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Senior Member
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Join Date: Jul 2005
Location: Richmond, VA
Posts: 1,110
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Senior Member
Join Date: Jul 2005
Location: Richmond, VA
Posts: 1,110
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Good explanation of things, folks. Bravo zulu!
I would add that the newer engines with "knock sensors" do indeed retard the ignition timing when preignition is detected. Most of the time this is not audible, but other times you can still hear your engine ping a bit.
If the only time your engine pings is under heavy throttle or pulling a load, then it is probably running normally. However, if it still wants to go around the block when you shut if off ("dieseling"), then you could have a problem. Anyone who has driving a pre-1990 car with a carburetor will have experienced this when using cheap gasoline!
While octane ratings are used to measure the anti-knock capacity of gasoline, diesel engines use cetane ratings.......which is virtually the opposite situation because you want it to "knock" all the time. That is how a diesel engine operates, and part of the reason for the noisy clatter. They work on compression ignition - rather than with a spark. (The compressed air & fuel mixture ignites itself when the pressure is great enough.)
Of course, that is why diesel blocks, pistons and rods must be so darn heavy. The compression ratio of most street cars is probably between 8:1 and 10.5:1, while diesels have a compression ratio of 17:1 on up to about 22:1. Yikes! 
Cetane ratings for diesel fuel in our area runs between a low of 40 and a high of 47, although my large International diesel truck seems to run the same no matter where I get fuel. With the recent tightening on the quality of diesel fuel by the government, it is all pretty good quality nowadays and must mee the ULSD (Ultra Low Sulfur Diesel) requirements of 15ppm sulfur content.
My V6 4Runner seems to run very well, no matter what fuel brand or grade I use. I think I have only heard it knock once or twice when climbing a steep hill at a very high rate of speed. The owner's guide and the stealership both say to use "87" unleaded, so that is pretty much what I do. For kicks and giggles, I'll dump some premium in once in a blue moon....but have not noticed a difference.
P.S. I once owned a 1964 Cadillac with a 429 V8. That car would knock on almost any pump gas, except for Sunoco leaded 93 octane....which probably has not been available for at least twenty years. It actually used to have a greenish tint to it!
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-- Former 2005 4Runner Driver --
Vehicles: 2002 Nissan Sentra & 2004 Chevrolet Silverado
Last edited by Jeff Kleb; 05-14-2007 at 08:19 PM.
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