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by
Terry Hayden
Contents
When I first got my
Max I erroneously believed that it had all the power I would ever
want. One month later, mostly due to the bad influence of all the
power junkies on the Vmax list, I found myself searching how to coax
a few more ponies out of the beast. Part of my search led me down
the path of unrestricting the intake. Being the “technoid” that I
am, simply buying a set of filters with jet kit was too simple.
Noooo...I had to research and design my own! Luckily I was able to
acquire a second set of manifolds and carbs. This provided me an
opportunity to modify, test and basically futz around with filters
and the airbox without affecting my main ride.
In theory, making
power is simple. Increase the amount of air the engine uses each
cycle (volumetric efficiency) by decreasing the intake resistance,
and voila, more ponies! After all Max has to breathe. Unfortunately,
if done wrong, it’s also an easy way to turn a smooth running bike
into a coughing and sputtering former version of its self. There’s
more involved than simply changing filters or cutting holes in
airboxes...but once you understand the concepts that are involved
and make them work for you, the reward of “dusting” your buddy’s
stock Max can be very satisfying. All it takes is a little
understanding of how fuel and air should be mixed for combustion.
The ideal air to
fuel ratio that provides for complete combustion is called the
stochiometric ratio. This is 14.7 parts of air to 1 part of fuel. In
the real world however, nothing is ideal. Maximum power actually
requires a little bit of excess fuel to help ensure that all the
available oxygen is consumed. So when trying to spank a Toyota
Supra, for instance, Mr. Max requires a mixture of about 12:1.
So we may be
tempted to set our carbs to always run 12:1 (or rich) to have the
most power on tap. Well, it’s not quite that simple. While a
slightly rich air/fuel mixture ensures all the available oxygen is
consumed, not all the fuel is burned. A continuous supply of
unburned fuel will quickly carbon up the engine’s valves and piston
crowns. Or worse, if too much fuel is delivered to the cylinder in
raw form (unmixed), it could wash down the walls causing all sorts
of wear and eventual grief. For cruising and looking cool
(pre-spanking mode), we want reasonable performance and maximum
economy. That means a air/fuel ratio of around 15 to 16:1, or
slightly lean. For starting and cold idling however, Max is extra
thirsty and needs a “Big Gulp” of liquid hydrocarbons, mixed about
6:1.
To deliver the
various air fuel mixture requirements as required, each of the Max’s
4 Mikuni carbs have a number of inter-related fueling systems:
- a float system
- a starting
enrichment system
- a pilot or idle
metering system
- main air jet
system
- needle jet/ jet
needle system
- main jet
All though the
factory carb set-up works “fairly” well, Yamaha’s engineers are
limited by a number of factors. First, they have to keep the intake
roar down to help meet engine noise standards. This is accomplished
with a restrictive airbox intake port. Secondly, they have to meet
tougher a nd tougher emission standards each year. Over time the
carbs have been adjusted to run very lean, which is the opposite of
what we need for power. Thirdly, they must have a system that works
reasonably well in different types of locations. These range from
the cooler, high mountain scenic roads of the Yukon (picture birds
chirping) to the hot, sea level car infested freeways of Florida
(picture sweaty tourists). So they designed a system that is not
overly sensitive to changing weather conditions. Unfortunately it is
also a bit slow to respond to engine needs. We, my friends, are not
bound by the same restrictions. EPA Standards! We don’t need no
stink’n standards!
With the thoughts
of any standards banished from my mind I set out to measure how much
of an improvement I could get with a variety of different airbox
mods and filters. Without going into the details (involving a
manometer and a plenum box)
I essentially bolted a set of manifolds, carbs, and various airbox
and filter combos together and then used a powerful vacuum cleaner
to suck air through the whole set-up. I measured the tiny changes
and differences in air pressure caused by the various airboxes and
filters.
For ease of
comparison I am presenting my findings on a scale of 0 to 100 with
100 being the stock airbox and 0 being, you guessed it, 0
resistance:
Stock box w/ filter 100
Stock box w/ K&N 100
Stock box w/o filter 100
Y removed w/ filter 75
Y removed w/ K&N 68
Trimmed Lid w/ filter 62.5
Trimmed Lid w/ K&N 56
Individual filters 44
Bellmouths 43
Open carbs 37.5
Manifolds only 37.5
What the
measurements tell us:
- The stock air
box, not the filter is the point of most restriction. Adding a
high flow filter and doing nothing else will be of little benefit
- A stock airbox
is 2.25 times as restrictive as individual K&N filters
- The stock
manifolds are as restrictive as the carburetors, therefore
internal carb mods will only be slightly beneficial without work
on the manifolds as well - one recommended change is to check the
fit between the manifolds and the carb boots and remove manifold
material that sticks out into the airflow.
However before we
all run out to our bikes and rip our airboxes off expecting big big
gains similar to my numbers, lets remember a number of things.
First, I was measuring changes in air pressure not total air flow.
Secondly, my rig does not simulate the real world, often violent,
air intake pulsations of an engine. Thirdly, the theory that power
is proportional to airflow relies on the assumption that the right
amount of fuel will be properly atomized into to the air and then
successfully burned. Assuming that the proper amount of fuel will be
delivered regardless of a change in the airbox or filters is where
many home do-it-yourselfers go wrong.
As intake air
passes through the narrow part of the carb (called a venturi) it
increases in velocity. As air increases in velocity it decreases in
pressure (Bernoulli’s
principle) . This allows the
standard atmospheric pressure (14.7 psi at sea level) to push fuel
out of the float bowl into the air stream that is feeding the
cylinders. The lower pressure in the venturi also causes the carb
slide, and attached jet needle, to rise allowing more air and fuel
in to the engine. Different air velocities, created by different
engine speed and load, draw proportional amounts of fuel. In this
way, the right (rich or lean) fuel air ratio is delivered as
required.
When an airbox or
filter is made less restrictive the pressure in the carb venturi
will be greater (than the stock set-up, for the same given engine
speed and load). With the air pressure in the venturi now closer to
atmospheric pressure, less fuel will be pushed into the air stream.
Max is now receiving a lean mixture and perhaps stumbling a bit. To
further compound the situation, the Mikuni carb slide is vented at
the carb throat (oval hole at the top). Since this slide works by
using the air pressure difference between the venturi and the carb
throat it will now not lift as high, making the mixture even leaner.
Sooo... what’s the
solution? The easiest way is to buy a carb kit that has been
specifically designed to work on the Max. You will pay $100 for
about $15 worth of jet needles and slide springs. Built into the
price, however, is the cost of all that dyno testing the
manufacturer went through trying to get the carb’s to deliver the
right amount of fuel. (Kit options and how to install them could
fill another article). Or if you are willing to experiment and be
patient, you could custom make a kit. It will require a lot of road
or dyno testing. FactoryPro has a good overview of the process at
their website. As a tip, it also helps to have an exhaust analyzer
to gauge your results.
And what are the
expected results of decreasing intake resistance? Well
interestingly, based on my collection of Max dyno runs from around
the world, the gain is approximately equal to the square root of the
pressure difference. Huh? In other words, decreasing the resistance
of a stock box set-up by 50% by replacing it with a well tuned
individual filter system will likely net you about 6-8% more ponies.
Good Luck with your changes... the important thing to remember is
that more power CAN be made by unrestricting the intake system...
when combined with an appropriate adjustment of the carburetor fuel
delivery systems.
cheers, Terry
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