Con-rods
Once
again, there are many rods available, your standard rods (use type 311, this
will be cast into the bottom of the big end) should be good as they are for most
engines up to 100 BHP and with a 69mm stroke crank. Once you go to a longer
crank you will either require these rods to be machined or to buy a set of
previously cleared rods, the advantage of buying a set of rods is that they will
have been balanced end for end and as a set, which will help your engine's
longevity.
Like crankshafts, con-rods are available with different journals, and like the
crankshafts you have no need to change from the stock VW 55mm diameter.
Rod length is also an area to look into as this can move the power band like a
cam, higher or lower in the rev range. The length of the rod and the length of
the crankshaft stroke give the rod ratio. Rod length (mm) divided by the
crank stroke (mm) = Rod Ratio. The lower the ratio the lower the power band,
the higher the ratio the higher up the range the power band will move. As a
comparison the stock 1600cc set-up is 137/69 = 1.98, now if you wish to hill
climb you will require a lower ratio (I had a ratio of 1.76) and if you wish to
drag race a high one.
Chevy 327 rods are 145mm long, are strong and cheap (in the US), but will
require some machining as the big end is only 51 mm instead of the 55mm of the
VW, this does save on changing the crank just to match the rods. These used with
a stock length crank will give a high revving engine with a high power band,
giving a rod ratio of 2.10, ideal for a small bore drag engine. The Porsche 912
rod is a little shorter than the VW, but when it comes down to it the VW is
stronger, not a bad length and it fits. The is the key to keeping the
price down, get things that fit each other.
This is the theory of rod ratio.
A long rod will give a slower piston speed, and therefore a longer life for the
rings and cylinder walls, but this also causes the cylinders to fill up slowly
when the piston moves down during the inlet stroke. Of course, once the rpm
builds this is overcome, hence the higher power band. A shorter rod will, of
course, fill the cylinder much faster and therefore give more power at lower
engine speeds, but be limited to a lower rev range.
There are many aftermarket rods available, but like crankcases this guy sees no
reason to change to a set of rods costing more than I sell a complete recon
1600cc engine for.
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