2003 Arctic Cat Firecat F5

A lesson in power to weight performance.
There has been much to do about Arctic Cat's lack of a 600cc performance sled this season. While we readily admit it does seem odd, we thought we'd see just how the F5 engine performs before we got worked into a knot.

Tech Notes
The F5 mill uses the bottom half of the 440 laydown race engine. It measures 71mm across with a 63mm stroke. Like the Sno Pro engine, it draws its air in through the beak of the hood, through the high tech hard foam airbox and into the engine's front side through heated TM 38 carbs.

The 500 laydown engine continues the usage of Arctic Cat's unique pipe temperature sensor in a consumer model. Cat's performance engine guru Greg Spaulding actually holds a patent on the EPTS with regards to using it to determine ignition timing patterns. While other brands use coolant temperature to adjust timing curves, the F5's exhaust sensor is said to be substantially quicker to react to what the engine is actually doing.

Like the F7 engine, Spaulding and his design team route the coolant system backwards from conventional engines. By sending fresh, cool water through the heads first and exiting through the crank case, they found they could maintain a more consistent combustion chamber temperature, which allowed them to take the port timing and ignition closer to the edge. This is how they developed the power they did.
On the Dyno
Again facing a print deadline without a delivered production engine from any factory, we turned to the Arctic Cat engineering department for a dyno run on a production pilot F5 engine.

We pulled 102.4 horsepower out of the new 500cc engine. The power band should be plenty big for most guys to tune their clutches; it stays above 100 hp for 400 rpm from 8300 to 8600. Torque on the engine is a moderate 63.5 pounds. But, again, it has a nice wide band to shoot for.

We did notice there is a difference in the way the Arctic Cat engineers pull a dyno run and our usual dyno guy Rich Daly does it. Cat made a long, three-minute pull, like a long Canadian lake run. Rich makes a quick pull, more like a blast down a drag strip. As a result, Cat needs to keep a very safe BSFC in the .6 range. If we had done the test at Dynoport, we could have leaned the engine down into at least the low .5s and lived. That would have given us a few more horsepower at the top end.
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