Connect four… These vital components
connect the pistons to the crank in our four-stroke engines (and make the title
of this feature make sense). Here’s the low down on them…
The two most common designs of con-rod are
the I-beam and H-beam, both of these take their name from the shape of their
centre cross section. Cut them in half and one looks an “I” and one looks like
an “H”; pretty self-explanatory, really.
Manley
releases Turbo Tuff I Beam Steel connecting rods for R35 Nissan GTR
As a rule of thumb, the I-beam type rods
tend to be the heavier and stronger of the two designs. This is due to its
thicker centre section which gives greater resistance to compression and tension
forces. The extra weight is normally the trade-off for the extra beefiness
though, and this can work against you if you are planning a very high revving
engine as the faster the engine spins the heavier the rod effectively becomes.
The I-beam’s extra torque capacity means they are more commonly used in forced
induction applications, where high torque is generated at lower rpm and the
extra strength is needed to prevent the rod from bending or failing all
together. Typically for faster and higher-revving engines, the H-beam is used
due to its lighter weight. It achieves this lightweight by having a thinner
centre section, which would normally reduce its strength to unacceptable
levels. But the H-beam has wide side supports, which give incredible rigidity to
the design whilst keeping the overall weight down. The amount of metal saved
may not look or sound like much when compared to its sister design, but when
you add up that saving over the amount of rods in the engine, which can
normally be anywhere from four to 12, it can make a significant difference to
the weight of the engine’s rotating assembly.
Owing
to the fact that there are so many different manufacturers of con-rods, all
using their own various different designs, grades of metal and production processes,
there are exceptions to the rules when it comes to choosing between I-beam and
H-beam, but the above should give you a good starting point when choosing your
engine upgrades.
Owing to the fact that there are so many
different manufacturers of con-rods, all using their own various different
designs, grades of metal and production processes, there are exceptions to the
rules when it comes to choosing between I-beam and H-beam, but the above should
give you a good starting point when choosing your engine upgrades.
Do you need to upgrade/what type do you need?
This is a very tricky question, to answer
it we’d need to know your engine type, the type of application you’re going to
use it in (road/race/ rally etc), the target power level, the target RPM, what
other modifications the engine has… the list goes on and on. So to answer this
would take longer than waiting for Mr Morley to stop talking about drifting
and/or boobs after a few cans of Heineken.
The
H-beam design allows for a similar or improved strength over the forged I-beam
standard rods with a significantly lower weight.
The main advice is that if you’re planning
on changing your engine from naturally aspirated to forced induction, or
revving it substantially higher, then think seriously about beefing up the
bottom before going much further. If you are in any doubt about upgrades, speak
to either a professional engine builder or aftermarket rod manufacturer, they
will be able to discuss your needs and guide you to what will suit your particular
engine setup.
