[Mk2] [Turbo] anyone running with a lightened crankshaft?

[TUBBY-DEUCE]

im just after someone with experience in running one, as to what advantage or dissadvantage are noticeable over stock.
what are the pros and cons?
on a forged block, with lightened flywheel is it overkill? will there be any running issues?

[RyanRs]

the more you reduce interia mass the easier the engine will rev and the less power losses you will get.

lightening the crank and flywheel in a forged bottom end is definatly worth doing so long as its all balanced correctly. also having a good balance and lighter overall weight will reduce stress on connected parts.

On the downside of reducing interia, the engine will decelerate much faster basically meaning much more engine braking -some people find this reduces drive ability. you may find also that the turbo will not produce quite as much boost in the lower gears.

[TUBBY-DEUCE]

thanks for the reply.
i kinda jumped feet 1st in buying it to be honest, im now doing an engine rebuild because of a spun bearing,im on a steep learning curve.
ive been reading up on the driveabilty, and to be honest thats something i can live with.
i know it wont effect my power (bhp) but how will it effect my torque? is that why the turbo will produce less boost in lower gears

[RyanRs]

it could effect your RWBHP as there will be less transmission losses with a LW crank + flywheel.. i belive torque will be increased over the whole rev range due to there being less component weight to shift before the wheels can start moving.

Turbochargers like something to sort of 'push' against. the engine will be under more load when there is more resistance and the turbo will receive more 'push' as a result of this. making the crank + f/wheel LW, will reduce the resistance.

[TUBBY-DEUCE]

great info, thanks for the input

[Century Motorsport]

The point that has been missed is that it is essential for material to be removed in the right places.

Lightening the crank can put extreme stress on the main bearings... if you just cut the counter weights off, don't expect it to be lasting long - no matter how well balanced it is.

Nathan

[Slarty]

Having seen a 3S GTE crank, I really don't think there's "that" much metal that could actually be removed. The counter weights are quite slim as it is. Please don't take this in a patronising way, but the counter weights are there to counter act the weight of the piston/conrod assembly. As such, by design they are equal to, and opposite, the masses of the piston and conrod assembly.

Any weight removed from the counter weight must be matched by removing weight from the rod/piston.

Fitting forged rods/pistons might make them lighter (I honestly don't know!) in which case you can remove an accordingly sized mass from the crank.

Other than that I'd just make sure the rotating masses are balanced evenly.

[TUBBY-DEUCE]

notes taken so does that mean no one on this site has had it done from recolection?
would the advice be not to fit it?
obviously il be getting the thing cecked, but i wouldnt have thought its the type of work a novice would carry out, surely its quite a specialised job to do.
its got me worried now, surely even a numbnut wouldnt just chop the counter weights off.
i mean im a numbnut and i wouldnt think of doing that

[Harry]

The point that has been missed is that it is essential for material to be removed in the right places.Lightening the crank can put extreme stress on the main bearings... if you just cut the counter weights off, don't expect it to be lasting long - no matter how well balanced it is.Nathan

Nathan:- I like your approach to engines-enjoy reading learning from your article replies.

[RyanRs]

got any pics of it mate? any ideas on who did the work?

[Century Motorsport]

The point that has been missed is that it is essential for material to be removed in the right places.Lightening the crank can put extreme stress on the main bearings... if you just cut the counter weights off, don't expect it to be lasting long - no matter how well balanced it is.Nathan

Nathan:- I like your approach to engines-enjoy reading learning from your article replies.

Thanks Harry - i try to help out where i can!

I have sat down and had a good look at the crank, and am in total agreeance (is that even a word??) with Slarty.

There really isnt much meat to be taken off of the crank, its even knife egded as standard (leading edge of the counter weight). If you do just pull material off left right and centre you loose the harmonic balance of the motor which puts immense stress on the main bearings.

It really isnt worth it unless its a drag motor - even then its questionable in my eyes..... reason being, the weight you have saved will be countered by the extra drag with the main bearing stress / resistance.

The ONLY way it would be worth it was if you were running titanium rods and lightened pistons.

Every bit you take off the counter weight, you have to take the same off the big end area.

Even though the crank will balance up on a machine the piston / rod balance will be a mile off.

Nathan

[arome]

there isnt a huge ammount to take off but there is enough to make a diffrence. i managed to get just over 1kg off mine with knife edging the lobes . this isnt a massive ammount but at 8000rpm and 43mm from centre it is a clear improvment. and you also have to think about the the reduced drag caused by the oil. i got mine balanced down to 0.127g one end and 0.15g the other. so there will be no issues there.

with your bulid dont forget to balance the rods and get all the piston/rod weights the same.

[TUBBY-DEUCE]

Any weight removed from the counter weight must be matched by removing weight from the rod/piston.

Fitting forged rods/pistons might make them lighter (I honestly don't know!) in which case you can remove an accordingly sized mass from the crank.

Other than that I'd just make sure the rotating masses are balanced evenly.

what about people who are putting forged rods and pistons on standard crank. why doesnt that effect the harmonic balace? i always thought that forge parts where considerably lighter than stock?

[arome]

crank shaft balance is totally seperate from piston/rod. pistons/rods counter balance them selves
i very much doubt that by lightening the crank will cause harmonics anywhere close to the 100hz mark. if its well balanced it will greatly reduce the instability that causes the harmonics to have less effect

[Century Motorsport]

crank shaft balance is totally seperate from piston/rod. pistons/rods counter balance them selves
i very much doubt that by lightening the crank will cause harmonics anywhere close to the 100hz mark. if its well balanced it will greatly reduce the instability that causes the harmonics to have less effect

The balance between the rod / piston and crank will be miles out. This is the issue with lightening cranks.

They will never be perfectly balanced, the piston / rod will always be heavier - thats why its even worse to be removing anything off the counter weights. After market lighter stuff can only help.

Nathan

[arome]

cranks are not in anyway balanced to the piston/rod assembly.
where have you got this farcical information from?

[Slarty]

cranks are not in anyway balanced to the piston/rod assembly.
where have you got this farcical information from?

Erm, from around 7 years maunfacturing crankshafts

The cranks aren't actually balanced with the rotating masses but the theoretical masses of the rods/pistons/flywheel (even the dowel that locates the flywheel) are known, and as such can be calculated into the equation.

Your point being?

[TUBBY-DEUCE]

cranks are not in anyway balanced to the piston/rod assembly.
where have you got this farcical information from?

Erm, from around 7 years maunfacturing crankshafts

The cranks aren't actually balanced with the rotating masses but the theoretical masses of the rods/pistons/flywheel (even the dowel that locates the flywheel) are known, and as such can be calculated into the equation.

Your point being?

on the sites ive been looking at, they offer a top and a bottom half balance.
top half consisting off rods and pistons and connecting parts.
bottom half consisting off crankshaft, flywheel and pullies etc (rotational)

im gonna be buying eagle rods and JE pistons, if i get the crank checked for balance etc.
are you saying the rods and pistons arnt gonna be balanced sufficiantly enough to match the crank?
i understand things need to be balanced etc, but i would have thought forge rods and pistons would be matched to quite stringent limits.

[Century Motorsport]

cranks are not in anyway balanced to the piston/rod assembly.
where have you got this farcical information from?

Erm, from around 7 years maunfacturing crankshafts

The cranks aren't actually balanced with the rotating masses but the theoretical masses of the rods/pistons/flywheel (even the dowel that locates the flywheel) are known, and as such can be calculated into the equation.

Your point being?

Amen.

I'm not totally sure if there is a way of fully balancing the counter weights to the rod/piston assembly.... the rod/piston is always going to be heavier unless you get extreme.

This is why when you cut the counter weights down to nothing your stressing the mains and resistance begins along with premature failure.

Nathan

[arome]

i dont know how to make it any easier. pistons are 180 deg out of phase so forces completly cancel each other therefore no counterweights on the crankshaft are needed. as for balancing the rod/piston assembly you just make sure there all the same weight. simple. if you dont know what your talking about dont give advice. especially if you intend people to act apon it.