mpgmotors.com Blog

Bikes and pipes

January 16, 2008 on 11:08 pm | In Maintenance |

The following makes no claims of legality of modifications or the affect on warranties. It is just a discussion of the effects on the engine after piping a bike.

Piping a sport bike or cruiser is the most common question we get about upgrades for our motorcycles. That question always makes me nervous particularly if the next question is, “How much does it cost?” Owners pipe their bikes but rarely do the other things they need to do it correctly. Bad things don’t automatically occur, but you can almost never just “change one thing”!

Carbureted or fuel-injected, it doesn’t matter. If you change the exhaust for more power, to change the sound, to make it look better - you probably changed the amount of back-pressure within the muffler and the gas flow through the motor. Whatever it was, whether you like it or not, you changed the flow through the motor.

The cylinder(s) in your stock engine originally got a certain amount of fuel and air mixed together and then burned to create, among other things, exhaust gases, noise and power. The engineer was looking to achieve a ratio of air and fuel somewhere around 14 to 1.

Let’s look at what’s changed by modifying or replacing a pipe… You put in a pipe that is almost always louder and usually makes a claim of reduced back pressure. Fine, it probably does. But what did we really do by installing a pipe with less back pressure? We allowed more gases to flow easily out of the motor. That will pull in more air and a small amount of fuel too. More air than fuel, however. Also, back pressure from the muffler can hold intake gases in the cylinder effectively richening the mixture at lower rpms. Either effect, reducing back pressure and better flow, will lean the fuel-air mixture. Leaning the motor (assuming it was adjusted properly to begin with) increases the combustion temperature and can actually cost you power or take you into a bad area during acceleration like stumbles or hesitation or backfiring on decceleration. You can actually increase or decrease certain pollutants. In extreme cases you can hurt the motor by raising the temperature in the cylinder particularly around the hot parts - like the exhaust valves.

Let’s talk about where we live. In the Philadelphia area (and many areas now) gasoline is a blend of traditional gasoline and ethanol. Ethanol also tends to lean out combustion. It has an oxygen atom that adds air to the mixture. We rarely see rich fuel/air ratios on newer bikes from any manufacturer. They are generally acceptable but on the lean side of acceptable. The ethanol/gasoline in the Philadelphia area just isn’t helping. Take a bike that was running lean with the stock pipe, add ethanol blended fuel and is then piped - you just got yourself a real hot motor.

Please take a look here:

 http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Most of your new bike spark plugs are going to look like #23 in this picture - before you put the pipe on!

I never see electrodes like 13 and 14 (some of the “best” spark plug pictures) on a bike unless the bike was made a while ago or has been rejetted already. What we would like to see is a plug like #18 in the picture.

To pipe your bike the right way, you need to consider the effect of the pipe and adjust the fuel supply to compensate for the increased gas flow through the motor. Your spark plugs give you the best indication of what is happening in the cylinder but you need to check them correctly and, to be really picky about it, at a number of different conditions.

Here is a good discussion of what this entails:

http://hmfengineering.com/forums/showthread.php?t=32

Your traditional carburetor adjustment for an aftermarket pipe is a jet kit. On a fuel-injection motor, you need a device to remap (reprogram) the unit. Both modifications will be working towards adding fuel to cover the additional air allowed into the cylinder by the pipe. The jet kits can include new jets and a different needle. It’s a job to figure out the right jet and needle when you have your new pipe on your own bike and it’s no better than an educated guess by the jet kit developer when the pipe can be one of many pipes out there - each with a slightly different character. This is why you have to do so many plug checks to get it right. “Right” can be affected heavily by the altitude where you ride, the temperature, the humidity, phase of the moon (just kidding about that one)….

There are a lot of different variables. You got to do this right or your pipe will cost you power, pollute more and maybe even hurt the motor.