Solid crank pulley

[andrew_mx83]

Gday gents,

What is the go with using a solid crank pulley rather than a harmonic balancer which has rubber in it?

the 5m crank i am using in my destroked 7m is approx 15mm SHORTER in length after the #1 main bearing than the 7m jobie. the plan at this stage is to get a custom crank pulley made with a 15mm "snout" on it to meet up with the end of the crank.
i am planning to rev the thing to the moon so lightweight is also an advantage but dont want the thing to shit itself from harmonics/vibration.

all bottom end components will be balanced weight for weight then i will likely also have the whole short block dynamically balanced (still gotta investigate cost of this one)


i can machine one myself on a normal lathe, or have access to a CNC jobie which might be an idea so it can be made spot on accurate.

[Billzilla]

I'm not 100% sure, but I think with a fully counterweighted 6-cylinder crank you should be fine.

[kickstartRA28]

The purpose of a traditional harmonic damper is to protect against crank failure from torsional movement. This is not necessary in most modern engines because of the many advances in engine design and materials

[petergoudie]

Dear kickstart, can you elaborate. I thought it was to absorb secondary dynamic out of balence forces on 6-cylinder engines. Four cylinder engines don't seem to fit these although I must admit I think some 8's have them..

[Sam_Q]

this link is relevant:

http://www.dinancars.com/bmw/technia...armonic-damper

that said there are quite a few people running aluminium pulleys on engines without a problem (yet)

[Billzilla]

Dear kickstart, can you elaborate. I thought it was to absorb secondary dynamic out of balence forces on 6-cylinder engines. Four cylinder engines don't seem to fit these although I must admit I think some 8's have them..

Back to front mate. A four-pot has the nasty vibes and really should have a harmonic damper on the front of the crank, a six-pot is perfectly balanced in the primary harmonics but I'm not sure about the secondary .... though I'm pretty sure they aren't significant.
This is why you still often see many car makers offer a straight-six in some cars, as they run so smoothly.

[petergoudie]

Yes I know about the nasty four cylinder vibes but harmonic balancaers are not on all four cylinder cars but seem to be on sixes.

(I was reading that Henry Ford did not like in-line 6's because of the 60 degree crank angles and only wanted 4 or 8 because of the 90 degree symmetry.)

[lexsmaz]

As kick start said its for torsional dampening ..

I would personally use one on a motor if it came out with one, it would be a lot cheaper for a Manufacturer to put a solid pulley on a 6 cylinder engine etc but they don't & in manufacturing every dollar counts so its on for a reason & a purpose ... The crank is not going to fail straight away with a solid pulley, but long term ?????

It amuses me when people second guess the extensive work that engineers & their computers have done in calculations & designs of things ..

[ed]

4's and 8's are not symmetrical

[petergoudie]

What I think Ford was talking about was having the crank positions at 180 degrees for each pair so there is some balence (irrespectve of the angle if it is a v)

[kickstartRA28]

Everyone knows basically what they do so if you have one replace it with one, if you dont have one then you probably shouldnt, the engine has been designed & balanced not to need it. How many 4 cyl cars in the last 15-20 years have come out with anything else but a cranshaft pulley.

[andrew_mx83]

Everyone knows basically what they do so if you have one replace it with one, if you dont have one then you probably shouldnt, the engine has been designed & balanced not to need it. How many 4 cyl cars in the last 15-20 years have come out with anything else but a cranshaft pulley.

the engine is being thoroughly redesigned.... there is not a lot about it that is not being changed. as i said it will also be well and truly balanced, if it gets dynamically balanced then that involes spinning up the WHOLE short motor (from crank pulley to clutch) to revs and balancing the harmonics out of it.... someone please correct me here if im wrong.

is the torsional stress on the crank going to be a big issue? its a race only motor that will see lots of revs but not lots of km's. i was hoping to run a fairy standard crank but forged slugs and shot peaned/polished rods. is likely to have ARP rod & main bolts also.

also, on the straight 6 issue, isnt this the most naturally balanced engine layout? i had read somewhere that they are naturally balanced in the 1st, 2nd and 3rd harmonic. they are definately the smoothest in my opinion, apart from a V12. i LOVE straight sixes, everything about them

[andrew_mx83]

this link is relevant:

http://www.dinancars.com/bmw/technia...armonic-damper

that said there are quite a few people running aluminium pulleys on engines without a problem (yet)

great link mate. +rep


now how to figure out my "critical rpms" lol

[Supra967]

Just bolting the front pulley on?

How'd all the timing gear line up?

[kickstartRA28]

Everyone knows basically what they do so if you have one replace it with one, if you dont have one then you probably shouldnt, the engine has been designed & balanced not to need it. How many 4 cyl cars in the last 15-20 years have come out with anything else but a cranshaft pulley.

But in your case, as you are doing some fairly heavy modifications, I personally would use one....may not be needed but lets just call it insurance.