The small blocks have a number of design advantages. One, the bore
size on a 350 is not much smaller than that on a 455, and the bore size on
a 403 is much bigger than a 455's bore. That means these two small blocks
can use valves as big or bigger than a 455 can, and with less valve
shrouding for the 403 than the 455. Usually, the most restrictive item in
the whole intake port of an engine is the valves; this means that a small
block Olds engine's heads should be able to flow about as many cfm as a
big-block Olds, and ultimately the maximum horsepower you can get from any
engine depends only on the airflow. This means that: Gasp! A 350 or 403
Olds should be able to make as much or more horsepower than a 455, albeit
at a higher rpm.
Want more? Okay, here goes. The 455 has a longer stroke, meaning that more
horsepower is wasted in internal friction, rubbing the pistons up and down
through that longer stroke; other things being equal, then, the
small-blocks should return better gas mileage, even if built to the same
peak horsepower as a 455. The rod length/stroke ratio is bigger for a small
block than a 455; this means more piston rod angularity for a 455, meaning
still more friction with the cylinder walls, and also a lower redline
before the connecting rods break from the strain (lower rod/stroke ratios
put more strain on the piston rods by accelerating the piston in a more
jerky fashion).
Still more? Okay. The small block is lighter, so your Olds will be slightly
less nose-heavy and will handle better. Also, your Olds is more likely to
already have a 350 in it - surely Olds sold more 350 powered cars than 455's!
It seems that Chevy 350 engines routinely exceed 350 hp in totally
streetable engines, and often get nearer the 400 hp mark with good cylinder
heads. I can't see why the same power levels shouldn't be achieved with an
Olds 350 or 403! The rpm capability seems to be there, with a shorter
stroke and better rod/stroke ratio than the Chevy 350; all it would take is
getting the heads to flow well enough, and then getting the rest of the
combo right. Unless you're driving a full-size Olds that really needs the
extra torque of a 455, or a drag-race oriented car, I would think that 400
honest hp from a small block would be enough for most folks who drive
mainly on the street.
So how about it, folks? Can we get a few of you to forget your 455
obsession for a little while, and think about what could be done with a 350
or 403?
The Big Block
rebuttal.
[ Thanks to John Carri
for this information ]
As for the crank, understanding your cost constraints, the best small block
Olds crank is the forged unit from the 64-67 330 motors. This will drop
into a 350, as main and rod journals are the same size (as is stroke). You
will need to rebalance and you'll need to use an early (64-67)
flywheel/flexplate due to the crank flange bolt pattern.
With the exception of the W-31s, all Olds small blocks used dished pistons.
The high compression pistons used a "shallower" dish than the low
compression pistons. This is actually a good design, as the ideal
combustion chamber is a sphere. The Olds dished pistons come a lot closer
to approximating this sphere than most motors with domed pistons.
Interesting side note: In a period test of the 1970 W-31 in Hot Rod
Magazine, they pointed out that the oversquare Olds 350 has the best
bore/stroke ratio of any GM 350 motor, with the result that there should be
a lot of potential in this motor.
Number 5 heads were used on the W-31s. These heads were used on all 350s
of that vintage, with only the addition of the larger (2.00" intake, 1.625"
exhaust) valves from the big block. You can have your heads machined to
incorporate these larger valves.
[ Thanks to Joe Padavano for this information ]
Windowed Mains
Gasoline blocks produced in 1977 and later years were cast with holes in the main
bearing web area, hence the term windowed mains, or windowed main blocks.
Some 1977 gas blocks have solid mains. All diesel blocks have solid mains.
These windowed blocks are less strong in this area, but that should not affect
street applications.
[ Thanks to for this information ]
Power Production Comparision
330 vs 350
Actually, I don't think that the surface area of the piston has anything to
do with the increase in power; the fact that each cylinder on the 350 is
inhaling approximately 2.5 cubic inches more of air/fuel mixture every
other revolution means that there is that much more energy twisting that
crank. The power characteristics (Torque and hp curves) of two
identical-sized engines running at the same efficiency will be different if
one is big-bore/short-stroke and the other is small-bore/long-stroke, but
as far as the actual measured power output, they would be nearly the same
in peak or average hp.
The stroke of the 330 and 350 is the same, 3.39". The 330 might rev
quicker than a similarly-built 350, due to less piston weight.
[ Thanks to Bob Barry for this information ]
350 vs 403
403 RPM Potential
There is no feasable way to make this engine survive really high RPMs.
The piston to rod
angle is OK, but the piston speed inccured to do this and the weight of the
pistons will really not allow it. Not to mention the oiling system and the
valve train. You may turn it to 7000-7500 a few times, but one time it will
protest loudly. Normally taking a good third of the main webs and the crank
with it. A 350 set up properly will turn 7500-7600, but costs some duckies
to do it. A 403 isn't the best high performance engine anyway. It is good to
do a street build up to replace a 260-307 smog engine and even a 350 if you
want it to look like one and have a few more cubes.
I seriously doubt that any open main web motor would survive long at above 7500
RPM. A more realistic top rpm would be maybe 7000 RPM [U.S.] -if- you build the
thing with al the best parts. Start with forged pistons. Price a set of them,
then see what your goal for RPM is.
[ Thanks to Ken Snyder for this information. ]
Stroking a SB
If you want to increase the stroke of an Olds small block,
start with a 350 diesel block.
The main bearings are the right size for the big-block crank, and it's
strong enough to contain it. A 307's lightweight block won't last.
[ Thanks to Bob Barry for this information ]
![[ Notice: ]](hand_rig.gif)
Please refer to the
Blocks section as well!