Posts Tagged ‘traxxas’

I saw this at http://www.beatyourtruck.com/forum/showthread.php?39168-Tuned-pipe-design and thought it might be of good use to a lot of you folks out there.

I put this as a post in another thread, but it was suggested that I put it here so everyone could see it.

Here is some more reading on the how and WHY of a tuned pipe.

Taken from this thread(http://monster.traxxas.com/showthread.php?t=261817) By BobR.

I was going to write most of this into my other post, but was being too lazy.

Tuned pipes – how and why
The need for a tuned pipe arises because of an inescapable flaw in two-stroke engine theory. Sure, these engines have outstanding power-to-displacement ratios, but only at the cost of fuel efficiency; they are far more thirsty than their frugal four-stroke counterparts. The reason for this lies in the crankcase & sleeve porting. On all two-stroke engines, the intake port(s) and exhaust port are open simultaneously. (A rear-exhaust engine, such as the TRX 2.5, positions the central intake port and the exhaust port in diametric opposition, thus increasing the velocity of the gases slightly.) As raw fuel enters the intake port, a large amount passes through the combustion chamber and out the exhaust port without ignition. There’s no way to solve this problem without utilizing a four-stroke design, but a tuned pipe will do a great deal of good for both power and fuel efficiency. It uses a series of expanding and narrowing cones to take advantage of the escaping exhaust gases and force raw fuel back into the combustion chamber (we call this process scavenging). Best of all, using a tuned pipe has no tradeoff whatsoever – a rare thing indeed with engines.
In addition to enhancing efficiency, the pipe plays a vital role in fuel delivery. As the engine increases in speed, its need for fuel increases exponentially. Most of the fuel is provided by negative pressure pulses from the crankcase, but to compensate, the engine must also have a pressure nipple on the pipe. This connects to the fuel tank, and as exhaust pressures build with RPM, fuel delivery to the carburetor is accelerated. As such, a proper tuned pipe will deliver more power, better fuel economy, and longer engine life.

I’ll describe your typical two-chamber (two-cone) pipe. Single-chamber and even three-chamber pipes are available, but two-chamber ones are far more common.

A) Header Adjusting the length of the header can make a substantial difference in the engine’s performance. A longer header will increase torque at the expense of throttle response and high-RPM power. A shorter header will increase power at higher RPM and speed throttle response, but decreases torque.
B) Divergent cone This is where the tuned pipe action starts. Exhaust gases flow through the header and into the divergent cone. As the pipe widens, the gases slow considerably (Bernoulli’s principle*). The slowing of the gases creates a low-pressure void behind the escaping charge, which sucks any remaining fuel vapor away from the exhaust port. A short divergent cone is good for torquey engines, whereas a long divergent cone is good for high-RPM applications.
C) Belly The belly of the pipe isn’t as significant as the other components, but its shape and size still have a considerable effect on engine performance and tuning. A short belly is ideal for high-RPM, and a long belly is good for torque. Certain pipes may have a small baffle in the belly to assist in building & redirecting pressure; if you see one, don’t bother modifying or removing it.
D) Convergent cone Now things get a bit more complicated. As the exhaust travels to the end of the belly of the pipe, most of it exits through the stinger (E). However, some strikes the convergent cone and begins to gain velocity as the pipe narrows (Bernoulli again). The pulse then reflects off the end of the pipe and flows back towards the header. The pulse returns the exhaust port (open at this point) and pushes any escaping fuel vapor back into the combustion chamber for re-ignition.
E) “Stinger” I’ve never seen a proper name for this little bugger, but “stinger” seems to be the common consensus. So that it shall be. The inner diameter regulates the amount of air exiting the pipe – too small, and the engine will stifle itself; too large, and it will allow far too much exhaust to escape during the first cycle, negating the effects of the convergent cone altogether. On a .12-.15ci (2.0-2.5cc) engine, the stinger’s inner diameter is usually about 5.5-6.5mm (~.25″) depending on pipe design.
F) Pressure fitting A small amount of pressure in the pipe is pushed through this fitting; it provides some backpressure in the fuel system, assisting in feeding the engine as RPMs increase. At higher engine speeds, exhaust pressure increases and pushes more fuel into the carb to compensate. Ideally, the fitting should be placed at the fattest section of the pipe belly, near where the two pressure waves converge.

So, what happens when you fail to use a tuned pipe? Since fuel delivery is regulated by the tuned pipe, without one tuning is impossible to perfect across the entire RPM range. As RPMs increase, the fuel mixture will progressively lean out, starving the engine of necessary lubrication and cooling – this makes the engine run more powerfully for a while, but quickly destroys it. Those “dual exhaust” systems sold on eBay are a death sentence for your engine. Although they all claim to have proper pressure characteristics, the science behind them is dangerous at best. No divergent cones (square “chambers” do nothing), no belly, and an improperly-placed fuel nipple all spell disaster. There’s a reason that no reputable engine manufacturer produces these things.

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