Revo Performance Set-Up Guide (part 3)
Engine, Clutch & Drivetrain
by Steve Slayden

Photo Courtesy of Cliff Hudson

Welcome to the third and final installment of my Traxxas Revo performance article series. Like my previous articles, I will be shedding some light on the tips and tricks that I perform on my own personal race trucks. In part 1, I showed you my complete guide to tuning the Revo suspension, and in part 2, I explained many of my steering and chassis tricks. In this installment, I will focus on the drive train along with getting the most out of your engine and transmission. Racing the Revo is a lot of fun. There are many ways to tailor the Revo to your driving style, so be sure to check out the previous two Revo performance guide installments located above to catch up on everything that I do to my own Revo race rigs.

The starter box in this pic is the Revo
starter box from RD Racing Products.

Converting to bump start
	The term “bump start” refers to starting the truck with a starter box. A starter box is a box with a motorized rubber wheel powered by one large 12V battery or two smaller 7.2V battery packs. The rubber wheel engages with the flywheel of the engine (while spinning) and turns the engine over. While the engine is turning over, a separate battery powered glow igniter is used to heat the glow plug.

Parts needed for bump start: back plate #5274R,
non p/s crankshaft #5219R, large flywheel #4142X

This starting system allows the removal of the EZ-Start onboard starting system so that weight can be reduced from the chassis while also reducing rotational weight from the crankshaft. This takes load off of the engine, which will increase acceleration and allow the engine to run more efficiently.

Install larger 1/8-scale Air filter

The stock air filter delivers exceptional performance keeping the air that enters the engine clean. However, on dusty tracks, the entrance into the air filter’s housing is a little small for long races. This area can become clogged with dirt, so I use a large cylindrical air filter like the ones found on 1/8-scale buggies. Check with your local hobby shop for the different 1/8-scale buggy filters that are available.

Use shoe-goo to bond the air filter housing to the Revo air filter base. There are also pre-filters available for this style of air filters which collect the bigger dirt particles while the main filter catches the fine stuff. Check with your local hobby shop for available air filter systems that will work with the Revo filter base.

Don’t forget to secure the base of the air filter to the carburetor with a zip-tie.

Single or dual chamber pipe
I generally use the dual chamber (stock) pipe for most applications for its smooth power band. The Traxxas single chamber pipe (#5490) is good for a little extra power in the lower to mid rage of the power band. This is useful on high–bite tracks or anywhere a little extra grunt is needed. Plus, it comes in this beautiful hand-polished finish.

I install my clutch shoes in the trailing edge (stock) configuration. However, the clutch shoes can be installed in reverse for a snappier bite and/or modified for a different engagement feel. The clutch shoes can be shaved for a later engagement. This can be useful for getting more punch out of your engine. Make sure that you take the same amount of material off of both shoes, and be careful not to take off too much material as this will soften the engagement too much and accelerate shoe wear.

Use a hobby knife to shave the material off of the clutch shoe. Only shave a small amount at a time and make sure that each shoe is cut identical to each other. This will level out the wear and provide better clutch performance.

Traxxas clutch shoes (#4146X) are already cut from the factory, but you can experiment by shaving off a little more material for later engagements. The stock shoes (#4146) are uncut and will engage a little sooner than the cut ones. These can be helpful on a slick track with a lot of loose dirt on top of the surface. Be careful not to remove too much material from the shoes, as this will leave the shoes too “light”. They will not engage the clutch bell firmly, defeating your purpose of getting more punch out of the truck. Trim a little, and then try them out in the truck to see what works better.

Here’s a tip: Trim off the corner of the inside diameter of the
assembled shoes. This will allow better clearance around the clutch
nut and will allow your clutch shoes to operate more freely.

There are quite a few different gear combinations available for the Revo. The chart below will give you a good idea of where to start for your local track. Remember, these are only suggested gear ratios. The actual track layout, jump specifications, surface conditions, etc., will ultimately dictate what ratio is best for your track.

Your gearing should properly match the size of the track for fast and consistent lap times. You want to be able to efficiently use all of the power available from the engine. Being over-geared will put more load on the engine and clutch causing the engine to run hot with less power out of the corners. The idea is to get the engine to clear out before you get to the next corner. The engine produces its best power when it is burning the cleanest mix of air and fuel (not too rich & not too lean). This is normally found in the mid to upper RPM range.

Big off-road tracks, such as Johnny Cool Guy Raceway in Euless, TX (shown above) is a perfect example of a modern, nitro off-road track that Monster trucks are raced on. Choosing the right gear ratio is very important on a track this size.

On larger tracks it is ok to gear a little higher because the truck's cornering speed is usually greater. This keeps the engine in the best RPM ranges as described above. If the truck is geared too low on larger tracks, the engine will spend too much time in the upper RPM range. This will cause the engine to run hot, increase the danger of fouling the plug, and will affect top speed.

The placement of jumps on the track can also have an impact on your gear choices. A lot of tracks will have a set of jumps coming out of low speed corners so good bottom-end punch is important to clear the set. What this means is that you may have to gear down to make certain jumps effectively. Your truck may be a tad slower down the straight, but it is now getting from apex to apex much more quickly. This set-up will be more desirable when your truck spends the majority of its time in the infield. Of course, this may change if you are on a more open course, with either smaller jumps or easier jump sets.

It is ideal to utilize a gear ratio that works best for the majority of the track. If the track has a long straight but a tight infield, then gearing up to be the fastest truck on the straight is not necessarily going to be the best move. You will need to maintain a fast pace through the infield as well. If you are coming up short on doubles, and bogging slightly out of the corners, then you can bet that all of the time lost in the infield will never be made up by the fraction of a second that you stand to gain on the straight.

Two-speed adjustments are necessary when changing the gear ratio between the clutch bell and the spur gear. Changing the gears on the clutch bell to a higher number of teeth and/or changing your spur gear to a smaller number of teeth will give you a higher top speed. This will also take away a little bit of low end punch. This will affect your shift point by making your two-speed shaft turn at a higher RPM, thus causing the two-speed to shift sooner. If you are going to a clutch bell gear that has a lower number of teeth and/or a spur gear that has a larger number of teeth, then the two-speed will react just the opposite. Turn the adjustment screw clockwise for a later engagement or turn the adjustment screw counter-clockwise for an earlier engagement, depending on the need. Adjust the two-speed in small increments. Never turn the adjustment screw more than 1/8 turn at a time before testing the shift point. This is a fine adjustment and can get off base very quickly if large adjustments are made.

This pic shows what you are actually inserting the driver into.
The 2-speed hub is located within the 2nd gear of the 2-speed assembly.

Notice the lever reaching out to lock into one of the 2-speed pins located
inside of the 2nd gear. The preload set by the 2-speed adjustment screw
determines the speed in which the 2-speed hub has to rotate to release
the lever. Lighter settings will allow the lever to engage sooner
and tighter setting will hold it back for later shift points.

The shift point of the two-speed is not only important for optimum performance on the track, but it is very important for long engine life. If the transmission is shifting too soon, power will be lost and the engine will over-heat quickly. If the shift point is too late, the engine will be allowed to rev too high for too long. This condition can cause many problems like accelerated wear between the piston and sleeve, plug damage, poor fuel economy, and very hot engine temperatures. The prime time for the transmission to shift is at the point before the engine is just about to “wind-out” or “peak”. You don't want it to actually reach its peak rpm, but you will want it to get close. This will allow the engine to clear out (burn clean) and continue to produce good power on through the next gear ratio.

Now that you know how important the timing of the shift is, it is now time to learn what is happening to the truck when it shifts and how it affects handling. When the two-speed shifts there is a noticeable “instant” boost in speed. This boost can upset your truck's handling before and through the turn. This can cause a greater chance of you over-shooting the corner, which adds seconds onto your lap times. For instance, if you're racing on a medium-sized track that has a long straight but the rest of the track consists of many turns and switch backs, then there is a good chance that first gear will be able to handle all of the infield duties on its own. There is really no need for the transmission to shift until you're on the straightaway. In a situation like this, allowing the transmission to shift in the infield will more than likely hurt your lap times by increasing the chance of the shift throwing your truck wide in a corner. If the engine is really revving out in an area or two within the infield, then it is ok to allow your transmission to shift; just use good judgment. You may also benefit from gearing up a couple of teeth on the clutch bell to get the rpm down through the infield portion of the track. This, of course, will depend on the track size and layout.

The second gear (lower middle gear) in the close ratio gear set is
molded with 40 teeth. This is one more tooth than the stock 39T 2nd
gear. This means that the engine will have an easier time pulling the
truck around the track in second gear. Note that the top gear cluster
must be changed to match the new 2-speed gear combo.
The close ratio gear set comes with this gear.

The best solution for getting smooth shifts is to install a close-ratio two-speed kit. This brings the second gear ratio closer to the first gear ratio. This helps with several things. Shift points are less critical in keeping the engine in the proper power-band. There is less of a ratio change when shifting which makes for smoother shifts. This is what attributes to better chassis balance when shifting through a corner. A small amount of top end will be lost, but for most tracks, there is still plenty of speed with the close-ratio gear set. Also, the attitude of the truck is much easier to control when airing out on high-speed jumps.

Removing the reverse system is a big step towards reducing rotating mass inside the transmission, along with reducing the weight of the extra servo that is used to shift the system. A lightweight single-gear shaft replaces the entire system and is located where the main output shaft used to be. This set-up eliminates reverse (which is not used during racing anyway) and provides quicker acceleration.

Look at what all is removed just from your transmission
alone when converting to the FWD-only set up.

Slip a 2.0mm driver into the slipper shaft that exits the front of the
transmission to lock the shaft for adjustment. Next, use the 8.0mm
tuning wrench to adjustment the slipper nut. Be careful to
only use small adjustment changes at a time. The slipper
clutch is very sensitive to adjustments.

High Bite TracksOn high bite tracks that offer a lot of grip, a properly adjusted slipper can help prevent the front end from lifting up when accelerating out of a corner. This will keep the front end down, preventing a push or under steer.

Low Bite TracksOn slick tracks or surfaces that do not offer much traction, the slipper can be loosened enough to limit wheel spin to keep the truck stable under acceleration.

Note the orientation of the bevel washers located between the slipper
nut and the bearing within the spur gear. The outer edges of the
washer should touch each other in the middle for proper operation.

A good starting point for the slipper setting on the Revo is threading the nut to where there is only about 1/16-inch of the slipper shaft sticking out from the nut. Experiment with different settings for your track conditions to obtain optimum traction with as little wheel spin as possible.

Caution: Be careful not to run the slipper extremely loose or tight. If the slipper is set too loose, this can cause excessive heat build-up and could result in a melted spur gear. If the slipper is set too tight, not enough slip will be allowed to protect the spur gear from violently engaging with the clutch bell. This can chip the teeth of the spur gear and will eventually strip all of the spur gear’s teeth.

The differentials of the Revo are filled with a thick lube to slow down the diff action. This will help prevent the diff from unloading when powering out of the corners. Any of the hobby-class differential lubes out on the market should be fine; the thicker the lube, the slower the diff action. Slowing the diff down will make the truck's traction more positive on throttle. This will typically reduce steering entering a corner but will actually increase the steering coming out of the corner. Heavier fluids are also beneficial for higher bite surfaces. Loose and bumpy tracks will want thinner oils in the differentials.

I’ll generally start with 10K in the front and rear. Experiment with different thicknesses to see what works best for your driving style/conditions. Thicker fluids can be used to mimic thinner fluids by not using as much of it in the diff case. If you only have 30K diff fluid in your pit box and you need the feel of 20K, then you can just fill your diff cases 40-50% full of 30K fluid to get the same diff action that 20K would give you.

Thicker fluids – high-bite, smooth, high-output engine

Thinner fluids – low-bite, bumpy, lower output engine

Traxxas offers diff lube in three different thicknesses:
10K (#5135), 30K (5136), and 50K (5137).
They also offer them in a 3-pack (5136X).

Photo Courtesy of Cliff Hudson