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

[Strange Magic]

There are significant gains in excellaration rates and excelleration means et and mph. The other benift is the mechanical reliability (if done properly) will increase significantly. I use this approach in engine builds all the time with the first focus being the increase in mechanical reliability and I don't mind the performance after effects either.

HP gains equivents to that of 75HP-80HP can be achieved when bob weight reductions of 350-500 are found. Bearings and the rod big end is multiplied by 2 during a balance proceedure.

It's the first and foremost area that I go after, if I can.

[rskrause]

There are significant gains in excellaration rates and excelleration means et and mph. The other benift is the mechanical reliability (if done properly) will increase significantly. I use this approach in engine builds all the time with the first focus being the increase in mechanical reliability and I don't mind the performance after effects either.

HP gains equivents to that of 75HP-80HP can be achieved when bob weight reductions of 350-500 are found. Bearings and the rod big end is multiplied by 2 during a balance proceedure.

It's the first and foremost area that I go after, if I can.

75-80hp? I am somewhat sceptical of that in a typical automobile application. Do you have an example, including how the hp was measured. And more importantly, an example of a car with significant 1/4 mile gains with a bob weight reduction. Also, 350-500gms is a HUGE reduction. Again, specifics would be helpful. It must have been an unusual set of circumstances and I would like to learn more.

Richard

[rustbucket79]

Although my combination change is too far from my previous engine to make any accurate determination I will add my .02 anyways.
Old engine, GM 400 crank, 4 slugs of mallory, int balanced, Scat 5.7" I beam rods that were beamed, lightened TRW forged pistons, about a 1780 GM bobweight. New combo is the Scat 3.75 stroke superlight crank, 2 slugs of mallory to internal balance. It has undercut counterweights. If your crankshaft P/N ended in a 3, you should have the same crank as mine. If it ended in a 2, it's their lightweight pro comp crank and wont have undercut counterweights. My new rods are Scat 6" h beam and pistons are domed Wiseco's, bobweight is in the 1840 gm range.
Like I stated, a definate apples to oranges comparison, but the new bullet revs like a 2 stroke compared to the old one, even with the heavier bobweight. Heads are the same on both engines.

[Strange Magic]

Sure Richard, I would be glad to.

I don't want you to think that the reduction of component weight actually shows up as HP during a dyno test because it doesn't. You can have two identical engines, one with a 2475 gram bob weight and one with a 1630 gram bob weight, and this example is not out of line when it comes to a basic BBC engine vs. a BBC engine that has attention to detail payed to it.

Your basic race engine that makes anywhere from 1.4 to 1.8hp per cubic inch sees about .10 for every 22-25hp of a gain (509-598cu). The smaller the cubic inch engines (330-420cu) will see gains of about .1 for every 18hp and the even bigger ones (600-815) will need almost 55hp to move .10. A simple weight reduction of about 130-140 grams on a piston pin combo only, will achieve gains of close to .10

My post was only to show how the gains would be equivelent to HP gains down a race track. Do remember with et comes mph.

[Howard Healy]

The engine dyno in our shop will measure these gains, as it measures acceleration not torque.
The dyno's fabricator had seen gains of 8hp per 100 grams of bob weight reduction years ago. His testing was over 20 years ago.

[Strange Magic]

That is a unique dyno Harold, quite unusual from the rest which measure torque. Pretty interesting.

[EngineTech1]

You can still see the results of changing rotating and reciprocating mass on standard water brake type dyno's like the superflow 901/902, people do it all the time. It will only show up much in the faster sweep's though. The 300 - 600 rpm/sec pulls won't let you see this as much as going from a 300rpm/sec pull to a 1000rpm/sec pull. Engines with less inertial losses will show less of a power gain going from a 1k rpm/sec pull down to a 300rpm/sec pull. Big pigs with heavy internals and such will show much more of a gain when you slow the acceleration rates down.

[Howard Healy]

Yes Strain Magic, it is unique. This is why many have come from far, to test.
This also contributed to the reason we were the first to run 1 second under, in comp.

[randy331]

Rick; is there a way to figure power or acceleration gain if you have dyno info on the same engine, with no changes except going from 600 rpm. per sec. to 300 rpm per sec. acceleration rate, and then change just bobweight? Will the following info tell you anything about the MOI, or just this engine is short on power?
My engine; @ 600 rpm per sec. 277hp. @ 4500 /334tq@ 4100 rpm
@ 300 rpm per sec. 283hp. @4600 / 347tq@ 3900 rpm

If the bobweight is reduced 65 grams and the required amount removed from the crank to rebalance, would there be a measureable gain on the dyno? If so how much?
Would a low powered engine like this gain more or less than an engine of 500hp @ 6600 rpm. with the same change?

Is there a formula to compute this?
Anyone else besides Rick360 with info is welcome to answer.
I would also be glad to here from someone with dyno experience on this subject.

Thanks; Randy

[Rick360]

Rick; is there a way to figure power or acceleration gain if you have dyno info on the same engine, with no changes except going from 600 rpm. per sec. to 300 rpm per sec. acceleration rate, and then change just bobweight? Will the following info tell you anything about the MOI, or just this engine is short on power?
My engine; @ 600 rpm per sec. 277hp. @ 4500 /334tq@ 4100 rpm
@ 300 rpm per sec. 283hp. @4600 / 347tq@ 3900 rpm

The full dyno curve from both dyno pulls should be enough info to figure MOI for the full rotating assembly as it spins on the dyno. This would include the dyno flywheel, shaft and absorber impeller. If the MOI of the dyno components had a known MOI you could calculate the MOI of the entire assy from the dyno numbers then subtract out the MOI of the known dyno components MOI. The dyno accuracy/repeatability would certainly get in the way of these calculations.

If the bobweight is reduced 65 grams and the required amount removed from the crank to rebalance, would there be a measureable gain on the dyno? If so how much?

If you know or can approximate how far from crank centerline the weight was removed, you can calculate the change in MOI and therefore the HP gain at a given rpm and accel rate.

Would a low powered engine like this gain more or less than an engine of 500hp @ 6600 rpm. with the same change?

A high rpm engine will gain more at the same accel rate from the same change in MOI. A 280ci 9500 rpm engine making 600HP would gain much more than a 555ci making 600HP at 6000rpm with equal MOI changes. How much time it gains in 1/8th or 1/4 is affected by the gearing and converter. If a car is overgeared, which increases the actual engine accel rate on the track, the power gain from MOI reduction will be more providing more ET improvement.

Rick

[SchmidtMotorWorks]

If you know th dimensions of the material removed from the crank, it would be easy to model it in a CAD system. Then with physics analysis you could determine how much power is required to accelerate it from one speed to another in a given time.

Horsepower can be measured on an engine running at a constant speed.

The abilty to accelerate faster does not mean the engine is making any more power. Acceleration and Power are complelty different types of calculations.

An engine with a lighter crank could accelerate faster while making the same or less horse-power.

[randy331]

rick360;schmidtmotorworks, thanks for the replies.
Lets say on a 3.48" stroke you reduced the small end of the rod, and the piston a total of 40 grams. Reducing the bobweight accordingly, then removing the required amount from the counterweights to rebalance the engine. Would this amount of reduction show up on a dyno at 600 rpm per sec.? (300hp. @ 5000 rpm. engine)
Also would the net gain/loss be more or less if some of that weight were taken off of the big end of the rod, assuming the total bobwieght reduction was the same?

Again Thanks Rick360 and Schmidtmotorworks.

Others are encouraged to post replies.

Thanks; Randy

[SchmidtMotorWorks]

Would this amount of reduction show up on a dyno at 600 rpm per sec.? (300hp. @ 5000 rpm. engine)

The change would be proportional to the moment of the rotating parts of the engine and the dyno.

For example if you had a heavy flywheel the effect would be smaller than if you had a light one.

If the dyno has heavy rotating parts, it will show a diferent result than a dyno with lighter rotating parts

[Rick360]

Reducing the bobweight accordingly, then removing the required amount from the counterweights to rebalance the engine. Would this amount of reduction show up on a dyno at 600 rpm per sec.? (300hp. @ 5000 rpm. engine)

Yes, a dyno would indicate more HP at a 600rpm/sec rate with lighter engine components.

The change would be proportional to the moment of the rotating parts of the engine and the dyno.

For example if you had a heavy flywheel the effect would be smaller than if you had a light one.

If the dyno has heavy rotating parts, it will show a diferent result than a dyno with lighter rotating parts

I disagree, the indicated HP gain would be the same, regardless of the weight of other components (dyno or engine) at a fixed accell rate. Remove a certain amount of weight that causes some lower MOI, it takes a certain amount of TQ to accellerate that difference of MOI at a given rate. Therefore the indicated HP gain at that rate would be the same regardless of the other components or the proportion of the change to those other components.

Again, the accuracy of the dyno may require a fairly big change to measure these changes.

Rick

[randy331]

Wouldn't Superflow be able to provide the inertia factor for their dynos?
Using that info, and the two different dyno pulls at different acceleration rates, couldn't you calculate the inertia factor for the engine? Then take bobweight reductions, and the stroke length (distance from center of rotation) and the weight removal needed to rebalance, and calculate the increase in available power.

If you have two engines of equal power, when dynoed on a steady state pull (not accelerated) the only difference is one has a heavier rotating assembly. Then you pull both engines on an accelerated pull, the lighter assembly would now show more power than the heavy one. (correct?)
Although both would now show less power than before, because some power is now consumed accelerating the mass of the rotating assembly.

Question; We have an engine that we have dyno info on. We now take the engine apart replace the 100 gram wrist pins with 200 gram wrist pins, and we drill the rod throws just enough that when we put our new heavier bobweight on and spin it on the balancer, no weight needs removed from or added to the counterweights.(it remained in balance.) In other words, all we did is move weight from rotating to reciprocating.
Assuming no additional crank flex, cylinder loading etc,
Would this engine show a power gain on a steady state pull?
Would this engine show a power gain on an accelerated pull?


Thanks; Randy