There are many ways to get more power out of you nitro motor.  For example, replacing the stock air filter with a low resistance filter, running a higher nitro content fuel, and swapping out the stock pipe for a tuned one will all help you get make your motor more powerful.  And let’s not forget, properly tuning your motor is probably the single most important thing you can do to squeeze every drop of power from your mill.

Additionally, there are lots of other things you can do to R/C vehicle to make it faster.  First and foremost, make it as light as possible. Some areas where weight can be cut is by replacing the heavy stock tires and wheels with lighter ones, remove the reverse gear assembly from the transmission, replace your heavy NiMH battery pack with a light LiPoly pack, and lastly if your vehicle is made by Traxxas, remove the EZ-Start.  Don’t forget to balance your wheels too.

So assuming you’ve done all this and our vehicle still isn’t fast enough for you, what else can you do? Well, short of dropping in a bigger, stronger, and more expensive motor, you can modify your motor’s crankshaft and ports.  By using the term “modify”, I am talking about removing any bottlenecks that may be restricting airflow through your motor’s internal components (induction port, crank face, intake port, and boost port).

You see, most mass produced nitro motors don’t have this “final step” of the manufacturing process performed.  The reason, of course, is cost.  All this work must be done by hand and is very labor intensive.

Now, before you start, I must warn you that if you don’t know what you’re doing you will probably destroy your motor.  Therefore, don’t attempt this unless you know exactly what you’re doing.  In addition, you may want to experiment on a motor with a piston and sleeve that’s nearly shot.  That way, if you screw things up you won’t be sending your motor to a (much) earlier grave.  Lastly, I can in no way be held liable if you destroy your motor based upon anything shown in this article.  Got it? Good, let’s continue..

OK, so you want to modify your motor??  Before you do anything, make sure your motor’s been properly broken in and has several tanks of fuel through it. 

Step 1. Disassemble Motor

Remove your motor from your vehicle and thoroughly clean it with nitro spray or denatured alcohol and blow off any remaining dirt with compressed air.  Proceed to disassemble your motor on a clean surface (I prefer using a bath towel). Set everything aside except for your crankshaft, piston, and sleeve.

A newly broken in TRX 3.3 with the head removed

Step 2. Modify your Crankshaft

Most crankshafts are very restrictive where the induction port meets the intake manifold.  Look at the crank from a Traxxas TRX 3.3 and you’ll see what I mean.  Using a Dremel tool fitted with a small grinding bit proceed to open up the “mouth” of the crank’s induction port so it is shaped like the horn on a trumpet. This may require a lot of grinding, so just take your time.  When done, use a polishing bit and smooth everything out.

The stock crankshaft. Notice the narrow and rough induction port.

The same crankshaft with the induction port opened up & polished.

Next, open up and smooth out the throat of your crankshaft’s induction port with a long grinding bit. You’ll probably notice that the throat is pretty rough, a side effect of the casting process.  Don’t remove too much or you’ll weaken your crank or throw it out of balance.  Your goal here is to make the throat as smooth and defect free as possible.  When done, use a polishing bit and make everything shiny.  Pay special attention to the “dogleg” area in the induction port (near the intake manifold).  Making things smooth helps the air/fuel mixture to flow with less turbulence.

Lastly, cut a “fang” onto the face of the crank (see photo for location).  The fang will help atomize the air/fuel mixture.  Polish this up too.  Additionally, some people like to polish the whole face and edge of the crank.  The theory is that a crank with a polished face will have an easier time moving through the air/fuel mixture in the crank case.  This theory may be valid.  Consider that a crank face with a circumference of 2.5 inches at 30k RPM is moving at 70 MPH. So why not, polish it up.

4. Stuff your Crank

Whether or not you need to do this step depends upon the angle of your crank’s induction port.  If your induction port’s dogleg is nice and curved, they you’re OK. However, if it’s not, then you may want to consider “stuffing” it. The induction port on the Traxxas 3.3 motor is awful. It’s nearly a right angle and no amount of grinding or polishing will fix it.  The drawing below shows the port (in white).  You can see how the dogleg is nearly a right angle.  By placing some epoxy here (shown in red) you can really improve the air flow.

The first thing you’ll want to do is make sure the area of the port where you plan on putting the epoxy is nice and rough. This will give the epoxy something to bite to. Then, use some degreaser (nitro motor spray works great) to remove any oil or grease present.  Lastly, using a tooth pick, place some epoxy in the dogleg. I recommend 5 minute epoxy so it will set up quickly.  You’re going to have to hold the crank and keep tilting it back and forth and side to side to keep the epoxy where you want it -you want a nice curve.  Doing this for 5 minutes until the epoxy sets is a little challenging, but the results are worth it.  The picture below shows the finished product.

4. Cut your Piston

Most pistons have a spot cut out of them where the piston’s sleeve meets the cylinder’s boost port. If yours doesn’t, carefully grind a small section out that will allow the air/fuel mixture unrestricted access to the boost port when the piston is at bottom dead center.  Be very careful not to scratch the side of your piston. If you do, then the piston is no good. The easiest way to accidentally scratch your piston is to unknowingly get a piece of grit on it from your grinding bit and then slide it across the piston’s side when holding it.  For this reason I recommend holding your piston by the top and bottom only (with the connecting rod removed).  When done grinding your piston, spray it clean with lots of WD-40.

5. Fang your Sleeve

Nearly all sleeves have four ports in them (the exhaust, the boost, and two intake ports).  As its name implies, the exhaust port allows the exhaust to flow from the combustion chamber.  On either side of the exhaust port you’ll see two intake ports.  The intake ports are the primary ports that allow the fresh air/fuel mixture to enter the cylinder.  Directly across from the exhaust port is the boost port.  The boost port directs a charge of air/fuel mixture towards the top of the cylinder thereby helping to force out the exhaust. In theory, the intake ports and the boost port should work together and point towards the top of the cylinder near the side furthest from the exhaust port.  In practice however, the intake ports usually aim the air/fuel mixture straight up into the cylinder.  The boost port, however often does a better job in its aiming duties.

For our purposes, we are going to leave the exhaust port alone.  We are, however, going to modify the boost and intake ports.  The first thing you’ll want to do is to grind down some of the outside of the sleeve that leads to the port.  You want a nice smooth entry into the port.  Next, cut two fangs on either side of the intake port. Make sure you aim the fangs towards the top of the cylinder opposite of the exhaust port. DO NOT change the opening of the port at all and make sure you don’t grind the inside of the sleeve at all.  Take care not to flare the fangs out so far that they go beyond the intake channels in the cylinder block.  Additionally, don’t make the fangs too deep or you’ll weaken the sleeve.  After you’ve finished with the intake ports, polish them so they’re nice and shiny.  Rough surfaces impede airflow.

Lastly, let’s turn our attention to the boost port.  Smooth the port’s entry and cut in three fangs (left, right, and middle) all pointing straight up.  Polish your work here too.

When you’re all done, thoroughly wash the outside and inside of your sleeve and crankshaft with WD-40. You don’t want any grit left on the inside of the sleeve or else you’ll ruin your sleeve & piston.  Before you put your piston back in the sleeve, coat the inside of your sleeve with 3-in-1 oil (or similar).

This graphic will give you a general idea on the angle of the intake port fangs.

Your intake port fangs should point towards the X

To put what we’ve done perspective, let’s consider how much air actually moves through your motor. Take the Traxxas TRX 3.3 for example. Its top RPM is 35,000 and it has a 0.2 CID (cubic inch displacement).  Simple calculations reveal that this motor can pump up to 7,000 cubic inches of air/fuel mixture through it per minute.  That’s 30.3 gallons.  At that rate, the fuel air mixture is traveling through your crankshaft’s induction port at speeds up to 216 miles per hour.  Going that fast, even the smallest imperfection will have a big impact on airflow. 

6. Test Drive

Once you’ve reassembled your motor and installed it back in your R/C vehicle, it’s time to test out your handy-work.  Success or failure should become quickly evident.  If you’ve done everything right, you’ll now have a motor with more midrange and lots more top-range power.  You’ll also notice that you’ll have to back out your high speed needle by up to one full turn (proof that you’re now pumping more air through your motor). Enjoy!

Drop me a line at webmaster@e-maxximumrc.com!