RE: <all> stroker engine

[Thi VanAusdal <tvanausdal@xxxxxxxxxxxxx>]

K.C.,

You are correct, sir.  The stock block is nearly zero deck
height (top of piston ~= top of block, or deck), so a stroker
motor requires something else to change.

The longer throw of the crank is offset by either shorter
rods (which increases rod angularity and decreases dwell
near TDC both of which affect RPM range, engine strength,
and so forth) or higher piston pins within the pistons
(new pistons = mo' money; and you're limited by how high
you can move the pin up), or both.

Yes, a stroker crank in an otherwise stock motor will push
the stock pistons on stock rods RIGHT OUT THE TOP OF THE
BLOCK.  This is hilarious, unless the engine is assembled
already.  (wow, that was Dave Barry-ish)

Example:
The late E21 320i and early E30 318i had a 1766cc motor:
89mm bore x 71 mm stroke
The 2002 and early E21 320i had a 1990cc motor:
89mm bore x 80 mm stroke

So the 2.0 has, in effect, a stroker version of the 1.8
(in fact, it was a de-stroked 2.0 in the later car, but
I digress).

The rods are the same between these motors.
How far will your piston stick out if you use the 2.0
crank in the 1.8?  9mm right?  No, 4.5mm!  Remember the
throw is half the stroke (it goes up, it goes down).
So since the distance from the crank centerline to the
deck is the same between the 1.8 and 2.0 (same block),
and the rod is the same, the 1.8 must have a piston
pin height 4.5mm lower than the 2.0.  

If you are building a HOT 2.0 (which you'd probably bore
out to 90 or 92 mm so you'll end up like 2.2 or 2.3 with
your stroker) you'd go with a stroker crank, limited by
the size of the crankcase - there's a point where the
crank throw/rod end hits the crankcase at the bottom of
the piston bores; longer rods to reduce rod angularity
for better revvability and reduced stress on the pistons
and cylinder walls; and higher-pin-height pistons to 
make the piston height correct in the block.  

Ooh, then you can pick your compression ratio, too (new
pistons, after all!), add a 292 or 304 cam (hotter cams
are tamed somewhat by larger displacement) and programmable
FI ...

At which point it costs more than a used E30 M3 motor. :)

 - thi v.

> Okay, a theory question, if you'll entertain it.
> 
> Using the M20/B27 as an example, when the piston is at TDC, there is
> precious little space in the cylinder.  If you add a stroker crank, I
> can see how it can eke out some additional displacement at BDC, but
> since the crank is symmetrical, it must also decrease clearance at
> TDC.  How exactly does this work?
> 
>    KC Boyce
>    '85 325e
>    E30 Eta Page: http://rikki.cc.colorado.edu/~k_boyce/

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