Hot Rods So, let's talk reciprocating weight on a crankshaft rotating assembly

The other advantage to bracket racing is seat time. You build your mighty motor, make a few passes and scatter it across the track and you are done until you can scratch up enough money to build a new one.
In bracket racing consistency counts, not speed. It doesn't matter what type or level of racing you are doing, without consistency it is almost impossible to evaluate changes.

 

Chris,
What does the Sedan weigh?
How quick and fast is it safe too?
Glenn

 

If you are racing where the engine rpm varies a lot, like a sprint car or 4spd manual drag car losing 2000rpm on each of 3 shifts, a 600hp BBC won't stand a chance against a 600hp SBC. It's basically due to the added weight in the BBC's rotating assy.
If your racing where the engine rpm varies very little, like an powerboat or Powerglide equipped drag car with 800-1000 rpm drop on a single shift, the performance difference will be much less.

Here's a comparison of two engines that were installed a street/strip manual 4spd car, only significant difference between them was different 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.

 

Who remembers Dons boxed rods.

 

There are times when saving weight can be overboard and rediculous. A case in point. We ran a sprint car with CRA many years ago. We were at a late season race at South bay Speedway in San Diego. We were wandering the pits, looking at the several cars brought out from back east to run with us since everything back there had stopped for the year. One big name driver ( since retired and who shall remain nameless) was in the process of applying two small dabs of RTV sealant to each of the dash openings for the gauges. I asked him about the silicone sealer and he replied he used it to hold the gauges in place to save weight. He was serious, I watched him slide the gauges back in place and put a piece of masking tape on to hold them until the sealer set up. I shook my head and left. How much weight savings can 2 back straps, 4 10-32 nuts and lock washers effect the weight of a sprint car? Gosh, if I had glues my gauges in, I could have gone 3 inches deeper into that last corner and won the main! Saving weight is important to a point and only you can decide what that point is.

 

Probably didn't even consider the steak and eggs he had for breakfast.

 

Back when I was in high school Aluminum flywheels were the thing . A 8.25/15 bias ply only has so much traction about the biggest tire available.

 

Sunbeam makes a great point. Piston speed (switching directions) makes a big difference on forces put to the connecting rods and camshaft's and when they come in. Reciprocating weight (piston, pin, rings, upper half of the rod) is divided by 2 (on most v8 engines)when calculating a bob weight. So shaving weight from these components has half of the effect produced by taking weight from the crank or the lower half of the rod. The main advantage you have with lighter piston weight is it makes the rods job easier.
The weight taken away from the bob weight with lighter components has to be removed from the crankshaft counter weight to maintain balance. It is a little bonus you get with a light bob weight.
Removing or adding weight from the flywheel, or converter, makes a bigger difference because it is further out from the center of the crank.

 

Sounds like a famous Ohio driver who refused to run nerf bars for weight reasons also.

 

It weighs 1880# Glenn. It will go to 10.0 without cert.

 

I took a lot of weight out of the fly wheel on my roadster. Makes great power but would be in trouble if the car was a heavier body style. Makes it a pain to drive at low rpm. The combination of a big camshaft and light rotating assembly is meant for high rpm and hard acceleration. Not cruising around the neighborhood. My opinion!