34 NATIONAL KART NEWS

YAMAHA PIPE TUNING

How Adjusting Flex Length Can Shave Lap Times.

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

When the WKA’s 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 clutch-es, 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 out-side of the exhaust port, allowing engine and exhaust builders the opportunity to fine tune to get the most out of Yamaha’s classic 100cc design. Additionally, the pipe classes give the technical minded racer one more avenue to tune and experiment for that unfair advan-tage over the competition.

However, over recent years, many kar-ters have moved to restricted exhaust class-

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 it’s too difficult to tune a pipe. Others say they have to buy too many pipes and head-ers to be competitive, and the final reason: “a pipe wears out your motor too quickly”. Questions we’ll 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 isn’t going to make much d i f f e r e n c e . Clutches used to be another

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).

Let’s dispel another myth about pipe rac-ing: 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 combina-tion; 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 pop-ular 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 dif-ferent “flex” lengths to find out which

works best for your class.

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Measuring the flex length with a tape measure.

YAMAHA PIPE TUNING

Pipe Basics and Terminology:

First, let’s define what is meant by “flex”. Flex is the tube/ring/or other material that con-nects the header to the pipe. Flex is an outdated term left over from the days when a flex-ible 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”.

Probably the best way to cut a length of flex tubing is with a plumbing type tube cut-ter. 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 karting’s fore-most expert on Yamaha exhaust systems. Verlengiere states, “The most common ques-tion asked regarding pipe tuning is, “where do I set my flex length?” But the answer is

not black and white (other than the check-ered, what is in motorsports).

Verlengiere clarifies that when a racer asks about needed flex length, they’re actu-ally 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 measur-ing point for the flex length is most often

from the exhaust side of the piston to the end of the length of flex.

When measuring the flex length, there are cus-tom 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 cen-ter by rotating the starter nut c l o c k w i s e with an Allen wrench.

Move the tape measure or tool into the header until it touches the back of the piston, preferably at the lowest pos-sible 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

36 NATIONAL KART NEWS

37FEBRUARY 2005

YAMAHA PIPE TUNING

requires a long flex tube, it’s 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 motor’s 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 tem-peratures. 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.”

Gearing, in relation to the flex lengths is a whole separate topic; however, it is important that we do touch on it here. Let’s 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.

“Before adding the teeth, we spent more time between 10,000 and 12,500, and we didn’t 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.”

A final thought on gearing and pipe tuning. When you add one tooth, and the RPMs increase by 200, your top MPH has not increased, but your rate of acceleration has. Conversely, if the RPMs do not increase at least 200 RPM, you have lost both top MPH and accelera-tion. A gain of 300 or more RPM proves an increase in both top MPH and acceleration.

So taking this knowledge to the track, we have essentially learned: the shorter the flex the more effective use of the top end motor’s power band. The opposite is true for longer flex lengths: they are used on tracks where more acceleration is needed. But how big of difference does an eighth of an inch make?

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.

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.

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 I’ve used the L-3 before was a nearly flat-out 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 sustained high RPM level (Rock Island, for example).

In conclusion, it’s not as hard as it sounds to run a pipe class. Remember the simple rules of tuning the flex: longer equals more bot-tom end use of the engine’s power band, and shorter equals more top-end use of the engine’s power band. However, looking at the numbers from our test, draw another conclusion: don’t waist valuable practice time on race morning testing a 1/8” difference in flex. Go big or stay home! Well, at least a 1/4” is the start of big.

Flex needed for RLV A-series (or older) headers (left); flex “rings” needed for

L-Series header (right).

Visibly there is little or no difference between

the RLV L-2 and L-3 pipes.

38 NATIONAL KART NEWS

YAMAHA PIPE TUNING

Just for Fun:After seeing ICAs and some other direct

driver karts: the curiosity got the best of me. Could we make it competitive for our local club’s Yamaha pipe class?

In every form of racing, reducing rotat-ing mass is the key to success. There’s a simple formula top race teams use: 1 pound of static weight equals 3.5 lbs of rotating weight, reduce that and reduce lap times. So our thought process was to eliminate the clutch for rolling start races, and find and/or tune a pipe to make it competitive.

We started with the same 11/80-gear combination we were using to test the pipes. Next, we installed an RLV DD87 direct drive pipe, the WKA mandated one for Formula Y at a 12 1/4” flex length. After a quick push start, we hit the track.

As I negotiated the quicker corners of the track, I thought we’d hit a home run. It definitely pulled faster from the 9,000 to 14,000 RPM range. Then in the lower RPM hairpins, when the power dropped below 7,500 RPMs, I realized we fouled out! No make that, struck out. It seemed to take an extra 100 plus feet to get the motor “going” in the power band with the direct drive versus slipping a clutch. By the time we hit the halfway point of the long straight, the motor was maxed out on RPMs, and stopped pulling.

Oh well, it sure was fun to drive! It was also nice not to have any worries about the clutch overheating, but it did require a little different style of driving in comparison to a clutch kart; more focus on keeping RPMs in the power band. It makes me hope ICA rac-ing catches on in the midwest a little more!

A STARS direct drive

ICA engine.

The NKN/CRG setup in direct drive configuration.