Originally Posted by ahmadkq (Post 221185)

Hi All, I have a choice betwen unleaded 90 Octan and unleaded 95 Octan. Whice one is the best?

Originally Posted by Mendel Leisk (Post 221211)

What higher octane gas does is resist pre-ignition better. This is solely needed for higher compression engines. If your car needs it, it will be spec'd. in the Owners Manual.

My '01 Porsche 911/996 C4 has a 300HP very seriously HIGH COMPRESSION engine. When I picked it up at the factory in the spring of 2000 for our travels thoughout europe the only caution I was given was to be very careful to not fuel it with leaded fuel. They would test it on my return and if I had used leaded fuel both catalytic converters would be replaced on my nickel.

When I asked about octane levels I was told the fueling with regular would not be problematic. I was told that the A/F mixture ratio would be adjusted automatically to accommodate the fuel octane.

That said, one instance where higher octane might be needed but not spec'd. in the Owners Manual: an older vehicle where the engine has accumulated enough carbon build-up in the cylinder head to actually reduce the air volume when the piston's at the top, in turn increasing the compression ratio.

That, coupled with all the possible ignition sources (the carbon will tend to have glowing points), might increase the odds of pre-ignition.

If higher octane gas is spec'd for your car, and you put regular octane in, the car's computer will likely detect pre-ignition starting to happen, and retard ignition timing a little, ie: make the sparkplugs fire a little later.

No, as I understand today's engine control computer firmware specifications the timing will only be retarded if the knock/ping is the direct result of the (too early) ignition timing itself. Otherwise the mixture will be enriched to alleviate the knock/ping or it is even possible that the automatic transmission will be "commanded" that a downshift is required.

This in turn will prevent pre-ignition, your engine will run a little cooler, but you will take a performance hit.

If I'm car shopping and see some particular car will need premium fuel, that crosses it off my list. Cars needing premium fuel tend to be high performance models, tuned more max acceleration, trying to get more horsepower out of a smaller displacement engine.

Are you aware that most gasoline engines in these "fuel-sipping" modern day HSD hybrid systems have an unusually HIGH compression ratio, 13:1....? And that will like go up to 15-16:1 once the problems of DFI are resolved.

The newest, 2010, HSD models even leave the EFFECTIVE compression ratio at or near 13:1 (DFI/15:1 coming soon) as long as the cylinders are to be only partially filled with mixture.


They're more costly to fuel and tend to sacrifice fuel economy. Also, I suspect they will be more prone to engine problems down the road.

Originally Posted by GreenRedTCH (Post 221291)

My 2003 GMC with the 6.0L (364 CID for us old folks) LS1-type engine has 2 knock sensors, and pulls timing when knock is detected. I can put up log graphs showing this, as well as the timing tables, trigger points, etc. if anybody is interested.

What if the knock/ping is the result of detonation, spontaneous mixture combustion prior to spark ignition?? In that case of what good does it do to retard the spark...??

And what about knock/ping due to lugging the engine?? Knock/ping as a result of the piston being "restrained" from moving downward as fast as the flame front advances....??

The reason the TCH engine has such a high static compression ratio is because it is an Atkinson engine, which has a very late intake valve closure point, which reduces the dynamic compression ratio to a normal level.