Using What We Have Learned

We have learned the formulas, disassembled, cleaned, lubricated, assembled and measured our engines and have properly broken them in. Now what? Well I'm affraid we have to repeat those procedures again. You see while breaking in that engine, you got it dirty and loosened up some parts. So get to the bench and start over from scratch including those measurements. Look closely at the parts, don't just wash them. Any scratches or cracks? Metal shavings? Having fun yet? No this is not as much fun as flying or driving, but you put in 3-4 hours at the field, now it's time to put 3-4 hours in at the bench.

Compare those old numbers to the new ones. See any difference? You should. When you break in an engine, you are doing just that, "breaking-in". During that period your connecting rod expanded and contracted and those bushings got worn. The piston also expanded and contracted along with the sleeve and case. Everything clean and lubricated again? How did the engine perform? Does it do what you want? Can you handle it? These are questions you need to ask before you modify that engine. What is the purpose the engine is used for? Everyday use, racing, all out speed, race course?

Using the Numbers

Finally huh? There are only a few formulas that you need to use right now. The first is to calculate the piston position in the sleeve (height) at a given moment in time (degree of the crankshaft). To do this we use the formula "Piston Travel vs. Crank Rotation" on the Formulas Page where:

   
   d = ((S/2) + L) - (S/2 cos X) - L sin[cos-1 (S/2L sin X)]   

   d = distance 
   S = Stroke (in/mm)
   L = Connecting Rod Length (in/mm)
   X = Crank Angle Before or After TDC (deg)

Lets say that our connecting rod is 1.385 in. in length from center to center of the bushings and our stroke is 0.70 in. from TDC to BDC and our

crankshaft has moved 90 degrees. Get out your calculator and solve the formula. Your answer should be -0.39495 in. or -10.03182 mm. You may need to use the alternate formula for some calculators. That is how far the piston moved with 90 degrees of crank rotation.

We have gone from degrees to inches, now lets do it from inches to degrees since distance is what we know. Try this formula in your calculator.

  D = acos (B^2+C^2-A^2) / (2*B*C)

  acos = cos^-1 for your calculator
  C = stroke/2
  B = C + A - (measured piston height)
  A = Connecting rod length

Now you should be able to determine the timing of your engine from the dimensions you recorded !!!!!

After calculating the position of the piston at 90 degrees, place your crank at 90 degrees and measure it. I tried this and got an entirely different measurement. Why? I don't really know and am looking into it. If you can figure it out

let me know. Here is what I have:

Bore:                    0.8520
Stroke:                  0.697
Connecting rod length:   1.38 

Calculated piston height:  0.393 in. @ 90 degrees
Measured piston height:    0.329 in. @ 90 degrees
Difference of:             0.064 in.

If we use the reverse formula to calculate a piston height of 0.329 in. we get 79.62599 degrees.

I have found why they were not the same. I did not use a degree wheel to set the timing mark of 90 degrees. I eyeballed it. Well I found out how easy it is to misjudge a crank angle and learned a valuable lesson, measure. Hope this teaches you too. Calculators don't lie, liars calculate. :-)

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