100%

If the only reason for light weight cranks was for static
weight, people wouldn't bother with the extra $2000 but,
as I've previously admitted, I have been wrong before.

realistically, what gain could one expect to see with an 043 ring pack and just header evac setup going to a vacuum pump????

Scott's p7 deal has me sucking 3rd boob

I have a big block crank here in the shop that is 84 lbs! Solid rod pins, full counterweights, solid mains, etc. That is about 20 lbs more than they usually are.

The circle track guy that I work with has a 434 SBC circle track motor with a 37 lb crank in it. That motor has Ti rods, light pistons, tiny pins, the whole nine yards. It is a motor that he leases to big buck boys when they need to win a race.

Just remember, there are limits as to what you can do with a Crank because it has to be in balance. So if you have it fully machined and you have a heavy rod and pistin, you will be ADDING weight to the crank Via Mallory metal to balance it. If you save 3 pounds in the rods alone by buying Titanium then you will have to take more weight out of the counterweight. So light pistons and rods compliment the crank and help remove more weight. Light pistons & Pins are easier on the rods and so the rods don't have to be as strong and so on.......Light parts really are good as long as strength is not sacraficed and they are used in the proper application.

Herb Adams in his book "chassis engineering" states that one pound of rotating mass is worth roughly 6 (or maybe it was 8? don't have the book handy) pounds of static weight. I remember years ago switching my car from Magnum 500 wheels that weighed about 26 pounds a piece to weld wheels that weighed about 10, the car lost 64 pounds of static weight but the COMBINATION of dynamic AND static weight reduction dropped the et close to 2 10th's. I was amazed and the car pulled faster through the gears (although the rotational reduction really only applied to the rear wheels) Using 1 for 8 that would mean a 12.5 pound reduction of crank weight (or suppose any rotational weight) would be worth about 100 pounds of static weight or a rule of thumb 10th.

The thing to remember is the crank can only accelerate as fast as the sum total of the rotational forces will allow, The cranks rotational mass is added to that of the tranny, the driveshaft, the ring gear and the axles and \most importantly the tiresagainst the ground so although you are dynamically lightening the crank and reducing the total mass of the drivetrain, it's not just the crank itself and when the totals are summed the percentage weight loss % is (quite) a bit lower than you'd see just revving the motor in neutral.

Also there is a flywheel effect to rotating mass as far as how it Hits the tires at the moment where static energy becomes kinetic. Like a lighter or heavier flywheel there is a trade off that is beneficial in most cases but not necessarily in all cases. I went with a heavy steel flywheelin my 427 CObra because although I'm sure it would accelerate quicker in the 1/4 mile with lighter mass, I wanted the proper big block "feel" when I dumped the clutch as low engine speeds and to offset the very light 1850 gram bobweight of the smallblock. To transfer equal inertia with lighter rotating mass you have to tranfer the load at a higher RPM which is usually an advantage for drag racing. But EVERYTHING is a trade-off, the big slicks you need to launch off the line take a lot more power to turn at the top end, so racing is always a matter of where and when to apply power
most effectively.
also I believe when comparing cranks the effective weight of the rotating mass is the sum of the crank plus the rotational weight of the pistons and rods. you have 2 cranks one weighs 50 and the other 60 it is 20% lighter, but if you included ONLY the rotating (big end) weight of 8 rods (lets say the same 10 pounds) the effective rotational difference is really 60 vs 70 or a net of 16.6% lighter.....now add the sum of everything elese that spins inthe drivetrain.

Interesting topic for sure!

Yeah Don......a 904 has 17 lbs. less rotating mass. 27 lbs. total.

one pound of rotating mass is worth roughly 6 pounds of static weight

Way too general - doesn't say radius (major factor), or relative speed (engine speed or less?).

the effective weight of the rotating mass is the sum of the crank plus the rotational weight of the pistons and rods. you have 2 cranks one weighs 50 and the other 60 it is 20% lighter, but if you included ONLY the rotating (big end) weight of 8 rods (lets say the same 10 pounds) the effective rotational difference is really 60 vs 70 or a net of 16.6% lighter

The actual math to calculate changes in inertia is a mess - the formula is easy, but figuring out the "mean center of gyration" isn't.
If the crank were lightened 20 lbs. by simply drilling the main journals out to paper thin (this never happens!!), the inertia would hardly change - because their mean radius less than 1-1/4". The rod big end is somewhat better since it's located (about) stroke radius distance. The really important part is the counterweight OD.

I was told that the biggest noticable difference would be in the 60 foot times
True - because the overall gear ratio is directly proportionate to inertia.

Good post Very interesting stuff, now where are the math guys to prove it one way or the other

Bob, I have some of that info from the Herb Adams book.

What I have shows a test between Static and Rotating weight of 15# Off the car, wheel, and crank.

And by Far 15# of the crank was worth about 15 times as much as 15# off the car/dead weight.

It appears Herb took a 3000# car and accellerated it from 64 MPH for a marked or timed distance. Dropping 15# first off the car, and then at the wheel and finally from the crank itself.

Then by comparing the MPH change they calculated the approximate HP change that would of taken place.

With a base line of 428 HP on a 3000# car the accelleration tests showed a gain of

2.14 HP equivalent from a 15# deadweight drop.

6.42 HP equivalent from a 15# wheelweight drop.

32.10 HP equivalent from a 15# crankweight drop.

This from the old Herb Adams Chassis book. As far as its accuracy, I dont know. But it sure does imply that the best place to remove weight is in the engine rotating assembly.

So with a 40# weight loss at the crank you should definately see and feel something, mike

Think about it this way. Above it is stated that you cansave 17 pounds rotating weight switching from a 727 to a 904 trans. Well you could EASILY lower that much going from stock pistons/rods/crank to good lightweight stuff. Probably more. Even if you considered gun drilling the mains static weight.

It has been proven that switcing transmissions is worth a measuarable gain.

I always turn the counter weights down to get it real
close on the balance for my light weight pistons and such

500" 1400hp N/A Prostock cranks weigh less than 40 pounds

It's worth everything, power, acceleration, windage, and just plain old weight off the car.

Sport,

So herb was saying 15:1?

I better go back and brush up....and then go drill some holes in my crank

I guess the point I was making is everything from the Balancer to the tire that spins needs to be SUMMED, a crank will only spin as fast as the load against it can let it......Light cranks from my experience are really felt above the torque peak and as "panic" (sorry, you'll always be Panic to me!) said it really depends on a lot of other variables. Where it can also pay off (in an obvious but not really mentioned way) is if the lighter mass allows the motor to more safely rev and be built/tuned to a higher HP peak and allow the motor for more RMP/POWER than it would otherwise be able to achieve.

That's probably also true and that a lighter rotating assembly would net more to the wheels with a direct drive/manual trans/clucth than with an automatic which would have more efficiency losses tranferring power to the wheels.

there are many ways to make a car go faster, especially if your using stock, OEM type parts. As already mentioned the Pro Stock guys have lightweight cranks, rods and pistons. I'm not sure if the honda 1.8? size rod journal size is still the hot tip or not but anytime you can reduce the mass, weight and drag internally the car will end up going faster as long as you can keep it hooked up. Pro Stock motors have real big bores(not sure exactly what size now) and relatively short strokes, 3.6 maybe? And they use the lightest weight parts they can buy or make in those motor on the rotating parts If you where to make two identicle motors like a NHRA legal stocker and a legal NHRA legal super stocker and used just the short block not including the SS cam, lifters and upper end and used the stocker top end in the same car the SS short block will be faster with the stocker cam and upper end than the stocker short block will be in the same car with the same weather and same all other parts other than the short block. Now don't ask me how much differences ther are in the bob wieght bewtween those two motors, especially if it is 426 Hemi motor