Piston position and timing

Changing the rod length will alter the position of the piston in the bore and ultimately the degrees of ignition timing.

So the question... is there a base position to start from for calculating a safe ignition setting, for instance 1 1/2" down the bore and then take an initial reading of the degree hub?

We bench race this subject in the workshop and came up with many different theories.

Cheers Ray

 

And therefore it travels faster between those two points.

 

ok, now i am really confused. seems like the shorter rod would move quicker between tdc and bdc. will please help the confused.
thanks

 

I think everyone is confusing rod length with rod ratio. The rod length will not change the speed of the piston travel, TDC or timing. The change in rod ratio will change TDC/BDC dwell which intern impacts valve timing issues.

 

There's only 360 degrees in a circle. If the piston is "parked" for more crankshaft degrees at TDC&BTC with a longer rod(which it is). Then the piston has to cover more ground per degree, in the remaining degrees.

 

The pistons physical position in the bore is differant at the same point in the crankshafts rotation(degrees BTDC or ATDC) with a long vs. short rod. When all other factors remain the same, the piston makes more progress up or down the bore with a longer rod. Piston speed is greater between TDC and BDC when rod length to stroke ratio is increased. (Longer rod with the same stroke).Remember a rotation is only 360 degrees.

 

Sorry for not replying to my original post but I was unable to access ITA, dont know why...

Mike, I see where you are coming from, when we did the bench test we didnt change comp height so we had a different point of reference. Liked your work on the plug reading info, very helpful and I have since purchased one of those plug reading light gizmo thingy with the long name..

engineman, I think this is what perahaps we were realy playing with.

So is there a point of reference for base ignition timing between 2 different rod/stroke ratio's?

 

what you guys are talking about is refered to as instantaneous velocity. remember, feet per minute doesnt mean anything if we dont also consider the amount of TIME a piston spends at a particular spot in the cylinder, and how fast it gets to or leaves that spot.

piston speed is measured in many different ways, some of which are entirely useless. when you hear about mean piston speed, youre looking at something that is entirely independent of rod length. all you need to know to calculate that is the RPM and the stroke. however, remember that a piston is traveling at ZERO feet per minute at TDC and BDC.

this is where it gets tricky. the longer the rod, the longer the dwell, right? as the piston leaves TDC, it increases in speed (i.e. accelerates) until it reaches a peak somewhere around 70-75 degrees ATDC. however, longer rods cause the piston to accelerate away more slowly, which in turn reduces the mechanical stress on the rotating assembly.

its all sounds like nonsense until you stop to visualize it. youve got pressure rising and falling on the top of the piston throughout its travel in the cylinder, but where this happens IN RELATION TO THE CRANKSHAFT ANGLE is what determines where your stress is at. a long rod allows the peak in pressure AND the peak in piston velocity to occur farther along in the travel of the crank as it leaves TDC. the rod is at less of an angle and combustion pressure is more directly applied to the crank. less piston side loading, more power.

this is why a short rod is prefered for torque. the average force exerted against the piston by the combustion process occurs when the connecting rod is at more of an angle vs. the crank. theoretically, a rod at a 90 degree angle to the crank will make the most torque possible from its combustion pressure.

whew.

 

I have an Excel program that I found on a Mopar website or some other website that will calculate this info for you.
I tried to include it here but I couldn't figure out how to attach it.
If anyone is intrested I can e/mail it to you or you can go to this website & download it.
Don't know if this is true or not, but I heard a long time ago Austin Coil (bear in mind this was in the Hustler days) used to change rod lengths to tune individual cylinders because it changed where the piston was in relationship to timing.

Bob Kraemer

Piston Motion