Two identical engines, other than rotating mass, will make the same power but you'll get quicker acceleration from the engine with less inertia.

Power = Torque * Velocity

Torque transmits through objects regardless of the weight. The weight of the rotating assembly doesn't change how hard the piston pushes on the crank (and resulting torque). If you can get both engines at the same speed, and they make the same torque, then they'll make the same power too. Acceleration is dependent on mass though. Heavier things get up to speed slower but will eventually get there.

Sometimes people get confused because they treat mass like friction. It's not. Friction consumes things like torque and power so that you get a loss at the output of the engine. Mass doesn't consume things. It's just there, always present, not giving or taking, just there. And the engine has to haul it around every time it wants to change speed. More mass makes it harder to speed up, and harder to slow down, but it doesn't change the engine's ability to produce torque at a given rpm.

 

^^^^ Best post in this thread.

 

With higher acceleration rates like 1st and 2nd gear with a stick shift and wider rpm windows the lower rotating mass has to make a difference. It sure does with flywheel and clutch mass. Circle track guys especially seem to care about this stuff.

With a high stall drag car and a 1000rpm operating window over 9 seconds or whatever I’ve argued the opposite point that rotating mass means very little for that combination since the engine is almost steady state.

 

I think what WE TODD DID is saying is the difference between rods are maybe insignificant as a percentage of the whole.

 

Crazy thing is Balancing, if rebuilt most I've heard go to a typical steel rod.
if free cool or @ a small cost, nothing to call home about.
Replace the thought 💭 of these rods, with a Center counter weight crank if you wanna Rev freely 👍

If it were that major Every or most builds would Include used Ls7 rods

 

Mast does re-sleeve also.....

 

🤔 You've gotta have some mass to Accelerate as it's transferred to the drive shaft then to the rear end.
Quote a post from the 8 sec build thread: ATI said they didn't recommend the smaller 10% to 25 % under driven pulley on 8k+ builds Harmonics and needing weight = Mass.😲 So mass/weight is needed afterwards 4 displacing the new found hp with less rotating mass at the *initial* point of Hp being made.
BTW you can try but can't Erase drive train loss .... Moving parts = friction/ loss

PBS....
Bet $ I'm correct... Teachers 🐕😂🤣

 

Grab a delorean with a flux capacitor and go back to the late 1980's (when i was 10 years old) and see how quick some 440 wedge mopars were running 900 grams each trw forged pistons. One ran 9.8 1/4's, naturally aspirated, stock forged steel crank, stock 'prepped' rods, forged TRW's. Factory iron heads even (hand ported to the max). The rotating weight of a diesel truck, yet the ones that were put together by people who knew what they were doing with the entire combination were definitely no slouch, even by todays standards but specially for 30+ years ago.

Also an engine builder old friend of mine once told me he thought that a heavy crankshaft and rotator accelerates quicker in the top end of the 1/4 mile, literally from gaining momentum. I think that's another reason why some of those old modified mopars could snap the speedo on the top end at a blink of an eye. Nowdays a lot of them mopars you see seem slow to what they used to be, especially the stroker ones. Maybe the china cranks are flexing at high rpm and eating HP, or the cam grinds are junk.

Rotating weight on a car doesn't do as much as you think it should, in a WOT 1/4 mile run. From my experience it does more for part throttles or hitting the throttle 'from a roll' sotp feeling. Static weight removal is where et gain is at.

If you have fallen for the rotating weight hype, get this..... i once sold a guy a full set of weld racing pro stars, with 26x9 hoosier back slicks, moroso DS2 fronts, and he swapped em over from a full set of steel wheels and heavier tires he had. It was like a 44lb weight loss total. He was a regular at the drag strip at the time and thought he would gain a heap going off of those rotating weight et calculators. He gained literally ZERO et, testing again and again, and was hooking as well as before. He wasn't happy about it and didn't blame me, i was as surprised as he was that the full set of lighter wheels/tires gained nothing. It was a head/cam 383ci ls1 stroker running high 10's.

You want to go fast, remove static weight, add raw HP/Torque. You are never going to gain anything with 150 gram lighter rod or piston. Or 5lb lighter driveshaft. It might feel a slight bit more peppy at best hitting it on a roll or at part throttles.

My 2c. Believe it or not, idc

 

🤔 same thought 💭 thread
https://www.yellowbullet.com/threads...nents.2383786/

 

I'd love to see all those who want to maximize driveline efficiency with featherweight clutches and carbon driveshafts come in here and rile stuff up lol.

 

My mountain bike is full carbon fiber! Its so zippy!

 

I don't know how much it is worth but you can't tell me zero unless we're talking a brake engine dyno with a controlled acceleration rate.

44 pounds off the wheels and tires is still 44 pounds off the car which should've been ~5 hundredths ET gain just from weight reduction. Rotating weight "rule of thumb" which I don't prescribe to as a rule - but it is claimed to be worth 3x more than static weight in the 1/4 mile. So .1 ET per 100 pound average static, .1 ET per 33 pound rotating weight average. Not usually referring to engine components - usually wheels/tires/brakes which also benefit from reducing unsprung weight.

Rotating weight gains generally won't be seen much on a dyno, but it all adds up in performance. A loaded dyno with a fixed acceleration rate won't show it at all. A dynojet or some inertia dyno - yeah it will start to show, just probably not in a 150+ mph dyno pull. Accelerate 1st - 4th on an intertia dyno and I think you'll see a difference.

How important is reaction time in a drag race? A tenth in RT at the starting line is over a car length at the quarter mile. What if by reducing inertia of the drivetrain you gain a tenth immediately on the hit from a roll race in response time of the powertrain? You get the jump and that might be your edge against two otherwise equally matched cars. C6Z's still punch above their weight class and are known to dyno and run much better than other platforms. You start looking for major differences to other LS engines it is the trans axle/differential, Ti rods and intake valves.

Also - it isn't always about drag racing or dyno pulls. A car that is repeatedly accelerating over and over again like road racing or oval short track you'll see a gain in ET even if you don't see gains on the dyno - maybe after 10 laps and it adds up to something significant. For some that is huge.

I still would love to hear GM's reasoning for the Ti rods as it is unlikely it helped them hit a power goal, and forged steel rods would've been cheaper, and gen 4 cast rods would've probably made it through validation just fine.

And I mean...of course an engine can overcome it's own inertia. It has to move a 2 ton vehicle. Stick shift drag cars back in the day used to have enormously heavy flywheels because it helped the smallblocks get off the line - they just needed an eternity to get the revs up at the starting line.

 

This reminds me of back when the ZX12R group would argue that the early ones would rev faster because of the lighter crank but the heavy crank ones said it would stay in the power band because the revs fell off slower. Hilarious....lol
If you're looking for gains from the lighter rods I surely hope you have already changed clutch, drive shaft, drilled your axles, lightweight seats, rid of AC, stock bumper supports should be gone, lightweight wheels and brake kits, lexan windows and gutted interior. Haha
Seriously though, I am sure it helps being lighter but how much is most likely insignificant.

 

I found this article a few years ago when researching my LS7 build. It's a really interesting read. See page 6 for info about the titanium rods. https://www.corvetteactioncenter.com...7-engine6.html

The article says titanium was used to reduce inertia loads at high rpm so to not exceed the film strength of the Mobile 1 5W30 oil. A heavier oil could have been used but that would have conflicted with some other design goals. And it was cool to have those bitchin' lightweight rods for better throttle response and to add to the mystique of the engine.

 

What some of you don’t realize in your theories, is there is rotating weight and reciprocating weight in the engine.

You are talking about a 300 gram bobweight difference, which equals about 2-5/8 pounds total.

Light weight bottom end parts are good up until a point that you’re hurting parts. Less mass = less stress, but then there is a point to where your part isn’t strong enough for its own good. You start scuffing pistons, burning pins, bending rods, eating bearings, etc.

If you took 3 lbs off the driveshaft, would the car be faster?

 

That does put it in perspective a bit more, but it is at least directionally correct lol. I couldn’t get behind stating that they make no difference. Thanks for taking the time.

 

I dunno, but Rich Bogart earns a pretty good living selling reciprocating weight reduction devices. They are a far cry in comparison from a set of connecting rods tho.

 

Let's assume it makes more power. Will your car put every extra HP to the ground? If you can't hook now more power won't make you faster.

I wonder if that plane ever took off from the treadmill. Hrm....

 

That would be correct. The only other advantage to the Ti rods, besides what you've pointed out, would be possibly less stress in an overrev situation. Kinda like the Ti valves offer a little more "headroom" in an overrev......

 

Why does GM still uses the same supplier (Linamar) that machined the LS7 heads incorrectly?
1: Because they're cheap.
2: Because they can
GM has made a lot of questionable decisions. What absolutely ticks me off is they continue to use the same questionable suppliers. Repeating the same mistakes, then expecting a different outcome, well,, we know what that action, or lack thereof, means!!