So I needed to get the pistons just a tad lighter :)

[1 fast z]

Ok, so I wasnt totally sattisfied by the amount of mass I had scavanged out of the 3.1 forged pistons, so I took them back to the bridgeport, and started some more work on them, there still not as light as they could be, but frankly I am just running out of time, and it takes ALOT of time to do these procedures correctly to 7 pistons. But yea got them all finished and balenced within a half a gram . Should finally be assembling this weekend! Ok, for the numbers, they are 320 grams, the pistons by them selves. Combining the pistons and pins I am right at about 150 grams lighter than a stock dished piston. thats what about 2.0 LBS out of the recipicating mass, WHOOOO HOOOOOO!!!.

 

 

 

[Gollum]

holy crap...

 

How much HP are these gonna actaully be able to take?

[2003z]

why 7?

[1 fast z]

Its amazing how everyone reffers to EVERYTHING to "how much HP can this or that take" Its not a matter of HP!!!!! Its a matter of FORCES!!!! Lighter pistons do MANY things, but I will touch on the main points. FIRST and MOST important is that it makes the ENGINE stronger and last longer. The lighter the mass on the end of that rod, the stronger the rod, block, crank, etc. are going to be, for the reason that you do NOT have to stop and accelerate so much mass. Kinda see what Im saying, say you take a rock on the end of a string, that has a mass of 10kg, and you swing it in a circular motion, to create a certain centripital force. Then you take a mass thats 7kg, with keeping all other varibles constant, and swing it in the same motion and direction, and speed, the centripital (NOT centrifigul) force will be reduced ALOT, wich in turn makes the string stronger. Sorta see what Im saying?

 

 

 

It also raises the red-line of the engine, makes it rev faster, etc.

 

These pistons are not PAPER thin, like they may look, they still retain all the critical thicknesses.

[1 fast z]

7 for one extra, and that extra was used for mockup, and first artical machining.

[Gollum]

Trust me, I don't assume there's a HP limit on metal. I'm not an idiot by any means. BUT, under NA or FI situations you can guage the HP limit of a bottom end withing a window. I understand that lightening the piston will have MANY benefits. The only con is possible weakening of the piston. If they're still well within the critical thickness I assume they'll be fine well within the HP limits of the L series motors.

 

Should definatelly be worth the work I'd love to hear some audio of that thing rev once it's put together.

[POSTM]

That area of the pin bosses is exactly what I was talking about in my previuos post. Looks real good. From where you are at now it would take many hours of work for minimal weight loss, unless you sacrificed some longevity. I can’t wait to here how it runs.

[1 fast z]

Sure, I get some footage of it. Sorry if I seem like an A$$ its just that everyone always ask that question "how much HP" BLAH. Its Ok though, I know your not an idiot by your post that you make, you know your stuff.

 

 

 

Yea, I could probably scavenge another 10 grams if I REALLY REALLY tried. Under the pin boss is sitll about .400 thick but I would need to plunge through the wrist pin hole, with a carbide drill, and If I had more time I would, but I need to get this motor together.

[Dane]

Please excuse me for reviving such an old thread, but I was reading this in the archives and made a few quick physics calculations based on the weight reductions made to the pistons, and I came up with some numbers that are surprising enough that I thought it was worthy to post. The numbers were so surprising actually, that I think I could have made a mistake and wanted some second opinions.

 

So here goes for the calculations:

 

Lets assume you have an L28 crank spinning 6000 RPM. That correlates to a maximum piston speed of 24.8 m/s. I used the equation v=rw, with one rotation being 2pi radians, r is half the stroke so 39.5 mm (0.0395 m).

 

(6000 Rot/min)*(2pi rad/rot)*(1 min/60 s) = 628 rad/sec = w

 

v = rw = (0.0395 meters/rad)*(628 rad/sec) = 24.8 meters/sec

 

By removing 150 grams (0.150 kg) from the pistons, and using the kinetic energy equation KE=1/2mv^2, the kinetic energy of a piston at 6000 RPM (24.8 m/s) is REDUCED by 46.13 Joules (J).

 

(1/2)*(0.150 kg)*(24.8 m/s)^2 = 46.13 J

 

The piston decellerates from full speed to a complete stop at TDC. This repeats for BDC. So for each engine rotation, the piston loses all of its kinetic energy two times. So in this case, the DIFFERENCE IN ENERGY CONSUMED by accelerating the pistons after weight reduction is 2*46.13 J = 92.26 J. At 6000 RPM, this happens 100 times per second, and with 6 pistons this leads to:

 

(92.26 J/rotation)*(100 rotations/second)*(6) = 55,356 J/s = 55,356 Watts

 

1 HP = 746 Watts, so:

 

(55,356 Watts)*(1 HP/746 Watts) = 74.20 HP ?!?!

 

Basically this is saying that the power required to accelerate 150 grams from rest to 24.8 m/s...repeated 200 times per second...and multiplying this times 6 for each piston...is equal to 74.20 HP. Thats the DIFFERENCE in power required at 6000 RPM after removing 150 grams from each piston.

 

Can this number be accurate? Judging by gut instinct, 74 HP just seems like A LOT of power saved by lightening the pistons by that much.

 

Is reducing the reciprocating mass really that important for building high performance engines? If somebody can see a mistake I made...please point it out. But if this is actually true...all I can say is thats FRIGGIN SWEET!!!

 

I could be making a mistake in my reasoning based on how the piston, crank and rods exchange kinetic energy as a system:

 

When the piston is at maximum speed halfway up the stroke, both it and the rod are traveling straight up and down at exactly that instant. When the piston stops at either TDC or BDC, the rod is swinging over with its max rotational speed. But the energy of the rod at TDC or BDC can't be greater than its energy at max piston speed...so that can't be what makes up the difference.

 

When the piston accelerates, its taking energy away from the crank. But when the piston decellerates...is it giving energy back to the crank? I think that could be the conceptual issue that this whole thing hinges on.

[1 fast z]

Sounds good to me. People often wonder why my engine revs like it does, and has the power output it does. But the MAIN reason I lighten pistons this much, is for strength. The less mass you have to accel, and decell, means stronger rods, rod bolts and crankshaft.

[Gareth]

Sounds good to me. People often wonder why my engine revs like it does, and has the power output it does. But the MAIN reason I lighten pistons this much, is for strength. The less mass you have to accel, and decell, means stronger rods, rod bolts and crankshaft.

 

That's a good point, although I'd say that because you have less inertia from the pistons moving at high speed, it is less likely for the crank or rods to break, because of the smaller forces that are generated during the crank's rotational movement. I suppose that a combination of lightweight internals (light forged pistons, titanium rods, knife-edged crank) with a normal (read: quite heavy, to compensate for less rotational weight) flywheel should make the engine both stronger (especially during high RPM operation) and streetable (that'd be because of the moderately heavy flywheel).

[JMortensen]

I am a math moron, but I'd say that the problem with the calculations is that it presumes that one is starting and stopping the piston in a linear fashion. Since the piston is connected to the crankshaft and the crank stops the piston while continuing to turn and not by absorbing the force of the piston directly, I would assume that the hp number would actually be a lot smaller. That's pretty crappily put, but that's the best explanation my physics ignorant brain can come up with.

[Flexicoker]

I looked through your math and didn't see any errors. But I know that lightened internals don't gain you any hp. Then I showed it to my roomate, and he pointed out that you didn't analyze the entire system. His theory (which I agree with) is that as the piston is reaching TDC its being decelerated, and if you draw a free body diagram you'll see that the force thats pulling down on the piston is also pulling up on the crankshaft... in the same direction its already going... accelerating it. Then right after TDC the crank is transferring its enery to the piston, accelerating it, and decelerating the crank. The entire process repeats at BDC.

 

So by lightening pistons/crank/flywheel your gains will be marginal at best, and solely due to less friction. HOWEVER, you have less mass you have to accelerate, so you will see an increase in the vehicles acceleration or on an inertia dyno. On a water brake dyno you will not see any gain other than those due to less friction.

[1 fast z]

I have to argue. Say you take a fifty pound piston in each bore. And it takes, X amount of WORK to accel and Decel that piston. Work is a direct relationship upon HP. You take a 1 lb piston, that is LESS work to accel and decel that piston at a given RPM, therefore more HP, as you dont have to use as MUCH HP to work that piston up and down, etc.

[bubbleguinea]

"People often wonder why my engine revs like it does, and has the power output it does"

 

at a 12.9 second 1/4mile with a car weighing at 2700lbs with driver with a 3.1 i wouldnt be caught dead wondering.

 

i would wonder however why it isnt running a low 12 second one with all these "super mods"

 

like i said before---my NA L28 was right on par with your NA L28 ITB setup with my stock cam... i shutter to think with some "better" headwork and a larger camshaft would do to my L28...probbaly low 13's

 

soooooo l28 low 13'2....L31 high 12's.....something doesnt make sense, or is my math wrong?

[bubbleguinea]

I have to argue. Say you take a fifty pound piston in each bore. And it takes, X amount of WORK to accel and Decel that piston. Work is a direct relationship upon HP. You take a 1 lb piston, that is LESS work to accel and decel that piston at a given RPM, therefore more HP, as you dont have to use as MUCH HP to work that piston up and down, etc.

 

completely agree....but a stock pistons doesnt weigh 50lbs.....

 

at that weight, yeah- its going to make a difference...

 

but when you get talking about taking grams off the pistons, its HARDLY the same thing as taking pounds off the piston....

 

all you did was work a LOT doing this, and "mabye" added 5k more miles to your bottom end.....congrats.

 

how long did this take you again? something tells me the input time to do this isnt worth the output of doing it.

[Flexicoker]

I'm not saying what you did was worthless... Force = mass * acceleration. You have less mass, the same force, therefore you will accelerate faster.

 

If you put your car on an INERTIA dyno before and after lightened pistons you'd see a hp gain, same as you would if you put on lighter wheels, a lighter driveshaft, etc. However if you put it on a water brake dyno and held it at a constant RPM you would NOT see a difference. Do you see the difference?

 

The relationship between the crank and rods is one of energy transfer. When the piston is slowing down, you are transferring its energy to the crank, when the piston is speeding up you are transferring some of the cranks energy to the piston. If you had a friction-less piston/rod/crank in a perfect vacuum it would spin forever, as energy would be constantly transferred between them. You could have a 100lb piston and it would still spin forever, provided there was no friction. Now, to accelerate this perfect system (increase RPM) it would take energy, and the heavier your piston the more energy it would take.

[Sparks280zt]

Bubbles, I like you and you seam like a cool guy, so please dont get anything started! Lol

[HizAndHerz]

Tony D, where are you? Bubbles is at it again

[Tony D]

i would wonder however why it isnt running a low 12 second one with all these "super mods"

 

Sniff, Sniff, I smell Jealousy at someone's DETT project firing up and running...

 

On the accelerative mass issue, the responsiveness of the engine is directly related to the weight of it's components. What should have been done in conjunction with the weight reduction of the pistons, was an equal reduction in the crankshaft counterweight as it does not now need such a large overhanging weight to 'counteract' the stock weight.

 

The pistons are 150 grammes less than stock, and of a stronger material. It is possible that this setup is more durable than stock!

 

It wouldn't be the first time material thicknesses were reduced without any effect on overall part longevity...and I can point out in some bearings where superior metallurgy was used to greatly reduce thicknesses of races with absolutely no reduction in B-Life.