gollum

iTrader: (1)

Why does LS7 have titanium rods? LESS ROTATING WEIGHT = QUICKER ACCELERATION.

Would using steel rods be more profitable for GM? OF COURSE.

DavidNJ

Well, the 4hp number is probably about right, however 1800gms is a very heavy rotating assembly!!! Maybe 1800gm bobweight (includes twice the rod bottom end).

If 4hp is worth $3-4k to you...that is the way to go.

Now, why does the LS7 have Ti rods? The same reason it has Ti valves and beehive springs. To spin a 4.125" piston at 7k rpm on a 150+k mile service cycle, they needed to get the stress down around the rod bolts. I doubt that there are a $1k set of rod bolts in an LS7. Note, for Ti rods they are very heavy. Racing engines have steel rods that are lighter than the LS7 Ti pieces.

Why do F1 engines have light pieces? Well, they are trying to get every last rev out of the engine. Lighter pieces can handle more revs. And at 20k rpm that 4 hp is more like 15hp, which is pretty significant in F1.

P.S.
Please note that rotational speed is calculated in radians/sec.

gollum

iTrader: (1)

Lightest LSx 6.125" aftermarket rod I found, Crower I beam.... 585 grams
LS7 beefy titanium rod 6.067"........................................... 466 grams

ls7 Ti rod part # 12586258 $78 at gmpartsdirect

Adrenaline_Z

It makes perfect sense to me. If torque is created by pressure applied to
the piston crown and transfered to the crankshaft through all associated parts,
then it would follow that less energy to move the mass will result in more
torque per stroke.

LOnSLO

iTrader: (15)

Simply put...lighter rotating assembly = faster engine acceleration = lower ET's / lap times. Any engine builder in the world will agree with that. Obviously there are power limits to lighter parts. My current rotating assembly cost well over $4500 and would only hold about 1100 hp I would guess. It is VERY light, which is what I need to be competitive in my class.
Being light IS NOT about gaining hp.

J-Rod

If I remember correctly the Probe I beams we're using are ~530-550 grams.

DavidNJ

If the dyno is held to an appropriate acceleration rate (say 600-800rpm/sec), the power seen is what is accelerating the car. There is nothing hidden. You are accelerating the engine at the same rate it will see in the car.

I thought the LS7 rods were 480gms. If you order Crower Maxilights with small journals (a key feature of light weight and add less internal friction to boot) you will be in a similar weight range. Piston-guided rods will also get you there. Keep in mind, a race setup is designed for a shorted duty cycle, and possibly higher revs. The LS7's 7000 rpm and 500hp would put it in the range of a 2bbl super late model engine.

Yes, as long as you realize you are looking at 2-5lbf-ft of torque, not more.

sciff5

iTrader: (1)

couldent any power lost by heavier internals just be countered by a 50 shot.. I'm lookin for the cheapest way to go fast not the most exotic

SScam68

iTrader: (20)

Lets take your example. Assuming you could cut 30% weight down on a rotating assembly what does that translate to in terms of E.T? MPH?

How much more are you looking at in terms of $$$$ compared to a "typical" forged shortblock. Within reach of the average joe.

With the loss of both rotational and translational energy at what power level do you overcome those losses on a stick car at the line? As J-Rod posted you would have to spin higher to balance the lighter assembly, but then again in another post it was established that another limitation is your valvetrain. At what power level is that not as much of a factor?

J-Rod

Less rotating mass is always better, Period.

One thing I do is look at what the motor is intended for. People put a rod that will handle 1000 HP in a car making 500. The rod has a bunch more strength, but its heavy. They put heavy forged pistons that can take a 300 shot of gas on a N/A motor. Again, uneeded weight.

Go look at qualifying crank in a Nextel motor. its what 35-38lbs...

As an example. The loss of an iron FW to an Al flywheel usually means raising launch RPM about 1000 RPM roughly. But in back to back testing, the switch in everything from a 13 second street car to a 9 sec race car all netted an across the board gain.

Here are some words of wisdom from Lawrence Conley.

I go find the hundreths. You find enough of them, they add up to tenths. You find enough tenths, and they add up to seconds...

Here is a term to go look up - Moment of interia

DavidNJ

If needed strength requirements are met, better, yes. But not necessarily cost effective. In Nextel Cup a field of 43 cars can qualify within 1 second or so on a 20-25sec lap. Within .4 sec on a 16 sec lap. Every hp counts.

Taking your GTO out for some bracket racing, is $4k in the rotating assembly better than the same money on a solid roller valvetrain, better piston sealing, a high vacuum dry sump, or more dyno development time? Or an ITB intake?

In a 7000rpm motor it is maybe 5hp, maybe as little as 2hp.

SScam68

iTrader: (20)

lol, JRod I'm a mechanical engineer, I'm perfectly aware what moment of inertia, area moment of inertia etc etc is hence where the terms rotational and translational energy come from.

What I'm getting at is if 1800gms is reduced by 30% what does that translate to in terms of track times?

UMD_Jesse

iTrader: (6)

Eagle H-beam 6.125 550gms. You can see it on their site

DavidNJ

The torque is assuming an acceleration rate of 600 rpm/sec. This is pretty typical 2nd or 3rd gear acceleration depending on the car. The power can be determined if you know the torque and the engine speed. Those assume 6000-7000rpm engine speeds.

There is a slightly bigger affect going from a 45# 11" clutch/flywheel to an 18"# 7.25" or 11# 5.5# clutch/flywheel assembly.

Changing to a Champ Car at 10000rpm with 800hp in an 1800# car or F1 at 20000rpm with 725hp in a 1500# car increases the rate of acceleration and the engine speed.

steve2001sh

The problem is that the "rotating assembly" is not accurately named. The crank rotates, but the pistons and the rods (sort of) recipricate.

Fixed Parts apply to linear inertia. F = M * A. Take the back seats out of your car, and it accellerates faster due to weighing less. There is no HP change.

Rotating Parts apply to angular inertia. This is applies to your crank, flywheel, driveshaft, wheels, etc. This equation is more complicated. Torque = m * r^2 * alpha. In short, it still counts toward accelleration like linear inertia does. It just counts more. Still no HP change.

Recipricating parts apply to linear inertia, but in a special case. Your pistons are acellerated from a dead stop to really fast 200 times a second at 6k rpm. That takes a lot of power just to make them do that (about 25hp). Saving weight here gives you actual HP, but not a lot, maybe 7hp for several thousand dollars.

I forget who asked for the quick HP, but if you want quick cheap results, a rotating assembly is not for you. Try: NOS, cut-out, or cam.

SScam68

iTrader: (20)

Stroker, still waiting for an answer.

Old SStroker

Herb Adams (also a mechanical engineer, BTW) devoted Chapter 15 of his book "Chassis Engineering" to rotating inertia. He includes some numbers on acceleration of the vehicle. You can usually find a copy on the shelf at a Barnes & Noble. Don't be insulted because it's written for the non-engineer.

Some engine simulator software can calculate the difference in flywheel output for various amount of inertia and engine acceleration rate, and you could plug that into a drag racing simulator to see the effects.

My $.02

DavidNJ

Adams treats all engine speed mass without regard to diameter or relationship of engine speed to vehicle speed, that is 1st gear vs. 4th gear. He uses a 15:1 ratio, which may be ok for a flywheel, but not for gundrilling a main bearing journal.

EAPro shows about a 1.5lb-ft² difference in inertia between the heaviest and lightest assemblies.

SScam68

iTrader: (20)

Thanks, I knew it couldn't be much but all the "little" stuff adds up as JRod posted.

I'll definitely check it out

BTW I've gone back to school to finish up my masters in mechanical engineering and lucky me they are offering a course in High performance engines. It's going to be interesting to see what they teach in class compared to what I've learned on this forum.

Old SStroker

600 rpm/sec is pretty close for a 3500 lb 12-13 second street car. 2000 rpm per second or more might be more representative of a 500 cube Pro Stock in 2nd and 3rd gear where they may spend less than a second in those gears. It would be much more important for the PS to minimize the rotating inertia than the street/drag Camaro. That's why they spend the kind of money they do to "free up" those extra hp.

Herb was referring to a single-gear circle track car in his example with a 5:1 overall gear ratio starting from 64 mph and 4000 rpm, as I recall. First gear with a 4.10 final drive might show even more gains with about 1000 rpm/sec and 12:1 or more overall gear

Drilling the mains on a crank doesn't change the rotating inertia much because of the small R or distance from center of rotation. However, you might remove 3 or more lbs of weight from the crank. In some applications that might be useful. For a street/strip Camaro it's probably not high on the cost/benefit list.

FWIW, if I were roadracing a Camaro I'd send some $ on keeping the rotating inertia of the engine down. I'd also opt for lightwieght rotaing/reciprocating parts to keep the loads down. Lighter parts help both ways.

My $.02