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Jetting

Bigger jets mean more horsepower, right?  Wrong!!!!  One of the prevailing fallacies is that larger jets mean more horsepower.  No No No No No No.

In the areas of motorcycle jetting there are two different areas of  issues:

Cruisers and Sportbikes

1.)    Cruisers

     This area is slightly more complicated from a hardware standpoint as the first thing done is the installation of loud(er) pipes or slip-on mufflers.  While slip-on mufflers are usually not as bad, the lack of baffle design in most cases means more noise but LESS FLOW.  Some of the worst offenders are Samson (racket) and Hardkrome (racket and power loss). On a VN-1500 Vulcan we put on a set of Hardkrome pipes and LOST 35% of the horsepower we had before.  I swear the design engineers must be deaf and stupid.  The only company that makes an effort in the Japanese cruiser pipe market is Vance & Hines.  Theirs make the best power and offer alternative, decently designed baffles.  A Harley   usually makes enough torque to roll right through the flat spots in the power band.

To add insult to injury, the obvious next step in the mind of the accessory salesman (sometimes the owner of the bike looking through a magazine or catalog) is a jet kit.  These invariably richen the mixture.  Now this, combined with the restricted flow through the exhaust, makes a bad situation terrible.  Fouled plugs, poor response, and excessive gas consumption result.  How can you cruise effectively when it sounds like the opening of duck season behind you?  Most of the Dynojet kits use a modification of the slide rise rate hole (done by drilling) to raise the needle faster.  This is basically an irreversible procedure, so be damn sure the jet kit is needed first.

What we recommend is consult with someone who is knowledgeable on exhaust.  This person can usually be found at a shop that has and USES a dyno.  We build our own exhaust here for our Ducati roadracers, Harley dragbikes, Alcohol Funny cars and choppers.  There are many books on the subject of exhaust design, but the one constant factor is that, especially on a single or twin, if the volume of the exhaust goes down, so does the power.  That is why there are those ugly bulging plenum chambers on factory twin cylinder exhausts.  Vance & Hines attempts to emulate this with their cross-over and most of the best-running Harley exhaust use a cross-over pipe somewhere in the system.  This tricks the motor into thinking the exhaust is bigger (volume wise) than it is and gives the exhaust pulse a place to hang out and cool off before it has to leave the system, thus reducing the flow demands on the baffling and allowing a tuning of the tone, not just decibels.

We recommend the dyno testing of the bike BEFORE any changes are made for two reasons.  One, if there are running issues they can be dealt with in within a known set of circumstances.  Two, if there are no running issues, an accurate baseline performance evaluation can be obtained.

Once the exhaust is installed a second test before any fuel system modifications are done is performed to see if the exhaust needs further tuning, or is a candidate for sawing into tinsel strips for the Christmas tree.  Hopefully your consultations with a knowledgeable seller will reduce the possibility of the need for a saw.

The shape of the curve will indicate where changes need to be made.  These changes can be in the fuel system, intake system or sometimes to the exhaust itself, to optimize the power delivery.

2.)    Sportbikes

Many customers in the past have simply followed their cruiser bretheren down the road of take off the stock, ugly, heavy exhaust system and replaced it with a complete aftermarket unit.  This would be followed by a K&N and the biggest jets they could find.  This was a marginal approach in the era of the 1976 Kawasaki

KZ-900 and completely inappropriate in the modern times of  powerplant platforms regularly producing 150 hp per liter (a Formula 1 car dream not that many years ago).  If you are not road or drag racing the bike (and I don’t mean

profiling up and down the main street) there is no reason to add anything but a slip-on muffler to the bike on anything manufactured after 1990.  The Japanese manufacturers have gone to great lengths to produce horsepower and still have a driveable combination.  Most of this driveability, oddly enough, comes from the configuration of the exhaust BEFORE the muffler.  Some of the biggest holes I’ve seen in power curves have come from complete 4-1 exhaust replacement.  These

aftermarket systems are often beautifully made and of very equal length in the header pipe area.  This is because the length is derived from equations and very little development time is needed to produce a pipe with the desired top end characteristics (cheaper to make).   It is easier to sell a pipe with an increase in top

end horsepower than one that is more productive (more area under the power curve in the rpm range of operation).  The factories are not constrained by the Federal Gov’t  in this area and have spent a LOT of time and money to get the exhaust right.  I know, having built enough pipes that were nice looking turkeys.

Very often the length, convergence angles and even diameters are different.  The most visibly apparent example of this is the Ducati 999 exhaust. Go take a look!

As in the cruisers we recommend a before-modification dyno testing, installation of the SLIP-ON muffler and retesting.  In well-designed muffler that has sufficient volume (notice how the mufflers have all been getting larger in the last 5-6 years?) the power delivery is generally improved and a nice tone obtained throughout the rpm range.  After the testing the need for a jet kit is then determined.  The hundreds of dollars needed to produce a few horsepower gain is seldom seen as reasonable.  The factories have been doing their homework and left little on the table in the way of productive external changes to the newest bikes. 

We hope this has been helpful in making your choice of both exhaust and installation facility helpful. 

© Bill Whisenant 2007
updated 11/26/2012

 

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