Rotating weight.....crankshaft....bobweight

[randy331]

Thanks Rick360, That's not as much as Greenlight had said. He had stated that an engine in the 7000-9000 range would gain about 8 hp. Is some of that power gain from reduced crank, block deflection?
Greenlight is the 8 hp you mentioned based on average dyno results?

Should this subject be moved to advanced tech, and I'll start browsing amateur tech????

Thanks; Randy

[SchmidtMotorWorks]

Not nearly as much as I expected. I went back and recalculated about 5 times thinking there was an error. I can’t find one, please anyone, let me know if I’ve made a mistake. I don’t think you could measure this on any kind of dyno.

I am not sure about this, but I think it may be nessesary to consider the mass of the moving parts. (maybe this is in your calculation and I missed it) The reason I think this is that removing 10lbs from a 40lb crank will do more for acceleration than removing 10lbs from a 60lb crank.

[Strange Magic]

The reason I think this is that removing 10lbs from a 40lb crank will do more for acceleration than removing 10lbs from a 60lb crank.

No.

It's all relative to exactly where the weight is removed. I can tell you for fact that by adding weight to a crankshaft in the correct areas will alter its harmonics for the better and that crank will make more power even though its heavier. If you can reduce the harmonics in an engine your fuel curve will be better and the engine will natrually make more power.

[Rick360]

I am not sure about this, but I think it may be nessesary to consider the mass of the moving parts. (maybe this is in your calculation and I missed it) The reason I think this is that removing 10lbs from a 40lb crank will do more for acceleration than removing 10lbs from a 60lb crank.

This may be true EXCEPT this is a FIXED acceleration, ie 600rpm/sec., controlled by the dyno. The difference in power shown on an accelerated dyno pull should be exactly the same (measurement error excluded). It takes a very specific amount of torque to accelerate a given weight or change in weight, at a given radius from center, at a fixed rate, regardless of the weight of the rest of the rotating assembly.

In the formulae for MOI and Torque, MOI is the rotating equivalent of mass and can be added together with different components. The Torque required to accelerate at a given speed can be calculated independently and added, so the mass of another rotating component has nothing to do with the torque required to rotate the component we are calculating.

In an actual run down the track the percent of crank weight removed may become more important as well as gearing and car weight etc.

Rick

[rmracing]

Where is the best place to get a crank lightened and who can get the most weight out of it ? Has anyone ever used shaftech ?

[TORQUE INC]

Duane and Nick do 7 of 10 cranks that go out of here and they do great work.
As far as ightening a crank they can do it but its time based once they get past the basic's like cutting down the counterweights and bullnosing and knife edging .

Id post some pics of the work they have done but the pictures are huge and would look like crap.

I would recommend them to anyone

JW

[Old School]

Back in the early 80's we did a back to back test on reciprocating weight. A fellow brought in a 426 hemi he was bracket racing. Steel rods, heavy factory pistons, heavy pins. The engine had the lastest technology roller camshaft and valve train and was running well for the early 80's.

We replaced the stock steel rods with aluminum rods, replaced the pistons and pins with lightweight units. The weight savings was very significant. The engine was reassembled exactly as it had come apart. The valve seats were lightly touched to make sure they were true.

We were expecting a couple of tenths improvement in et. We got five hundredths. That was all. We were devastated, the owner was also in shock because everyone he had talked to indicated there would be quite a bit of improvement. Maybe we missed something, I don't know. I accused the owner of changing the tune but he swore it was exactly the same. To this day I am still baffled by the lack of performance we got for removing so much bobweight.

After this removing weight from the crank/rods/pistons/pins falls under it can't hurt category. I will not tell you it will go faster but I don't think it will slow up.

[jimivice]

I'm new to this lightweight crankshaft theory. An 1850grm bob wt.is a stock 350 bob wt. If you have a short stroke light bob wt.(1400grms - 1600grms). Then lightening the crank by pendulum cutting might be helpful. Lightening a crank for lightening's sake and paying no attention to bob wt. can be detrimental. If weight is removed it has to be added somewhere else when balancing -heavy metal. Pendulum cutting tries safely to remove weight but it takes away from the structual intergrity of the crank. It will have the tendency to flex. Everyone treats the crank as an entity. What about the flywheel or converter filled with oil, they have inertia or stored energy. What is going to happen when the tires bight and has to launch a 3000 lb mass. You can not treat the crank by itself. It is part of the drive train. If you have a heavy car, you will need a heavier flywheel. So the lightened crank means nothing. Yes a lightened crank might at first look might seem great when you are only considering a piston and connecting rod, if you do not have to propel a car. It seems lighten cranks are now in vogue. Has there been any significant gain in H.P by lightening a crank 5lbs? I don't know, but I bet ther are alot more broken cranks.

[mod911]

I can tell you exactly taking weight off will do for you it will jump 1 car legnth away from you off the corner even if you hit the gas first. That is about 5500 rpm when I pick up the throttle and it is frustrating as hell but even if I am first to pick up throttle I get jerked. But as far as after that initial gain I see no benefit.I can be under them again next corner and get jerked again,and again. I have 29 years at 40 plus nights a year at least and I have asked santa for a lighter rotating assembly this year. About 12 feet in .2 seconds on a 1/4-3/8 track. After that a horse power only deal and I am equal. On a similar note a guy with a super light motor he turns 9100 on a longer track can drive away from 5500 to 9100 like your car stalled every corner and dissappear. You can hold on or gain after the flagman but that initial jerk sucks. Help me santa. Old age and treachery with a quick motor will be hell on the young uns.

[maxracesoftware]

Thanks; Rick and packard;

With the power needed to accelerate a given weight,a given rpm. is a fixed amount.
Is there a formula to calculate the tq. or hp. required to accelerate 100 grams 600rpm in one second at 1.74" from the center of rotation?

Or is the formula one that I would need a calculator with the formula allready in it, so I just enter weight,distance from center,and acceleration rate and it gives me the answer?

Thanks; Randy

Excusse the spelling as my spell checker is at school.

I finally had time to work this out.

First convert units to metric
100 gram bobweight change x 4 rod journals = 400gms = 0.4 kg
Crank radius to center of bobweight = 1.74” = 0.044196meters
Accel rate of 600rpm/s^2 = 62.83185 radians/sec^2

MOI = mass x radius^2 = 0.4 x .044196^2 = 0.000781 kg.m^2

Torque = MOI x Accel = 0.000781 kg.m^2 x 62.83185 rad/sec = 0.049092 Nm

Convert back to American/British units 0.049092 Nm = 0.036208 Lb/ft of Torque

HP at 5000rpm = (5000 x 0.036208)/5252 = 0.03447 HP to accelerate 400grams total bobweight at 600rpm/sec at 5000rpm

HP at 7000rpm = (7000 x 0.036208)/5252 = 0.048259HP HP to accelerate 400grams total bobweight at 600rpm/sec at 7000rpm

Not nearly as much as I expected. I went back and recalculated about 5 times thinking there was an error. I can’t find one, please anyone, let me know if I’ve made a mistake. I don’t think you could measure this on any kind of dyno.

The calculated total rotating dyno (engine and dyno) MOI is approx 0.303 kg*m^2 using the HP difference (6hp @ 4500) from your 300rpm/s and 600rpm/s.

Rick

MOI = mass x radius^2 = 0.4 x .044196^2 = 0.000781 kg.m^2

Rick , did you forget the Shape Factor ?
or you using 1.0 for the Shape Factor ?


MOI = mass x radius^2 x Shape_Factor

the Shape_Factor for Crankshaft w/Bobweights should be approx=.667313

another example is a Flywheel
a Flywheel's Shape_Factor is usually considered equal to = 0.5
in the Polar Moment of Inertia equation

i calculate
400 grams = .881834 lbs
PMI= .004616 inch-lbs-sec^2
.023008 HP @ 5000 RPM 600 RPM/SEC for 400 Grams @ 1.740" Radius
.032211 HP @ 7000 RPM ..................................................................

HP diff= 0.009203 from 5000 to 7000


.024168 inertia torque @ 5000
.024168 inertia torque @ 7000

[David Redszus]

After far too many black beers with a guy who designs and builds clutches, the topic evolved to the benefits of reduced rotating mass in an engine.

I decided (the next morning) to run the calcs to see what benefits might result from a reduced rotating mass assembly. We included the rotating masses from the crank, flywheel, clutch pressure plate, drive shaft, rear axle and rear wheels at various engine and vehicle speeds.

The critical factors are: total mass, rotational speed and polar moment of inertial of each component. Those were measured (or estimated) fairly accurately.

Here is what we learned.
A reduction in rotational mass is always beneficial.
The required kinetic energy goes up with the square of the speed. Doubling the speed (of any component) requires four times the kinetic energy.
The distance of the mass from the axis of rotation is very important. A reduction in mass close to the center axis has little value. But a reduction in mass in a component with a large diameter is very important.

In our examination, a reasonable reduction in rotating mass, properly removed, resulted in a reduction in required kinetic energy of over 30%.
That goes straight to the bottom line, or rear wheels, and its relatively cheap and rarely fails.




Those who do not read have not advantange over those who cannot read.

[maxracesoftware]

MOI = mass x radius^2 x Shape_Factor

the Shape_Factor for Crankshaft w/Bobweights should be approx=.667313


how about trying a Crankshaft

Crankshaft = 40.0 Lbs @ 1.740" Radius 600 RPM/SEC @ 7000 RPM
-vs-
Crankshaft = 60.0 Lbs @ 1.740" Radius .....................................


40.0 Lbs= PMI= 0.209316 Inertia HP Loss= 1.461109 @ 7000 RPM
60.0 Lbs= PMI= 0.313974 Inertia HP Loss= 2.191661


if it were Flywheels instead->
Lets say 14.0" dia , so 7.000" inch Radius
0.5 Shape_Factor

600 RPM/SEC
40.0 Lbs= PMI=2.538281 Inertia HP Loss= 17.7 @ 7000 RPM
60.0 Lbs= PMI=3.807422 Inertia HP Loss= 26.6 @ 7000 RPM

reducing PMI's of things like Flywheels/ Converters/Pressure Plates/
Tires, ETC pay off huge in comparison to Bobweights

[williamsmotowerx]

40.0 Lbs= PMI= 0.209316 Inertia HP Loss= 1.461109 @ 7000 RPM
60.0 Lbs= PMI= 0.313974 Inertia HP Loss= 2.191661

What is PMI?

MOI is moment of inertia, right?

The above quote, are you saying there's a HP loss of 1.461109? Or gain?

And I don't know, what happened with the aluminum flywheel?

Is it correct terminology to say the crank stores energy with it's rotational inertia? There has to be an optimal weight for a given engine.

[maxracesoftware]

40.0 Lbs= PMI= 0.209316 Inertia HP Loss= 1.461109 @ 7000 RPM
60.0 Lbs= PMI= 0.313974 Inertia HP Loss= 2.191661

What is PMI?

MOI is moment of inertia, right?

The above quote, are you saying there's a HP loss of 1.461109? Or gain?

And I don't know, what happened with the aluminum flywheel?

Is it correct terminology to say the crank stores energy with it's rotational inertia? There has to be an optimal weight for a given engine.

PMI = Polar Moment of Inertia
same thing as saying Moment of Inertia

yes theres a HP Loss as you try to accelerate rotational mass

thats why majority of time,
like on Engine Dyno you change RPM/SEC test acceleration rate
from like 300 RPM/SEC to 600 RPM/SEC you will loose HP and TQ
as you accelerate the entire rotational mass at a faster rate

vice/versa

you will gain HP and TQ going from 600 RPM/SEC to 300 RPM/SEC
or any lower RPM/SEC Rates or all the way to
non-acceleration steady-state type Dyno test like in old Days
from a 600 RPM/SEC Test Rate

What about a RaceCar like a Modern ProStocker w/5 spd Trans



same
40.0 Lbs= PMI= 0.209316 Inertia HP Loss= 1.461109 @ 7000 RPM
60.0 Lbs= PMI= 0.313974 Inertia HP Loss= 2.191661

at 6000 RPM/SEC in 1st Gear at 8700 RPM
becomes
40.0 Lbs= 18.16 HP loss
60.0 Lbs= 27.24 HP loss

and surprise->

40.0 Lbs 14" Flywheel = 220.02 HP loss
60.0 Lbs 14" Flywheel = 330.3 HP loss

[williamsmotowerx]

Ok, I get what you're describing with your math now.

Going back to the "stored energy polar inertia (or momentum)", there has to be some merit of that somewhere. To light, can't be good for power?

What was with the aluminum flywheel in EMC?