Subject: Re: crank pulley


On Mon, 28 Nov 1994 tlowery@seamail.nos.noaa.gov wrote:

> >I have had the 3.1L up and running for about 6 mos or so and am having
> >an exhaustive amount of trouble with the crank pulley.  First all the
> >pullies that I have had on it before (3) have all separated where they are
> >joined.  That is to say it is the type with two pully grooves, and where
> >the two pullies are joined they come apart (they are put together with rubber
> >don't you know).  Anyway, I finally found one that didn't separate and it

> It is my understanding (second hand info here) that such problems may be due to 
> lack of balance especially in the crank.  I assume your 3.1L is a stroker that 
> may have some radical rod angles setting up additional vibration.  The reason 
> why I happen to be able to relay this is that I'm about to build (with help 
> from experienced builder) a 3.05 L engine.  My builder recommended putting a 
> fluid harmonic balancer on my stroker in addition to balancing everything 
> excessively. Otherwise, he says it could shake the harmonic balancer off and 
> possibly self-destruct due to some weird harmonic disruption  stuff I don't 
> understand.
> 
> Anyway, a well balanced crank and reasonable rod angles may solve some of your 
> problem.

The other thing is to get a lighter flywheel. On th 2.8 race motors they 
had problems with the crank breaking and the solution they came up with 
was a light flyheel and a heavy damper (harmonic balancer). The rubber is 
ther so that the thwo parts of the damper can move independantly (a 
little). The innertia of the outer pulley will resist the 
acceleration/decceleration pulses of the crank, while the innner pulley 
is locked with the crank and must floolw those pulses. The innertia of 
the outer pulley helps to counteract those pulses . If the pulses are of 
sufficent magnatude, they will put more load into the rubber than it can 
stand, and the rubber will fail. You have a much larger pulse (due to the 
added stroke) and the rubber may just not be able to handle it. I would 
go with a Fluid Damper, or one of the other aftermarket performance 
dampers, but know that they are expensive. By lightening everything, 
(rods, pistons, flywheel, clutch, crank if possible) and then getting it 
all properly balanced (and this is hard, for motors where balance is 
critical we have to ship our stuff to Portland which is 2 hrs away, there 
are local places that will "balance" but they just don't do a good enough 
job), you will reduce the strain on the damper. Oh yeah, if you're 
running an aluminum flywheel, we've seen these fail when the flywheel to 
crank bolt washer star to imbed themselves into the soft aluminum. As a 
fix I recommend using the center plate from an automatic. It has the 6 
bolt pattern in it and if you use it instead of the individual washers, 
you will spread the loading out better. That way the plate won't imbed 
itself in the aluminum and everything will hold together better.

Marc Sayer
Performance Engineering
1070 W. 2nd Ave. 
Eugene OR 97402
(503) 484-0904  Fax 746-0863
msayer@efn.org