Lighter rotating assembly power gains

[toad490]

Hi, I went with 7.135 long aluminum rods for my 4.25 stroke RB. This took 1048 grams off the rods and piston from stock lenght steel rods. 34 grams per rod and 97 per piston. Any ideas on what kinda gains would've shown on a dyno with a swap like that? Every thing else staying the same.

[TK]

well, from experience, you didnt gain any torque.

[franklin]

hey toad sounds like a stout rb. the lighter reciprocating and rotating weight might not show up on a load cell type dyno, but it will show up on the drag strip or inertia dyno.how much is beyond my very limited knowledge. btw. have you been to the auto wrecker behind the big truck wreckers,they have some neat old mopar stuff there worth checking out.also what does the compression height on your pistons work out to.

[toad490]

Franklin, the compression height should be 1.447. It'll depend on how the block cleans up, I'll know that shortly and then I'll order the pistons.

Is that auto wreckers the one with all the Land Crusiers? I'll have to check em out.

I hope to have it ready for Ashcroft in June, depends on how many more times Visa will up my limit. I've pretty much maxed out the wife limit, if you know what I mean. So if you see the Cuda and a old, grey bearded guy with a big grin on his face, say hi.

[toad490]

Originally Posted by dodgetkboy78

well, from experience, you didnt gain any torque.

Any time you reduce intertia or friction from moving parts of an engine you increase both torque and horse power.

[TK]

well, no, i dont believe so, what you did was lighten the rotating mass, wich will, make the engine capable of hitting higher rpm's, providing that it is built to utilize the ability, and you lost the shock torque you had by the heavier reciprocating mass (inertia). you did not change the friction inside the motor.
the good news, it will wind a hell of a lot faster, wich will help you at the strip, and that is what you want. i wouldnt think it will show on a dyno, if so, very little, but will show in your ET's! and thats what you wanted.

[dwc43]

dodgeboy's right on this one. IT wont show on the dyno at all. If it was possible to time from idle to max rpm and then rebuild with you light assembly you would see that it accelerates faster to max rpm, but will make the same torque and HP. That's why we try to reduce rotating mass on our oval track engines. It does not make any mroe power, but it gets us off the corner faster.

[toad490]

Well guys, one of these days we're bond to agree on something, just not today.
I'll try to explain it as it was explained to me. (thanks D.) The pistons and rods have to change direction twice each rotation, the heavier they are the more power it takes to change their direction. Yes the higher the the rpm the more the benifet, but I think we'd be suprised at how soon it would be noticable. Swing 1000 grams around on a 7 inch string at 3000 rpm and your dealing with a lot of weight. From what I understand it will drop a few more rpms at launch, but will recover faster than a heavier asseymbly. I know that the difference won't be big, but I got the rods for $335 delivered to me up here, alot less than I could get steel H beams. So I figured why not, they'll last me years.

DW, if its getting off the corner faster and reving quicker, isn't it making more of something? Franklins point on the type of dyno is probably relevent to your point.

[franklin]

I don't remember seeing any land cruisers there,it was located behind a commercial truck dismantler and beside another auto wrecker which also had some neat mopar trinkets.what kind of heads are you running,hows the pick up tube clearance with the aluminium rods, are they ron parrs rods? sorry about all the ?'s i've only played with la.'s and need to grow in the future.knowledge is power and i'll be sure to say hello.

[toad490]

Franklin,they're R&R rods, I'm not sure on the clearance yet but they are nicely profiled. With 4.25 stroke it should be okay, 4.5 you have to go to a external oil pump.

[dave571]

Pretty basic physics....

Less energy (hp / torque) used to turn the rotating assemnbly, means more used to turn the wheels.

Try strapping some lead weights on your feet the next time you ride your bike.

Then tell me it wasn't more work to ride. Horse power is the numeric value of the physics measurement doing work. Force acting on and moving an object.

There's no debate that removing the fan bade and using an electric will save a few horse power, right?

It uses energy that could be used to turn the wheels, because it is a mass, being turned by the motor.

Take 6 pounds off the rotating assembly of a big block (ie 451 vs 440) and there is definately a hp gain. How much? A good guy to ask would be Andy F who wrote the 451 manifesto, at www.arengineeering.com

I agree there are other benefits too,(better reving, less stress on the internal engine components etc.)

[toad490]

Now thats what I was trying to say. I'm going with the Romac damper for those reasons. Its only 5.3 lbs.

[23T-Wedge]

Toad,

No matter what the dyno says, in the real world, you will definitely feel the difference in the seat of the pants.........................

Hope you're right about the pickup tube clearance, at least with your rods. I had to grind about half of the tube boss away to clear Carrrillo rods with my old 4.25" setup. The GRP aluminum rods that I got to replace them actually appear to have more room than the Carrillo's, mostly due to rod bolt length on the cap side of the rod.

[toad490]

23T
The rods are just over 1" from the inside to the edge of the cap head. I took a measurement off a assembled short block today and your probably right, but it dosen't look too bad. I don't think I'll have to go to a external oil pump.

I shouldn't have said dyno it kinda threw things off track. I don't really care what a dyno says, I just thought it might make a interesting thread on all aspects of lightening a assemby, damper, crank, piston milling ect. Maybe I should've said that, might of helped.