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This is all guessing on my part. I believe on the up stroke the grove helps direct the squish velocity to cause better mixture of the gas air and flame front movement. At the time after squish velocity has peaked, which is still BTDC we have a pressure differential where the squish area is lowering quick and the cylinder is 30 degrees or so into its burn. At this point the movement is back into the squish area. How deep do you cut your grooves?
Lets look at how much piston movement we have from 15 degrees BTDC to 15 degrees ATDC. In this test engine Bore = 4.0", Stroke = 3.25", Rod Length = 5.7" and ci = 326.73. The piston moves from .070908" down in the hole to TDC and back to .070908" down in the hole during this very important time.
Bore.=.4.0...Stroke.=.3.25...Rod.Length.=.5.7...RP M.=.6500
Wrist.Pin.Offset.=.0.0
Rotation.Time.of.crank.per.degree.in.Milliseconds. ...0.0256410
Rotation.Time.of.crank.per.rev.in.Milliseconds.... ...9.2307692
Crankshaft.rev's.per.Second....................... ...108.3333333
Crankshaft.Degrees.at.which.Rod.and.Crank.are.90.D egrees.74.0878
Crank......Piston.....Crank.......Rod....Cylinder. ..Cylinder
Angle......Travel......Rod.......Bore.....Volume.. ...Volume
Degree.....Inches.....Angle......Angle......CI.... .....cc
-ATDC.
345.0000....070908.-160.76852..-4.23148...0.89106...14.60184
346.0000....061842.-162.04523..-3.95477...0.77713...12.73492
347.0000....053382.-163.32305..-3.67695...0.67082...10.99275
348.0000....045532.-164.60191..-3.39809...0.57217....9.37619
349.0000....038295.-165.88173..-3.11827...0.48123....7.88602
350.0000....031676.-167.16241..-2.83759...0.39806....6.52297
351.0000....025678.-168.44390..-2.55610...0.32268....5.28771
352.0000....020303.-169.72610..-2.27390...0.25513....4.18085
353.0000....015554.-171.00895..-1.99105...0.19545....3.20293
354.0000....011433.-172.29235..-1.70765...0.14368....2.35442
355.0000....007943.-173.57622..-1.42378...0.09982....1.63575
356.0000....005086.-174.86050..-1.13950...0.06391....1.04727
357.0000....002861.-176.14510..-0.85490...0.03596....0.58926
358.0000....001272.-177.42993..-0.57007...0.01598....0.26195
359.0000....000318.-178.71493..-0.28507...0.00400....0.06549
360.0000....000000.-180.00000...0.00000...0.00000....0.00000
Crank......Piston.....Crank.......Rod....Cylinder. ..Cylinder
Angle......Travel......Rod.......Bore.....Volume.. ...Volume
Degree.....Inches.....Angle......Angle......CI.... .....cc
-ATDC.
....0000....000000..180.00000...0.00000...0.00000. ...0.00000
..1.0000....000318..178.71493...0.28507...0.00400. ...0.06549
..2.0000....001272..177.42993...0.57007...0.01598. ...0.26195
..3.0000....002861..176.14510...0.85490...0.03596. ...0.58926
..4.0000....005086..174.86050...1.13950...0.06391. ...1.04727
..5.0000....007943..173.57622...1.42378...0.09982. ...1.63575
..6.0000....011433..172.29235...1.70765...0.14368. ...2.35442
..7.0000....015554..171.00895...1.99105...0.19545. ...3.20293
..8.0000....020303..169.72610...2.27390...0.25513. ...4.18085
..9.0000....025678..168.44390...2.55610...0.32268. ...5.28771
.10.0000....031676..167.16241...2.83759...0.39806. ...6.52297
.11.0000....038295..165.88173...3.11827...0.48123. ...7.88602
.12.0000....045532..164.60191...3.39809...0.57217. ...9.37619
.13.0000....053382..163.32305...3.67695...0.67082. ..10.99275
.14.0000....061842..162.04523...3.95477...0.77713. ..12.73492
.15.0000....070908..160.76852...4.23148...0.89106. ..14.60184
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