Somewhat repeating myself here, but I've spent my dollars on DV and one has to assess both his strengths and his weaknesses of not just his understanding, but in the time and space available for his explanations. There's a reason one gets much more out of an HP Book than an SA Book. There are other things going on but
lets get back to the topic of picking of cams and the role of LSA.
Rod stroke makes a difference because it changes the time the piston will dwell at the top vs bottom of the stoke. The valve openings and closings ought to work with piston dwell and take advantage of it rahter than fight it.
Those are interesting observations regarding reversion and chosing valve closing and opening points from a guy in the trenches involved with everything from motorhomes and commercial trucks to motorsports. Because a guy who focused on combustion and tuning mostly for racing (but also to meet Aussuie emissions rules) explained that same thing although from a slightly different perspective.
Shrinker wrote:
A wide LSA creates high vaccum at idle for sure but it also does other things. I prefer to think of LSA in the old ways of overlap because at least calling out the overlap amount at TDC defines the actual event rather than you having to calculate it. The overlap of the valves allows the exhaust to supply gases to the intake system depending upon the physics of what is happening in the exhaust at the time. I have done tests that indicate that large cam motors actually run on substantial amounts of air inducted through the exhaust backwards into the intake manifold. Decreasing the overlap reduces this and allows better controll of the mixture. Quite a lot of the time the exhaust system functions as a uncontrolled carburetor, in as far as it has combustible gases in it that are drawn into the engine.
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How this all relates to idling is that the LSA is the un-throttled connection of the inlet to the exhaust. If the exhaust gas is going gang busters and doing all super scavenging stuff and being nice and not blowing up the intake then you can get high manifold vacuum. High manifold vacuum helps you to turn the fuel into a gas before it gets compressed.
Theres not much gas being compressed inside a cylinder when the engine is idling so you dont get much compression energy. An engine wont run at all unless the fuel is turned into gas and its the amount of gas at ignition time that determines the quality of the burn and the quality of the burn is what causes the exhaust to behave properly or be a pain in the butt.
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Measuring it as LSA is a modern thing that I think confuses the issues of the overlaps effects. Ed Iskendarian got it right when he said its the fifth cycle [overlap]. And thats the best way to understand it, you have to know the ramps and imagine the effects etc. I fully realise that you can have different durations with the same LSA and therefor you get a different overlap.
I was talking in terms of varying the LSA without changing the duration. We were discussing the idle vacuum verses duration which is of course not the most important influence on the vacuum; its predominantly the overlap that affects idle vacuum. As the overlap is an "unthrottled" inlet to the inlet manifold.
LSA is a useless number in my opinion as you have to calculate what the overlap and bottom fills are but its convenient to use for advertising reasons i suppose and its whatever you get used to.
So this is a deeper explanation of why I think LSA can be useful for comparing cams but the valve events for the specific engine should be the main focus. Also very very important is what the engine needs to do. A circle track cam can focus on a narrow rpm range whereas other applications are going to want a much broader power range.
