Date: Wed, 25 Feb 1998 08:58:12 -0800
From: Robert Harris 
Subject: Long Rods - NOT for street
To: Dave Williams 

For many reasons, after Heywood, Jenkins and long thought, I have come to
the conclusion that longer rods DO NOT "MAKE" MORE power - they have LESS
loss's.  Shortening the rod has many things going for it and in fact should
make more low and mid range and even upper end power than longer rods -
except for the loss's.

Assume only the rod length changes - all weights, bore stroke, offset etc.
are identical.

The results are "trends" as Jenkins asserts - meaning that over a number of
engines, the results will trend to follow the pattern - depending on how
near optimum the original rod length was and if the induction and cam are
properly matched. I now understand how you can deliberately mismatch to
either peak or broaden power range.

First, changing the length changes the velocity distribution, with longer
rods slowing the velocity near top dead center and shorter ones slowing near
bottom dead center.  Acceleration is a power factor of mass and velocity.
The short rods have to "give up" more energy as they approach TDC, also
increasing tensile load (rod stretch) and absorbing engine power to stop,
whereas the longer rods "return" more energy as they approach BDC.  The
accelerations away from both TDC and BDC are a wash.

Next, the friction loss including slap is relative to the crank angle.
Reducing the angle reduces the loss's - advantage long rod.

From all that I have seen, the longer the rod, the less the LOSS's.

But, after giving the longer rod the lower loss's - which are exponentially
related to rpm, Let us look at why shorter rods make more power.

Power is made by the expansion of combustion gasses. The higher the
expansion ratio, the longer the expansion time (in degrees), the more power
is made. Slowing the piston near BDC increases the expansion time
significantly. Opening the valve at the same point will increase the
blowdown time and reduce the pumping loss's.  Delaying opening the valve
will increase the effective expansion ratio, and if about the same as
before, still have the same exhaust pumping loss.  At the tail end
(scavenge) portion, the higher piston speed near TDC will have imparted a
higher exhaust gas velocity, thus improving scavenge effects.

For the intake, as the valve is opening, and the piston passes TDC, it sees
a higher piston speed and since air flow is directly related up to piston
speed up to a high fraction of Mach 1 (the limiting speed of air flow), it
pulls a larger, faster charge in. Conservation of energy, inertia, ramming
continues to fill the cylinder near BDC up the compression stroke till the
valve closes.  Note that the cylinder is at maximum volume (highest vacuum)
for a longer period of time allowing better filling and more ram advantage.

The short rod accelerates compression near TDC.  The slower initial rise and
steeper ending rise result in the charge spending less time under high
pressure, greater compression induced swirl and less likelihood of
detonation.  The greater turbulence improves the burn speed and
characteristics - making more power.

So what we have is better filling, better combustion, better expansion,
better scavenging at the cost of increased HIGH RPM loss's.

Next some compensation for the "loss" of dwell time at TDC.  Some very large
percentage of the power is made in the first 15 to 20 degrees after TDC and
most of the gas is burned also. So, simply raise the compression ratio such
that the chamber volume at this point is the same. That's actually overkill,
but what the heck.  If the volume is the same, the pressure applied to the
piston is the same si?  The shorter rod likes more compression anyway, and
more geometric compression means more expansion and more compression and
faster compression makes a more better fire - so no biggee.

The shorter rod MAY allow for a thicker ring stack, so take advantage of
spreading the rings out and reducing the rocking and loss associated.

Offset boring can be used to wipe out the angle and slap loss's, leaving us
with only the acceleration loss's.  Strange how long rods were not a "thing"
until heavy cross pollination from NASCAR big ovals arrived?  Could it be
more related to that tensive,compressive thing and weight of heavy pistons
for endurance brought the numbers to a significant level?  Maybe for the
rest of us, a titanium piston pin and a light weight piston would erase the
difference.

Then we could start to optimize the cam timing intake etc.  Heywoods has
been a help - if you read it and keep an open mind and not self delude.  He
tends to lay out a lot of stuff - professor like, and you need a non dweebie
mind to see how it fits with what you really know.



Date: Thu, 26 Feb 1998 10:30:13 -0800
From: Robert Harris 
Subject: RE: Long Rods - NOT for street
To: Dave Williams 

Heywood defines friction to be the difference between the work delivered to
the piston while the working fluid is contained within the cylinder (i.e.
during the compression and expansion strokes) and the usable work delivered
to the drive shaft.

Three parts - pumping work

Motion loss's

Accessory loss's

all of which are dissipated as heat.

He then backs you up by saying

"An approximate breakdown of rubbing and accessory friction is: piston
assembly 50%, valve train 25%, crankshaft bearings 10%, accessories 15 %."

Can't be too unhappy with a man who believes as you do and tries to get the
dweebs to agree.

Pumping loss's greatly increase by increasing compression and throttling,
which is why just adding "dead" air EGR hepps a bunch.  The loss is in
"sucking" it in, as most of the compression of it is returned during
expansion.  More argument for part throttle massive EGR - it greatly reduces
pumping loss's associated with throttling and allows closer running to
diesel like manner.

Jenkins on rod relationship's  Shorter rod ratio needs Larger Intake Volume
and Greater Lobe Angle.  Understand the intake volume part, still trying to
understand lobe angle.

Using the 302 as a baseline of "acceptable" rod length, it works out 1.66
Rod to stroke or 3/5 ths stroke to rod.  The 3.8 with a 6" rod has a longer
ratio.  Stroking it to 3.6 inches and retaining the 6" rod would give it the
same ratio as a 302.

So far, Buick 3.8 pistons fit, Olds 307 V-8 1980-1990, Chevy 250,292
(3.875), Buick 350 V-8 - 68-81, Chevy 262 truck and 265 V-8 - oversize 3.75,
Chevy 305 V-8 3.736 (+60), Chrysler V-6 3.5 liter 3.7795 with dome high
compression (90's), Chrysler V6-230 3.775, Ford V-8 -260, Ford V8 -312,
Buick/olds V6-181 (82-87).  Not a real large selection.

Would be nice if it took a 3.875 piston - or even better yet a 4.00 but
.....

Thinking stroking the shit out of a V-6 and shortening the rod, using light
piston and titanium pin.  Get piston Moly Coated to reduce loss's  (really
like the piston shape I saw on an Olds Piston I saw).  Compression and turbo
the shit out of it.


Let me know when the head comes off. Really interested in the chamber
design.  Any clue if it will take a bore of 3.875???

After you get the book, let me know then I can describe next generation
exhaust.

-----Original Message-----
From: Dave Williams [mailto:dave.williams@chaos.lrk.ar.us]
Sent: Wednesday, February 25, 1998 5:33 PM
To: bob@bobthecomputerguy.com
Subject: Long Rods - NOT for street

-> For many reasons, after Heywood, Jenkins and long thought, I have
-> come to the conclusion that longer rods DO NOT "MAKE" MORE power -
-> they have LESS loss's.  Shortening the rod has many things going for
-> it and in fact should make more low and mid range and even upper end
-> power than longer rods - except for the loss's.

True.  But cylinder-to-wall friction is the greatest single power loss
in the engine, unless you Believe in the "lost energy out the tailpipe"
crap.  Prior to the 1960s most engines were running rod ratios of 2:1 to
2.5:1, with my oldest engineering references claiming 2:1 to 3:1 as the
optimal range between practicality and side load.