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The Traxxas Revo. What can be said about this truck that hasn’t been said already? It single-handedly turned the world of nitro R/C on it’s head and sent all the other manufacturers running back to the drawing board. It’s that revolutionary.
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Being such a great truck, does that mean there’s no room for improvement in the Revo? Certainly not! I must admit, though, there are far fewer items on Revo that cry out to be upgraded. Any changes that are made to Revo are more like ‘tweaks’.
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Let’s look at some areas where Revo can be improved.
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1. Exhaust Header Mod: Both The TRX 3.3 and 2.5R motors are truly marvels of engineering. For their size and displacement, they are two of the most powerful, reliable, and durable nitro motors anywhere at any price. Does this mean there’s no room for improvement? Certainly not! One of the most popular mods for these motors is the ‘header mod’. Quite simply, all that needs to be done is to cut the flange off of the header (the last 3/8”).
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Once you’ve completed this modification, you’ll notice that your motor has lots more low end torque. It will be difficult to keep your trucks front wheels on the ground, especially when going up hill. In fact, the performance will be similar to a truck running with a lean condition, except that the engine’s temperature isn’t raised and it’s not running lean.
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At this point you may be tempted to ‘reflange’ the header’s end – don’t. I’ll explain why below.
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After you’ve put the header back on, you’ll want to take care when attaching the exhaust coupling. Make sure you’ve got a nice tight seal and that you’ve slid enough of the coupler over the header. Without the flange, it will be much easier for the coupler to come off. If you haven’t figured it out yet, the purpose of the flange is to give the coupler something to ‘bite’ to.
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At this point, you may be wondering how cutting the flange off the header improves performance? There are two theories as to why this is so.
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The first theory postulates that by shortening the combustion chamber / tuned pipe distance, you’ve shortened the length of time it takes the pulse wave to reflect back to the combustion chamber. Now the motor gets ‘on the pipe’ at a much lower RPM. This theory makes sense except for the fact that by getting the motor on the pipe at a lower RPM, the motor would loose high RPM power. This is not the case. High RPM power stays the same. Additionally, many people have replaced the stock coupler with a longer one, keeping the length of the exhaust system the same. To me, this last point discredits this theory.
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The theory that I feel makes more sense is what I call the “Reverse Venturi Effect” theory. By removing the flange, the exhaust system is now a consistent diameter all the way from the motor to the pipe. If you are familiar with fluid dynamics, you’ll know that gas moving through a pipe will experience an increase in pressure and a decrease in flow rate if the circumference of the pipe increases and just the opposite if the circumference decreases (the venturi effect). I believe the flange causes a reverse venturi effect that messes up both the exhaust flow and pulse wave at low RPMs. Removing the flange eliminates the reverse venturi effect and allows the motor to realize its full potential.
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2. Single Steering Servo: The Revo is one of the only monster trucks that does not require a steering servo upgrade. The set-up designed by Traxxas provides plenty of torque. To achieve this, Traxxas utilizes two servos. Installing two 80 oz/in servos (160 oz/in total) provides the torque needed to turn the massive wheels while keeping the Revo affordable. That’s right, it’s cheaper for Traxxas to install two medium torque servos rather than one high torque servo.
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So why would anyone want to switch from two to one servo? Well, there’re three reasons. The first reason is to save weight. If you’re a racer, every ounce saved is worth its weight in gold. For bashers, though, saving weight, while nice, isn’t as important. What is important is long run times! That brings us to reason number two. One servo uses less power than two. The most power hungry device on your Revo is your steering servo(s). Power savings in this area will extend your battery pack’s charge by an estimated 15%. I don’t know about you, but more time bashing and less time charging is a very good thing! Lastly, when running two servos, they tend to ‘fight’ with each other. You see, your servos won’t always have exactly the same amount of power and speed. One servo will always be holding back the other. This results in lowered efficiency. This problem surfaces over time as the servos wear.
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When selecting a replacement servo, you’ll want to find one that has at least the combined torque of the stock servos (160 oz/in) and the same transit time. Next, the type of servo to select (analog or digital) is important. Digital servos, while having many advantages, have a big disadvantage – they’re power hungry. So, if you are looking to save power (and save money), stick with an analog servo.
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A word of caution, be very careful if you use this servo with the stock Traxxas radio gear. This servo has a greater range of motion than the stock servos causing it to want to over extend your steering linkage. If this happens, you may strip and/or burn out your servo. You’re better off using this servo with a system that has programmable end point adjustments (e.g. JR XR3i).
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At the other end of the servo spectrum is Hitec’s Titanium super torque series of servos. The top dog is their HS-5955TG. This servo cranks out 333 in/oz of twist (that’s over 20 lbs. of twisting force at one inch, people!) with a transit time of 0.15 seconds (@ 6 volts). It’s coreless, it’s digital, it has a built in heat sink, it’s totally over the edge (which means it’s just about right). On thing it’s not is cheap. The retail price hovers around $114. But, if you want the ultimate servo in your Revo, then there’s really no other choice (IMO). Below is a picture of this sweet servo.
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Well, back to the servo installation. As a final step, remove the unused half of the steering arm . It will save you a few grams and will give you one less thing that can get caught on debris.
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3. Tight Turns Modification: This is a simple modification that will allow your truck to turn so tightly that its front bumper will hit the back bumper. Well, not really, but your turning radius will be reduced. The mod is simple enough. All you have to do is remove the steering stops from the steering arm mount (and have a servo with a longer range of motion than the stock servos (like the Blue Bird BMS-630MG)).
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4. Air Filter Mod: Think about it, an internal combustion engine is basically an air pump. Being an air pump, the easier the engine can pump air the more power it will generate. Therefore, it’s a good idea to look at your engine, from air filter to tail pipe, and see if any overly restrictive items are robbing your engine of power. When conducting this exercise on the TRX 2.5R, one item jumps out at you – the air filter.
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Traxxas equips the Revo with an air filter that’s really good at removing dirt from the air. One thing the filter isn’t good at, though, is allowing large quantities of air to move through it. Why?? The amount of surface area that’s exposed to the air is pretty low. For regular bashing, the air filter is fine. But if you plan on racing or your only desire is to squeeze every last drop of performance out of your motor, it’s time for a filter upgrade.
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When considering an air filter upgrade, one brand rises to the top: Motor Saver. This filter not only allows air to flow into the element from the top, it also is fully open around the whole cylinder. Plus, the filter is protected at all openings by micro-screening.
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The stock filter does have one big plus, though. The intake manifold. The intake manifold has a large internal circumference, is made from heavy duty rubber, and it fits perfectly around the spur gear and into the servo tray’s filter holder. The Motor Saver’s intake manifold, on the other hand, does leave something to be desired. The rubber isn’t that thick and it doesn’t fit as well as the stock manifold. What to do?? Simple, swap the Motor Saver’s manifold with the stock one.
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Making the swap is easy, but take your time and do it right. If you mess up, you may end up ruining your new filter or not having a perfect air filter/manifold seal.
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The first thing to do is to cut the cylindrical opening off of the bottom of the Motor Saver filter housing. Next, enlarge the hole on the bottom so it is the same diameter as the stock filter. Once you’ve done this, thoroughly clean the filter housing and intake manifold with soapy water or denatured alcohol. When it’s clean and dry, fit the manifold into the hole in the bottom of the housing and securely glue the two items together with CA glue or epoxy. This step is very important because the Motor Saver filter housing and manifold don’t mate as tightly as the stock filter and manifold. Not only do you not want the filter housing to come off during a run, you don’t want any air leaks. So don’t rush this part and do it right.
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Lastly, take the cylinder you cut off the bottom of the Motor Saver filter body and using fine grit sand paper, sand it down so it is more of a ring and less of a cylinder. Place the air filter “seating ring” into the throat of the intake manifold and look at how much it raises above the top of the manifold’s throat. Sand the ring down enough so the top of the ring sits flat with the bottom of the air filter housing.
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Finally, glue this into the throat of the intake manifold. Why?? Because the Motor Saver filter is a little narrower than the stock filter. That means it only contacts the top of the manifold throat and not the bottom of the filter housing. I don’t feel this ‘ring of contact’ is nearly enough to provide you with a good seal. Remember, air, like water, will follow the path of least resistance. You don’t want the intake air to take a shortcut between the bottom of your filter medium and the top of the intake manifold. Gluing the ring into the intake’s throat creates a larger surface area for the filter to sit on.
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As a final step, smear some grease around the bottom of the air filter cylinder (where the filter foam sits). That way, when you install the foam filter you will have the tightest seal possible.
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5. Seal the servo box from dirt: The purpose of the servo box is of course to protect the servos from dirt and debris. However, dirt can easily find its way into the box via the throttle, break and shift linkage linkage holes. The best way that I’ve found to seal these holes against dirt but still allow the linkages to move without restriction is with heavy duty felt. The type of felt I’m referring to is used to line the tank for snakes and reptiles, among other things. It’s very heavy duty and water doesn’t affect it. To make the felt water resistant, simply spray it with WD-40.
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The first thing you’ll want to do is cut out pieces of felt to be placed on the wall of the box and on the underside of the box’s cover. Glue the felt in place with epoxy glue or something similar. Lastly, brush the felt with a wire brush to make it fuzzy. It is this fuzzyness that will ultimately keep dirt from getting into your servo box.
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6. Balance your wheels: The Revo has big wheels that rotate at high speeds. If your wheels are out of balance, and they probably are, you are needlessly converting some of your wheels rotational energy into vibration energy. That means your truck’s top speed isn’t as high as it could be. To learn how to balance your wheels, click here.
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7. Install a fail safe: The Revo, like all nitro powered trucks, really should have a fail safe installed. What’s a fail safe? It’s a small circuit that lets up on the throttle and applies the breaks if it detects either a weak battery pack or a weak transmitter signal. Without a fail safe, your nitro truck can become a crazed, out of control vehicle that stops only when it runs out of fuel or hits something (which do you think is likely to happen first?). I happen to like the model by OFNA. It’s small and inexpensive (around $23). Here’s what their sales literature says about it: CPU fuzzy logic system with low battery warning, prevents accidents by detecting low battery voltage, low radio battery voltage, no radio signal, and signal interference. Stops vehicle by applying brake servo.
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8. Get 100%: It’s critical that you ensure your carb is mounted properly or else you won’t get 100% from your Revo. What do I mean by this? If you carb is mounted so it’s angle of rotation makes it’s slider shaft move exactly parallel to the motor, you may find that your slider won’t slide enough to allow your carb to open 100%. From the factory, the most mine would open is 80%. Of course, this killed my motor’s top end. I tried everything (playing with servo end points, adjusting the servo horn, everything) without success. I just couldn’t get my carb to open fully. Then I tried rotating my carb. I found if the carb is rotated counter clockwise just slightly (i.e. the 11:30 position from the 12:00 position) suddenly my carb would open 100%. As you can imagine, with my carb opening fully my truck suddenly has lots of top end!
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9. Bore out the Stinger: Being essentially an air pump, anything that can be done to allow your nitro motor to move air more efficiently will, in theory, enable it to generate more power. The increase in power comes from the motor using less energy to pump air through it.
Boring out the pipe's stinger is a very easy modification. Enlarging the stinger's diameter from 13/64” to 15/64” (0.203” to 0.234”) doesn't sound like much, but that's an 13.25% increase. This will lower the exhaust's back-pressure enabling the motor to more efficiently purge the cylinder of exhaust gasses. Your motor will also use less energy to move the exhaust gasses out of the pipe. You may be tempted to bore out the stinger a little more, but I advise against it. Not only will the stinger be very weak, you may eliminate too much back-pressure. Remember, the exhaust back-pressure is what pressurizes the fuel system. Too little back pressure and your fuel system will not ’push’ fuel to the carb.
I don't recommend using a power drill for this procedure. You could too easily destroy your pipe. Rather, use vice grips to hold onto the drill bit and carefully bore out the stinger by hand. Make sure you don't drill out beyond where the stinger mounts to the pipe. The stock Revo pipe has a cone in the bottom that's used to reflect the pressure wave back to the engine. Drill too deep into the pipe and you'll drill through the cone.
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Don’t hit the Cone with the Drill Bit!
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I’ve drilled in about 1/8”. See the difference?
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Here’s the finished job!
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10. Get rid of the EZ-Start: Don’t get me wrong, I’m a huge fan of Traxxas’ EZ-Start system. It’s far and away the easiest way to start your R/C vehicle. However, if you are looking to squeeze the most performance from your Revo by shedding weight and lowering it’s center of gravity, then it’s gotta go.
If you’re a racer, this is a no-brainer; use a starter box. But if you’re a basher, you probably don’t want to lug a starter box around with you. That leaves you either with a pull start or a remote starter. I’m not a fan of pull starters. To start your vehicle with a pull starter, you need to remove your car or truck’s body. For me, a remote starter is nearly as convenient as using the EZ-Start. I’ve used a few remote starters and my favorite is the TigerDrive. TigerDrives work really well with the Revo due to it’s inclined motor and because you can access the TigerDrive through the wheel opening. The TigerDrive’s 90 degree angle of approach is what makes it so versatile.
If you decide to use a TigerDrive, I recommend you also get the TigerDrive starting wand (unless you already have one you know will fit). TigerDrive can be used with a cordless drill or a hand held starter.
So, by using a remote starter, like a TigerDrive, you’ve substantially lowered your Revo’s weight (making it faster) and you’ve lowered it’s center of gravity (making it handle better). Click here for more info on TigerDrive.
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