Short and Dirty Rod Length Discussion Starter

["John Stricker" <jstricke@domain.elided>]

I'm catching up on some digests and someone asked about how and why rod
lengths affect power.  Well, here's a short and dirty crash course of my
understanding of it.

Now, keep in mind that while I've played with this, and seen some results,
we are delving into the mythical, almost magical range of engine design
subtleties.

I'm quite sure that over 3/4 of the list members could sit down with a clean
sheet of paper, draw an engine, take it to a CNC machine (after CADing it)
and it would run.  No big trick.  But will it be the 5.7L GM Diesel flop or
will it be the venerable small block Chevy?  That's where the subtleties
come in.

I get a lot of this from reading a TON of history on the small block Chevy
and it's first design form.  That engine was my bread and butter for quite
some time and I tried to understand it as best I could.  Be that as it may,
you can't beat physics and it just so happens that for quite some time it
was (and in many cases still is) "right".

As the little Chevy grew in size to the 327, GM needed a racing engine for
the road race circuit and the used the 4" bore with the 283 crank (3"
stroke).  This gave them a very high revving engine with a b/s ratio of
1.33:1.  The reciprocating mass was quite low.  The kept the same rods
(length wise) of 5.7".  BTW, all the SB Chevy's use the same rod length,
except the 400 oddball.

This gave them a rod/stroke length of 1.9:1.  The 327 has a stroke of 3.25",
and with the same rod length a ratio of 1.75:1.  Now at the time, if you
built a 327 and a 302, with the exact same parts except the crank and
pistons (to correct deck height) the 302 would put out the same or more
power with a FLATTER torque curve.  (The high rpm power reputation of the
302 was deserved, but mostly because of 1) the shorter stroke and 2) the
induction systems fitted at the time.)  The 350 uses the 3.484 inch stroke
crank giving a ratio of 1.636:1.  The 350 is a renowned performer, but since
it was a common hi-performance candidate, naturally there was
experimentation.  Many thought that if we could get back to that "magic"
1.9:1 ratio, un-known power could be found.  You can now buy rods for the
engine as long as 6.25" commonly (1.795:1).

Instead of increasing power, what they found was that the longer rods would
typically flatten the torque curve.  And here is why I think it happens.

Picture a single crank arm, with your hand pushing down on a connecting rod
that is very short.  Say a 4" Stroke with a 3" rod.  Now as the crankshaft
rotates on your push (the power stroke) the rod is initially almost straight
up and down but as it moves to 90 degrees rotation, what's the angle?
Pretty severe.  The downward force you are applying is being redirected with
a very pronounce outward component. This is lost power and at a specific
point in time.  Now, take off the little rod, and put on a 12" long rod.
Push down again and see how much more of the force at the 90 degree point is
now downward and not outward.

The interesting part of this is that they found that this was really only
important at certain RPM points for certain strokes lengths.  The total
power with the long rods was not a great deal more, but because the power
was more evenly distributed, it tended to flatten the torque curve.

The problem with this is that you still have only a fixed height block deck
so the piston pin location needs to be changed to maintain the deck height
of the piston.  And you can only go up so high and leave enough room for the
rings, etc.  So, there have been some interesting trade-offs.

For example, you may or may not know that the Big Block Chevy truck engine
has a taller block than the car engine.  Care to guess why?  Of course,
longer rods to flatten the torque curve.  This makes it a VERY popular block
in tractor pullers and marine engines, but is also bigger, heavier, and
requires different intake manifolds, distributors, or adapters for their
use.

As I said, these are subtleties and only really looked at when we're talking
about trying to get an extra 20 hp out of a 600hp engine (3%) which may be
enough to win the race.

A quick point here as to how we get the engine sizes we have.  For GM, the
original SB was a 265.  How did they get at that?  From what I've read, they
simply decided they needed something over 250, (their 6 cyl) and with a 3"
stroke crank being easy to jig up in the prototype, a 3.75" bore worked out.
The 265 was a little anemic, with many 250 6's out powering them so the
bored it 1/8 inch.  As time went on and the OHV V8 competition grew, they
punched it out again to 4" and upped the stroke to another round number of
3.25".  The 302 came about from needing an engine for a particular class
racing.  The 350 as an answer to the 351 Ford and 340 Mopar.  They didn't
want to retool the block so stroked it to a 350.  For all the credit we give
the "brilliant" engineers, the pattern of what is a standard size and what
is expedient for production shines through from the beginning until they
came out with the 400 in 1970 which required a complete redesign of the
entire bottom end.

If I haven't been very clear here, yell at me and I'll try to rephrase as
this is a pretty commonly misunderstood subject and for our purposes one
that is not really that big of a deal.  Sorry for the length of this, but I
think it's interesting to note how and why things come about.  The answers
are sometimes simpler than we believe.

John Stricker

jstricke@domain.elided

"I didn't spend all these years getting to the top of the food chain
just to become a vegetarian"