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Small Engines For High Performance
The history of business has been one of continually increasing competitiveness, higher overhead and lower margins. Very few businesses have the luxury of such a strangle hold on their market that they can charge whatever they like for the product(s) or service(s) they provide. It has been this way since man traded his first fish for two coconuts.
By Ken Weber
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Most businesses have the same
problems and each must look for ways to remain competitive by
reducing costs and exploring new markets for existing products
And so it is in engine building. There is always
someone that is able to provide more features per dollar than
you (or at least make someone believe they can). Therefore, to
continue in business, a shop must continually make more efficient
use of its equipment and operating expenses.
More and more shops today are actively exploring
alternative markets for existing shop equipment and employee expertise.
One of these markets is the junior dragster classes that National
Hot Rod Association (NHRA) started a couple of years ago. An "all-out"
Briggs and Stratton motor can cost $4,000 and up, and a super
light chassis with all the "gee whiz" items can double
that. Add in designer driver's apparel, an enclosed trailer with
an upscale truck to tow it, and the total investment could top
However, since this is bracket racing, complete
with break outs, the aspiring Kenny Bernstein "wanna-be"
can also start at the grass roots level with a bone stock 5 hp
Briggs, centrifugal clutch instead of torque converter drive,
home built chassis, and haul it in the back of the old Chevy pickup.
This approach allows you to be as competitive
as the next guy and can probably be done for $1,000-$3,000 (if
you already own the pickup). The difference is that the $4,000
Briggs can be expected to turn 1/8th mile elapsed times in the
nine second range while the stock 5 hp will probably be in the
mid to high teens, depending on the total weight of car and driver.
One benefit of the junior dragster market is
that the parts and components for the driveline are, for the most
part, borrowed from the go kart market. So if you build these
Briggs engines, you can market in both the junior drag and karting
Another offshoot is the new Bandolero full-bodied
spec car program which seems to be patterned after the successful
legend cars. These vehicles use the Briggs Vanguard V-twin and
are reported to be sanctioned by the same INEX organization. In
karting, another engine comes into the picture, the Tecumseh Star
motor. Bigger in both physical size and displacement than the
Briggs (21.8 cu. in. versus 12.56 cu. in.) it is capable of about
20% to 30% more horsepower. However, in junior dragsters NHRA
rules specify only the Briggs be used.
Peripheral equipment is also a large segment
of the market. With every engine there must be some means of transmitting
the power to the rear wheels. Clutches or torque converters, belts,
chains, sprockets, axles and hubs are all necessary components.
Other support equipment includes a battery-powered starter instead
of the rip cord if a highly modified engine is used. The list
goes on and on. This segment of the performance market is growing
rapidly and could offer profitable business for your existing
The other day I was talking to a dad that was
thinking about getting his child a junior dragster. He had been
talking to someone who told him that "they" were getting
75 horsepower out of a Briggs motor. "Oh yeah?" I asked,
"Who's building those motors?" I then questioned him
if 6 hp per cubic inch from a 7 to 1 compression air cooled flathead
motor sounded realistic? And if it were true, I'm sure one or
more of the major automotive companies is probably conducting
around-the-clock surveillance of that person's shop.
Imagine roughly two-and-one-half times more
horsepower per cubic inch than the best Dick Maskin pro stock
motor is capable of. WOW! Think of it; a 500 cu. in. flathead
air-cooled pro stocker making 3,000 hp, or a 2,100 hp Winston
Cup car. I'll bet I could win every race of the season with that.
Jeff Gordon move over!
Realistically, though, a Briggs with a .200"
over bore and a .500" stroker (17.6 cu. in.), on a 30% load
of nitro methane will make low to mid 20s hp at 9000+ rpm. A Tecumseh
can make over 30 hp at about 8000 rpm. To get to that point in
the Briggs, however, requires major surgery, and the only original
part in either motor will be a severely modified block.
To put a .200" over piston in the Briggs
requires boring the block almost into the head bolt threads and
resleeving it. Then with the stroker crank, the rod hits the cam,
the lifters, and the bottom of the cylinder, and the rod dipper
hits the front of the crankcase requiring more custom fitting.
When you install a high lift cam, you must clearance the bottom
of the lifter bores to allow the lifters enough room to reach
maximum lift, and the bigger lobe makes for more conflict with
the connecting rod and crank counterweight.
The current trend for increasing horsepower
in these motors is to weld angled intake and exhaust runners to
the block. This is necessary because the original layout consists
of a pair of cylinders intersecting at a 90° angle with one
cylinder offset from the axis of the other; it's designed to promote
a very high swirl to the mixture. There is not enough material
in the block to simply grind away what you don't need and have
a good port.
You can achieve dramatic results by using some
epoxy in the intake ports, but to get that last 4 to 5 hp requires
the welded runners. Of course this elevates the cost of the engine
considerably. On the bright side, Briggs & Stratton Motorsports
has just announced its "Blockzilla," a new cylinder
cast out of T356-T7 alloy with heavier casting sections and angled
ports that will offer more performance potential than the production
Porting a flathead is a different can of worms
than dealing with overhead valves. In the overhead valve layout,
the name of the game is to get the mix turned from the axis of
the port to the axis of the valve so that you utilize as much
of the valve window as possible. In the flathead, it doesn't do
much good to turn the mix because the roof of the combustion chamber
sits right on top of the valve; it's like pointing a hose at a
The approach here is to simply make the short
turn smooth so that it works well during the low lift part of
the valve opening where the mix can get around and over the top
of the valve, between it and the roof of the combustion chamber.
For the higher portion of the lift, as the valve approaches the
head the flow path over the top of the valve is cut off. Consequently,
you work to get the mix to slide across under the valve and shoot
across the channel between the valve seat and the bore.
The exhaust port works the same way except
in reverse. Fairly often, in motors that have had some porting
done by the owner, you'll see that the first thing they've done
is to remove the exhaust pipe threads. There's no measurable benefit
to doing this. The most that should be done is to knock the peaks
off the threads on the bottom (long side) of the port.
In both the Briggs and Tecumseh engines, the
"as cast" exhaust port is too large. You can achieve
success with a half-round shoe in the roof (short side) to reduce
port size and effectively straighten the center line of the port.
This results in a "D" shape with the flat of the "D"
at the top. It seems to be beneficial, but it's still questionable
if the rewards are worth the time and effort to do it because
most of the development efforts have really been in other areas.
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