Drive it forever
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Q. Please explain the difference in the fuel-rating systems RON and AKI.
A. Gasoline engines can be prone to igniting the air/fuel mixture prematurely, whether it's due to too much ignition advance, too much compression, or a lean-burn condition. This premature ignition is called knock.
The standard hydrocarbons used to determine an antiknock rating are octane and heptane, with octane molecules having eight carbon atoms and heptane molecules seven. Heptane is prone to knock, octane is not. Octane rating is the knock resistance of a sample of gasoline compared to a mixture of octane and heptane, so 95-octane fuel is as knock-resistant as a mix of 95 percent octane and 5 percent heptane.
"RON" stands for "Research Octane Number." Fuels rated on this scale are used to operate a specialized engine with a variable compression ratio to determine the octane rating of each fuel experimentally.
"MON" stands for "Motor Octane Number," which is measured in a similar fashion, but under conditions that more closely resemble a running, modern engine: leaner mixtures and hotter cylinder-head temperatures. A fuel's MON rating is usually eight to 10 points lower than its RON.
So what is AKI, the number displayed at the pump? It stands for "Anti-Knock Index," and it's the average of the other two numbers.
Q. I have a 2002
When I returned home, I pulled the spark plugs. In the number one cylinder, the spark-plug arm was completely bent, touching the center electrode. I did a compression test, and the numbers were fine. The next day it backfired again, and now I have a loud tapping noise.
Any ideas?
A. A bent plug electrode is indicative of a foreign object in the cylinder. What comes to mind first is that some object, such as a nut or a stone from the road, has been ingested by your engine.
Another possibility: Aftermarket turbos are notorious for overboosting the engine to the point of detonation. It's easy to blow in more air than the fuel injection can match, causing a lean condition. An engine running too lean can make chunks of piston crumble like my grandma's buttermilk biscuits.
Either way, it's time to pull the cylinder head. Actually, I'd pull the entire engine: When the piston has squashed something into the head, I like to replace the connecting rod and magnaflux the crankshaft to look for cracks.
Dear Readers:
Nobody has written in about this particular issue, but it's well worth your attention if you find yourself needing to raise your car with a jack.
Standard operating procedure calls for placing the jack under designated points near each wheel. And that's fine, if you're using the scissors jack they threw into the trunk at the factory.
But we're Saturday mechanics, and we need to work on our cars. That means using a floor jack and some proper safety stands, so that we don't become something resembling a pizza if the car should slip off the aforementioned jack.
Floor jacks are great, but they can bend the pinch weld because they have a flat lifting surface rather than the formed fitting on the stock jack. The resulting bent pinch weld is ugly and leaves a place for corrosion to start.
Most floor jacks have rubber pads, but the pads don't last and aren't really spongy enough. So I use a chunk of 2-by-4 with a saw kerf in it to keep from mangling the pinch weld.
Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com. 
