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now, Speed’s Performance Plus might be a familiar name. If you’ve
been to any major event around the country you’ve probably seen
the Speed’s mobile bike shop and tuning center, and about a year
ago we introduced Speed’s right in these pages. Wayne Hanson, “Speed” himself,
travels the country coast-to-coast with his sons Jamie and Jason and
they’ll set up and tune anywhere from 3,500 to 5,000 motorcycles
a year. These guys see it all and know how to make it all run like a
top. In the last issue, Speed and the team laid out their performance
formula for a strong running FI “Stage I” tuning combination
on a stock-displacement Harley TC engine. There are all sorts of intricacies
and nuances involved in the successful tuning of these modern fuel-injected
marvels and if you haven’t reviewed Part 1 in this series we suggest
you do. This time we’ll go big. What follows is the Speed’s
Performance Plus formula for success on a big-inch FI bike, the performance
setup for one of those increasingly popular 103-, 107-, 117-, 124- or
131 cubic inch engines.
As the saying goes, “There’s no replacement for displacement,” and
these big-inch V-twins are no different. These engine conversions aren’t
cheap, though, costing anywhere from $5,000 to $10,000 or more depending
on the parts chosen and how they’re used. It only makes sense to
get the most from all of that and here’s how Speed’s Performance
Plus does it.
The first thing to address as you move into the world of big-inch engines
is to make sure you’ve got everything you need for the intake-air
delivery system. Big engines are deep breathers and you’re going
to need lots more air as you feed lots more fuel into them. There’s
just no way around it. You’re now going to have to use at least
a 50 mm throttle body on the 103- and 107-inchers, and something in the
range of 56 mm, 57 mm or even 60 mm on the larger displacement engines,
the 124- and 131-inch motors. There are plenty of these larger throttle
bodies readily available from a number of suppliers and Speed’s
Performance Plus has even developed one of their own, offering it in
sizes from 48 mm to 56 mm, with special orders up to 60 mm. That’s
step Number 1. We’re dealing with a much bigger engine now, with
much bigger pistons, better cylinder heads with bigger valves and better
ports and a much more aggressive cam grind. All that needs lots more
air to perform to its full potential.
 The Dynojet
Power Commander, probably the best-known FI management system
on the market, is perfectly capable of properly and accurately
setting the air/fuel levels in these big-inch motors.
Going hand-in-glove with that larger
throttle body is the need for an air filter big enough to keep
up. A standard Stage I style high-flow air filter isn’t going to be sufficient to service a big engine
with a larger throttle body. It simply won’t move enough air. You’re
going to need something much more efficient, something like one of those
forward-mount filter kits with the big, tapered elements clamped on the
business end. Your 2006 FatBook has a lot of options to choose from.
At the very minimum you’re going to want that big engine breathing
in through a filter element that’s at least 3-inches thick. Large-displacement
engines draw large volumes of air, and without a properly sized filter
at the intake that just-installed oversize throttle body will be severely
restricted and choked down.
 The next consideration is the fuel injectors themselves. Just like that
throttle body, you’re going to want bigger ones than Harley-Davidson
mounts as stock. With the airflow now dramatically increased, a factor
of both the increased displacement and the larger throttle body with
a bigger, free-flowing filter, the OE injectors aren’t large enough
to supply all the added fuel now needed. Along with those larger injectors
the big-inch V-twin needs a stronger fuel regulator as well, a regulator
able to set the fuel pump pressure considerably higher. That stock Harley
fuel pump itself is fine; a new regulator allows the line pressure to
be raised from the stock 40-PSI range up to the now required 60-PSI or
so.
Those changes pretty well set up the intake side of things. The next
area to address with these big-displacement FI engines is the need for
a suitable, and matched, exhaust. You need a good stepped-header exhaust
system on the back end. Putting together a 124-inch motor, as an example,
and then running it through a standard 1 3/4-inch exhaust pipe–whether
it’s part of a 2-into-1 or a set of standard drag-style pipes–isn’t
going to be enough pipe to let the engine flow at the new levels it needs
to. The answer is that stepped-header system, a set of pipes starting
at that 1 3/4-inch diameter, or better yet a 1 7/8-inch diameter, and
then stepping up to 2- or 2 1/4-inches and then opening up even more
into either a collector or a pair of big, free flow mufflers, depending
on the system design. Once again it’s all about airflow. When you
open things up on the intake side it makes no sense whatsoever to choke
it back down on the exhaust.
As far as the actual performance tuning of these
big-inch FI engines go all of the fuel management devices on the market still
apply and they all work great. With these fuel-injection hardware and software
packages the performance-tune of the fuel delivery through those oversize injectors
operating at a higher pressure begins with the idle mixture and continues
on from there at 2-percent of throttle, 5-percent, 10-percent, 20-percent
and so on right up the scale, just as always. Precise adjustments are
possible at 250-RPM increments right up to 6,000 RPM and beyond giving
that big-inch engine exactly what it needs where and when it needs it.
With some of the available adjustment systems changes can simultaneously
be made to the ignition timing, also crucial to top performance.
Speed's Performance
Plus is one hoppin' place at the rallies!
One added wrinkle comes into play with these big-inch engines and their
fuel/air requirements, and that’s the startup situation. When you
begin inching up to those 124- and above displacement engines you must
have the ability to adjust the startup fuel. This doesn’t seem
to be so much of a problem on the 103s and the 107s, even the 116s, but
once you get into that 124-inch and larger range the startup fuel delivery
becomes an issue. In cooler weather the bike will really struggle to
get running, and quite often with these larger displacement engines you
might even end up having to manually play with the throttle a little
during startup, cracking the throttle open just a little to let more
air in to compensate for the added startup fuel the ECM will now deliver
through those bigger injectors. Without this manual manipulation, if
the throttle was left untouched, as is normally the case on a stock FI
bike, the starter motor will just crank and crank and crank and never
start the engine. Most of the better fuel-management tuning devices all
offer adjustment for startup fuel, however, letting you dial it back
accordingly.
What’s happening here, by way of explanation, is that the heat
sensor is telling the ECM that the bike is cool and understanding that
information the ECM automatically richens the injectors up accordingly.
Along with this, a small air-bleed opening in the throttle body is automatically
pulled open to a certain degree to compensate for that added fuel. Well,
quite often that extra opening doesn’t let in enough added air
when the injectors are larger and the fuel pressure is higher. That’s
why it’s so helpful to be able to dial back the level of added
fuel during the startup mode. All of this startup action, incidentally,
is controlled by one of the most important sensors on the engine, the
heat sensor in the cylinder head. That sensor can cause you more problems
during the life of the motorcycle than anything else you might encounter.
Hanson says he can’t tell us how many times a bike comes up to
the shop not running well or the rider is complaining about this or that
problem, and it all stems from a faulty heat sensor.
There are
plenty of choices for forward-mount filter kits in the 2006 FatBook,
like this D&M Custom Cycle Air Cleaner Kit shown here.
There’s one final big-inch tuning/setup consideration: You have
to have compression releases on these engines, and at roughly $150 or
so there’s no reason not to. On a bigger engine you’re going
to be running a lot more compression, you’ll have a lot more piston
to turn over, and a stock starter motor just won’t survive without
some kind of compression release to help it along. There are many designs
available, everything from the simple manual push-down style to the more
elaborate electronic versions. Speed’s Performance puts a set in
most every high-compression engine they build, and not only the large-displacement
ones. Any V-twin running with a compression ratio of 10.0:1 and above
requires the releases. Those compression releases really come into play
during a hot afternoon ride when, say, you just roll in off the freeway
for a quick gas stop to splash in some fuel and hit the road again. That
right there is the toughest situation you’ll ever encounter getting
one of these big-inch engines running again. Let that bike sit and cool
off and it’ll fire right up, but restarting a hot bike without
those compression releases seems just about impossible.
Big-inch engines are lots of fun. Properly equipped
and tuned they’re
rocket ships giving new meaning to the term “roll on power.” Modern
fuel injection makes it even better, and the trick, if there is one,
is all about breathing. There’s lots of swept volume here. Fill
it with the right mixture of fuel and air and then get it out quickly,
repeating as needed over and over again, and you’ll be smiling
all the way. Ride on! 
Larger
throttle bodies are available from a number of different suppliers,
including this one developed by Speed’s Performance.
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