Dial A Boost For the Wastegate

The Secret Weapon

Wastegate adjuster helps the XN85 reach its full potential

Suzuki XN85 Turbo owners have long been envious of their Kawasaki GPz750 Turbo brethren. Not only was the Kawasaki as fast as a striped ape right out of the box, but there's a shopping-cart load of tricks available from Horsepower Unlimited if more power is what you crave. Low production numbers and the mysterious last-minute power-ectomy Suzuki performed on an engine that was painstakingly engineered to handle gobs of extra horses all but conspired to keep the performance junkies away from the Suzi-T.
   Short of designing a high-performance chip to slip into the computer there' s precious little one can do to unleash those huddled ponies yearning to breath free.
   If you're one of the lucky ones your XN's turbocharger will give you about 9 psi of boost before the wastegate opens and it's lights out, party's over. Unfortunately, many XN wastegates would short-change you and open at an anemic 7.5 psi. That's why many owners--myself included--
would never see the boost gauge fully lit in all its glory. Andy Morris of Long Island, NY, also experienced this "psi envy," if you will, with his own XN85. Andy decided not enough was not enough and set out to do something about it. But What?
   The service manual says that when the boost pressure reaches 12.5 psi the computer sends a pink slip to the fuel injectors informing them that their services are no longer required until things calm down in the boiler room. So to modify the wastegate to stay closed beyond that limit is fruitless unless you can design that aforementioned performance chip. But in between that cutoff point and the standard wastegate opening pressure lies approximately 5 psi of "gray" area. So Andy put his gray matter to work.
   The result is what Andy calls the "Secret Weapon." The "Secret" refers not to the science of the device, which is actually quite simple, but to its hidden location in the storage pocket in the right side of the fairing. What the device does is no surprise to those in the turbo "know." It simply bleeds off excess wastegae pressure allowing the turbocharger to build boost closer to the 12.5 psi "kill" point, increasing power in the process.
   But the true beauty of the device lies in its simplicity. The Weapon consists of an adjustable valve (Fig. A) and all hoses and hardware needed to install it in less than 15 minutes. That's right, less than a quarter of an hour. The only modification, if you can call it that, is to ream out the existing hole in the bottom of the fairing pocket to allow the hose to squeeze through where it attached to the sole outlet in the valve. With a little skill you can install in so the adjusting knob on the valve stays upright in the pocket. Mine just flops over and lies upside down in the compartment. Oh well.
   Don't go hunting for Andy Morris and begging for a Secret Weapon. He's long since gotten rid of the XN85 and moved on to more modern hardware. But, you can easily (and cheaply) replicate the kit yourself. Here's what you'll need:

• Parker 14 R11OF regulator (or similar)
• 125 HBL-4-4 (Hose barb)
• 3 1/2 ft. hose (1/4" I.D.)
• 4 hose clamps
• 3-way "T"

If the boost reaches 12.5 psi the wastegate pressure sensor will cut-off the fuel injectors as advertised. If this happens to you Andy instructs to turn the knob (pull it out to adjust, push it in to lock) in increments of 1/8 counterclockwise rotations until you can see full boost before the rev limiter does just that and cuts the ignition (about 9,500 rpm). If the rev limiter does do its thing before you reach maximum boost then you must twist the knob in 1/8 increments clockwise. You might have guessed by now that it’s a little difficult watching the tach with one eye, the boost gauge with the other, and looking out for the local uniformed revenue enhancers with your third. But it’s easier than it sounds, trust me.

Which brings me to my experience with the Secret Weapon. Initially I reached 12.5 psi before the rev limiter could cut the juice. Strangely enough I didn’t noticed my boost gauge reaching what Andy refers to as the “forbidden zone” (that 7th segment of the gauge indicating full boost that never seemed to light). I stopped the bike and gave the knob the prescribed 1/8 turn counterclockwise twist. Va-roooooom...back out on the road and once again the rev’s dropped before reaching 9,500 rpm. Stop and adjust 1/8 turn. Same thing.

OK, how about a quarter turn?

Eureka! I’ve found it! (curiously enough the motto of Andy’s home state). In third gear I saw the XN boost gauge’s forbidden 7th light wink briefly just before the rev limiter woke up at a promised 9,500 rpm. With a grin I took the XN on the NJ Turnpike to repeat the thrill and was not disappointed, the 7th boost gauge segment showing its approval. Hey, it works!

Andy claims that installation of the Secret Weapon yields a louder exhaust, and makes the bike rev faster and pull harder. I didn’t notice an increase in decibels but the remaining claims appear valid, at least by my seat-of-the pants measurements.

Andy cautions not to run at full boost for more than 10-20 seconds at a time unless your running racing fuel. No problem here, Mr. Morris. He also warns that temperature and humidity can affect the valve settings. In other words, future tweaking may be necessary.

Keep in mind that fuel mileage will suffer slightly with the device installed—but hey, that’s the price of performance.

One last note: After returning to my garage after testing the personality-enhanced XN, I noticed a faint blue haze coming from the exhaust. It went away after about 3 minutes of idling but reappeared every time I achieved maximum boost— for about a week, or about 200 miles of riding. Then it never occurred again. I guess the turbocharger just decided to give up its smoky protest to the added power. Va-roooooom!

Buy one Air Valve from your local Pepboys or other preferred store.

Splice the hose into the existing wastegate plumbing (Fig. B).

Run it along he bottom of the fuel tank (Fig. C).

Insert the tube through the underside of the pocket hole you reamed out some (Fig. D).

Attach the "Secret Weapon", tighten the clamps and the "hard" work is done. Now for the fun part!

Once installed the bike must be road tested and the Weapon adjusted to insure the wastegate isn't allowed to stay closed long enough to reach 12.5 psi. Andy's hand-drawn but well-executed instructions suggest turning the knob on the regulator "clockwise" until it bottoms out (Andy marked the knob with a crude but easily visible reference point).

Now find a long, desolated stretch of road and run the bike to redline through the gears (this proved to be the hardest part for me living just outside New York City).

To make maximum boost and power without triggering the pressure sensor ignition cut-off (12.5 psi), you must locate that limit with the regulator and "back off" by turning the knob 1/8 turn counterclockwise. Begin by turning knob all the way clockwise. Valve is now wide open, turn it 8 times counterclockwise and run the bike wide open to 9500 rpm through the gears. If ignition cuts out (briefly) you've exceeded 12.5 psi and need to "back off" another counterclockwise turn. If ignition DOES NOT cut off after the initial 8 turns counterclockwise, find the limit by turning the knob in 1/8 turn increments clockwise until ignition cuts out, then you "back off" 1/8 turns counterclockwise until ignition is normal again.
You have now found 12 psi boost and 10-15 extra horses, which should bring you close to Suzuki's infamous promise of 85 hp (but you knew what the "85" in XN85 meant anyway, right?)