Straightline Performance Inc. (SPI) said last fall that its head and pipe would add 10 hp to our 2008 Ski-Doo MX Z TNT demo sled, bumping it into the 115 hp range. Excellent, we thought. But we weren’t so concerned about quantifiable gains; we wanted to feel the improvement rather than read about it on a dyno chart. After running the mods for almost 2,000 miles, our impression was that the modded 597cc Rotax performed better than stock.

The sled had 500 miles on the odometer when the parts were installed, so the engine was broken-in and we had a good baseline impression to compare the stock setup to the hopped-up version. With the mods, we felt a stronger pull from the low-end through the mid-range and throttle response was meatier so it was more likely to stretch a driver’s fingers.

Top-end power made the machine run with other sleds that outclassed it in terms of horsepower. In fact, our TNT ran dead even with a 145 hp Cat and 125 hp Polaris in a drag race up to 80 mph.

This little modified sled ran like a top last winter with one hiccup early on – which might have been caused by user error – and a cooling problem late in the season.

We were told to run 91-octane fuel because of the compression boost caused by the head and more air flow from the pipe. After installing the head and pipe, we added a few gallons of premium fuel into the tank of 87-octane gas and figured it would be OK. But after about 50 miles into our first ride with the aftermarket parts, the engine shut down while running about 60 mph, perhaps due to detonation.

There might have been too much regular fuel left in the tank when we topped it off with premium, which means the fuel didn’t meet the engine’s requirements. We let the engine cool down for a few minutes and then restarted it.

After nursing it home, we siphoned the gasoline and filled the tank with pure 91-octane gas and wrote “91” on the cap to make sure the mistake wouldn’t happen again. An inspection of the engine’s internals showed no damage to the top end or spark plugs nor any signs of detonation.

Late in the season the engine overheated, which cooked the cylinder head o-rings. Or, perhaps the o-rings failed and caused the engine to overheat. Which came first: the chicken or the egg? It’s hard to say. We installed new o-rings and ran it without any further trouble.

Torque is what you feel when you squeeze the throttle lever. Boosting torque output from an engine, and therein horsepower, typically means its peak rpm will increase. This is why clutch modifications are necessary for modded engines. But in this case with SPI’s pipe, it was designed to make more peak torque at 7600 rpm, which is about 400 rpm less than the stock setting. Unfortunately, a communication breakdown between SPI and us led to us running with stock clutch calibrations all season even though the drive pulley should’ve been loaded with more pin weight. We left some power on the table.

Despite that the calibrations didn’t fully harness the modifications, we felt gains over the stock setup. The sled was more responsive and had a generally livelier attitude on the trails. Top-end speed was unchanged compared to stock, but SPI owner Jason Houle said TNTs he tuned with this kit and another 3 to 4 grams of pin weight in the clutch picked up 6 to 7 mph.

This test proved that clear communication is important in order to take full advantage of sled mods. Our TNT’s performance improved over stock with the $390 head kit and $425 pipe, but it might have been better if we had double-checked the setup with SPI.

Is this package worth the money? Absolutely. All test riders were surprised at how powerful the engine was and how fun the machine was to ride without the extra noise that some aftermarket pipes make. It has a mild rasp around 3000 rpm, but is otherwise unnoticeable from stock.

If you buy SPI’s pipe and head for your TNT, make sure you pick up the $27.50 clutch pin kit to dial it in.

Straightline Performance Inc.
Forest Lake, Minnesota
651/466-0212
straightlineperformance.com

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