premium.

My opinion is you can't damage the engine driving the car on 87 from A to B with any amount of mileage....and I recognize I represent a minority opinion.

But here's why:

The PCM will retard spark (essentially throwing combustion heat down the downpipe and losing fuel economy as a result) in response to ESC (knock) events.

A single knock event doesn't hurt anything, it's been likened to sandblasting your fingertip with a little shot of air, it's a "hot" sensation but lasts just a second and there's no damage. But...line up the knock events when the crankshaft is rolling over at 6000 rpms (50 combustion events *per second*) ...or with the analogy a nearly constant flow of sandblast media onto your fingertip....and you can see the problems with heat building up on the piston face. And that's where the trouble begins. When it gets to a point where the PCM can't command enough responses to the ESC events you get into base-engine trouble...and IMHO that is also likely coupled with other systems such as lubrication and cooling not being up to "factory fresh" specs either.

Generally if you are trying to produce power the PCM is in a "power enrichment" mode which drops back to around 12.5 to 13:1 A/F ratios for max power and also helps damp out knock events....other factors such as relative humidity, air temp, etc all factor into what happens in a combustion event.

The 3800, being an old-school iron-head engine is knock-limited...in other words you can never actually get to MBT spark advance before the ESC events start. The tradeoff is that the engine is very EGR-tolerant and makes very good fuel economy numbers for the displacement.

I would suggest doing your own "engineering study" with 87 and 89 (and 91 if you are so inclined) to find out quantitatively which is the better economic choice. Run several (say 10 tanks) of gas thru mixed driving modes and average economy numbers and see what's best. I would also state as a general trend that the more easily you drive (milder application of throttle) the better you'll do on 87 because then you're not "wasting" heat thru spark retard. If you're a hot-foot :D, the greater octane numbers would make sense as above as a low cost compared to base-engine trouble.

I dont know Crazyhorst....

You sound like a very intelligent person, but pistons will get hairline cracks in them and chunks will even chip off them and fly around the combustion chamber from knock.

I dont think we should risk it.

Top ring land is the most susceptible (weakest) spot...this is the aluminum surface on the cylindrical side of the piston (all the way around) before the intersection with the flat top.

The crevice there is a hiding spot for unburned fuel which quickly adds to hydrocarbon emissions...every engine manufacturer in the world is trying to reduce the volume of that crevice...of course balanced with the strength of the piston.

Of course if you've ever seen the valve cutout reliefs in a DOHC piston you'll know why they get into "piston protection" overfueling much more quickly. The sharp edges "concentrate" heat.

A relatively flat dished piston as in the 3800 variants is reasonably robust. However, you'd be amazed at the progression of piston designs over the years (and how much mass has been "carved" out of them) on an engine that's been relatively constant in displacement (ala Chevrolet 350cid V8 and the "Buick" 231 cid V6 we know and love for a couple of examples)

I would suggest doing your own "engineering study" with 87 and 89 (and 91 if you are so inclined) to find out quantitatively which is the better economic choice. Run several (say 10 tanks) of gas thru mixed driving modes and average economy numbers and see what's best. I would also state as a general trend that the more easily you drive (milder application of throttle) the better you'll do on 87 because then you're not "wasting" heat thru spark retard. If you're a hot-foot :D, the greater octane numbers would make sense as above as a low cost compared to base-engine trouble.

:rolleyes:

You're entitled to your opinion and yes you are in the minority. What long term personal experience do you have with L67 SII 3800 engines and what "engineering studies" or testing have you performed?

Most of us have already done that. BTDT like 10 years ago. Fuel economy is superior with 91 octane or better premium unleaded period!!! as is overall driveability!!!! :grinyes:

Naylor, I understand your skepticism when a cat shows up out of nowhere and comes off like he once worked at GM's V6 engineering center.

Well, simply, I did.

I was there when the directive was issued for the next generation of the L27 to take up the packaging space of the old 3300 ironhead V6, have a 6krpm redline, and make 200bhp naturally aspirated...and was there for about 6 years after the first running prototypes were built. It was first dubbed the ERV6 "extended range V6" and later the S2 that we know today.

I've literally beaten 3800s and the 60 degree conqueror 6-ways-to-Sunday, dyno cell to towing trailers in Death Valley in August.

But, I've never owned one until someday soon this year. I surely don't have all the answers, but personal opinions are still always free.