Baldy201
New Member
WHAT IS THE ROD RATIO IN INTEGRAS?
i/ LS or SE (B18A/B18B)
connecting rod length = 137.0 mm
stroke (determined by crankshaft) = 89.0 mm
rod ratio = 137/89 = 1.54
ii/ GSR or ITR (B18C1/B18C5)
connecting rod length = 137.9 mm
stroke (determined by crankshaft) = 87.2 mm
rod ratio = 137.9/87.2 = 1.58
DEFINITION OF ROD RATIO
The geometric relationship between the rod and the rod journal is one not generally understood by many mechanics but plays a key role in the motor's breathing characteristics and overall power characteristics.
This rod ratio is calculated by dividing the connecting rod length by the stroke length.
The rod's length is measured from the center of the piston-pin opening to the center of the big-end bore. Rod ratio greatly affects the way an engine performs and how long it lasts.
RANGE OF VALUES FOR ROD RATIO
How big or small can a rod ratio get? There is a small range of rod ratios for most conventional piston engines:
The rod is between roughly 1.4 and 2.2 times the stroke length.
It's not possible for the rod to be the same length as the stroke.
Rods much longer than twice the stroke make the motor very tall and these engines cannot fit inside the tight engine bays of modern aerodynamic cars with low hoodlines. They are impractical except for racing engines.
WHAT IS THE BEST ROD RATIO?
A rod ratio value of 1.75 is considered "ideal" by many respected engine builders, if the breathing (i.e. cylinder head port sizes and IM size) is also optimized for the design.
Notice that the Integras have a low rod ratio.
CONSEQUENCE OF ROD RATIO
The ROD RATIO dictates the ANGLE at which the piston travels up and down the cylinder.
Lower rod ratios produce a larger or steeper angle between the rod and the crankshaft. The rod angle must not be too steep such that it encourages excessive friction at the cylinder wall and piston skirt as the piston travels:
- A lower rod ratio (eg. B18B) , resulting in a steeper angle at which the piston travels, can be made by installing a shorter rod or by increasing the stroke (crankshaft).
- A higher rod ratio (eg. in a B16A) , resulting in a less steep angle for the piston travel, can be made by using a longer rod or a shorter stroke (crankshaft).
a low rod ratio produces a steeper rod to crankshaft angle and MORE PISTON SIDELOADING FORCES against the cylinder wall, as it travels up and down the cylinder. The consequence of a low rod ratio and resultant larger rod angle is MORE WEAR & ENGINE VIBRATION as the engine revs higher and higher: you are eventually forced to use a lower redline in order to prevent engine damage.
Secondly, when the rod ratio value becomes smaller, it has other effects: mechanically, on breathing ability, and on how you set your spark timing.
I. MECHANICAL EFFECTS OF A LOW ROD RATIO
Motors with low rod ratios (like our 2nd & 3rd gen. integras ) typically exhibit the following characteristics (compared to high rod ratio motors):
- shorter pistons, measured from the pin center to the bottom of the skirt.
- higher level of vibration
- greater wear on piston skirts and cylinder walls
- slightly higher operating temperature & oil temperature due to friction
- physically shorter engine (more oil pan, header, and air cleaner clearance), allowing for a lower hood line.
- lower block weight.
II. LOW ROD RATIO AND CHANGES IN SPARK IGNITION TIMING
- earlier timing (MORE IGNITION TIMING ADVANCE) is required with a low rod ratio, since the combustion chamber volume is larger (i.e. the piston is farther from TDC) at the same point of crankshaft rotation compared to a high rod ratio motor.
- the motor may also be less knock-sensitive with a low rod ratio, as the chamber volume increases more rapidly ATDC, lowering cylinder combustion pressure (this is useful for nitrous & supercharged motors).
III. LOW ROD RATIO EFFECTS ON CYLINDER FILLING AND VOLUMETRIC EFFICIENCY (HOW WELL A MOTOR BREATHES)
- with a lower rod ratio, intake vacuum rises sooner ATDC and higher flow speeds can be reached earlier in the rpm range: this allows you to have larger cylinder head intake ports & intake manifold (IM) plenum volumes without the loss of throttle response. This fact is important for headporting and IM selection.
- a too small or badly shaped port will "run out of breath" sooner in the upper rpms with a low rod ratio motor.
- piston speed or motion away from BDC is slower. The result of this is a trapping a higher percentage of cylinder volume. This, in turn, makes the motor less sensitive to late intake valve closing (i.e. later ABDC intake valve closing with longer duration "wilder" cams like Skunk2 Stage 2, Jun 3, or Toda B/C).
i/ LS or SE (B18A/B18B)
connecting rod length = 137.0 mm
stroke (determined by crankshaft) = 89.0 mm
rod ratio = 137/89 = 1.54
ii/ GSR or ITR (B18C1/B18C5)
connecting rod length = 137.9 mm
stroke (determined by crankshaft) = 87.2 mm
rod ratio = 137.9/87.2 = 1.58
DEFINITION OF ROD RATIO
The geometric relationship between the rod and the rod journal is one not generally understood by many mechanics but plays a key role in the motor's breathing characteristics and overall power characteristics.
This rod ratio is calculated by dividing the connecting rod length by the stroke length.
The rod's length is measured from the center of the piston-pin opening to the center of the big-end bore. Rod ratio greatly affects the way an engine performs and how long it lasts.
RANGE OF VALUES FOR ROD RATIO
How big or small can a rod ratio get? There is a small range of rod ratios for most conventional piston engines:
The rod is between roughly 1.4 and 2.2 times the stroke length.
It's not possible for the rod to be the same length as the stroke.
Rods much longer than twice the stroke make the motor very tall and these engines cannot fit inside the tight engine bays of modern aerodynamic cars with low hoodlines. They are impractical except for racing engines.
WHAT IS THE BEST ROD RATIO?
A rod ratio value of 1.75 is considered "ideal" by many respected engine builders, if the breathing (i.e. cylinder head port sizes and IM size) is also optimized for the design.
Notice that the Integras have a low rod ratio.
CONSEQUENCE OF ROD RATIO
The ROD RATIO dictates the ANGLE at which the piston travels up and down the cylinder.
Lower rod ratios produce a larger or steeper angle between the rod and the crankshaft. The rod angle must not be too steep such that it encourages excessive friction at the cylinder wall and piston skirt as the piston travels:
- A lower rod ratio (eg. B18B) , resulting in a steeper angle at which the piston travels, can be made by installing a shorter rod or by increasing the stroke (crankshaft).
- A higher rod ratio (eg. in a B16A) , resulting in a less steep angle for the piston travel, can be made by using a longer rod or a shorter stroke (crankshaft).
a low rod ratio produces a steeper rod to crankshaft angle and MORE PISTON SIDELOADING FORCES against the cylinder wall, as it travels up and down the cylinder. The consequence of a low rod ratio and resultant larger rod angle is MORE WEAR & ENGINE VIBRATION as the engine revs higher and higher: you are eventually forced to use a lower redline in order to prevent engine damage.
Secondly, when the rod ratio value becomes smaller, it has other effects: mechanically, on breathing ability, and on how you set your spark timing.
I. MECHANICAL EFFECTS OF A LOW ROD RATIO
Motors with low rod ratios (like our 2nd & 3rd gen. integras ) typically exhibit the following characteristics (compared to high rod ratio motors):
- shorter pistons, measured from the pin center to the bottom of the skirt.
- higher level of vibration
- greater wear on piston skirts and cylinder walls
- slightly higher operating temperature & oil temperature due to friction
- physically shorter engine (more oil pan, header, and air cleaner clearance), allowing for a lower hood line.
- lower block weight.
II. LOW ROD RATIO AND CHANGES IN SPARK IGNITION TIMING
- earlier timing (MORE IGNITION TIMING ADVANCE) is required with a low rod ratio, since the combustion chamber volume is larger (i.e. the piston is farther from TDC) at the same point of crankshaft rotation compared to a high rod ratio motor.
- the motor may also be less knock-sensitive with a low rod ratio, as the chamber volume increases more rapidly ATDC, lowering cylinder combustion pressure (this is useful for nitrous & supercharged motors).
III. LOW ROD RATIO EFFECTS ON CYLINDER FILLING AND VOLUMETRIC EFFICIENCY (HOW WELL A MOTOR BREATHES)
- with a lower rod ratio, intake vacuum rises sooner ATDC and higher flow speeds can be reached earlier in the rpm range: this allows you to have larger cylinder head intake ports & intake manifold (IM) plenum volumes without the loss of throttle response. This fact is important for headporting and IM selection.
- a too small or badly shaped port will "run out of breath" sooner in the upper rpms with a low rod ratio motor.
- piston speed or motion away from BDC is slower. The result of this is a trapping a higher percentage of cylinder volume. This, in turn, makes the motor less sensitive to late intake valve closing (i.e. later ABDC intake valve closing with longer duration "wilder" cams like Skunk2 Stage 2, Jun 3, or Toda B/C).