Light Vs heavy pistons (ie: ACL specs)

[Sam_Q]

thats from the silver to the ACL, but I later corrected myself by comparing the ACL to the 4AGZE piston which is what my choice is down to. Also please work it out for 7500 revs if possible

thanks

[-GT-]

Can do, but it won't be tonight.

I can re-do my calcs above for 7500rpm no problem, but i'm not convinced they're an accurate representation of the gains you'll get. I'll do it anyway and we can adjust once we get some input on where i've gone wrong.

[Mr Revhead]

iv got $100 that says on the road ya butt dyno cant tell the difference

there is no doubt lighter internals = better

but the gain/difference depends largely on the use and spec of the engine

[-GT-]

Yeah, i agree, more so for NA and high revs.

[Ben Wilson]

Have you ever driven an early and a late 4AG back to back?

The early ones are much more rev happy and responsive, they have a lighter crank and flywheel. Lighter pistons would make an even larger difference in my opinion.

[Mr Revhead]

the flywheels are the same

the cranks are lighter. not sure on the rods as they are quite larger, i suspec tthe bluetop ones are lighter though
pistions...... pretty much the same i think only difference being the gudgeon pin size

[Poggy]

repco is a dealer of ACL, the trade price I have received so far is $790

i got quoted 690 for the Forged turbo acl 20v ones

[love ke70]

lighter internals will not make a difference to peak hp from memory. it just lets you get there quicker.
soo...im out of my depth here, but the way im seeing it is that its possible that the 7.5KW or whatever it is the energy saved in accelerating the piston and thus the revs will climb faster to the point of peak power.
whether using heavy or light pistons the same power will be made at the same revs, its just the acceleration that will be improved...

same with lightening flywheels etc, theyre just revier

disclaimer: im only hypothesising, i dont know for sure...
cheers, andrew

one also wonders how this will affect driveability, much like a lightened flywheels...

[fixeruperer]

the les power it has to use to spin the more power it can make or say less power is taken from max power.
it all makes a diff.

[Sam_Q]

i got quoted 690 for the Forged turbo acl 20v ones

how did you get such a cheap price? mine was a special trade price

[Jasper]

I'm convinced our calcs are accurate enough, but struggle to believe the value, there's got to be something in the theory i'm missing!

The kinetic energy from a slowing piston is redistributed (minus friction losses) to the accelerating ones.

[Hen]

I concur with what Jasper is saying.

The pistons are decelerating as the crank moves from 9oclock to 12oclock (looking from the front of the engine). Through this section the deceleration force will pull the crank clockwise (the direction it is already spinning), so the piston will be accelerating the crank. This is reversed from 12-3 oclock, occurs again 3-6 and is reversed 6-9 oclock.

So with no friction (and no fuel/explosions etc...) a spinning crank with rods and pistons attached will spin forever, no matter how heavy they are. Therefore no power loss due to the weight of the parts.

However if you work out the total kinetic energy of the whole crank, pistons etc at 6000rpm then the lighter internals will have less. Less energy needed to get there means it will happen faster, hence be more "revvy" or whatever you want to call it.

Hen

EDIT - But I also think the main point of lighter internals is lower load on the crank and rod bolts. Therefore a higher safe rev limit.

[Mr Revhead]

EDIT - But I also think the main point of lighter internals is lower load on the crank and rod bolts. Therefore a higher safe rev limit.

yup, which is why imo its not worth it in a road engine, or even a lot of race engines.

[-GT-]

Yes, but power is rate of work, it's per unit time, ie 1W = 1N/s, so if something happens faster, it either used more power to do it, or required less force. If it used less force, that's more that's available to spin the wheels.

So if it revs faster due to being lighter, it has freed up some power initially lost to inertia.

What jasper is saying is on the money, but remember the combustion energy is too, as that only happens at 1 cylinder at a time, and is distributed across all. The whole crank/rods/pistons has a mass, and therefore inertia.

Hen is kinda agreeing with me there, he's on the money with the kinetic energy theory, but he just hasn't made the final statement. The lighter internals have less kinetic energy at a given revs, and due to the formulas involved, this grows exponentially as revs increase.

I've got to correct you on the no power loss due to the weight of the parts bit though. You're right that if there's no friction or any other force acting on it, it will continue to spin at the same speed forever, no matter the mass, but power has nothing to do with this situation, as power only exists when work is done (or attempted? Once a force is exerted anyway)

It's only once you try and change the rpm, ie exert a force on the system, that the mass comes into play. But if less energy is required to change rpm (ie the work done to increase the speed of the rotating system) then there has to be a difference seen in the power used, and hence lost from the combustion energy generated. This in turn is seen as a gain at the flywheel.

I think things a couple of you have said have helped me see why the calcs are wrong (i think?!) I was oversimplifying with considering a 1 cylinder system then just multiplying by the number of cylinders. I did this because i thought i was only considering 1 power stroke, but you need to look at the whole system due to the counter-rotating masses (pistons). The acceleration and deceleration (and acceleration and deceleration) of each piston is all energy required, but each phase is done by different cylinders as the fire their own piston (ie cyl 4 firing accelerates piston 4 but decelerates ... two other ones

The total answer is 7.5kW.

[Sam_Q]

7.5kw is still heaps if you ask me if that is right


also I changed the topic title of this thread to suit the content more