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Cutting down Harmonic Crank Pulley
jimgiblett - 26/3/10 at 10:46 AM

My SR20DET engine has a triple groove pulley the inner (nearest the block) runs the water pump and the alternator the other two are for powersteering and aircon pumps

As I won't be fitting powersteering or aircon I could dispense with these grooves by chopping them off on the lathe.

I am told that the pulley wheel is harmonically damped (I assume there is a rubber sleeve in there even though it is not very visible if it is).

So the question is if I remove roughly 50% of the rotating mass will it in any way adversely effect the harmonics/damping?

Cheers

Jim

britishtrident - 26/3/10 at 11:40 AM

It is the rotational inertia of the mass of the outer ring of the pulley in conjunction of the elasticity the rubber that creates the torsional damping.

The rotational inertia of the damper and elasticity of the rubber bonding ring are chosen to match the critical vibration frequency (i.e. critical vibration RPM band) of the crankshaft.

Reduce the rotational inertia and you will increase the frequency at which the damping is effective putting it above the rpm where it is required.

jimgiblett - 26/3/10 at 11:47 AM

Ah okay I think I understand this. Will just wire brush it and give it a coat of paint.

I just hate carrying around un-necessary weight but this necessary

Thanks

Jim


quote:
Originally posted by britishtrident
It is the rotational inertia of the mass of the outer ring of the pulley in conjunction of the elasticity the rubber that creates the torsional damping.

The rotational inertia of the damper and elasticity of the rubber bonding ring are chosen to match the critical vibration frequency (i.e. critical vibration RPM band) of the crankshaft.

Reduce the rotational inertia and you will increase the frequency at which the damping is effective putting it above the rpm where it is required.

tegwin - 26/3/10 at 12:07 PM

Is it not possible to machine a solid pulley to spec and ditch the damper?

flak monkey - 26/3/10 at 12:17 PM

I ditched the damper on my duratec when making an ali crank pulley...

Lots of the aftermarket engineering companies do the same for a lot of modern engines

Worst I have heard is of people snapping the nose off the crank on some engines, but that seems extremely rare.

The old pinto didnt have any form of torsional damper, neither have most engines over 15yrs old that I have seen.

Its certainly become more common over the last few years and I wonder if its due to the shock loads that cranks see in modern cars as things like the alternator and air con are ECU controlled and can just switch on at high rpm sending a significant shock through the belt?

richardlee237 - 26/3/10 at 12:23 PM

You must also remember that the rotating system includes all the components driven by the engine pulleys. the fact that you have already discarded the power steering and aircon pumps will change the rotating masses in the system.

cliftyhanger - 26/3/10 at 12:26 PM

I believe 4 cylinder cars are not too bad, but 6's are a nightmare if the harmonic damper is changed.
Saying that, I have little faith in many of the "tuning" specialists out there, they seem to make stuff with little thought for if it actually works. Or on race cars where longevity of engines is not an issue....
I would err on the side of caution

MikeRJ - 26/3/10 at 12:39 PM

quote:
Originally posted by flak monkey
Its certainly become more common over the last few years and I wonder if its due to the shock loads that cranks see in modern cars as things like the alternator and air con are ECU controlled and can just switch on at high rpm sending a significant shock through the belt?


Anything powered by a rubber belt will have some level of damping that will adsorb shock loads. I suspect the move towards cheaper nodular iron cranks compared to expensive forged items in older engines has meant more attention needs to be paid to fatigue life.

jimgiblett - 26/3/10 at 12:39 PM

quote:
Originally posted by tegwin
Is it not possible to machine a solid pulley to spec and ditch the damper?


I have been advised that I should keep the damper and once I go too far down the machining / balancing route I may as well buy an aftermarket harmonic damped crank pulley for c£250-300.

- Jim

MikeRJ - 26/3/10 at 12:40 PM

quote:
Originally posted by cliftyhanger
Saying that, I have little faith in many of the "tuning" specialists out there, they seem to make stuff with little thought for if it actually works.


As long as it's anodised in a pretty colour, drilled for lightness with a hefty price tag it must be good, right?

jimgiblett - 26/3/10 at 12:45 PM

If this is the case then the tension and load on the the belts will be quite critical.

The drag will be higher and lower whether the accesories are loaded or not (eg turning a corner or switching the aircon on. I cannot see how this changes rotational mass of the crank pulley and if it does it is going to be very variable.

I am confused again.

- Jim



quote:
Originally posted by richardlee237
You must also remember that the rotating system includes all the components driven by the engine pulleys. the fact that you have already discarded the power steering and aircon pumps will change the rotating masses in the system.

Russell - 26/3/10 at 01:34 PM

Is there a poverty spec version of this engine used in another model? You can often get a pulley that doesn't have the section for the aircon. May be trickier to lose the power steering though.

BenB - 26/3/10 at 01:46 PM

quote:
Originally posted by MikeRJ
quote:
Originally posted by cliftyhanger
Saying that, I have little faith in many of the "tuning" specialists out there, they seem to make stuff with little thought for if it actually works.


As long as it's anodised in a pretty colour, drilled for lightness with a hefty price tag it must be good, right?


Oh yes! Was reading the Merlin motorsport catalogue last night. Loved the billet ali fuel filter holder. Chunky thing. Probably weighs more than a couple of large jubilee clips and some rivets and costs £45 to boot. Looks fast though.....

franky - 26/3/10 at 02:05 PM

I've got the same issue on mine, i'm just going to clean up the standard one, as I was going to put it in my lathe and remove the bit thats not needed with a parting off tool.

I've removed aircon/pas/clean air pump.

Also I wonder how much bhp this will liberate? another 5-10?