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Volumetric Effiency

Try this Excel spreadsheet, I had to zip it to upload:

The v constant is the theoretical optimum velocity for best fuel delivery. On calculating I enter the bore, stroke and the size carb I think I want to use. It then tells the optimum RPM for efficiency. The engine will perform best at the calculated RPM +/- 1000 RPM
 

Attachments

  • carbsize-20090819.zip
    1.1 KB · Views: 59
Volumetric efficiency is the engine's ability to breathe and its a dynamic thing: it goes down and up (and then down) as the engine gets on cam. if you just treat the engine as a pump and were to measure the compression you would find that the compression changes dynamically with RPM. A engine with say a 10:1 compression ratio and a VE of 80% really has an effective compression ratio of 8:1.

The inertia of the air and mechanical bits means that you have to open intake valves in advance and leave exhaust valves open later. Any cam with any performance will have a period of overlap where both valves are open - this helps when the engine is 'on cam' and hinders when it is off - hence changeable cam timing.

Add shock wave tuning and you can actually get a VE higher than 100% when charge is rammed back into the combustion chamber.

So what number do you enter? Its all a crap shoot and it depends on the bike you want to produce: do you want a bike that is carbureted for WOT or for going to the corner store. Pick 65% for the corner store and 95% for the drag racer.
 
Great stuff for mental exersize.

Using the size of my carbs I see that the .xl program tells me an RPM that is close to the max I run at Bonneville. I realize that MANY factors determine what my max RPM/HP/gear/Speed will be, but this program offers some interesting perspectives.

Thanks to all.

ALex
 
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