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Old 10-05-2012, 09:36 AM   #1
Dr. octagon
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Default Lightened Rotating Mass Questions (2.1L)

1. What are the benifits of having a lighter rotating mass

2. Would it be advised to take it a step further and lighten the pistons and rods further (via machine-ing)

3. Can you get to a point where having too light of a rotating mass is an issue/problem.

4. Again would there be any benifit as far in a billet crankshaft, as this would further decrease rotating mass.

*The goal is to when it comes time for the build to do everything to obtain the lightest rotating mass while the motor is out.

Plan:
This in terms of 2.1, shimmless buckets, rev limit of 8k rpm, og fp red, kelford 264's,. Daily driven, back roads, highway pulls (on ramps & such), double d ports. I will not see the track.
5mt for now; it has seen stock, 18g, og green on stock clutch.

I have no knowledge on this subject
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Old 10-05-2012, 10:04 AM   #2
mekilljoydammit
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Quick answer; I'm sure others will chip in.

Lighter rotating (and reciprocating; pistons and rods don't really rotate if you think about it) mass, broadly, does a couple things. First it means that less of the engine's power is going into changing the speed of the rotating assembly - so especially in low gears where the engine speed can increase more rapidly, more will be getting to the wheels. Secondly, it means that the rotating assembly can lose speed faster when it's not generating power - so it can drop speed faster during gear changes. Lastly (off the top of my head before coffee) a lighter reciprocating assembly means that there's less inertial forces on everything at high RPM.

That said, unless you know what you're doing, I really wouldn't machine the pistons and rods. They're both under a lot of stress and it wouldn't be hard to take metal out of places where it's important. Pick the lightest you can for a strength level, sure, but not machine yourself.

Too light of a rotating assembly mostly would get into issues of standing starts - same effect as lightweight flywheel (which is going to be a big part) in that since there's less inertia stored in the rotating assembly, when you engage the clutch it's going to change speed a lot faster... easier to stall, in short. Also, if you went really gonzo with weight reduction on rotating assembly, you might be running into strength issues.

If you don't want to talk to an engine builder about custom stuff, I'd look into the Carillo A and SA rods and lighter aftermarket pistons, but leave them as delivered. If you want to be more serious about it, probably be worth the time to talk to Micah of 3MI Racing as I know he can get a hold of stuff that is custom made to specs that will hold up. Probably others too, just he's one I have talked to myself about some things.
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Old 10-05-2012, 10:28 AM   #3
CatfaceType-R
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Dave buschur says the most power he has ever made was always with the lightest rotating assemblies, take that for what it's worth. Engine building is engine building, regardless of platform. The lightest crankshafts are spendy; I would say a high rpm proper rotating assembly balancing is also very important. Good luck, heavier doesn't mean stronger, it is usually cheaper, though.
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Old 10-05-2012, 11:05 AM   #4
Dr. octagon
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appreciate the insight, and forgot to add he lightweight flywheel and light crank pulley into the equation.

I am looking for the lightest possible rotating assembly
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Old 10-05-2012, 11:23 AM   #5
RaceFaceXC
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Keep on mind that a light rotating assembly makes for poor street driveability, as was mentioned previously about standing starts. If it's possible, I would recommend driving something with a lightened drive train to see if it's really what you want. I would be a shame to spend extra $ and then regret it after.
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Old 10-05-2012, 11:34 AM   #6
JhnBrackett
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Great advice so far but wanted to put a few Rules of Thumb on the effect of reduced inertia/mass.

Removing mass from the rotating engine assembly is 15X more effective than removing weight from the vehicle on a pound for pound basis.

Reducing the torque multiplication gear (transmission, driveshaft, differential, axleshafts, hubs, rotors, wheels) will be 5X more effective.

This is due to the Area Moment of Inertia having values to the 4th power in regards to the moment arm of the center of gravity. No linear relations on this one. That being said, listen to the advice you've seen. Unless you've got some sick FEA software (like moi =P), then you're going to weaken the wrong spots on the units. Balance the assembly as best you can, don't bother going crazy with removal.

My setup has a lightened crank, flywheel, rods & pistons compared to stock. Revs crazy quick. Especially compared to my Jeep with its 50 lb flywheel.
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Old 10-05-2012, 11:37 AM   #7
KOJAK-02-WRX
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You also run into the ecu throwing codes when you lighten everything up but it sounds like there are ways to disable that now:

Misfires are detected by measuring crankshaft acceleration using the crank trigger (PIP) wheel. Based on RPM, pedal position and engine load a certain acceleration / signal is deemed a good combustion event; anything below that is binned a misfire. Generally if you have more then a 1% misfire rate it would cause you to fail emissions and therefore the CEL is required to be illuminated.

In OEM land it is required to be sensitive enough to meet emissions thresholds which usually means taking into account every source of rotating mass in the drivetrain. In my experience I have seen OEM's re-certify their misfire monitor to the EPA for things as small as a change in the durometer of the engine mount.

Changing a flywheel can push you towards the edge of the detection envelope depending on how light it is. Generally disabling the code just makes the misfire monitor a non-CEL, but the monitor still runs in the background and counts misfires. A properly trained person with the correct tools can pull misfire rates for each cylinder that are stored in the PCMs' memory.

This is only the tip of the iceberg for the misfire monitor. To develop this monitor for a new vehicle usually takes 1 engineer with 2 cars at their disposal a solid month.

Hope this helps.
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Old 10-05-2012, 12:01 PM   #8
the suicidal eggroll
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1 - Less stress on the bearings, especially at high RPM. Less energy wasted in rotational inertia, more energy delivered to actually accelerating the vehicle. Think of it like a 2000 lb car with a 200 hp engine vs a 3000 lb car with the same engine. The same energy is delivered, the two cars will have the same momentum at any given point, but the lighter car will have more of that momentum in speed, while the heavier car will have more of that momentum in mass. Remember momentum is mass*speed^2. Same deal here, except you're working in angular momentum instead of forward momentum.

2 - I wouldn't. As mekilljoydammit said, it would be very easy to accidentally compromise the structural integrity of the part in this quest for less weight.

3 - Kind of. Beyond the "easy to stall" bit that mekilljoydammit mentioned, a light rotating assembly will accelerate and decelerate faster with each cycle in the engine. Each combustion will accelerate it more, each compression will decelerate it more. The rotational inertia acts as a damper to keep acceleration smooth through all combustion cycles in all cylinders. With too little inertia, the ECU can start to think that the engine is misfiring because it accelerates much more rapidly than the ECU expects. 2.0L engines with lightweight crank pullies and flywheels often have these phantom misfire issues. I've never seen a 2.5L do it though, my car included (I have an ACT Prolite flywheel and ATI harmonic dampener pulley which is about half the weight of stock).



edit: damn, left the post up too long before I decided to respond. Looks like I was beat to the punch.
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Old 10-05-2012, 12:11 PM   #9
mekilljoydammit
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Oh yeah, #4 on billet crankshafts... generally most aftermarket billet cranks aren't going to have that much weight taken out of them. For the most part, aftermarket billet cranks are billet just because it's more affordable for small production runs than making forging dies, not because they're doing anything exotic that the OEM ones aren't. There's a few exceptions - Tomei and Cosworth and maybe a couple others have light weight versions that leave a couple counterweights off which on a boxer you can kinda more or less get away with.
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Old 10-05-2012, 01:37 PM   #10
Dr. octagon
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Thank you all for your responces, this helps tremendosly.
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Old 10-08-2012, 12:32 AM   #11
Zee Biker
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I don't think you'll spool at 5. With that said, I don't think you *need* double d ports either.

Good stuff in this thread though; it helps to know that focusing on mass might not be the best for a DD on OS with all the worries about the computer freaking out.
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