Tech Notes: Power and efficiency
Cylinder size matters, but in this case smaller is better
By Olav Aaen
Published: Friday, March 06, 2009
Many sleds from the 1990s created more power than today's machines, thanks to their 3-cylinder engines like the one in this 1998 Yamaha SRX 600.
Ski-Doo has always packed power into its Mach Z models. This 1996 Mach was no exception with its 774cc liquid-cooled Rotax triple.
Yamaha packed plenty of power with its 3-cylinder models. This particular engine is from a 1998 model.
Polaris runs this 600cc 2-stroke twin in several of its current sleds.
The 1997 Arctic Cat Thundercat was another speedster with a honkin' triple.Dear
The Polaris XCR 800
AmSnow,
"I am disappointed in the new sled technology; it does not seem to help much. My old triple sleds were faster and often got better fuel mileage than my new twin. Why are we going backwards in so many areas?"
-Disappointed Sledder
We've heard this complaint many times during the last several years, and there is a sizeable group of snowmobilers who still swear by the older triple "muscle machines." Polaris XCR, Ski-Doo Mach Z, Arctic Thundercats and Yamaha SRX, SX and Vipers are still popular with the performance crowd.
There's a good reason for this, a 1000 Cat came stock with triple pipes and 182 hp, still more ponies than you can get from all but Cat's new Turbo 4-stroke. A good older triple can be had for $5,000 or less, if you can pry it loose from its owner. Most owners also modified their triples with pipes, big bore kits, and larger or bored-out carbs, upping the horsepower to the 200 to 250 range, depending on the mods.
There also are a number of changes in the chassis technology that have influenced mileage. Newer sleds have much more suspension travel and more aggressive tracks. This new setup requires a lot more power to turn than the older 7/8-inch ribs with 8 inches of travel.
A lot of groomed trail and lake riders today don't care for all that travel, but prefer a lower set-up that hugs the corners rather than a tall snocross-inspired bump-runner that tips up on the outside ski when you go around a corner fast. Without all the long travel suspension technology, the older sleds were also lighter than many of the newer machines. With free-turning tracks and less weight, performance was better on lakes and groomed trails, and this influenced fuel mileage.
Earlier sleds also, had simple ignitions that made it possible to rev engines higher without the ignition timing and power dropping off. With the advent of TPS (throttle position sensors) and 3-D multiple maps, manufacturers took the opportunity to install a number of limiters in order to cut down on engine damage and warranty claims. This often means that if your stock engine is tuned with a power peak at 7800 rpm, the factory may retard the engine sharply at 8200 rpm. This dramatically drops power. If you are a little off on the clutching and revs are up, power is reduced.
With the introduction of knock sensors and fuel injection, the computer richens the fuel mix and retards the ignition at the slightest sign of engine knock. Unfortunately the sensors are calibrated way too sensitively on many new machines. This results in sleds seldom running at maximum, and some 800 twins may easily be off by as much as 10 hp.
With stringent emission requirements, and additional heat sensors, there are a number of new sleds that are actually calibrated too rich, because some warning systems retard ignition and richen up fuel supply before the engine ever gets up to the heat load it was designed to run at. This is something we probably will have to live with for a couple of seasons until the engineers get the fuel injection sorted out, and it may take another generation or two of both software and hardware before the new sleds run as good as an old machine with a single ignition curve and normal carbs you could calibrate yourself.
Cylinder efficiency
These are some of the reasons why you have less power today, but there's another fundamental reason why triples are more efficient, and that is cylinder efficiency, both in how the cylinder is filled with fuel charges and how that charge is burned. The basic engineering formula for cylinder efficiency goes like this:
Displacement means the total displaced volume of the engine in cc in one revolution of the crank. RPM is how many times the volume is displaced in one minute, and BMEP (Brake Mean Effective Pressure) is an expression for what the average pressure on the piston would be per stroke in psi.
Brake Mean Effective Pressure is figured backward from the torque measured on the engine brake (dyno), its cylinder size and rpm. It would be the equivalent of the constant pressure that would have to be exerted on the piston through its full stroke to produce the torque output measured on the dyno. BMEP actually becomes a measurement of how efficiently each cylinder is filled with fuel and how well it is burned.
How much charge you can get into a cylinder depends on the size of the cylinder and the time you have to fill it. Larger cylinders are harder to fill and even more so as the rpm is increased. The advantage in efficiency therefore goes to the smaller cylinders of a triple, and with the shorter stroke it can even be revved higher. Typical BMEP values can vary from 130 psi on an old piston port engine to 200 PSI on a full race 340 mod engine.
A typical number for a 140 hp 800 twin revving at 7800 rpm would be 143.5 psi. An 800 triple producing 160 hp at 8200 rpm would produce a BMEP value of 156 psi. The difference is in the cylinder size, the 800 twin has two cylinders at 400cc each, while the triple 800 has three 267cc cylinders. Since the triple has a shorter stroke, it also can be easily revved at 9000 rpm or higher without losing much in cylinder efficiency, which will increase its output to 175 hp. If you want to try this yourself on some of your engines, here is the formula for BMEP in psi.
Improvements in port design and combustion chamber shapes have improved on the cylinder efficiency on the new breed of 800s. With a claimed power of 150 at 8000 rpm the new 800 twin engines actually are very efficient with a BMEP of 150 psi.
There is a good reason why factories are discontinuing their 1000 twins and going back to 800 twins. The large 500cc cylinders were just too hard to fill efficiently at higher rpm and didn't produce the expected benefit. In addition, the larger strokes produced difficult to tame vibration and torque reactions that were hard on the chassis, drivetrain and riders. So what is the maximum power you can expect from an all-out racing engine?
The highest BMEP value we recorded on a race engine was 198 psi with a 340 Sno Pro full-mod Ski-Doo rotary valve engine producing 105 hp at 10,000 rpm. If you want to find out what a full-mod 600cc snocross twin is producing, here are some guidelines we picked up. Most snocross race engines are revving above 9000 rpm with twin pipes, some are claiming as high as 9400 rpm. With high compression heads, ported cylinders and twin pipes a realistic BMEP figure for the 300cc cylinders of a 600 twin race engine would be 175 BMEP. Throw this in the formula,
This is actually pretty close to the numbers we're hearing through our sources on the race teams. If you go to an ISOC snocross race, listen to the full-mod sleds as they fly by, they are definitely revving those engines to the nines.
So they are getting 154 hp out of the mod 600 engines, that's more than a stock 800 twin! With smaller cylinders running at higher rpm and with better efficiency, the power is now matching the 800 mods of just a few years ago.
If you want the most power and efficiency, smaller cylinders and higher rpm are your tickets. That's why a triple will outperform a twin in power and efficiency and give you a smoother running engine to top off the bargain.
Olav Aaen is a long-time contributor to AmSnow.
As a mechanical engineer and president of Aaen Performance, Olav has been heavily involved with snowmobile performance since 1968. Aaen Performance is best known for pioneering performance pipes and introducing the roller clutch to the snowmobile market.
