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      03-04-2010, 04:47 PM   #9
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Drives: 2011 M3 COUPE
Join Date: May 2008
Location: NEW YORK

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Originally Posted by TrackRat View Post
Octane is a rating of the fuel's resistance to detonation. It is not an indicator of power at all. More octane does not produce more power.
This I knew.

The information you were given is incorrect. Slow burn produces less power every time. The peak cylinder pressure needs to occur at a very specific point in the crank rotation. To get that peak cylinder pressure (bmep) at the correct point the ignition timing must be advanced on slow burning fuels. When you advance the timing to get the peak cylinder pressure at the correct point then the cylinder pressure before top dead center has to increase, causing negative work as the increased pressure tries to push the piston down before it reaches TDC which effectively tries to turn the engine backwards or stall the piston's upward motion. This is why slow burn fuels never produce as much power as fast burn fuels. In addition slow burn fuels can increase end gas burning which can increase spark knock or detonation depending on the chamber shape.
This is new info to me. I was under the impression that quicker burning fuels were spent long before the piston reached the bottome of its power stroke, therefore there is a period during the power stroke (while piston is traveling down) where no more power is being created. Hence the additional power that can be harnessed by slower burning fuels while keeping all other variables (i.e. timing) the same.

For all practical purposes at medium to high engine speeds, the useful power stroke is over as 90 degrees past TDC. Early cylinder blow down is used to reduce pumping losses because the change in cylinder volume vs. the expanding gas rate, means the push on the piston is minimal.

I have no idea what chemicals Liqui Moly is using. Read the label for contents. If you check many newer "octane boosters" they are primarily methanol or ethanol. They will increase octane some but so will E85 for a lot less though you can't use much if your fuel system wasn't designed to handle alcohol fuels.

FWIW, any college automotive engineering text such as Taylor and Taylor's "The Internal Combustion Engine in Theory and Practice", will help you understand the facts on engines, fuels and the combustion process.