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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #41  
Old 3rd August 2021
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Quote:
Originally Posted by Ferrous Head View Post
With a fresh battery an electric leg should be fast enough to get you a correct reading. Kicking is always "Problematical".
Kicking over is slow, put her into top gear and push for a few yards with the usual hot motor, wfo, choke off if you have one etc. (tape your plugs together if you have a magneto).
…….Davi.
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  #42  
Old 4th August 2021
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So many comments...

CCP is only close to static CR X atmospheric pressure by accident, there is no linear mathematical relationship.
VE at cranking speed is 100%.
The "dynamic" CR is based on cylinder volume at the intake valve closing point ABDC divided by the actual chamber volume (displacement of one cylinder divided by CR-1).
I've made a longer explanation on my site here: http://victorylibrary.com/tech/cam.htm
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  #43  
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VE at cranking speed is 100 % ?

I would have to think carefully about that. Especially with very late closing intake cams.
To get 100% efficiency the valves would have to be fully closed at BDC.
At slower speeds some of the charge is going to be pushed back out of the combustion chamber.

The VE is going to change over engine RPMs as sometimes inertia of the incoming charge combines with a pressure wave happening at just the right time in the inlet tract to achieve effective VE higher than 100%.

At lower rpm's when the pressure waves are 180 degrees out it will work against you and reduce VE.

If you drop below the required VE the engine won't even start. You have to kick faster.

VE is only an approximated number. Static compression is an actual number but doesn't really tell you that much. And even with the best set up for checking cranking pressure it doesn't tell you the whole story as the pressure will rise in a running engine at high revs.

Of course, all of this is only my opinion and others may disagree.
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Blair separates VE and captured charge very nicely.
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  #45  
Old 5th August 2021
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Quote:
B2 × SE × .7854 (1 cylinder) [always less than “VN” , above]
The "VN" referred to is cylinder volume, nominal.

But I would disagree with that.

Tuned length exhausts and intake tracts are able to push VE above 100%.

On the intake side the combination of pressure waves and intake charge inertia will increase the internal pressure above what would be expected by normal atmospheric pressure. In effect, supercharging the cylinder.
Exhaust pressure waves timed correctly help lower the effective pressure in the pipe at the moment of valve opening and help in extracting the spent charges giving the intake a lower chamber pressure to fill.

This is exactly why we run open megaphones on single cylinder bikes along with what may seem overly long intake tracts.

And that's all great as long as you keep the engine in a very narrow rev range.

Again of course, this is only my opinion.
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