Piston travel question

[Turn4fun]

Was never that good at geometry so I have a silly question. Does a piston travel the same distance forward with every degree of crankshaft rotation or does it travel further towards the ends of the stroke than it does mid stroke?

 

[pnwboat]

Re: Piston travel question

Mid stoke is when it moves the most. As the piston approaches TDC or BDC it begins to move less with each degree of crankshaft rotation. When the piston reaches TDC it actually pauses momentarily. Same thing with BDC. The longer the connecting rod, the longer the pause at TDC and BDC.

 

[Frank Acampora]

Re: Piston travel question

And that's why longer rods require less timing advance than shorter ones.

To expand a bit: In order to produce the most power, an engine requires the fuel/air mixture to burn approximately the same time before TDC and after TDC, Since gasoline takes a finite time to burn, as the engine speeds up, the mixture must be ignited earlier. Since with longer rods, the piston stops for a very slightly longer time at top dead center, there is slightly more time to burn, so less timing advance is needed to have an equal burn before and after TDC. (only about a degree or two though)

AND: That's why Chrysler and Force engines require 32 or 30 degrees advance. When the engines were first designed, the pistons had three rings and two needle bearings carrying the wrist pin. This necessitated moving the wrist pin hole down in the piston skirt and using a short rod. They (Chrysler and later US Marine and Mercury) never changed the design even after they went to two rings and needle bearings in the rod small end.

To my knowledge, the only Force engines to use longer rods is the Mercury built Force 70 and 75. While I don't particularly like those engines, I must admit they run well.

The Chrysler 55,60, and 65 was a new design which used long rods and (for the period) cutting edge design. These engines were very powerful and robust. Unfortunately, Chrysler put a substandard lower unit on them. The engines were so strong that it was quite easy to strip the lower unit gears if under propped.

And I have prattled on enough.

 

[Bifflefan]

Re: Piston travel question

As a side note,
Long rod motors make more torque than a short rod with the same displacment.

 

[Turn4fun]

Re: Piston travel question

Thanks for the replies. That's what I thought but I got into a debate with someone on in and the more we debated, the more I started to second guess myself. Basically, I showed my homemade timing tool to a friend who argued that I should be able to calculate the piston position at full advance for any engine simply by dividing the length of stroke by 180 and then multiplying by the number of degrees of advance. This could only be true if the piston travel remained constant with every degree of crank rotation. Later I was able to disprove his theory on one of my engines. My 35 HP chrysler has a 2.540" stroke and the WOT timing is 28 degree BTDC. So by his theory the timing would be set with the piston .395" below TDC (2.540/180*28= .395). I put a degree wheel on and set it at 28 degrees and whala, the piston is only .250" below TDC.