EngineBuilding
Will 200gram lighter rods amount to any significant power/ fuel mileage gain? Same part. The thinner rod is newer model year, forged, around 500g, big rod is over 700g. They did switch to bearings without locating tabs, but I'm not sure if the hassle is worth it.
Discussion
What engine, what car?????
Are they both forged, or is the big one powered metal or cast?
I'd probably not switch to that skinny thing. As the Italians say, 'if it's beautiful, it will work'. The right rod is not beautiful.
Less rotating mass the faster the engine will rev and the less power it takes to move the parts. As long as the rods are rated for your horsepower then go with the lighter rod.
It will not amount to a significant gain in measured output, at least on a 300/600 rpm/sec test. Installed in a car, I dunno. Most people are not that sensitive.
The difference in weight between the two will largely show up in the reciprocating portion of the balance, which on a v8 is usually 50% of the weight, so a 100 gram difference. The rotating weight is largely unchanged. You didn't give any info, like a bobweight total. But, if you have an 1800 gram bobweight with the heavy rod, and you switch to the light rod, you will have approximately 6% less in bobweight. Enough to balance, but not enough to make a huge difference otherwise.
You could get a more noticeable improvement in power and fuel efficiency with 1/2-1 point increase in compression.
Have you seen the engine masters dyno test where they test an engine back to back with a normal flywheel and then with a flywheel that's basically non existent, just enough to bolt it to the dyno hub? The engine with the non existent flywheel makes more HP, and the faster the ramp rate on the dyno the more HP it makes over the engine with the standard flywheel. It's pretty interesting.
I have not. But you are talking about a flywheel that's maybe 14 inches in diameter, with a lot of mass. The difference on the bobweight with these rods is not a lot.
Yeah it was a large flywheel. But different to what's happening here. It was just interesting to see the difference it made on a large scale.
Probably quicker reving. But you do you
What are we building here ?
Con rods
Zinggg!
Nope.
The advantage is an ability to rev higher.
Surprisingly, there's nothing to gain in efficiency. This is counterintuitive - you're reciprocating a heavier weight, it has to start and stop a heavier thing fifty times a second, how does that not waste energy? Well I'm here to tell ya, what gets spent accelerating the rod, you get back when you decelerate it. So it adds up to zero.
Having lighter rods is a lot like having a lighter flywheel: you'll feel it but it won't magically increase power, because that's not where your power is getting wasted.
When the engine is revved way up, the lighter weight will reduce g forces and may become a benefit if you're building a race engine.
Coming form a 4x4 angle I can tell you a heavier flywheel can make for lovely relaxed crawling over things as the momentum of the rotating mass helps avoid stalls.
Indeed! I’ve built two of the same model of engine, one with heavier connecting rods and more rotating mass than the other, and guess which one delivered smoother power and a better overall drive. The flywheel is there for a reason. It’s heavy for a reason. Lightening up the other bits of rotating mass like the connecting rods, won’t make anything more efficient. It makes the engine better at high speeds, but not so good at low speeds. It’s a trade off, just like anything else.
There are friction losses in the bearings related to the weight of the rods, etc.. while I doubt it is above the noise in the measuring method, it will probably mean more efficiency.
But it might also mean more abrupt power delivery, so...not guaranteed.
Agreed; I think those losses do exist but are negligible in the grand scheme of things.
The piston might be affected by the slight increase in side to side forces, in fact it seems quite likely, but again, I’m not sure the difference will even be measurable.
The bearings themselves should see an increase of literally zero: If there’s frictional contact at those surfaces, there damn well shouldn’t be. The bearings are pressurized and work like the puck on an air hockey table, with the two sides of the bearing not contacting one another at all because there’s a thin pressurized film of oil between them. There’s an energy cost to that but the oil pump has to pressurize the oil regardless of load, lest the bearing make contact and really mess up your day.
But the g forces of the reciprocating mass can become destructive at very high engine speeds, which is why lightweight racing parts exist.
If OP is looking for a place to increase efficiency, it seems to me this isn’t the place to find it. The difference is, as you say, probably nonzero but almost certainly negligible.
It makes a few more hp depending on how much weight reduction you do combined with rods.
But it helps the engine rev faster. Being its biggest benefit
The lighter rods will require balancing/weight removal from the crank counter balance. There wouldn't be any additional power or torque, but the engine will rev to max in less time.
Reducing the weight of the rotating mass actually makes a very big difference.
I'll get downvoted for this, but it's true.
Same for sprung and unsprung weights for handling.
Geared and non geared weight for acceleration.
Also, if you go forged rods and pistons, you need to get it balanced. It's very common to have to Mallory balance.
I would assume there is a great rpm benefit, aluminum rod engines sacrifice the fatigue resistance for weight.
they also find it suitable to give up quench to run the al rods for a lighter rotating assy so the benefit of reducing rod weight in a race motor outweighs fatigue resistance and tighter quench values.
Better accelerarion-but you will need to get your rotating assembly balanced, or it will shake like hell, and only spin up a few times before it grenades.
Lighter reciprocating and rotating mass will allow for higher rpm and an increase in horsepower. It must be noted that balancing the crank is a must after modifications, do not skip the balancing!!!
A lot of math goes into calculating the small gains. It takes energy to spin up an engine and the more mass the rotating assembly has the more it saps. That being said not all mass is the same. Mass further from the center of the axis of rotation is significantly more impactful. Light weight flywheels that remove weight from the outer edge will be significantly more impactful than removing that weight from say the counterweights on the crank.
For rotating components in a geared system it's even more impactful. The moment of inertia (an objects resistance to a change in rotation) is multiplied by the square of your total gear ratio. So removing say 1 pound from the outer edge of a 12" diameter flywheel with a transmission with say 4:1 first gear and 3:1 final axle ratio (total gear ratio at launch of 12:1) then the moment of inertia decrease is 144 times greater at the wheels in first gear. So one pound six inches from the axis of rotation at the engine would be equivalent of removing 144 pounds 6 inches from the axis of rotation at the wheels. Meaning it would be the equivalent of removing that much total weight from the rims (assuming the weight reduction stays at the same distance)
This is all to say that rotating assembly weight matters a lot more than you might think, but when dealing with small amounts of mass and mass very close to the center of rotation, being multiplied still doesn't add up to much. Plus the added complexity from the fact connecting rods are calculated the same since they don't rotate in the same way as everything else.
Tldr: if you are going with several light weight rotating assembly pieces the mass adds up and It can be worth it but if not then just this alone won't be noticeable in a real world application. It's also not a bad thing so if they are close to the same price go for the light weight. If it's significantly more expensive then its probably not necessary. Also consider that while light weight components will help increase your safe redline you still will be limited by airflow. If you aren't cammed for that high rpm then increase revs might not even matter. All depends on application.
I have never done or seen any A/B testing. However I wouldn't expect any power gain necessarily. Maybe a mileage gain but very minimal.
I would however expect it to rev quite a bit faster and possibly higher.
If it revs higher while producing the same torque, that's a power gain.
That's why I said no gain necessarily. All things being equal it should run out of airflow at the same point it would with the heavier rod. It will just get there faster.
True but you'd have to alter the redline in the ECU and hope that everything else in the engine can cope with the increased rotational speeds.