RB25 pulleys

[rev210]

simonster,

my flywheel is 5.2kg lighter than stock and has less mass on the outside edges as well (custom chromemolly).

The flywheel is very streetable. I'd take an even lighter version in a second. The 'streetable' description is very subjective. Some will work around low rev and high load vibrations others want to drive like nanna. 5kgs lighter is still fine for nanna btw.

[caminperth]

Paul - have you remebered?

[GTS-t VSPEC]

Shit *Paul blushes* I'll go there this arvo, promise:D

[EnricoPalazzo]

excellent

[sidewaymambo]

PAUL............

[GTS-t VSPEC]

Just when you thought it would never happen... I went and saw them:D

I talked to the owner, who I already know, and he seemed quite keen to help us. I need to get him a set of example pulleys and then he'll be able to work out what needs to be done, and give us an approximate cost:D

If anyone is mechanically minded do they want to suggest which pulleys need to be made bigger or smaller, as I have no idea.

I'm hoping to borrow the pulley's of my mates engine. Keep reminding me to get this happening:D

[benm]

You have been reminded

[Simonster]

REV210

Good to hear you had a positive result with your flywheel lightening

[GTS-t VSPEC]

Benm, Thanks for the reminder, but I think reminding me in 1 week might be more helpful:p

[silverskyline]

nismo, osgiken, hks, trust all make them, go to www.nismo.com.au, i think theve got pictures of them

[GTS-t VSPEC]

They have ones for the WRX and 300XZ, and alos have:

Ultra Street Set Lightened Underdrive Pulley Sets

Available to suit most late model Japanese cars.

$1,350.00 Per Set

Does someone want to contact them to see if these are suited to the RB25DET?

[EnricoPalazzo]

Wow thats expensive!!

[Steve]

$1350? I am quite sure it would be cheaper to buy some billet and get naked 16 year old virgins to gently wear them into shape by rubbing them on the insides of their thighs.

Hmm did I just write that or think that:D

[Gradenko]

Hear, hear! I'll get the billet, steve, you organise the rest .

[GTS-t VSPEC]

Yeah, I thought it was damn expensive too, so I'll keep looking into get some custom made:D

[rev210]

Paul,

10% underdriven ratio would be good. The important thing is to have the pulleys made light.

[GTS-t VSPEC]

Dave, Ok, I'll get the stock ones weighted, and see what what difference we can expect to get.

[the phantom]

Hey all,

Harmonic balancer operation appears to be a very missunderstood subject. Its actually quite scientific in detail, with entire volumes of theory for those that way inclined. Unfortunately very few are exposed to this and all sorts of interpretations and missconceptions come along.

Firstly, harmonic balancers have nothing at all to do with engine balance. For this breif intro assume that the crank is 'perfectly' balanced, and that all the pistons/rods are 'perfectly' weight matched.

When a cylinder fires it causes a massive shock on the crank pin. So much so in fact that the crank pin physically deflects a measurable amount every power stroke. The crank shaft then acts like a spring and the crank pin deflection bounces back and forth until it comes to rest in its natural position. This happens AS THE CRANK IS ROTATING, and results in the crank twisting back and forth along its axis producing what is known as torsional vibration. This bouncing back and forth is akin to a tuning fork tuned to a certain 'note'. Similarly the crank has a given 'note' that happens to be a function of its manufacture. This is determined primarily by it weight, shape and material stiffness. Just as a tuning fork will ring its note if placed next to say a piano playing the same note, so too will the crank vibrate if the power pulse frequency happens to match its particular 'note'. This phenomenon is called resonance, and results in a build up of amplitude rather than it just dying away quickly before the next wack of the crank by the firing pulse. This ultimately leads to hammered bearings or a snapped crank. Usually the actual resonance frequency is beyond the redline of an engine, but what complicates things is that multiples of the resonance frequency still cause significant vibratory excursions. Remember this is a simple explanation in order for the point to be made. In reality the multitude of firing strokes and other complex interactions results in the crank 'ringing' uncontrollably at various RPM's determined by multiples of the resonance frequency throughout the RPM range. Lots of peaks and troughs in torsional vibration occur throughout the RPM range. The harmonic balancers job is to dampen out and reduce the amplitude of the torsional vibrations. The flywheel can be considered as a solid fixed point that supports the crank at one end. This is also as a result of the rest of the drivetrain acting against that end of the crank. Therfore the effect of the torsional vibrations end up at the 'free' end of the crank, so this is where the balancer is placed.

Now for an explanation of how a standard harmonic balancer does its job. Recall that a OEM type job has a hub that attaches to the crank, with a heavy circular ring surrounding it. Between these components is a rubber material. Contrary to popular belief, it is the large inertia of this ring that dampens vibrations. The large inertia tends to oppose sudden changes in rotational speed. When the crank snout starts to twist as a result of the aformentioned power pulses, the large heavy damper ring doesn't want to change its speed due to its inertia. With the rubber seperating it and the crank, the shearing action tends to limit the extent which the crank snout can move torsionally and so the extent of the excursion (and vibration) is reduced. The energy of the vibration is instead dissipated as heat within the balancer rubber.

So the moral of the story is that a heavy damper DOES NOT increase the stresses on the crank due to vibration, but in fact reduces them. (ever noticed the size of the GTR700 balancer compared to the stock one ???) Additionally the balancer itself is tuned to match the vibratory tendencies of the crank itself. Making matters worse is the long crank of the inline 6 cylinder engine, making it more prone to torsional twisting. This is why some 4 cylinder engines can live without a balancer, as the crank is physically stiffer due to its shorter length. Note however that even so, lots of 4's still have them. SO DONT MESS WITH YOUR BALANCER!!! Do you think that the factory wouldn't replace this bulky heavy and consequently expensive device with something cheap and nasty if it was beneficial. Dont underestimate the work of hundreds of engineers...they do know what they are doing.

Another interesting point is the so called power saving of underdriving engine accessories. You can actually work this out mathematically, and in reality does not amount to any more than a handfull of KW at best.

phantom

[rev210]

thanks phantom for adding that.

I was thinking the large mass (and therefore inertia) of the factory harmonic balancer and flywheel that will excite this tortional vibration frequency. So by dramatically reducing the weight and inertia of the crank pulley, the amplitude is reduced and frequency of the crankshaft is moved and its ability to self-excite is greatly reduced. So in fact it is the harmonic balancer's own weight that necessitates the dampening. An alloy pulley of 0.8kg or less may not supply an exciting force significant enough to damage the crankshaft in some cases, also its outer diameter being far smaller.

If I understand you depending on the engine and its application a harmonic balancer at the front may not be required or the original will become inadequate if various engine characterisitcs are changed.

"if the manufacturer put it there, it must be there for a reason". However, if you look at it from the car manufacturer's point of view, casting pulleys from steel is very cheap and easy, because they can be produced in large numbers and there is no waste (as opposed to machining them from billet). But because the resulting pulley weighs significantly more than one made from aluminium alloy, it requires dampening.

I had the oppertunity to fang around in an S2000 on the weekend, no harmonic balancer it had a billet alloy pulley and a 9000rpm redline.

Power wise there is no 'power' gain to changing the weight of the front pulley. The reduced inertia however will increase the acceleration significantly in lower gears. Making the lack of extra power a non issue given you are quicker anyway.

Where power may be gained is in the reduction of the ratio's of the accessories. A modest amount but the reduction is primarily aimed at giving the alternator an easier life at high rpms and stopping the water pump cavitation at sustained high rpm.

From experience I am quite keen to machine up a front pulley for the RB25 for street/strip use.

[the phantom]

Hey rev210,

I disagree with you on a number of points. The sole source of the torsional excitation is from the impulses of combustion. The haramonic balancer is a driven device, and through its inertia is only a store of rotational energy delivered by the crank. As such it can not generate any excitation whatsoever, unless of course it is in a state of dynamic imbalance, which it is not. Weight of the balancer has nothing to do with excitation. Simply put a lighter balancer will have a lower ability to attenuate torsional vibration.

The OEM type balancer is significantly more expensive to manufacture than alternatives with its multiple sections/compositions and assembly. In many circumstances the balancer itself needs to be dynamically balanced as the outer ring cannot be positioned with total accuracy over the flexible rubber hub interface.

Yes the S2000 has no balancer. But it is an inline 4. One cant compare this with an I6. A inline 4 has a significantly higher crank resonant frequency to an I6. This is a direct result of a significantly shorter and therfore stiffer crank, and this is the major factor in torsional vibration susceptability. In addition the I4 recieves 2 excitation impulse per revolution whilst the I6 receives three.

What I do agree with is that a lighter balancer will posses less inertia and allow the engine to build revs significantly quicker. However one must balance this with crank/bearing lifetime which may be significantly reduced.