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:

1.) Cruisers and 2.) 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