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Article from National Kart News, the Nation's Leading Karting Magazine Since 1986

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YAMAHA PIPE TUNING How Adjusting Flex Length CanShave Lap Times.story; pipe engines traditionally were much harder on a clutch. However, the popularity of dry single disc clutches in the Can classes has probably tipped the reliability scale in favor of the pipe classes, (which for the most part still use multi-disc wet clutches). Lets dispel another myth about pipe racing: You have a trailer full of pipes. Probably 80% or 90% of Yamaha pipe racers in the U.S. use an RLV pipe and header combination; retail price is under $150 for the set. The others are using custom pipes made by other local motor builders. For this test and article, we used the popular RLV L-2 (a good all-round pipe) and L-3 (a slightly more top end pipe versus the L-2) exhaust pipes and the RLV L series header. The most notable advancement of the L series header, versus the A series, is the use of a hidden/internal flex ring. For the most part, this ring is between 1/4 and 1 in total length making it easier to store, transport, and most importantly measure for flex length. Performance wise, the L series header is supposed to produce more low-end power than the A series, but sacrifices a bit of top-end power. Finally, tuning a pipe. This can not be summed up in a paragraph or two, but the basics are this: ask your engine builder or exhaust retailer which pipe is best for the track(s) you run, and test a range of different flex lengths to find out which works best for your class.

hen the WKAs Manufacturers Cup set up shop in the west last year, they brought Yamaha Pipe classes back to California, under mixed reviews. Many people believe this is a dead or dying class: hard on clutches, tires and it requires karters to have a large number of pipes in their trailer. These myths are no more extreme for pipe classes than they are for any other class. For karters seeking a medium horsepower class, a class where they have another avenue to tune their karts, or a second class for added track time: The Yamaha Pipe class may be the answer. For years, the KT-100 Yamaha pipe classes were among the most popular in U.S. karting. The rules basically end at the outside of the exhaust port, allowing engine and exhaust builders the opportunity to fine tune to get the most out of Yamahas classic 100cc design. Additionally, the pipe classes give the technical minded racer one more avenue to tune and experiment for that unfair advantage over the competition. However, over recent years, many karters have moved to restricted exhaust class-

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es, or Can classes. Although the SSX (four hole) and YBX (three hole) classes are quite competitive, they take away either a little imagination or engineering by eliminating the exhaust tuning. So why are many karters shying away from the pipe classes? Many racers believe its too difficult to tune a pipe. Others say they have to buy too many pipes and headers to be competitive, and the final reason: a pipe wears out your motor too quickly. Questions well try to find the answers to in the remainder of this article. The pipe wears out your motor too quickly excuse is rather false. If most pipe motors operate at a maximum of 15,000 RPM and most Can motors 14,300 RPM max, common sense tells us that 700 RPM isnt going to make much difference. Clutches used to be another

Use a tubing cutter to cut flex lengths easily and evenly.

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Pipe Basics and Terminology:

First, lets define what is meant by flex. Flex is the tube/ring/or other material that connects the header to the pipe. Flex is an outdated term left over from the days when a flexible piece of tubing was used to bridge the gap between the exhaust header and the exhaust pipe. Today, the most common material used is 1.75 outside diameter steel tubing, and with the long RLV L series headers the piece of flex can be as short as 1/4.

Measuring the flex length with a tape measure.

from the exhaust side of the piston to the end of the length of flex. When measuring the flex length, there are custom tools available. It is just as easy to use a small tape measure too. First, remove the exhaust pipe from the header or exhaust flex. Assure the piston is at or near top dead center by rotating the starter nut clockwise with an Allen wrench. not black and white (other than the checkered, what is in motorsports). Verlengiere clarifies that when a racer asks about needed flex length, theyre actually asking the distance from the back of the piston to the start of the divergent (or first) cone of the pipe. The divergent cone is the first cone of the exhaust pipe from the intake side. It is also recognizable as the cone that expands or widens before the bend in the pipe. Often, most pipe competitors do not concern themselves with the beginning of the divergent cone. The recognized measuring point for the flex length is most often Move the tape measure or tool into the header until it touches the back of the piston, preferably at the lowest possible point. The measurement at the end of the header (L series) plus the length of the metal ring, flex, equals the length of the pipe as referred to by most karters. For an A series, or any other short header that

Probably the best way to cut a length of flex tubing is with a plumbing type tube cutter. Available at any hardware store, this tool allows you to make a straight cut evenly and effectively each time. Art Verlengiere of RLV Tuned Exhaust Products is probably kartings foremost expert on Yamaha exhaust systems. Verlengiere states, The most common question asked regarding pipe tuning is, where do I set my flex length? But the answer is

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requires a long flex tube, its easier to keep the flex tube on the header for the measurement. The flex length measurement is important for two reasons. One it adjusts the power band of the motors peak performance. Secondly, the flex length can affect the operating temperatures of the engine: shorter flex equals higher temperatures, and longer flex will lower the temperatures. These temperature differences are more easily noticeable with an exhaust gas temperature sensor. Lengthening or shortening the flex length does not necessarily make any more or less power. More accurately, we are tuning the pipe to a higher or lower RPM number which generally shifts the entire power band up or down, stated Verlengiere. Verlengiere continues, The adjustment of the flex length is to tune the pipe to match the power requirements and RPM band for the track.

Visibly there is little or no difference between the RLV L-2 and L-3 pipes.

Flex Test:We took the new Official SSC/NKN CRG Blue Demon test kart to our local track for a shakedown and decided to test some flex and pipes while we were there. We started with an 11/80 gear, which we knew from previous experience would rev about 14,500 RPM using an RLV L-2 pipe and L series header at 7-1/4 flex length. This combination did exactly that. Max revs were 14,510 RPM; however, the EGT was a little high at 1,173 degrees. In a five-lap run, this combination produced a best lap time of 1.06.75. For the next run, we increased the flex length to 7-3/8. This was quite a minimal change; so slight I could feel no difference at all. There was some improvement in lap times, nearly .2 seconds (which could be credited to many factors: driver, wind change, etc.); however, the greatest effect was losing 20 degrees of EGT temperatures. Gearing, in relation to the flex lengths is a whole separate topic; however, it is important that we do touch on it here. Lets begin with a 7.5 flex length which produces 13,600 peak RPMs and the clutch is engaging at 10,200 RPM. By adding X amount of teeth to the rear gear, the rate of acceleration will increase, and the max RPMs will increase to 14,000 RPM. With the increased acceleration, we will move through the power curve much faster and spend more time in the 12,500 to 14,000 RPM range, states Verlengiere. We went up to a 7.5 flex length for the third five-lap run. This time, the results were a little odd, but satisfying. Lap times dropped another .3 seconds; however, so did the maximum RPMs to 14,408. The EGT also again dropped to a comfortable 1,130 degrees. At the 7.5 flex length, you could feel a difference in acceleration versus the 7.25; most notably the clutch did not seem to slip as long. The final run with the L-2 pipe was at a 7.75 flex length. This yielded some change versus the 7.5 length: 14,276 max RPM, and a fast time of 1.05.99; EGT was relatively unchanged.Flex needed for RLV A-series (or older) headers (left); flex rings needed for L-Series header (right).

Before adding the teeth, we spent more time between 10,000 and 12,500, and we didnt need to move the power. But, keep this in mind: if we move to a shorter flex, we are able to match the power band more closely to the gear we just added.

As the clouds began to form on the horizon, we left the 7.75 flex length in and switched to the L-3 pipe. Knowing the only place Ive used the L-3 before was a nearly flatout street race, I knew keeping the longer flex in would make it a lot more competitive with the L-2 at a shorter length. The L-3 was about a half second slower than the L-2 was at the same flex length. Maximum RPMs were only 14,268, while the EGT was only two degrees lower. It was easy to feel the difference between this L-3 pipe and the L-2. When the L-3 got out of the power band of the motor, it was easy to feel the delay before the pipe would again put the power to the ground. However, as I said the L-3 is an excellent pipe for racing at a sustai