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"Piston Position vs Crank Angle vs Rod Length" Graphic

Engine Building Fundamentals

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Looking at your "Piston Position vs Crank Angle vs Rod Length" Graphic and considering the same crankshaft used, I do not understand how a 100mm conrod and a 150mm conrod have the same piston position at -180 degree, zero degree and +180 degree. Are "Rod to Stroke Ratios" the same for both situations despite Conrod Lengths? In this graphic when we increase the Conrod Length we decrease the Crankshaft Throw?

Would you explain how is this possible, please? Thanks in advance!


The position of the gudgeon/wrist/piston pin that joins the connecting rod small end to the piston is higher for the long rod - it is closer to the top.

Sometimes it's called piston height, sometimes compression height, sometimes compression height - this article may help - https://qph.fs.quoracdn.net/main-qimg-eaba51fdb7c8398cf6ddbc9c30a709b7 - otherwise this video explains it tather well https://www.youtube.com/watch?v=2pEPTIX8qPE

Some vehicles, like the old small block Chev' used the same connecting rod in different stroke versions of the engine - the 302, 327 and 350 CID engines all used same deck height (distance from the crankshaft centre line to the head gasket surface, 4" bore, and 5.700" long connecting rods - and, IIRC, used a 3.00", a 3.25", and a 3.48" stroke, respectively, for the different capacities. The main problem with this is that if the wrong piston is chosen (one with the same bore but different stroke) the piston could be quite far down the bore, or sticking out, at TDC, or come to far out the botton of the bore, possibly hitting the counterweights.

On the other hand, some manufacturers would use the same pistons but change the connecting rod length, or use different height chylinder blocks, or a mixture of the various options.

While there is a possible risk of getting the wrong parts, sometimes one can benefit from mixing and matching - a short stroke crank and tall block could allow a cheap-ish long rod engine to be built for top end power with cheaper off-the-shelf forged pistons and relatively cheap aftermarket rods. Others may be more interested in a larger capacity to their current engine, in which case a long stroke crank may be used in the short block with aftermarket rods and/or pistons.

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