3 steps to more ipoweri

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We're always thinking project sleds here because we know you like to see how we, and then you, could improve a sled.

This year the editors suggested I do something with a Ski-Doo REV, the industry's top seller the past few seasons. Since we have three customers that are asphalt dragging their REV 800s it seemed a good choice. Plus we have a lot of good data on this model, both from the dyno and the asphalt drag strip. We have been both grass dragging and asphalt racing with an '03 REV 800 as well as hitting the trails hard in the winter with long trips both in the western mountains and far into Canada.

The sled - now owned by racer Butch Veltum - was updated with an '04 engine last season courtesy of Monroe Powersports, Butch's racing sponsor, and this is the model we have been basing our development on for the past two years. Here are the changes we made to give the 800 a 21- horsepower boost.

Stage 1:
Adding a new pipe
Pipes are one of my firm's main products, and testing different configurations takes up most of the development time. That's especially true when working on a Ski-Doo, since they are known to have a good stock pipe.

Add to that our development of "Trail Quiet" technology aimed at meeting the newly mandated 88- decibel (dB) sound levels, and developing new pipe models becomes a constant process. Luckily we had a good jump on the pipes because we began building "Internal Stinger" pipes in 1978, so we knew how to tune them.

Internal stingers bleed the exhaust from the pipe's center section, where the pressure and temperature is lowest. As a result, sound pressure waves are reduced by 50% (6 dB) before the power pulses reach the silencer. With the sound level cut in half compared to a rear outlet pipe, we can use a smaller double core silencer and save considerable weight, about 15 lbs. Cutting weight is a big step in making a sled faster.

Internal stinger pipes also tune differently than conventional pipes. The return pulse builds power quicker because there is no bleed at the end of the pipe's rear cone. Throttle response and acceleration is quicker, and the pipe also builds more peak power as a result of the stronger return pulse.

That said, the latest stamped version of the internal stinger pipe we added here is producing 8.1 hp more than the stock pipe, up from 140.2 hp at 7,600 rpm stock to 148.3 hp at 7,800 rpm.

Back-to-back tests at the drag strip with a good clutch set-up netted an ET of 11.114 seconds at 108.22 mph with the stock pipe, compared to 10.840 seconds and 112.69 mph with the new internal stinger pipe. But that was just the beginning of our search for more horsepower.

Stage 2:
Porting the cylinders
The next step was to port the cylinders and machine the motor's head.

The 800 H.O. engine's porting can be improved if you don't over do it. We raised the transfers and back port 1 mm, and widened the side exhaust ports, while leaving the main exhaust port the same. The porting by itself resulted in a 4 hp increase, and the head machining complemented that with an additional 3 horses, for a total gain of 7 hp.

Many REV owners prefer to do the head machining only, getting better response with more torque in the mid-range and that's certainly a reasonable option. With the ported cylinders, machined head and the new pipe we moved the sled up to 155 hp at 7,900 rpm. The result was a 10.631 second run at 116.23 mph on the ¼-mile asphalt at nearby Great Lakes Drag Strip.

Stage 3:
840 big bore kit & pipe
Butch then wanted to go a step further and coaxed me into building a big bore kit.

After some research we found an 84 mm Wiseco piston originally used in a 780 Sea-Doo watercraft. The piston is 2 mm over stock, which calculates out to 839cc, which results in a solid 5% increase in displacement. Add 5% to 155 hp and you could see as much as an 8-horse gain over just the porting.

While the stock piston has just a single ring, the Wiseco piston has a top L-ring and a lower straight ring. With a twin ring set-up you are going to lose about 1 hp per cylinder to friction, but I much prefer the twin ring design, because it provides a larger margin of safety against blowby.

For our sled, the cylinders were bored out and ported, and then re-nicasiled (NSC) by Millennium Technologies. Machining the head required quite a few changes because the dome on the watercraft piston is taller than most others. As a result, the squish angle is steeper and the combustion chambers had to be re-machined and enlarged considerably to achieve the correct compression ratio.

Butch runs the thinnest Ski-Doo base gasket when running on asphalt, and plans to install the thickest base gasket for his winter riding. Back on the dyno, the big bore kit pumped out 161.2 horses at 7,900 rpm with 107.5 foot-pounds of torque. That's a gain of roughly 21 hp.

All our power numbers are based on dyno tests made during the summer and corrected to SAE automotive standards. In the winter, a snowmobile engine produces considerably more power because of the dense, cold air, so as a result, the 161 hp in August translates to somewhere around 170 horses at the crank in January. There are not many other trail sleds out there that can boast that sort of number!

Track tested results
Back on the asphalt Butch clocked a ¼-mile run in 10.301 seconds with a top speed of 119.18 mph. The increased torque also cut a tenth off his 60-ft. time.

Butch later clocked a personal best of 10.180 seconds at 119.34 mph, but this was in heavy night air with a monster hook-up. Consistent hot day runs stay in the mid 10.3-second range when he uses slightly less aggressive clutching for greater consistency during bracket racing.

Final stage:
The look
To make the sled look racier, Butch took his REV to the local custom shop, Exclusive, in Fox Lake, Ill., for lettering and a silver reflective flame job. Now the REV looks as speedy as it really is!

Naturally it's possible to get even more out of an 800 H.O., but that involves reprogramming the ignition box and revving the engine up to 8,500 rpm, with bigger carbs and twin pipes. However, Ski-Doo's race department warns against revving the H.O. engine above 8,700 rpm, as Doo has experienced crank problems on its race sleds beyond that point. Yet with the new and stronger crank in the 800R motor, the rpm limit should be moved up considerably (see related story, www.amsnow.com/sno/default.aspx?c=a&id=2762.)

All our mods to Butch's machine produce power below 8,000 rpm for reliability on the trail, plus we stayed with the stock ignition box.

So, with 21 additional horses and more torque in a light, easy-handling REV, Butch is ready to take on the best of the 1000cc sleds on the trails this winter. A good performance shop could do the same for your REV!

2004 Ski-Doo 800 H.O. Project Sled
STOCK: 140.2 SAE hp, 7,600 rpm, 1/4-mile time: 11.114 sec., 1/4-mile mph: 108.22
STOCK & PIPE: Stage 1, 148.3 SAE hp, 7,800 rpm, hp over stock: 8.1, 1/4-mile time: 10.840 sec., 1/4-mile mph: 112.69
PIPE & PORTED-HEAD MACHINE: Stage 2, 155.2 SAE hp, 7,900 rpm, hp over stock: 15, 1/4-mile time: 10.631 sec., 1/4-mile mph: 116.23
PIPE & 840 CC BIG BORE KIT: Stage 3, 161.2 SAE hp, 7,900 rpm, hp over stock: 21, 1/4-mile time: 10.301 sec., 1/4-mile mph: 119.18