By: Charles Salyers
With the introduction of Bombardier’s DS650 and Yamaha’s new Raptor 660R, Honda is going to need to step up and release a big bore sport ATV. Today Hondas biggest sport 4-stroke is the TRX400EX, which is also the most popular 4-stroke sport ATV available.
The combination of a good handling chassis and a reliable 4-stroke engine are the main reasons it has been such a great seller for Honda. The owners of the 400ex’s wanting more power are left with finding the extra cc’s from big bore kits and other aftermarket power accessories.
Owning a 400ex myself, and doing a lot of Dune playing I decided to check out what is available for my 400ex. To start I contacted one of the best engine builders in the 4-stroke field, Rick Ritter of Ritter Cycle in St. Jacob IL.
After speaking with Rick on what options are available for the 400ex to gain more power. He explained how to get more engine performance from this model without reducing the reliability of the engine.
High performance usually refers to power output of an engine, by using high quality replacement parts, durability can also be associated with high performance as well. To start with a 440 kit was chosen.
When Rick receives the engine he starts spinning the wrenches to take a look inside. He thoroughly inspects it, including the transmission to see if any gears are worn or for other parts worn to be replaced during the assembly. With the motor now completely cleaned and inspected, he begins the machine work.
When installing a 440 kit the factory sleeve is removed and the aluminum block is bored to accept the larger sleeve. After the sleeve is installed it again goes to the boring bar to be properly sized.
He uses a Kwik-Way boring bar, it is a quite accurate piece of equipment. Doing this properly is a must!
While the cylinder boring is going on, he also checks the crankshaft runout on the ends. If the crank would need replacing now is the time to do it. The Connecting rod is also checked.
After the cylinder boring is done and the new sleeve installed he checks all clearances to insure a proper fit of the new 89mm JE piston that comes with the 440 kit.
Next the cases are reassembled , all bolts torqued to Honda factory specs. The cylinder and piston are installed. The next step is the Head / Valves / Cam /Rockers. Head porting DECREASES how much power is needed to bring air into an engine.
When the engine can get air easier, it can send more power to the tires. Good 4-stroke porting will not harm reliability in the least.
Next step on the Head is to do a 3 angle valve job on the freshly ported head. The most frequent and common failures in a 400ex engine are the intake valves. He replaces them with high quality Black Diamond valves.
The exhaust valves are also available, but many times the exhaust are not in need of replacement. When Rick replaces the factory cam with a more performance orientated cam, he also recommends installing a set of RcR Super Rockers.
These hard welded and reground rockers greatly increase the service life of the 440ex engine. Rick always says “High performance, can also mean high RELIABILITY”
With the cam in place using a special assembly lube the engine can be completely assembled, timed and the valves adjusted. The new 440cc engine can be expected to run and perform at a much higher performance level than a stock 400ex .
Total gain in power is 50% over stock.
So to gain a 50% increase and not loose any reliability the 440 kit makes for a smart choice for the people who love their 400ex but want to make sure those big bore newbies can’t leave you sitting in their dust.
It’s the kit of choice. My next step is to test this 440ex against a stock 400ex and a mild bore 415ex results to come soon, so please stay tuned.
First posted on ATVSource.com on 9 March 2001 by Charles Salyes.
