The flat six and the Roots blower is a match made in heaven.

These images show the LaMonster kit with the old style manifold.  Technical information on LaMonster’s blower is available from Eaton, the OEM manufacturer of the supercharger, and Magnacharger, the OEM for the LaMonster kit.

Lamont was able to coax 128 HP / 111 LBS-FT at the real wheel with this second-generation system, using a glasspack-modified stock exhaust, with Lamont predicting additional horsepower gains with additional tuning.

Lamont found some of that additional horsepower (up to 138) by switching to a Thunder Manufacturing exhaust and having some headwork done.  The glasspack modification adds a couple of ponies, but still retains the skinny headers and lack of true collector of the stock system.  I believe that replacing the stock exhaust is a key to unlocking the potential of the supercharger system.

My suspicions regarding the stock exhaust as a bottleneck seem to be confirmed by V-Man, who has reported a whopping 142 HP on his stock 28,000 mile Interstate with the only modifications being a TBR slashcut exhaust system.  That is nearly a 50% power gain!  Very impressive results for a non-intercooled, Roots-type supercharger.  Click here for details.

Jeff Kozlowski recently posted 165 HP with VMC pipes and heads!  Once again, extremely impressive results for a non-intercooled, Roots-type supercharger.  Click here for details.

I am contemplating construction of an air-to-water intercooler, like the one pictured here, for the LaMonster kit.  Although designed for a larger Eaton blower, the design principles should be the same.  This article explains why an intercooler would be worth the effort.

Supercharger Basics

Here is the theory on superchargers.  Fuel makes power.  You can pump an unlimited amount of fuel into an engine.  However, you can only mix a given amount of fuel with a given amount of air (ideally 14.7 lbs. of air per 1 lb. of fuel, but there is a wider range of acceptable values).

Therefore, the key to getting power from an engine is the amount of air you get through it -- AN ENGINE IS AN AIR PUMP.  A supercharger merely shoves more air into the engine, allowing you to provide more fuel.  As succinctly stated by Downing Atlanta, makers of an Eaton M62-powered supercharger system for BMW's:

"What exactly is a supercharger doing to help make more power? Your engine, as designed by BMW, relies on the downward movement of a piston in a cylinder to suck fuel and air into the combustion chamber. The limits of power come from how much mixture can be sucked in given the time allotted by an open intake valve. By pressurizing the air, more can be forced in during a given period of time. Matching this with the appropriate amount of fuel makes more power across the board."

Normal atmospheric pressure is about 15 pounds per square inch (PSI).  So, theoretically, if a supercharger is providing 15 PSI of boost, other things being equal, the engine should make about TWICE the horsepower!

The problem is that other things are not equal.  First, the process of supercharging creates heat.  For example, even at perfect adiabatic efficiency (100%), 8 PSI of boost will heat the air 75 degrees (remember, diesel engines create combustion from the heat of compression alone).  For an explanation, click here.  However, compressors are far from perfect.  The very best centrifugal superchargers have about 85% peak efficiency.  Roots-type blowers are about 50% efficient.  The Eaton M45 blower in the LaMonster kit, which uses helixed and plastic-coated rotors, is about 60% efficient.  The net result is intake air temperatures exceeding 200 degrees!  For a chart showing the temperature rise (or "Delta T") of the M45, click here.

This temperature rise reduces charge density, which in turn reduces power (less dense air means less fuel can be introduced to the engine).  As reported above, The LaMonster kit is presently producing approximately 35-50% horsepower gains.  This is outstanding, indicating that the LaMonster kit is performing near the maximum theoretical performance for a Roots-type blower at 6-8 PSI.

Roots-type vs. Centrifugal

A Roots-type blower is a so-called "positive displacement" device, using twin parallel lobes to pump air.  Roots-type blowers are well suited to a street-driven, high torque application like the Valkyrie.  Drag racers of stock-block machines typically use centrifugal superchargers (since no one currently produces a large enough centrifugal, Roots-type blowers are still the superchargers of choice for monster top fuel motors).

Centrifugal superchargers use a turbine-type compressor.  As described above, centrifugal superchargers have superior adiabatic efficiency, which means they can provide more horsepower per PSI of boost (their higher charge density allows more fuel and their cooler air allows more ignition advance).  Additionally, centrifugals provide a relatively linear boost curve, meaning they typically provide greater power gains than a Roots-type blower at the upper RPM ranges.

However, centrifugal superchargers, like turbos, don't provide much power right off the line.  Centrifugals make linear boost, meaning that boost rises linearly with RPMs.  On the other hand, positive displacement blowers are usually delivering nearly full boost by about 2,000 RPMs, giving that "big-block" kind of torque feeling (great readings on the "butt dyno").  This early delivery of boost makes a Roots-type blower ideal for cruising in the Valk.  Instant throttle response and handfuls of torque -- the perfect power adder for the perfect cruiser!  Once again, Downing Atlanta's description is right on the money:

"The key to acceleration performance is the area under the torque curve. With a boost level coming in so low and staying strong up to redline, the entire torque curve is moved drastically upward with [an Eaton] supercharger. At every rpm point, the engine can do more "work" (i.e. accelerating the mass of the vehicle). When you only spend a few seconds in each gear, making each rpm segment do more work means much greater and more satisfying acceleration. You will never need to wait for your engine to "get up on the cam" with this sort of system. Exiting corners from a hot apex become an instantaneous exercise in motion. You go forward right now, right when you open the throttle. Launching from a standing start begins strongly from the first rotation of the tires. Passing on the road occurs immediately - you are up to speed instantly."

The Problem of Heat

As shown above, even theoretically perfect compression creates heat.  Real-world, imperfect compression creates even more heat.  While this heat does not, in and of itself, damage the engine, it produces numerous power-robbing side effects.  If the boost is so high that autoignition (AKA "pinging," "detonation," "preignition," etc.) is occurring, there are several ways to deal with the issue, individually or in combination, but you must deal with it!  There is some evidence that at least one person running the LaMonster kit has sustained severe piston damage from autoignition.

First, one can reduce the ignition advance ("retard the timing").  This, however, reduces power.  Nonetheless, an autoigniting engine is not making optimum power, so the net result of reducing the timing may actually increase the power on a blown engine.

Second, you can switch to a higher octane fuel, which provides more resistance to autoignition.  Since, however, almost all supercharger systems already require the use of premium, this means upgrading to racing fuel, which is prohibitively expensive and not easily available.

Third, you can use water injection.  The extra plumbing and storage tank required, however, make this option a nonstarter on a motorcycle.

Fourth, you can simply reduce the boost until the intake temperature gets back down to a level not causing harmful levels of autoignition.  This obviously has a negative impact on performance.

Finally, you can intercool . . .

Intercooling Basics

Intercooling is an ingenious way to reduce the heat of compression.  The idea is simple--introduce a heat exchanger into the airflow between the supercharger outlet and the engine intake.  These heat exchangers look and operate much like a radiator, with the compressed air being flowed through the exchanger, which sheds heat with fins.  The radiator part of an intercooler system is usually called the "core."

There are two types of intercoolers.  The first type is air-to-air.  In an air-to-air intercooler, the outflow from the supercharger is routed through a large core which is exposed to ambient airflow.

The second type of intercooler is air-to-water.  In an air-to-water intercooler, the air from the supercharger is cooled by flowing water, rather than ambient air, around the core.  This water is contained in a closed loop.  On the other end of the loop is another heat exchanger, which is cooled by ambient air flowing across it.  So the heat from the supercharger is transferred to the water, which is then transferred to the air by the second exchanger.

Air-to-air intercoolers are typically more effective than air-to-water.  The reason is that the effectiveness of an air-to-water intercooler is a product of the individual effectiveness of the two heat exchangers required.  Air-to-water intercoolers are preferred in two applications: (1) drag racing, where the water can be substituted with ice water, and (2) where there is not enough space for an air-to-air in the engine bay.  Otherwise, air-to-air intercoolers are superior for street use.

The Benefits of Intercooling

The benefits of intercooling are multifold: (1) the lower intake temperature results in higher charge density, which makes more power; (2) it allows increased ignition advance, which makes more power; and, (3) most importantly, allows the use of higher boost levels, which – you guessed it – makes MORE POWER!  Even without increasing the boost, a properly sized intercooler on the Valk should be good for 10-15% more horsepower and allow full stock advance.  With an increase in boost from a smaller blower pulley (to, say, 10-12 PSI, as measured at the intake manifold), there may be another 10-25% horsepower increase.  So, the introduction of an intercooler might reasonably be expected to add another 20-40% (19-38 HP) gain over the stock LaMonster kit, bringing the total HP up to somewhere between 145 and 163!  Recall that 180-190 HP would be a doubling of horsepower.

The primary engineering challenge will be to provide maximum cooler core surface area in the manifold while not raising the blower up so high as to squeeze the carb into the bottom of the tank.  The core must be sandwiched in between the blower and the manifold.  As shown in the photograph below, there is not much room.

Compare the LaMonster manifold with the intercooler core made by Thrasher EP for the GM supercharged 3.8 (which, BTW, also uses an Eaton blower):

Although the 3.8 is a much larger engine, this photo gives an idea of the minimum core surface area required.  Perhaps a slightly taller manifold (or a spacer) with integral cooling cores could be designed.  Note how the Allen Engineering kit below uses a cutout in the manifold rather than a spacer like the Thrasher kit.

A second engineering (and aesthetic) challenge would be location of the second heat exchanger.  A small electric pump would provide circulation to the external heat exchanger.  While it could be added behind the stock radiator, this would expose it to heated air.  It could also be added underneath the stock radiator, but this may not be sufficiently attractive.  Perhaps it could be located underneath the bike, in the swingarm area, maybe adding a small deflector to direct air into the cooler.  Another possibility would be to place the second heat exchanger in front of the timing belt covers.  You can also see from the image above that the water-to-air exchanger does not have to be all that large (the Allen system is for a 4.6 Ford V8).

The cost?  Probably on the North side of $1,000 (higher to retrofit an existing installation).  However, if an intercooler could be designed to actually produce 20-30 additional HP, the cost per HP would be far less than the blower itself.

UPDATE:

Lamont recently posted some intriguing photos of a different manifold:

More photos of the new manifold can be found here.  Although this new manifold looks great, it would probably be even more difficult to retrofit an intercooler into.  It looks like a spacer between the blower and manifold with a core inside would be the only way to go, but the vertical limitations still apply.  Perhaps the most elegant solution would be to scrap the carb and go with EFI.  The throttle body would be smaller than a carb and could be mounted in some location that would allow enough extra vertical clearance to accommodate a spacer.

Dick Brandt has engineered an EFI system for his blown Valkyrie.  Very nice.  Dick and others are currently working on some sort of intercooling (or perhaps pre-cooling) solution.