Traxxas 4902, Owner'S Manual

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T-MAXX 2.5R

Terms to Know

You’ll find these Nitro R/C engine terms throughout this section 
of the manual.

.15

- .15 or “15” refers to the displacement of the engine. The TRX 2.5

Racing Engine is .15 cubic inches or 2.5 cubic centimeters (cc). The name
“TRX 2.5” is derived from the cc measurement. 

ABC

- Abbreviation for aluminum, brass, and chrome. Refers to engine

construction that consists of an aluminum piston that slides in a chrome-
plated brass sleeve. The TRX 2.5R uses ABC construction.

Air filter

- The air filter sits atop the carburetor and prevents harmful dust

and dirt from entering the engine. Dirt ingestion is the number one
cause of premature engine failure so the engine should never be run
without the air filter in place.

BDC

- Bottom dead center. The bottom-most position of the engine piston

stroke.

Break-in

- Break-in is the procedure for running a brand new engine

according to specific instructions. This correctly prepares the engine for
normal running. The break-in procedure can be different for different
makes of engines. Follow the Traxxas directions for break-in exactly. 

Carb

- Abbreviation for carburetor.

Carburetor

- The carburetor atomizes (mixes) the fuel with the air so that

the engine can burn it. There are two types of carburetors; slide carbs
and barrel carbs. The TRX 2.5R uses the superior slide carburetor design.

Clean-out

- Cleaning-out is a condition that occurs when the engine is

accelerating and the fuel mixture becomes sufficiently lean to allow the
engine to continue into its upper rpm power band. It is usually
characterized by a noticeable decrease in blue exhaust smoke and a
dramatic increase in engine speed. 

Combustion chamber

- The combustion chamber is machined into the

bottom of the cylinder head. This is where the glow plug ignites the fuel.
The shape of the combustion chamber is designed to promote more
efficient burning of the fuel.

Connecting rod 

- The connecting rod transfers the piston motion to the

crankshaft. The TRX 2.5R Racing Engine uses a “knife-edged” connecting
rod. The aerodynamic, sharpened edges allow it to “slice” through the
pressurized air/fuel mixture inside the crankcase.

Crankcase

- The engine’s “body” that contains all of the running

mechanical components.

Crankshaft

- The main shaft of the engine that holds the 

reciprocating assembly.

Cooling fins

- The cooling fins are milled into the cylinder head and

crankcase and cause heat to be drawn away from the engine. Heat is
removed when it dissipates into the air passing across the cooling fins. It
is important to keep the fins clean of dirt and debris for maximum
cooling efficiency.

Cylinder head (head)

- The finned aluminum part on top of the engine

that is responsible for dissipating most of the engine’s heat. The
combustion chamber is machined into the bottom of the head. 

Dyno

- Abbreviation for dynamometer. A precise piece of testing

equipment that accurately measures engine power and torque output
over the engine’s entire rpm range. 

EZ-Start

- Traxxas on-board electric starting system. The system consists of

a hand held starter control unit and an on-board gearbox with an electric
motor to spin the engine. 

Filter foam

- The oiled foam element inside the air filter housing. The filter

foam in the TRX 2.5R must be thoroughly cleaned and re-oiled after
every hour of run time.

Fit

- Usually refers to the fit of the piston and sleeve. If the fit is tight, the

piston will feel very tight at top of the sleeve (top dead center), and the
engine will have good sealing and compression. If the fit is loose,
compression will be low and both the piston and sleeve should be
replaced. 

Flame-out

- Occurs when the engine stops running at high rpm. Usually

the fault of an excessively lean fuel mixture or glow plug failure.

Fuel

- (10%, 20%, 33%) The TRX 2.5R must have model engine fuel to run.

Traxxas Top Fuel

™

is recommended. Fuel is sold in quarts and gallons

from hobby dealers. The 10%, 20% and 33% labeling refers to the
percentage of nitromethane contained in the fuel.

Fuel mixture

- The ratio of fuel to air as determined by the needle settings

of the carburetor.

Fuel tubing (fuel line)

- The thick silicone tubing that carries fuel from the

fuel tank to the carburetor. 

Glow plug

- The glow plug is located in the cylinder head at the top of the

combustion chamber. It contains an element that glows red hot when
voltage is applied. When the engine is being started, the heat from the
glow plug ignites the fuel mixture and starts the combustion process.

Glow plug driver

- This tool clips onto the glow plug and supplies the

required voltage to light the glow plug element. It is also called an
igniter. EZ-Start equipped engines do not require this separate tool.

The TRX 2.5R Racing Engine