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Old 03-02-2011, 10:23 AM   #70
Damaged goods
Weaverman's Avatar
Joined: Jul 2009
Location: Mid-Michigan USA
Oddometer: 60
Yea, I mis-spoke myself on the piston speed issue. The max velocity remains the same. I sure wish I still had the kinematic spreadsheet I made years ago when I was heavily into this stuff. It would have told us everything, including the forces on the components, including bearings, etc.

The initial acceleration of the piston off TDC is slower, but the rate increases to beyond that of a shorter rod configuration as you approach 90 ATDC. The burn is still happening well past 90 ATDC, BTW, particularly at speed. That's why you see the flame out the exhaust port. That's a negative to a longer rod actually because the pressure is diminishing more than it otherwise would at the point where the piston has greater leverge over the crank. But as we know, many factors will determine whether the diminishing pressures are hurting the power output. There's a lot to consider, obviously. Again, it's all about the area under the curve! This time it's a cylinder pressure curve relative to CA degrees.

Airheads could likely be an engine design that likes longer rods. With the low RPM ceiling they have, you sure wouldn't think the mass increase would be much of a concern, especially with their heavy pistons. And their cylinder head design makes for some very abrupt 90 flow direction changes. Supershaft, your experience here is good. Would like to see some data.

Where do you find the Carillo/Venolia combo? Sounds interesting, and I'd sure like a higher CR for the riding I do. Plus, I don't have to burn soft coal where I live! Remember, BMW was considering many factors when they made their choices.

This is an interesting read, mostly when you scroll down to the "Connecting Rod Influence On Power" part...good ol' Google!
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