Traxxas TRX-15 PRO Engine Modification
by Eric Perez

This is the mother of all technical articles. Folks, the mods here will make the TRX-15 PRO scream like a flaming demon! Note: The car really needs to be setup before it can handle the extra power. These engine modifications are not for the feint of heart. More on setup -If you are still running the plastic tuned pipe and the stock clutch this mod is probably not going to do you any good.

Quick overview of the TRX15-PRO:

Why modify your Traxxas engine?

If you are a sport user or a backyard basher you might give more importance to fuel economy instead of brute HP. If you're into racing it really makes sense to increase your engines performance. Especially if you where considering sinking $150 + or more to get a "racing" engine. I got a new TRX15-Pro using the engine exchange offer for only $70 with pull start from my LHS. That's not too bad, 8 sets of tires w/rims and inserts cost a bout the same. But the engine will last an entire season. No comment about the life span of the "new" tires...

Why is the TRX15-Pro not already optimized?

I'm not entirely sure why the designers of the 15-Pro did not exploit the full power of the engine. I can only comment on this, but they probably didn't want to give the "beginners" too powerful of an engine. They would rather have better fuel economy than a power house engine that would only run for 5 min on a tank of gas. How did the designers of the TRX-15 pro limit the engine power? A lot of the tooling for the TRX12-Pro was probably re-used in the newer design. so they made as little changes as possible to produce the newer engine. The ports on the .15 sleeve are longer than the previous .12 sleeve. But to keep fuel efficiency high, the ports machined into the engine case don't go all the way up to meet the sleeve ports. They actually fall short by about 1/4 of an inch. This makes it more difficult for fuel to reach the port on the sleeve, and thus the engine's power was reduced. -See image below-

In the rest of this article I'll teach you how to match the case porting with those on the sleeve. Along the way we will also increase engine efficiency in other areas to gain back some of the fuel efficiency that was lost by increasing the fuel flow into the sleeve. Engine modifications usually require work on different parts of the engine. There is a balance between air and fuel that must be maintained for the engine to work properly. Increasing one with out the other will only make a marginal impact on overall engine performance.

Legal Stuff:
Let me start by saying this. If you can't afford to mess your engine up don't try this. You must know how to operate a dremel and understand how to set the speed for the bit that you using based on the material that your cutting. Always wear eye protection when using the Dremel. Follow all safety procedures for operating the items/tools needed to modify your engine. once you port an engine you can't undo it - especially when you modify the engine case. Follow these instructions at your own risk.

Engine modification Intro:

Most people start by modifying an older engine to get a feel for how to do the mods, later they move on to newer engines that can benefit more from the modifications. This is not rocket science, engine's use fuel and air to make power. The available power is related to how much mixture the engine can suck in, this available power is reduced by the work needed to draw the mixture in through the carb and out the exhaust. If you are an "expert" engine tuner you can increase power by as much as 15%. Even if this is your fist engine mod -you'll see the difference at the track. Once you modify an engine you can't go back. Not even with a new piston/sleeve.

Tools of the Trade:

To modify your engine you need a mulit-speed Dremel tool. Depending on the bit that you are using you will need to either increase or decease the RPM. When using the cutting tools on aluminum you need to increase the speed until the bit does not jump or chatter. If the RPM is to high the tool will be hard to control and it will want to wander inside the engine -not a good thing. The grinding stones can be used at lower RPM, this will vary depending on the force being applied to the tool.
Dremel Tools Needed for Engine Modifications 
1. Round coarse cutting
2. Round de-burring
3. Cylindrical cutter 
4. Conical de-burring
5. Round Grinding Stone
6. Conical G. Stone
7. Tear-drop G. Stone
8. Sanding Drum Medium
  9. buffing drum
10. Wire wheel
11. Buffing wheel


Basics of Engine Modifications:

The power band is the range of RPM's that the engine will deliver max power. 2 Stroke engines make their peak power at the upper end of the RPM spectrum. This doesn't mean that you have to live with your engine's power band. It can be partially remedied by the use of a two speed and proper selection of the tuned pipe. Generally large volume pipes increase bottom end, while smaller diameter pipes increase top end. Most of the engine mods on this article are geared to increase the available mixture that the motor has available at all RPM . Normally the engine can't handle this increase in fuel unless we can get more air in the engine. On the TRX15-Pro a non modified engine will run for 6-8 minutes depending on fuel and carb settings.

Other engine modifications are geared toward reducing the work required to pump air and fuel into and out of the engine. This increase in efficiency is achieved by reducing the drag between fuel and moving engine components. The typical modifications include streamlining the connecting rod, beveling the crankshaft fuel outlet port and rounding out the header inlet. Basically we want to reduce fluid friction.

Other ways of improving the engine efficiency is by helping direct the fuel toward the glow plug and the exhaust down toward the header. this modification is done by grinding grooves into the outer part of the engine sleeve. These grooves serve two purposes they increase the available port area for the mixture and they promote mixture swirl inside the combustion chamber. The swirl helps prevents the air/fuel charge from going out the exhaust port. If the swirl is not optimized then fuel will be blown out the exhaust at a higher rate than the pressure wave coming from the pipe can compensate for, thus fuel efficiency will suffer.

Special warning:

Note: the inside of the cylinder sleeve is plated with chrome. If you attempt to grind on the inside of the port you will damage and possibly flake off the chrome -basically your engine will be ruined. Once again only grind the outside of the sleeve.

I tried to keep it simple by only working on the aluminum and brass components of the engine. These are easier to work with and are more forgiving on bit RPM than the other hardened steel parts. Your Dremel bits will also last longer :) The article is divided in a step by step approach to modifying your TRX-Pro engine.

Performance of the TRX15-Turbo:
The Trx-15 Turbo has tremendous torque and an almost unreal top end. After the engine mod I put the optional 15 and 19 tooth clutch gears on my Nitro 4-Tec . the car had a higher top end than before with the stock clutch bell gearing. The downside of the engine mods are that fuel efficiency goes down, A tank of fuel will last for 5 minutes with engine temps around 220-250 F. The flip side to this is that after the mods you will be able to challenge the Novarossi's. Before the engine mods the engine was tuned for racing. After the engine mods I needed to lean out the high speed for 1/2 a turn. Once again more evidence that the modifications reduced the restriction of fuel flow inside the engine.


It is all in the details:

The Traxxas engines are not "cheap" as in poor quality or materials. Traxxas uses top notch components and manufacturing methods. why don't they cost more than an O.S. or Italian based engine? Traxxas skips some secondary machining operations. Most of these are small details that sport users will never notice. That doesn't mean you have to live with them. A few small needle files and some patience can correct those small flaws. engines are mostly made of cast aluminum and steel components. Not all the surfaces have been milled. some don't need them, others are critical for a top performing engine. I'll illustrate this with an example of the crank-shaft that came from a box stock TRX15-Pro that has never been run. The intake port on the crank has some flashing from the casting process. This flash, could of broken loose with time and damaged the sleeve to piston fit. A quick and light file work and it's as shinny as an O.S. or Italian made crank-shaft.

Paying attention to the details -even at this level, separates the casual sport racers from the die-hard racers. R/C racing doesn't have to be expensive, you just need to pay attention to the details.

Porting the engine case:


Before you work on the case you need to remove the engine bearings. Bearings don't work to good when their full of metal shavings ;) Remove the bearings by heating the engine case to 250F in an oven for 15 min. Remove the bearings by inserting the crank-shaft partially into the bearing and pulling back smoothly. Remove the bearing from the crank-shaft and remove the remaining bearing in the same way. Let the engine cool down to room temperature before attempting to work on it. After you're finished modifying the case, re-heat the engine again to 250 F and reinstall the bearings in their original location and orientation.

The dremel work on the engine will be done using bit #1,5,7,2 in that order. Make sure there are no burs left from your engine work. Use bit #2 to smoothen up the ports. FLush the engine between dremel work with WD40. Use Brake-parts cleaner for the final rinse.

TRX15-PR0 Engine Case Porting 

The front port is the port closest to the carburetor. We use the dremel and a round coarse carbide bit to raise the opening. We basically align the top edge of our new port to the top edge of the sleeve port opening. We then use a round grinding stone to smooth out the port profile. 
The next port that we will modify is the port opposite to the exhaust port. Also called the side port. You'll need to do most of the Dremel work through the exhaust port opening in the case. Make sure to turn off the Dremel when you're going in and out through the exhaust to avoid damaging the face of the port.
This is another view of the side port. 
The back port is worked on from the top of the sleeve. Work slowly as it is hard to judge the height of the port from the top. After a little grinding rinse the engine with WD40 and insert the sleeve, continue to increase the port height until the top of the case port and the sleeve port are perfectly lined up.

Working the Connecting Rod:


Streamlining the connecting rod will increase the efficiency of the engine. Fuel has to be drawn up from the crank case and then it's pushed into the ports when the crankcase closes and the piston slides down the sleeve. The connecting rod is bathed with the air-fuel mixture after each revolution. By putting an edge in the connecting rod you lower the resistance of the rod as it is moving at high speeds through the mixture. Some say that a profiled connecting rod helps balance the fuel flow across the ports. Use a cylindrical shaped cutting tool # 3. Smooth it out with a fine sanding drum. You can use wet 400 grit sand-paper or a steel brush wheel to polish it up. I back this with a polishing felt wheel and some metal polish. When your done you should see your reflection on the rod. While you here make sure that the oil hole is clean and chamfer the outside hole going into the brass bushing, this will allow more oil to enter the bushing.

 Modifying The engine Sleeve:


The engine sleeve needs some attention as well. We will start by grinding the lower part of the exhaust port at 45 deg. Don't go too far we don't want to grind the inside of the port opening or we risk damaging the chrome plating. When your done with the sleeve polish it up with bit #11 and some liquid metal polish.

The next step is optional for the TRX15-Pro because the largest restriction for fuel on the engine was the case porting. Increasing the lead into the sleeve port when you have already increased the case porting by over 1/4 of an inch doesn't leave room for a lot of improvement. Try the engine as is -If you still have 30+ seconds of fuel to spare at the end of a 5min qualifying heat, then if you wish proceed with the extra sleeve modifications.

next we'll remove some materials from the bottom side of all three remaining ports (option: A). when possible we will try to induce more swirl by charging half of the port (option: b) instead of the entire area under the port. The modifications shown in the pictures give you max power, but if you need to retain a little more fuel economy, you might opt to go with the half groove method. The sleeve in the TRX15-Pro is already skirted. Meaning, that the outside edge has been ground to reduce the cross sectional profile for the air/fuel charge. When your done with the sleeve polish it up with bit #11 and some liquid metal polish.

I hope that you have enjoyed this Tech Article. writing this article has taken many more hours, than the time it takes to modify an engine. I'm by no means an engine "expert". These mods require more a steady hand with the Dremel than a Ph.D. in engine Combustion and Design. I would like to tell you that my Combustion class in College gave me all the necessary info about Engine design... It didn't, but at least I was able to model the combustion process. To say the least, engine Design and combustion is an entire field of knowledge in itself. Hopefully this article will get those aspiring engineers all revved up...

Whoop some r/c butt!!!

Eric Perez, 1998, 2011
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This page last modified: 07/26/11