Airbox mods and High Flow Filters

by Terry Hayden



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.

Stochiometric ratio

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:

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!

Airflow Tests

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:

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.

How to avoid going wrong - Carb basics 101:

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.

Making it work right

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