Polaris and Bombardier are using detonation sensing technology on some of their engines to help alert us while  riding or dyno testing,  and to  try to protect sled owners who experience detonation on their engines.  They utilize  engine noise pickups bolted to the cylinder head surface and some computer tries to recognize some new loud vibration as possible deto, and adds fuel and/ or retards timing to reduce combustion chamber temperature. Lower temperature (and reduced HP) should eliminate the detonation and all is well.

Bombardier is smart, and has programmed their newest computers to recognize and remember detonation, even to the point of going into a permanent, dealer-only removable fail-safe mode if severe engine knock occurs. This way they might have the option of avoiding responsibility if the owner of a new XP800R detos a piston/ cylinder on 75 octane gas unknowingly used by the sled owner.

75 octane?

I use that low octane rating rating as an example since 75 was the lowest octane test result done by Dateline NBC on 85 samples of "name brand" high test gasoline purchased in California. In that sample group, 11 were substandard, well below the octane posted on the pump.

Since then there have been more eye-opening reports of octane fraud at the pump. Car and Driver magazine reports that of 2816 samples of "high test" gasoline tested the previous year in Michigan by the Bureau of Weights and Measures, 217 failed  because of "low octane". Just last week the Today Show did a segment on NY State gasoline cheating, stating that 15% of gasoline tested this winter in NY was substandard. Roll the dice.

Digesting that information, we can assume that 10% of the time, when we fill our sleds with high test, we get boned with 87 or worse. This is no problem for most stock sleds which have safe BSFC and excellent cooling systems. But some high performance stockers that have marginal cooling systems (to save weight and cost) seem to be susceptible  to detonation. My opinion is that the new Arctic Cats which have "reverse cooling" (heads get the cold water first like modern automotive performance street and race engines) are most resistant to detonation, and can run higher HP on pump gas than most others. I have no actual  hard data, just feedback from many 100's of dyno users who run powerfully on the lakes and trails.

Now that we can enlist SkiDoo dealer Jim Cooper to monitor engine conditions during dyno testing SkiDoo engines, we see even light to moderate deto on his computer before things get bad (either going into "safe mode" or actually eroding a piston. We let Jim's deto meter go up to maybe half way before detuning. Richard Lavanant brought "93" octane gas from PA before doing stage tuning on his new XP800R. Bone stock with bone stock jetting Richard's engine made Jim Cooper's deto meter rattle hard to the right on his gasoline. Richard filled a jug with 93 from the gas station near the dyno, and that detonation was gone.

This same problem occurred with  an XP860R big bore tested on my dyno a few weeks ago. This anonymous owner brought  five gallons of "93" from his uncle's gas station in Rochester, and that gas pegged Jim Cooper's deto meter instantly with "safe" jetting! That problem also went away with gas from around the corner from the dyno. Now the uncle knew that his nephew was coming to dyno his sled, and I would think that if he was knowingly selling 87 for 93 he would have suggested that his nephew use different gas. In that case I'm betting that the gas wholesaler could be the culprit. Think of the extra $ people make for selling 87 as 93! How about the bar on the trail with above ground tank with "93" written in Magic Marker on the pump?

I get calls/ emails too often from people who have tuned closely for max "93" performance, wondering about mysterious, sudden detonation after many miles of dandiness.  Always, something has differed from those good times.  Leaner mixture?  Hotter engine? Longer  time at WOT? Usually it's the gas.

So what are we to do? The deto protection on Polaris' and SkiDoos is better than nothing, but not perfect. Extended periods of light occasional detonation, seemingly deemed acceptable by the sleds' computer, can gradually overheat pistons as the random "hits" of deto scour away the cooling boundary of air protecting the aluminum pistons (1200 degree melting point) from the 5000 degree flame. Pistons grow and sieze on "four corners", or actually melt just before they seize, because of the loss of that cooling boundary of air. New piston, new nickasil, all because some greedy wholesaler or retailer wanted to pocket an extra $2 per sled tankful.

So what can we do? Octane boosters so far have proven useless to me. My own motor octane tester is a 2000 Silverado 5.3 with twin  GT25  Garrett turbos and completely stock EFI system. This truck has 112,000 miles all with turbos, and it knocks on what they call "tip in" as throttle is depressed, before the computer senses the MAF is in the twilight zone compared to what is normal. This one second rattle is annoying, but harmless in eight years. With 100 unleaded, there is no audible knock at all. Some octane boosters have been tried with no success. I even tried some name brand stuff that's supposed to make "race gas" out of pump gas. You'd think I knew better, but this was a free sample for testing by Terry Paine. A quart to five gallons did nothing, two quarts to five gallons did nothing but make us laugh.

Leaded race gas or 100LL Av gas is in my opinion the only reliable octane booster for pump gas. On a few occasions I get to ride a 1000 HTG triple "pump gas" engine with 200-225 HP depending on whether power jets are set for .70 or .57. I like to use 100LL Av because it's cheap and always fresh and good enough. And I can water it down as I go and still have fun until dilution makes me afraid. Then I just cool it or give 1/2 turn to the pj's. I respect that thing and get great, reliable enjoyment from it.  People who run high HP on the trail need to respect their engines, and expect to be running 87 octane on occasion. So that means tuning for 87 if you must run 100% pump gas. That means running enough fuel, with relaxed timing if necessary to get you around .70 lb/hphr for hot rod two-strokes. And, it means you must have a cooling system that will keep up with your HP/ riding style. How can a Firecat cooling system designed for 130 HP be expected to keep up with a 170 HP big bore if you ride like a madman? Coolant temp should be monitored, and when engine coolant gets to be much over 120 degrees F (like you might experience on hard packed roads or lakes) you must take it easy or risk deto since hot engines are most likely to detonate.

Rejetting for doing lake runs can be a nuisance, so I like those adjustable power jets like used on Lectron carbs. A quick 1/4 or half turn twist can take you from .70 to .60 with a similar bump in HP when you know you have sufficient octane. I have limited experience with Holtzman float bowl pressure tuners, but I like what I've seen for ease of adjusting fuel flow.  EFI owners love Boondocker EFI controllers, since you can have one map dyno programmed "up nord style" for mystery gas, and one for short blasts on what you know is decent octane gas. And maybe another for race gas dragracing. Just a click of the Boondocker and you go from race to ultra safe. Couldn't be better.

Kevin Cameron has devoted lots of time providing technical information on detonation in the DynoTechResearch archives. Understanding what we're buying at the pump, and understanding the causes and effects of deto is important if we expect to avoid it while having reasonable fun with our sleds. This most recent realization that we're being "hosed" at the pump should make us all afraid.

2/20...After comparing the SkiDoo 600 dyno test data between  Etec and SDI, we are hunting for the missing 5 HP. While part throttle cruising A/F and BSFC are admirably low, BSFC at peak torque and HP RPM is disappointingly normal. The Etec and SDI single pipes have the same part number. So the lower HP throughout the powercurve is probably either retarded timing, lower compression, lower airflow CFM or some combination of the three.

Since I had no airflowmeter during the Etec test I relied on the Innovate wideband to report A/F. I may try to back into airflow numbers utilizing the wideband A/F ratio and mechanically measured fuel flow lb/hr. If airflow seems to be low compared to carbed and SDI models, it may be the lack of any fuel vaporizing in the engine's crankcase and transfer ports. Vaporizing fuel absorbs heat, and it's possible that Etec crankcase temps might be elevated, causing airflow to be lower than carbed and SDI.

Or it may be ignition timing. Jim Cooper's ETEC software doesn't yet have timing control like he has with other SkiDoo engines. He is lobbying with SkiDoo to obtain the control of timing on the Etec engine. Otherwise we may resort to an offset ket to add a few degrees overall. But that might mean excessive timing at part throttle (how low can those mid .30's BSFC go?).
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When Ed Sedlmeier was dyno tuning his SkiDoo SnoX racers last week, he had with him a most dandy battery powered "Pro Vision 618" fiberoptic viewer to look at piston domes. Skinny enough to fit thorough plug openings, it's perfect for viewing tough-to-see pistons in sleds like SkiDoo and Arctic Cat F twins. I found the best deal on the Pro Vision 618 at www.tooltopia.com $170 free shipping, and ordered one today.