Use premium fuel in your Super Buick

BY Jim KerrBackyard Mechanic . Aug 26 04:00 am

Question: Recently, I had a discussion with another Buick owner. I have a 2008 Buick Allure Super and the other party has a 2010 Buick Lucerne Super. Both have the V-8 engines with added performance built by the GM Performance Division. Although the Lucerne has the Northstar engine, in both of our owner’s manuals it states the same thing with reference to gasoline octane. Quoting from the Allure manual: “If your vehicle has the 4.6L V-8 engine, use premium unleaded gasoline with a posted octane rating of 91 or higher. You can use regular unleaded gasoline rated at 87 octane or higher, but your vehicle’s acceleration could be slightly reduced, and you might notice a slight audible knocking noise, commonly referred to as a spark knock.”

The Lucerne owner has a family member that works for one of the oil companies in Alberta and they indicated it is not necessary to use premium fuel, yet gave no scientific reason. Who is right? If (and I emphasize if) using regular gasoline is really questionable on these types of engines, then it is more harmful to the engine itself as a result of actual or potential spark knock, simply a compromise in acceleration, or both?

 — Kevin

Answer: Your Lucerne Super is designed to run on premium 91 octane fuel. While the owner’s manual does state you can use 87 octane regular fuel, it could possibly cause problems. Fuel with an octane rating too low for the engine design tends to pre-ignite or detonate. This means the fuel starts burning before it is supposed to and the sudden rise in cylinder pressures can cause damaging engine vibrations we refer to as pinging or engine knock.

The engine does have knock sensors and if a knock signal is sent to the engine computer, the computer will retard the ignition timing to try and prevent the knock from occurring. When you use 87 octane fuel, especially in hot weather, engine knock occurs and the timing is then retarded. If the engine can’t retard the timing enough to control the knock, engine damage can occur.

Retarded ignition timing will help prevent knock, but it also decreases engine performance and fuel economy. Once you fill back up with premium fuel, the knock will go away and the timing is automatically advanced. On performance engines, I often find using premium fuel is more economical because you get better performance and fuel economy when the knock isn’t occurring. If you could use regular 87 octane fuel without any engine knock, then that would be even more economical, but isn’t realistic given the requirements of your high-performance engine. I would use premium fuel in your cars and I expect you will find it provides better fuel economy because of the engine design.

 

Question: I have a 2002 Dodge one-ton truck I use on my acreage and the clutch disc destroyed itself. I am fairly mechanically inclined, but have never seen a clutch disc in so many pieces. The centre of the disc was broken completely out of the friction material. My question is, what could cause this to happen? I have only had the truck for a few months but it has been driving well until now.

Answer: It doesn’t happen often, but when I find a clutch disc has broken it is usually due to high and sudden off/on loads. There are torsional springs in the clutch disc to absorb these sudden loads and they may have failed or fatigued first. Once it is broken into may pieces, it isn’t easy to tell what happened first.

I suspect on your truck that it had been used for hauling heavy loads and the driver wasn’t very experienced driving a manual transmission vehicle. There was probably a lot of jerking from the drivetrain when the driver was starting to move away from a stop. This may have cracked the metal of the clutch disc hub and it just got worse as you continued to drive it, even though it is now being driven smoothly.

This is a situation where you just replace the clutch assembly and continue to drive smoothly. A new clutch should last you many years.

 

 james.kerr@saktel.net