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Stage Tuning – Ski-Doo MX Z/Summit 800

Stage Tuning – Ski-Doo MX Z/Summit 800 with Rich Daly – Dyno Port

Last season, Polaris got the jump on everyone with the introduction of their 800 twin offered in the RMK 800. By the time January 2000 rolled around, Ski-Doo had built a very limited number of MX Z 800s and Summit 800s. This version of the Rotax 800 was put on the Dyno Port dyno and produced just over 134 HP. At that time, we were told by Ski-Doo that it wasn’t a true representative of what the actual 2001-build 800 engines would be producing.

Now in full production, the 2001 version of the Rotax 800 is found in the MX Z 800, Summit 800, and the Lynx RAVE 800 Special (only available in Europe). Based heavily on the 700 Rotax twin, the 800 twin is a cylinder-reed “Series 3” engine with RAVE power valves, Digital Performance Management (DPM), Mikuni TM-40mm flatside carbs, and a tuned single pipe exhaust. With an 82mm bore and 75.7 stroke, the displacement is right up to 799.2 cc. The cases and the crank are similar to the 700, with repositioned connecting rods and a longer stroke. Both the exhaust port and RAVE valve are a full 5mm wider. The eight-petal reed cages are mounted on the cylinders, allowing low engine positioning.

The 800 looks different from the 700 with its 440 style head design. The cylinder heads are a two-piece sand cast design for more precise tolerances and increased head strength. A wide squish band and hemispherical combustion chamber maintains consistent (safe) power with these huge pistons. Coolant flow was optimized with a symmetrical design to the heads that cool the exhaust ports first. A thermostat allows for quicker warm-up times and consistent engine temperatures. Nicasil cylinders also help to keep the temperatures in line.

New moly coated single-ring pistons have quite a series of “grooves” under the ring. We’re told this feature is for improved oil retention and reduced wear. Larger diameter connecting rod pins on the crank should improve rigidity and reduce vibration too. The MX Z and Summit engines are the same, except for the RAVE springs. The MX Z RAVE spring is a bit stiffer, while the Summit spring is shorter which allows the valve to open easier at high altitudes (where less power is produced).

The very late release of the 800 twin has kept most of us guessing as to exactly how strong the package would be. We heard it could come with different porting, a different pipe, stronger bottom end, and possibly ignition tweaks. With the 2001 version of the Polaris 800 twin producing over 133 HP and the Arctic Cat 800 pulling over 38 HP, all eyes were on the Rotax.

For Stage Tuning the Rotax 800 we turned to Rich Daly and Dyno Port (315-258-5618). Dyno Port is an experienced and established Ski-Doo tuning center. Just this past Fall, Rich won two Pro Stock 1000 grass drag events, posting 111.8 mph in 500 feet with his Pro Stock 1000 (Formula III based) Ski-Doo.

Dyno Port spent many hours dyno testing a MX Z 800 (a for-real 2001 production model). After the engine was run for just over eight hours to get past the “break-in timer”, the engine was then dyno tested for a stock baseline, producing 135.0 HP at 7700 RPM.

Stage 1 – Dyno Port Canister Silencer

The Bulky, suitcase style stock silencer does an incredible job at silencing the exhaust note of the 800 twin, and is pretty heavy. Would there be any big power gains from a silencer that isn’t as restrictive? It looks like Ski-Doo did their homework, as the Dyno Port lightweight silencer provided just over a one-horse gain. The main attraction here will be for those looking for a weight savings and no loss of power. The Dyno Port canister weighs about five pounds less than the stock unit, and the noise level is increased only slightly; really.

Slight RPM differences from one stage to the next shouldn’t be a concern; this is more likely caused by slight differences in pipe temperature than an actual operating RPM change.

Stage 2 – Dyno Port Single Pipe & Silencer

The Dyno Port single pipe had provided a nice improvement on the 700 twin, how would a similarly designed single pipe perform on the 800 twin? Many hours were spent on this next stage, with the result being once again a big volume single pipe and canister silencer. The net gain was 4.6 HP, spinning the big twin right at 7800 RPM. The Dyno Port single exhaust and silencer package provides nearly a 10-pound weight savings over the stock pipe and silencer. This single pipe also works with the stock silencer! Rich commented that the factory Y-pipe is very efficient, as they found nothing to be gained here after several hours were spent on this area.

When we analyzed the fuel consumption numbers for all of these 800 twin runs, we questioned if they were a bit lean. Rich pointed out that the Rotax 800 is very much like the other Rotax Series 3 engines in that it is capable of making power at lower than what we’ve-become-used-to fuel consumption. Rich credits excellent engine cooling along with the piston and cylinder design that doesn’t rely on extra fuel as a coolant to keep combustion temperatures in line.

Stage 3 – Porting and Head Work

Stage 3 is where the big twin started to make some power. The engine gurus at Dyno Port trail-ported the cylinders and changed the port timing by installing cylinder shims to raise the cylinders. This required a head cut to make it all work again, so while they were at it they gave it a bit more compression (about 8-10 pounds) than the stock package, but still safe for 92-octane pump gas. The result? Another 6 HP, now up to 145.6 HP @ 7900 RPM. By itself, Rich estimated the cylinder shim kit and head combo with stock cylinders should be good for about a two-horse gain.

Stage 4 – V-Force Delta II Reed Cages

Once the airflow of an engine has been increased, the stock reed cages can become a significant limiting factor in the overall pumping capacity of the engine. So for stage 4, Rich modified a set of Delta V-Force II reed cages (for a 2000 MX Z 700) so they would fit the flatside carbs on the 800. The addition of the reed cages broadened the torque band. Typically we see a more significant throttle response difference in the field than what dyno results would indicate. We expect the actual 2001 production reed cages to perform better than these modified cages.

Stage 5 – Bored Carbs

If there were not any huge airflow restriction in the reed cages, would the package flow more air with the TM-40mm carbs bored? Rich bored the carbs out to 42mm (2mm is about the maximum you can go with flatsides) and increased the mains to reflect the airflow increase. This time the big twin hit 150.6 HP!

Beyond Stage 5

What about twin pipes? In the middle of this dyno session (between stages 2 and 3), Dyno Port obtained a set of factory Ski-Doo snow cross racing twin pipes. They pulled 137.9 HP and 87.3 foot pounds of torque at 8300 RPM and were still climbing, but the stock ignition had a rev limiter that kicks in at 8350 so they were not able to determine the full potential of the pipes.

Rich pointed out that the added cost and weight of twin pipes would be tough to justify if they only provided a couple more HP, adding that the single pipe really works well with a stock engine or one with trail porting. Since the rev limiter kicks in so low, twin pipes could likely require reprogramming of the ignition to realize the full benefit. As the season progresses, we will all learn more about the potential of this new Rotax 800 twin.

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