Lextreme II
Active Member
Rod ratio =
(Connecting rod length, center to center) ÷ crankshaft stroke
5.700/3.750 =
1.52
The figure determined in rod ratio calculations looks interesting, but can it possibly mean anything? You bet. Theories in regard to connecting rod ratios have been discussed, tested, and argued about for almost a full century, and there are probably more theories on the subject than there are connecting rod lengths. Currently, connecting rod ratios in the neighborhood of 1.8:1 have grown in favor in racing circles, and some people would have you believe that a Chevy engine can’t function with a ratio under 1.5:1 or over 2:1. Sufficient evidence is available to disprove both of these ideas, but the varied information that has fueled the rod ratio debate is ample enough to fill a volume twice the size of this issue of HotRod.com. Suffice it to say that changes in the powerplant rod ratio have an effect upon piston movement at both top dead center (TDC) and bottom dead center (BDC), which in turn creates changes in the effect of camshaft timing events and the timing of the intake and exhaust charges. As an example, a 302ci small-block works out like this when it comes to the l/r ratio: 5.7 inches (rod length) divided by 3.00 inches (stroke) equals 1.9:1. For the most part, the larger the l/r, the higher the rpm peak torque that is produced. In a mild street-strip application, a low l/r such as we have in our hypothetical 383 small-block is perfect; we want low rpm torque to move the vehicle (which likely isn’t a flyweight package). For the sake of simplicity, let’s leave the rod ratio debate at this point and use the formula for comparison’s sake only.
1UZFE
5.745" ÷ 3.25" = 1.768"
2UZFE
5.745" ÷ 3.31" = 1.736"
3UZFE
5.745" ÷ 3.25" = 1.768"
(Connecting rod length, center to center) ÷ crankshaft stroke
5.700/3.750 =
1.52
The figure determined in rod ratio calculations looks interesting, but can it possibly mean anything? You bet. Theories in regard to connecting rod ratios have been discussed, tested, and argued about for almost a full century, and there are probably more theories on the subject than there are connecting rod lengths. Currently, connecting rod ratios in the neighborhood of 1.8:1 have grown in favor in racing circles, and some people would have you believe that a Chevy engine can’t function with a ratio under 1.5:1 or over 2:1. Sufficient evidence is available to disprove both of these ideas, but the varied information that has fueled the rod ratio debate is ample enough to fill a volume twice the size of this issue of HotRod.com. Suffice it to say that changes in the powerplant rod ratio have an effect upon piston movement at both top dead center (TDC) and bottom dead center (BDC), which in turn creates changes in the effect of camshaft timing events and the timing of the intake and exhaust charges. As an example, a 302ci small-block works out like this when it comes to the l/r ratio: 5.7 inches (rod length) divided by 3.00 inches (stroke) equals 1.9:1. For the most part, the larger the l/r, the higher the rpm peak torque that is produced. In a mild street-strip application, a low l/r such as we have in our hypothetical 383 small-block is perfect; we want low rpm torque to move the vehicle (which likely isn’t a flyweight package). For the sake of simplicity, let’s leave the rod ratio debate at this point and use the formula for comparison’s sake only.
1UZFE
5.745" ÷ 3.25" = 1.768"
2UZFE
5.745" ÷ 3.31" = 1.736"
3UZFE
5.745" ÷ 3.25" = 1.768"
