some guessing

Fred /
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Aaah well... different story then.

There is a lot of guessing here >

I believe the huge range of advance and amount of advance outboards run is partly because they have rather inefficient and large bore combustion chambers (compared to what we know of as high performance 2 stroke streetbikes.. (old ones.. of course))... and the requirements of an engine running through a single speed power transmission, pushing an object with a lot of drag is quite different from an engine pushing a lightweight low resistance vehicle with a 5 or 6 speed.

In other words... the huge advance curve is necessary for these "tractor (boat)engines" with their large low compression combustion chambers... and these engines are also expected to idle around all day long getting halfway decent gas mileage and not overheating or fouling plugs.

The high performance motorcycle 2 strokes run in a narrow range at much higher efficiency (hp to cc) ratio... and a large advance curve is no advantage whatsoever.. it would be a detriment.

Correct Ignition timing in any one situation is determined a lot by the largeness of the combustion chamber and the burn rate of the mixture, and the RPM you are running at... It is like a fuse... if you want the bang to occur at the most advantageous time... you have to light the fuse early enough.

The larger the combustion chamber... the earlier you have to light it for the flame front to travel across the chamber and generate its peak pressure right about TDC... and as rpm goes up you have to light it earlier to let the burning be done in time for a near TDC peak.... and yet in these boats you still need to idle around all day with no trouble... That... is why I think they need the large range of timing advance... Boat 2 stroke pistons are way bigger bore than bike 2 stroke pistons (in general)

This single post is part of a larger thread. Start from the top or view this post in context.
Subject Written By Posted
  Fred, HELPTopic by: Ron Brown Ron Brown 06/12/01 10:16AM
  not much helpRe: Fred Fred 06/12/01 02:37PM
  Re: not much helpRe: Ron Brown Ron Brown 06/12/01 02:51PM
  some guessingRe: Fred Fred 06/12/01 03:39PM
  Re: some guessingRe: Ron Brown Ron Brown 06/12/01 03:54PM

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