So... I was arguing with a friend about the significance of rotational mass in the drivetrain.

I completely understand the laws of physics and that of moments of inertia vs tq... I also completely understand lightweight valvetrains and their benefits.

Lightweight pistons will keep stress off the rods... I get that.

But would lightweight pistons, rods, and cranks... be worth it? Im yet to believe that saving a couple pounds here and there in your rotating assembly will net any substantial gain.

The way I see it... the rotational moment of inertia of the engine is almost insignificant compared to the moment of inertias of the rest of the drivetrain (rims, tires, brakes, driveshaft, flywheel/clutch or TQ converter w/ fluid, rear end....) those all have a significant MOI much greater than that of the engine.

Then compare to that of a 2500+ pound car and those 2 pounds you saved on TI rods seem rather silly.

Any thoughts?

 

The inertia loads on the crank is one big reason for things like Ti rods, also the reason that Pro Stock and NASCAR put a minimum mass on rods, pins and pistons just another way to limit RPM.

You would be suprised as to the on track performance that light parts get you, namely the crank and flywheel/clutch assembly (or converter/flexplate) The light rods and pistons go hand in hand with the light cranks, you need them to produce a lower bobweight so you can take more off the crank and stress it less since most times the really light stuff is flimsy.

Bret

 

One thing that really bothers me is when people claim that a lighter rotating assembly will "rev faster." That just gets under my skin. Do they not understand that you have to produce TQ for your car to accelerate? The car is directly connected to the engine and for the engine to "rev" the car must be accelerated, along with the entire drivetrains moving parts.

I also understand inertia loads and how you want to keep the piston mass down to allow rods and cranks to last longer.

Do you have any idea to the average MOI of an entire drivetrain for a typical drivetrain (ballpark street vs race)?

I just dont see how a lightweight flywheel would be any more beneficial that lightweight brakes or lightweight wheels, yet nobody seems to think about that (or maybe I am arguing with idiots).

 

Lightweight wheels and brakes are not being accelerated to 7,000 rpm over the course of a race. Think about how many times a wheel rotates over a quarter mile compared to how many revolutions your engine makes. Acceleration eats energy, your engine is accelerated a lot quicker than your tires. Rolling mass, however, does weigh substantially more.

In conclusion, if you have the option to cut 40 lbs off your ride, lose it from the fastest spinning part.

Nick

 

As mentioned above, the most effective place to reduce the polar MOI is on the parts with the highest angular acceleration. This is why reduced flywheel mass works well. The effective mechanical impedance of the engine goes up with the square of the angular velocities. The first gear of an M6 is 2.66:1 so its 7.1 times more effective to pull mass from the engine to increase your angular acceleration (help it rev) than to pull mass from the output shaft or driveshaft of the car. At the wheel angular velocity, its hard to gain much at all. Light rims are probably more useful as a straight mass reduction than as a way to lower MOI.

Another parallel is valvetrain masses. Making a light lifter does very little because its on the wrong side of the rocker arm. At a 1.65:1 ratio, its 2.7 times more effective to pull a gram out of the valve than from the lifter.

I have measured an equivalent loss of torque in my car of about 15% in first gear compared to 4th gear due to drivetrain inertia. You can see a plot here 1st gear is wavy due to wheel spin, I was using an accelerometer to log the data.

 

Doing 1st gear pulls vs. 1:1 pulls is really where you see this, you see the changes of lowering the MOI in the setup much clearer when you do the fast accelaration pulls in lower gears.

Bret

 

I was doing this on a country road, that was coming to a T intersection. I was already at 115+ in 3rd. No chance to use 4th!

 

As has been said, lighter weight stuff is needed for higher engine speeds to keep things together. So, when you you turn 9-10k rpm, this is critical Even at 8k.

However, at 6.5k, less so. Which is one reason you don't see too many light weight parts being offered.

The flywheel and clutch assembly, because of the large diameter, have a huge affect. The crank, because of it small diameter (the outside of the throw is within a 3" or so radius).

As also mentioned is bobweight. Lightweight components may be necessary to internally balance components. Some cars use external balance. Lighter weight parts would give the flexibility for an internal balance.

On the pistons/rings/rods/pins/crank, on a 350cid engine, you are looking at 2-3 lbf-ft of torque at 600 rpm/sec. If you can manage more than 600rpm/sec, the effect is greater. At lower acceleration rates, less. If you are running a larger displacement engine with heavier parts (800gm rods, 80# crank) and long throws (4-5" stroke), the effect is larger. 2-3 lbf-ft of torque is 2.5-4 hp at 6500 rpm.

GM Ti rods are relatively cheap ($250?). Generally, lightweight steel rods will cost $1200-2000. Ti rods (from billet) $3000-4000 (Bret probably has actual prices). Crank is $2k-3k). Custom pistons $800-1000. Pins can be anything from $150-500 depending on material and coating.

If you are running a sub-7000rpm street hydraulic roller, it is probably not a big issue. If you are in a closely competitive class with restricitve rules that allow it, it is helpful.

The Ti intake valve allowed GM to use a 2.20 valve with an aggressive hydraulic roller cam. Why they chose to use Ti connecting rods is not clear to me. Maybe they got a good price from China of Russia.

 

For a given torque, a lighter rotating assembly will have a greater angular acceleration
(T = Ia (where a is alpha), the rotational analog of F = ma).

 

Also, reduced rod/piston mass means reduced stress at a given rpm, and (ignoring the valvetrain and other considerations like the rod caps/bolts) the rotating assembly can be now spun at a higher rpm before the rod sees the same stress level.

$0.01

 

Yea, it just clicked to me last night about the flywheel being geared... the engine is spinning much faster than the wheels.

Yes, I know all about F=MA and T=Iw

 

Just to make sure I have it straight....

I thought w (omega) is angular velocity;
isn't the relation T=Ia where a (alpha) is angular acceleration...?

(sorry, don't have greek chars.)

 

Sure you do: ΘΦΩΓπμτΔαβγεω

Yes, it reduces inertia, but not very much...because of the small radius. To reduce inertia, change the flywheel & clutch first.

 

the more weight that is removed from the rotating assembly is definitely worth it a light flywheel will show u a great gain your 60 foot will drop with this mod and therefore your speed should go up 2-5 MPH in the 1/4 which is a very good improvement, but this depends just how much the flywheel weighs.. my dad races round track and he went from a 20lb flywheel to a 8 lb and picked up from 95MPH in the straigh way to 105mph so it worked for him..

 

Light flywheels are not generally preferred for manual trans drag cars as launching is difficult. On a road race car they are great.

 

There is only one idiot in this discussion...listen to the wisdom disco...just cause we do not understand a concept does not mean that it is not correct.

 

Lightweight rotating mass is certainly beneficial....both for additional reliability at higher RPM and freeing up some power that can be directed to the rear tires versus absorbed in accelerating the engine's own internal (heavier) components. A buddy of mine who is big into circle track racing says without a doubt that with no other changes except Ti rods and a lighter rotating assembly (same car, same induction, exhaust, etc.) that the car definately wanted to zing to redline a little faster coming off a corner and simply felt stronger everywhere. He chopped like 200 grams of his bobweight though which is HUGE when he made the move to the much more expensive lighter components.

My next build will be of interest to this discussion as I have worked extremely hard at reducing the mass of most of my rotating assembly components.

I will probably have about 24 hours involved in modifying already light custom pistons down to about 352 grams each (they started at 392). Was that a complete waste of time?...probably but I felt the need to do it anyway. What will it be worth?....I wish I could tell you but optimizing was my focus here and I couldnt leave well enough alone. My current 383 has a bobweight of 1754....kind of typical for a steel rod motor with a 4" stroke (not heavy...not light). My new engine which will be 415 CID and have an even longer 4.125 stroke has a bobweight of only 1652....a significant weight savings not to mention much harder to achieve considering the larger 4" bore piston (My smaller 3.905 bore 383 piston weighed 373 grams which is fairly light in its own right....a stock LS1 piston weighs 425 grams).

Bottom line is my 383 engine tached to redline in a blink especially in the lower gears when I blew the tires off the road so I'm really curious what the larger, even more optimized 415 will be like. Of more significance is the weight reduction of my crank....A stock crank weighs around 51 lbs....my 383 Eagle crank weighed about 55 lbs, and my profiled new Lunati piece which just arrived a few days ago (with an eigth inch more stroke) weighs in at 49.6 lbs. There are very expensive exotic cranks that weigh less, but all things considered thats pretty good and significantly lighter than the 383 crank it replaces (5.4 lbs.)

Unfortunately with my current workload I wont be able to share any actual results with you guys till probably close to the Holidays, but none the less its in the works and nobody is as curious as me regarding the final outcome. Besides the typical attention to detail regarding actual assembly, the agenda of this combination was to feature lightweight rotating components and also use some engine coatings to try and free up as much power as possible ultimately further maximizing the end results.

Hoping to get close to 600 RWHP with this combo with only a medium sized cam kinda typical of the other builds you guys have seen me involved in (which makes it a bit more difficult but ultimately a much better rounded package for a true street car).

Tony M.

PS....Something also to note is that reducing the weight of the connecting rod/piston assembly means not only are you reducing inertia losses (purely based on the weight savings), but you are also reducing the weight of a component that changes direction and goes from 0 to 4000 ft/per minute and back to zero in the blink of an eye 125 times a second at 7500 RPM

 

David, thanks, now I have no excuse...

 

Tony, you don't even want to know about my project then, 1400g range for the bobweights in a LS2 block ;-) Hell I just put a customers LT1 355 together with a cast crank that came in at 47lbs and a 1590g bobweight, that's pretty helpfull on a track day car.

I found this looking around another forum

"I have plenty of experience with light weight crank assemblies, and I can say with confidence that they make a big gain in drag racing performance. I will admit, I only recall from years ago , about changing from a heavy to a very light flywheel. Most of the gain in E T came from way more traction off the line, but improvements of 2 or 3 tenths were common.The ultra light crank and piston and rods can make an improvement of 2 tenths on a high rpm setup." - Joe Sherman

Joe is one of those old guys that knows his stuff. I have a lot of respect for him, he makes stuff inexpensive and FAST!

Bret

 

LOL....Sounds good Bret but I'm guessing that rotater had the benefit of aluminum or Ti rods....what stroke and bore size btw??

Also....I know Joe Sherman pretty well....usually cruises into AFR once a month or so in the traditional Sherman sweat pants attire....LOL

He's a good guy and him and Ken Sperling (who started this company 35 years ago) go waaaaay back. There's an old picture of the two of them we stumbled on going thru some of Ken's old stuff (Ken succumbed to cancer about 10 years ago) that we saved because it was a great picture of the two of them that had to be 25 years old.

If I had nothing but time and was independently wealthy I would build two identical engines with the exception of a big decrepancy in the bobweight to try and evaluate what the lightweight components are really worth....It would make for a pretty cool test but it would probably best be evaluated on an inertia style chassis dyno that doesn't controll how quickly the engine accelerates (like a typical engine dyno).

Bottom line....lighter parts that are strong enough to get the job done are a desirable thing....whether there worth all the time and money is questionable but if your looking for that extra edge....it certainly couldn't hurt to focus there.

Tony M.