[DeTomaso] 408 Stroker Life Expectancy

[Daniel C Jones]

I've been asked to respond to the 351C stroker post.  I don't have
the time rght now to go into a lengthy post on every issue raised
but here's a quick take on the issue:

There's little difference in strength between production 351C and
351W blocks.  The blocks weigh about the same with similar wall
thickness (as tested by a local engine builder and Engine Masters
contestant who coincidentally entered the EMC with a production
351C block).  Given the stuff that needs to be changed to switch
to a Windsor block or Windsor engine, don't bother with a production
block.  If you need to switch blocks because you are making too
much power or turning too much RPM for the stock block move to a
hybrid race block.

The 351W has a 0.3" taller deck than a 351C which allows it run
a 0.1 to 0.15" longer stroke than a 351C for a few extra cubes,
all else equal.  4" stroke with 6" rods for 408 cubes in an 0.030"
over 351C or 4.1" stroke with 6.2" rods for 418 cubes in an 0.030"
over 351W are reasonable limits.  Race blocks may permit larger
bores for more cubes.

Duty cycle is as important as the peak power.  If the engine is
used in a street car that uses the peak power/RPM for only brief
periods, the engine will last much longer than one than a race
motor that spends it's life at WOT or one that sees drag slick
shock loads.  If you run it like a race motor, you have to maintain
it like a race motor.  Doesn't matter whether it's a 377, 393 or
408.  Keep an eye on detonation, oil temperature, bearing wear and
do a leak down test from time-to-time.  Tune the carb with a wide
band, examine your spark plugs (all of them) under a lighted
magnifying glass for specks of aluminum, check your oil filter
element for bearing wear, etc.

There is no one magic stroke.  From 3" to 4" stroke, it's just a
point on a continuum.  Pick what works best for your particular
application.  Lets put things in perspective here.  It's easy to
calculate the piston speed:

 MPS = STROKE * RPM / 6
 RPM = MPS * 6 / STROKE

where:

 MPS = mean piston speed in ft/min
 STROKE = crank stroke in inches
 RPM = engine speed in revolutions/minute
 6 = conversion factor (one-half of one foot expressed in inches)

For a 4" stroke at 6500 RPM:

 MPS = STROKE * RPM / 6 = 4 * 6500/6 = 4333.3 ft/min

For a 3.7" stroke, that piston speed equates to:

 RPM = MPS * 6 / STROKE = 4333.3 * 6 /3.7 = 7027 RPM

So a 408 at 6000 to 6500 RPM is like a 377 at 6500 to 7000 RPM.
Not a huge difference for most of us but may be important for
someone running open road races at elevated RPM.

In the Dember 2005 issue of Popular Hot Rodding ("The Outer Limits",
page 73) David Vizard suggests the following maximum Mean Pistons
Speeds:

 Factory cast iron cranks             3750 ft/min
 Aftermarket cast-steel cranks        4500 ft/min
 Factory forged cranks                4600 ft/min
 Budget aftermarket forged cranks     4800 ft/min
 Typical race aftermarket cranks      5500 ft/min
 High dollar custom endurance cranks  6000 ft/min
 Pro Stock Mountain Motors            7500 ft/min
 Formula One                          7500+ ft/min

The rod to stroke ratio for the 408C with 6" rods and a 1.2"
pin height is = 6.0/4.0 = 1.5.  A r/s of 1.5 is on the low end of
production Detroit V8's (slightly less than a Chevy 454 but slightly
more than a Chevy 400), so should be fine for most applications.
For reference, here are a few production V8's in the same range:

                stroke  rod length  r/s
Olds 400-455    4.25    6.735       1.58
Ford 460        3.85    6.06        1.57
Pontiac 455     4.21    6.625       1.57
Ford 300 Six    4.00    6.21        1.55
Chevy 454       4.00    6.13        1.53
Chevy 400       3.75    5.56        1.48
Ford 351C       3.50    5.778       1.65

piston pin height = deck height - (rod length + crank stroke/2)

For a 408C with 6" rods, the pin height iss 1.2".  The typical 347
SBF strokers have pin heights in the 1.1" region and I'm told (but
have not verified) that Ford's modular V8's have pin heights in
the 1.2", so all of the above pin heights should be fine.
The ring intersecting with the wrist pin is no big deal but if you're
worried about, drop back to a 3.85" stroke.  If RPM is more important,
do a 3.7" stroke or stay at 3.5" (or even drop back to the 3.0"
stroke of a 302C).

Longer stroke will tend to wear the cylinders more quickly due
to higher side loading and higher piston speed at a given RPM.
However, you can make the same power at a lower RPM which can
offset the effect.  Wear at the bottom of the cylinder won't
necessarily be any better with a thicker cylinder wall.  The wear
comes from piston rocking.

Dan Jones