Headers and Spark Plugs

Bob Evans

Grumpy Old Admin
Staff member
A good header will mean more to you for improving exhaust flow than any porting you can do on the Briggs. The Briggs exhaust port is really too large from the factory for maximum performance. This is why you will see some open motors using exhaust port reducers(not legal in WKA stock classes). Changing headers for different track conditions or lengths is one of the most neglected areas by most karters. Don’t be afraid to try several headers for a given track. A simple change from one diameter pipe to another can move the torque band of the motor up or down in your RPM range to help you with your starts, coming off the turns or top end. Longer tracks may lead you to have a top end pipe while on a shorter bull ring , you would want something with a lot of bottom end.

The primary header tube diameter helps determines where in the RPM range peak torque for a given motor will occur. I find most people tend to “over” tube, choosing a pipe with too large a diameter. I guess it’s the old “bigger is better “ theory or “less back pressure”. While a large pipe will possibly help the motor on top end, stop and think about your racing and the tracks you frequently run on. Do you need torque at the starts or coming off the turns? If your getting killed on the starts, look closely at your clutch engagement RPM and diameter of your pipe.

The general rules are: The bigger the pipe in diameter the higher in the RPM range the peak torque will occur. Small pipes help on the bottom RPM range if all other factors remain the same. Pipe length also plays a part in the torque range. The longer the pipe is, the more it will benefit the lower RPM range. Shorter lengths help the top end. Ever noticed how short and large the Modified headers are? What we are doing here is moving the torque around in the RPM range and almost always not increasing the torque. These two variables(length and diameter) work in harmony. In theory we take a given tube size / length and go to a longer but larger diameter pipe , we have changed little in what the motor sees. One thing to keep in mind in this discussion is that I have found the longer and bigger the pipe, the less throttle response you seem to have so don’t get carried away if you start experimenting with pipes on your own.

Multi-Stage or Step Headers: What’s going on here? Step headers have become very popular in all forms of racing these days, including Briggs classes. they address the issue of trying to build a motor for a wider power band than would be possible with a single size/length pipe. A bit later , I will attempt to explain some of the theory behind header tuning based upon sound wave created by the exploding gases and the real reason the step headers work. In simple terms, using a header that steps up(or down) in size at points along it’s length will help to broaden the torque band of a given motor. If you start with a relatively small initial diameter pipe and work your way up in diameter along the pipe you will get the best of both worlds. Good low end coupled with the top end performance to need to keep you up front.

Loop Headers: Loop header attempt to use a longer length provided by looping the pipe to aid in the timing of exhaust scavenging or the ability of a header to help pull the gases from the exhaust port. If we can’t extend the end of the pipe past the bumper(most rules) then loop it to gain length. That’s right pipes help pull the gases out of the cylinder and in many cases also help start the initial fuel/air mixture into the cylinder. The Competition Cams Gecker loop pipe was the first loop pipe of recent manufacture to make this style popular. Sheldon Gecker of Comp Cams is trying to use this longer length combined with a fairly large diameter pipe to create the proper reversion wave(sound wave) going back to the exhaust port. This reversion wave, if timed correctly, will actually help draw out exhaust gases and reduce the pumping work required to expel the gases. This pumping work is the work required by the engine to push the gases out of the cylinder. If we can help this along we will leave more net power for the next engine cycle. There are quite a few loop headers out on the market today and if setup correctly they all seem to work very well. One of the best loop pipes available today is from Dover Power in Concord, NC. This pipe is very forgiving as far as setup is concerned. The Gecker works well IF you follow their setup instructions(jetting/fuel flow). RLV also makes a very popular loop pipe.

How does all of this work?? Well I may bore you here so feel free to skip to the last portion of this chapter for my general recommendations on pipe selection for a given class of motor. Still here , well here goes. Any incomplete removal of exhaust gases during the exhaust cycle reduces our overall potential power in a couple of ways. First, if we leave exhaust gases in the cylinder they will dissipate any potential intake charge, thus lessening the overall power we are capable of making during the power cycle. More fuel/air = more power. It is also possible that pressure left in the cylinder can actually push back against the intake charge when the intake valve opens. Remember we have a short period of time when both the exhaust valve and intake valve are open(overlap). One closing and one opening. Both of these problems will hurt the intake charge being pushed by the atmosphere into the cylinder.

The basic theory behind a pipe’s design involves high pressure sound waves called finite- amplitude waves. These waves are very high in frequency and tend to react a bit differently from the sound waves we can hear as humans. Without getting too involved here, the basic principles are as follows: 1) These high frequency waves move rapidly from the valve to the end of the pipe. They move at a speed much faster than the exhaust gases being expelled from the port and tend to help push the exhaust gas out of the tube. These are called positive waves. The best analogy I can think of it a series of waves in water pushing an object along such as a stick or limb. If push the water with your hand and create a series of waves aimed at the stick they will push the stick along. 2). When the positive wave reaches the end of the pipe (or to a lessor degree a step in a Multi-stage pipe) a negative wave is produced. These waves travel back toward the valve and you would think they hurt performance but..... Negative waves PULL gases rather than pushing them as positive waves do so they also help extract the gases from the cylinder. This negative wave arriving at the cylinder will create a drop in pressure. If timed correctly it can actually assist in the intake flow from the carb(remember overlap). This affect is commonly called scavenging. Two things contribute to the timing and strength of these waves. Header size and length.

Tubing size is a measure of the overall exhaust system volume. It determines the amount of restriction we have in the exhaust system. Large pipes produce lower pressures. Lower positive pressures create lower amplitude waves reducing the scavenging affect. Smaller pipes do just the opposite. They create a higher wave and help more with scavenging. The smaller also creates more restriction to flow, so you see that like most everything, it’s a trade off. All of this can get pretty complex so thank heavens we only have a single cylinder to worry about.

Tubing length directly affects WHEN the negative wave will arrive at the port. Longer tubes delay this timing and thus are good for the lower RPM ranges(restricted motors). Shorter tubes return the wave quicker and help the faster turning motors and upper RPM ranges. Keep in mine that as we turn these motors harder, the relative timing (time span) for all of the cycle events gets shorter. We have less time to fill and empty the cylinder. What Multi-stage header do is to create lower pressure negative waves at different intervals as the positive wave travels along the pipe. Something a single stage pipe will not do. It’s waiting to generate a negative wave when the positive wave reaches the end of the pipe. These minor(smaller) negative waves help the tune the pipe for a wider RPM range verses a single stage pipe. As you might suspect the timing of the larger negative wave created by reaching the end of the pipe is happening at a vary narrow RPM window. Multi-stage pipes help over come this limitation.

It gets even more complex when you factor in cam timing events(IVC, EVO, etc.), but enough for now. Follow the general guidelines below and don’t be afraid to try different pipe combinations for a given track. As with cams, no one piece is right for every situation/combination. Trying these different sizes and lengths is something you can feel in the seat of your pants and you stop watch, so you don’t need a dyno.
General Rules on Headers:
• Small initial diameter pipes will help the lower RPM torque curve

• Larger initial diameter pipes help the upper RPM ranges

• Longer pipes will aid the lower RPM curve

• Shorter pipes will help the upper RPM torque curve.

• Multi-stage pipes help spread the torque band over a wider RPM range.(more forgiving)

• Use .880 or smaller for purple plates, .910 for gold and stockers needing low end power.

• Changing cams/carbs will often require a look at the header you’re running.

• Work with jets on a new header when you get it on a given motor, as jet size changes Up/Down can be the key for a given combination.

•The best muffled pipe I’ve seen for a stocker is Robertson’s 1T-MC-XX curved pipe or their new V pipe. Dover and Rapp curved muffled pipes are excellent. As good as the Robertson. Forget the loop pipes for a muffled application.

• For un-muffled application try the Dover 02 Mini - loop

Spark Plugs:

The good thing about our methanol motors is that other than getting the correct heat range they tend not to be very sensitive to the plug chosen. There are many good manufacturers out there with plugs in the heat ranges we need. Autolite, ND and NGK are the big three you will see available at Kart shops. The Autolite 411 and ND W24 - 31FSU plugs are the most popular. I personally like the NDs as they have more plugs in our heat ranges. We have also had good success with a surface gap plug used on Mercury outboards from NGK. The BU8H. The plug is right in the 411 heat range and does not seem to foul as easily as some other surface gap plugs you can find at Wal/K Marts. The Autolite 2852 or 2892 are also a good plug of this style. With a surface gap plug you also do not have to worry about the intake valve slapping the gap closed with most of these new Slapper cams! If you choose to run a conventional plug point the open end of the gap toward the exhaust valve side of the cylinder. Simply mark the gap on the body of the plug with a Magic marker so you will know where it’s pointing when tightened down. Varying thickness gaskets are available from kart shops to help index the plug toward the correct location. Very frustrating when you forget to do this one.

Heat range of a plug is simply it’s ability to dissipate heat from the electrode. The colder plugs simply do it faster than the hotter plugs. The trick is to run the coldest plug you can without running into a fouling problem. If you are fouling plugs on new motors then go to a bit hotter range. Fouling is tough to see coming on a Methanol motor as methanol by itself leaves NO deposits at all. The nice brown/black color you see is our oils being burned in the combustion chamber. Methanol fouling is sort of a glazing of the plug electrode and insulator. Best bet is to change your plugs often. I always put in a new plug for the features and then relegate it to heat races or practice from there. Cheap insurance to make sure you will crank and run at top performance. I have cleaned plugs in the past, but you have to be very careful to remove any glass bead matter left by the cleaner. Don’t want this in your cylinder scratching up the walls!

A cylinder heat temp gauge is a good guide to getting the correct heat range plug. I personally like to run all of my motors any where from 375-390 degrees CHT. I know I’ll get a lot of arguments here from people who claim that a methanol motor runs best above 400 but...... have they ever compared the HP put out by a motor running 380 verses 430 on a dyno??? If I can make the motor run at 380 then it will not warp the cylinder as quickly as 430 and will keep it’s seal(compression). More HP folks. Just my experience.

Changing jet size, exhaust closing point, and spark plug heat range will affect your CHT. Oils come into play here also, as well as, a poor performing kart(tight/pushing). Remember if you’re going to change jets do it in .002 increments with methanol.

The absolute best way to tune a motor is with Exhaust Gas Temperatures(EGT). No real numbers here but the basic principle is to shoot for the higher number you can get on a given day. Most of the time with a Briggs this will be around 1230 degrees or slightly above.