what flywheel

I had two engines that were pretty close to identical but with different component weights. The weight of the flywheel itself didn't change (both used the exact same 17lb steel flywheel and RAM PP), but i did change the "effective flywheel weight" of the entire crankshaft/flywheel assembly with lighter crank/rods/pistons...

...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 balanced to a 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 balanced to a 1492g bobweight.

Both had flat tops with nearly identical quench and compression. Same intake and carb, same carb calibration. Exact same cam installed on the exact same intake centerline. 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.

Here's the observed rates that the engines gained rpm WOT...

1st gear 2000 to 4000 rpm- engine #1 1634 rpm/sec........engine #2 1910 rpm/sec (276 rpm/sec difference)
1st gear 4000 to 6000 rpm- engine #1 1975 rpm/sec........engine #2 2217 rpm/sec (242 rpm/sec difference)
2nd gear 4000 to 6000 rpm- engine #1 1070 rpm/sec.......engine #2 1116 rpm/sec (46 rpm/sec difference)
3rd gear 4000 to 6000 rpm- engine #1 535 rpm/sec.........engine #2 541 rpm/sec (6 rpm/sec difference)

As you can see, in the higher gears where acceleration is slower there is not much difference in acceleration rate. But in the lower gears, the differences in rate of acceleration progressively increase.

There also seems to be a quite a difference in no load acceleration rate, but the conditions of the comparison were not the same. I had a clutch linkage experiment go bad in 2014 with engine #1, which resulted in a free-rev condition during a 4000 rpm WOT launch in which that engine gained rpm at an 8500 rpm/sec rate. I was also experimenting with pulling timing at the time for an upcoming no-prep race, but my records don't indicate that i was pulling timing on that particular launch.

Under NA free-rev conditions, engine #2 gains rpm at a 11,515 rpm/sec rate.

All acceleration tests are basically inertia dyno runs, even if that test is a no-load test where the only inertia resistance is supplied by the weight of the rotating assy itself. What most people lack is a way to collect accurate comparable data. Even though these tests were conducted almost 2 years apart, the car itself was basically a time capsule...engine #1 blew up 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 a couple years later. I was just picking up where i left off with regard to developing the car. To me the part that said the gain was because of the lighter rotating assy is because the closer the two engines got to steady state power production, the closer the power outputs were. By 3rd gear, there was only a 6 rpm/sec difference. Sorry, no 4th gear data available to compare from engine #1, that test area just isn't long enough.

Something to think about- if measuring reduced inertia acceleration gain from 1000 rpm, it would be hard to tell much difference. But the gain is exponential...any acceleration gain you get from reducing the weight of the rotating assy at 1000 rpm is magnified 49 times by 7000 rpm. Spin it to 8000, that gain is magnified 64 times.

In my street/strip car i want as light of an "effective flywheel" as practical, but even though a lighter aluminum flywheel would allow my engine to accelerate faster, i feel there is more overall gain available in using a lite steel flywheel. Basically, more thermal capacity allows slipping the clutch longer without overheating...which in turn allows me to raise the engine's average rpm...which makes it possible for the engine to produce even more power in a tighter time frame. More than enough power production gain to offset the loss of using a heavier flywheel.