Here's a comparison of two sbc engines of mine that were installed in a street/strip manual 4spd car, only significant difference between them was different internal component weights...
...Engine #1 was 4.04" x 3.48" w/ 5.7" i-beam rods, hypers with gas ported spacers and 1.2mm rings (12lb oil), 49lb crank and heavy 8" balancer, 1863g bobweight.
...Engine #2 is 4.03" x 3.48" w/ 6" aluminum rods, forged pistons with lateral gas ports and 1.5mm rings (14lb oil), 42lb crank with pendulum style counterweights, drilled rod journals, 6" balancer, 1492g bobweight .
Both had flat tops with nearly identical quench and compression. Exact same intake and carb, same carb calibration. Exact same cam installed on the exact same intake centerline. Exact same flywheel and pressure plate installed in exactly the same car, same weight, with exactly the same gearing and tires. Even though these tests were a couple weeks shy of 2 years apart, both tests are on the same location with zero tire spin and conditions were very close to the same. The car itself was basically a time capsule...I lost engine #1 a few weeks after the test, and i had other irons in the fire so the car sat until engine #2 was ready to install...just picking up where i had left off with regard to developing the car. Here's the average rates that each engine gained rpm WOT thru the gears...
1st gear 2000 to 4000 rpm- engine #1 1634 rpm/sec........engine #2 1910 rpm/sec (276 rpm/sec difference) = 18.8% gain
1st gear 4000 to 6000 rpm- engine #1 1975 rpm/sec........engine #2 2217 rpm/sec (242 rpm/sec difference) = 12.2% gain
2nd gear 4000 to 6000 rpm- engine #1 1070 rpm/sec.......engine #2 1116 rpm/sec (46 rpm/sec difference) = 4.2% gain
3rd gear 4000 to 6000 rpm- engine #1 535 rpm/sec.........engine #2 541 rpm/sec (6 rpm/sec difference) = 1.1% gain
No 4th gear data available for comparison.
The quicker an engine sweeps thru a gear, the more you will gain from lightweight components. These engines might both make around 500ft/lbs each running steady state NA, making engine #2's 12.2% gain in 1st gear from 4-6k roughly equal to around a 60hp advantage over engine #1.
Sometimes it helps to think about what happens in opposite extremes...
A given engine has a maximum acceleration rate that it can gain rpm without any external load at all...like a neutral free-rev. At that point all it's power is being used to accelerate itself, and no power is left over to do external work. The lighter an engine's rotating assembly, the easier/quicker it is to accelerate. Sweeping thru the heart of it's torque curve, engine #1 in my example above could gain rpm without a load at the average rate of 8500 rpm per second. Engine #2 could gain rpm without a load at the average rate of 11,515 rpm per second.
On the other end of the spectrum if a car accelerates and works it's way thru the gears, it eventually reaches a point where the engine can no longer accelerate the car. At that point all the engine's power is being used to overcome friction/drag, and there is no power left over for acceleration. This is also the point where the weight of the rotating assy no longer has any effect at all on the power output of the engine. All the torque the engine is making is reaching the transmission's input shaft, no power is being absorbed by the rotating assy as inertia. Operating WOT against maximum load, engine #1 and engine #2 both make the same power.
Looking at these two extremes makes it easier to understand how acceleration rate can have such a huge effect on dyno data. The two otherwise identical engines will make about the same torque when operating against maximum load at a constant rpm, but if engine #1 were dynoed at an acceleration rate of 8500 rpm per second, it would make zero torque on the dyno. Engine #2 still has power left over to move the needle.
If you are running wide open across the ocean, less crankshaft weight will probably hurt you more than help you. If you are a dirt track sprint car on the pole during a re-start, less crankshaft weight is going to be a “BFD”! Most of us here will fall somewhere in between.
Grant
