Suzuki RMX450Z Off-Road Project, Part 2
The stock Suzuki aluminum muffler is pretty light, but is very restrictive and makes the bike run hot and slow. If you remove the quiet core and spark arrestor for closed-course racing the bike feels pipey for a 450F but runs cooler and does pull harder.
Plug a Yoshimura Cherry Bomb EFI reprogramming gizmo into the wiring harness and the uncorked RMX runs better, but the problem is excessive noise ... when you uncork the RMX muffler the bike becomes obnoxious.
We nearly got kicked out of our local motocross track because by running the uncorked stock muffler, even though every other bike on the track was a full-blown 250 or 450F motocross bike.
The Yoshimura RS-4 full-exhaust system is the best answer we found. Constructed from stainless steel and aluminum, with a carbon fiber end cap, the RS4 comes with a 96dbA spark arrestor insert. We got the optional 94dbA insert as well. It is shipped with the Yoshimura Cherry Bomb and simply plugs into the RMX wiring harness and re-maps the fuel injection system for closed-course use with an aftermarket or uncorked stock exhaust.
The complete Yoshimura RS-4 system weighs in, according to our local postal scale, at a shade over two pounds lighter (7.9 vs. 10.2 pounds) than stock. It installed easily and fit perfectly. With the 96dbA insert the RMX was much quieter to the ear than the uncorked stock Suzuki muffler was.
We experimented with the 94 and 96dbA inserts. With the 94 the RMX ran about the same as it did with the uncorked stock muffler. The 96 gave more bottom end and smoothed-out the powerband. We can’t lie though ... the RMX runs best with the Yoshimura muffler in noisy “motocross” form. The bottom-end power and torque are stronger and the mid to top-end pull is smooth and strong. With the “mx-spec” Yoshimura pipe and the Cherrybomb our RMX loses almost no ground to a 450 MX bike through the gears, and at the track third gear starts are easy.
“I did not have an issue with the sound of the Yoshimura RMX exhaust,” West said after racing the bike. “At least it never came to mind while I was riding, so I certainly was not annoyed. It just seemed to have a nice ‘barky’ 450 sound. But with that much power on tap at any time I would certainly try a quieter insert for an extended ride.”
While the Yoshimura-piped and programmed RMX pulls great, the power delivery is a little too abrupt down low for tight trails, tiring the rider and making traction difficult to find at times. Suzuki didn't put a really heavy flywheel on the RMX, so it spins the tire pretty easily and will stall if you get sloppy.
The solution might be a Rekluse clutch, which we will be trying soon. Rodney Smith also recommended gearing the bike taller by going down two teeth on the rear sprocket, both to make the power more controllable and to allow the RMX to pull second gear longer in the woods. We'll be trying those things soon, as well as fine-tuning the EFI mapping beyond what is capable with the simple plug-in Yoshimura Cherry Bomb.
The WER-tuned suspension is amazing out back. With input from WER's Drew Smith we've been fiddling with compression settings and oil level on the forks, and they are getting better. Our fix for 'too soft or too hard' forks just might be Fasstco Flexx Bars.
The 10-degree sweep/Moto-hi bars we chose from Fasstco (there are many bends available) got rid of the RMX’s slightly low front end. The Flexx bars don’t feel weird, and with the stiff red dampers you only notice them when you do something like come up short on a double or plow into a log and don't get a sharp jolt through your wrists. They do add weight over the stock Suzuki aluminum handlebars, but they are about the same weight as old-school steel handlebars.
We raced the Fasstco bars with the two “hard” red dampers, but we have since been experimenting with softer compression dampers. Our hope is to be able to run the forks a stiff enough for moto but still keep trail junk from beating us up.
We used AME Grips because they’re comfy and last a long time. The Enduro Engineering Roost Deflectors are quick to install and will withstand a much harder hit than normal motocross-style handguards without being too big and heavy like wrap-around aluminum handguards. Fuel capacity on the RMX is minimal. We love the slim cockpit of the RMX, and the aluminum tank looks cool, but we'll be installing a bigger IMS fuel tank soon.
We blinged-out our ‘Zook with graphics from Factory FX. Although they were designed for the RMZ motocross bike they fit the RMX just fine after careful trimming. In part-three of our RMX project we'll focus on smoothing out the power delivery with refined EFI mapping, gearing changes and a Rekluse clutch. We'll try the bigger IMS tank and install a heavy-duty Scorpion skid plate. We'll lower the center of gravity by using a mini battery repositioned under the skid plate where the stock radiator overflow tank used to live, and will be swapping to a lighter and better handling 19-inch rear wheel. Then when things freeze up this winter we'll find some serious horsepower for ice racing!
Thank you AME Grips, Clare’s Cycle, Enduro Engineering, Factory FX, Fasstco, Scorpion, SSR, Suzuki, WER and Yoshimura for your help with this project!