Re: stroker cranks

[Dan Bubb <jdbubb@ix.netcom.com>]

Cr=(D+V)/V
Where Cr = compression ratio
D=cylinder SWEPT volume (displacement of one cylinder, e.g 496cc for a
VW 2.0)
V=volume of the combustion chamber with the piston at TDC. This includes
volume in the head, volume of any cutouts in the piston and volume due
to gasket thickness.
Rearranging the above equation we get:
V=D/(Cr-1)

So, if a 2.0 has a Cr of 10.8 then V=496/(10.8-1)=50.61cc

If the displacement is increased to 2042cc then D=2042/4=510.5cc
Then Cr=(510.5+50.61)/50.61=11.09Cr

As I said this assumes the piston height is unchanged at TDC after
stroking either by using a shorter rod or raising the pin height in the
piston and that the piston crown shape does not change.
Dan



"Fry, Larry" wrote:
> 
> Dan-
>   I haven't done the calculations, but only .3 increase in CR doesn't sound
> right.  Are you sure?
> How did you calculate?
> 
> Larry
> 
> -----Original Message-----
> From: Dan Bubb [mailto:jdbubb@ix.netcom.com]
> Sent: Thursday, November 08, 2001 10:11 AM
> To: Fry, Larry
> Cc: Jonathan Pack; Randy B
> Subject: Re: stroker cranks
> 
> There is no doubt that stroking a VW engine without changing rod length
> or piston pin height will NOT work. The piston comes within .040" of the
> head on an 8V engine. Accounting for rod and piston stretch at 6500 RPM
> will take  this down to about ZERO. Having a minimal piston to head
> clearance is generally a good idea if your combustion chamber has squish
> bands (and both 8V and 16V do) or the squish bands will not work to
> promote turbulence.
> So, you will have to get shorter rods (standard length is 146 I believe,
> so length would need to be 144.65) and make sure the bottom of the
> piston clears the counterweights at BDC, or raise the pin height in the
> piston by 1.35mm. With no changes in deck clearance or piston crown
> shape and assuming the CR is 10.8 stock, the CR strictly due to change
> in displacement will be 11.1.
> Dan
> 
> "Fry, Larry" wrote:
> >
> > Jonathon's method is the most correct.  Potterman's doesn't factor in
> number
> > of cylinders, so you couldn't use it universally.
> > And, yes, Scott, stroking an other-wise-unchanged engine will most
> > definately increase the compression ratio, probably to right up there with
> > the diesels IF, IF, IF the pistons don't hit the head.
> >
> > Larry  sandiego16V
> >
> > -----Original Message-----
> > From: Jonathan Pack [mailto:pack.39@osu.edu]
> > Sent: Wednesday, November 07, 2001 9:36 PM
> > To: Randy B; Scott F. Williams
> > Cc: scirocco-l@scirocco.org
> > Subject: Re: stroker cranks
> >
> > First, I would be afraid of piston-to-head interference.  The stock
> pistons
> > come all the way to the top of the cylinders with 92.8 stroke.  Once they
> go
> > 1.15mm farther...I don't know.
> >
> > Second, this would substantially increase CR.  If you know chamber size
> and
> > the volume of the cutouts for the valves on the pistons, you can calculate
> > how much defference it will make.
> >
> > Third, the formula for volume of a cylinder is pi*h*r^2.  In engine terms,
> > this means pi*stroke*(bore/2)^2 is displacement of each cylinder in mm^3.
> > Divide that by 1000 to get displacement in cm^3.  Multiply by number of
> > cylinders and you've got it.  Somebody else posted a formula that squared
> > bore (diameter) instead of (bore/2), which is the radius, and what you
> > square to find area of a circle.
> >
> > Fourth, TDI cranks are pretty spendy.  Kudos to you if you have the money
> to
> > buy one.
> >
> > Jonathan
> >
> 
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