January 2016 Ask the Experts

Answers to your tech questions about fuel, studs, belts, compression ratio, timing and more
Polaris snowmobile stud traction patterns
Performance or safety? Makers of traction products often have multiple stud patterns available depending on the desired sled use.
Stud Clarification
Back in 2010, American Snowmobiler studded a 2009 600 Dragon with one-inch studs. Was that a misprint? That sled came with a 1.25-inch track. – polariscometSamagalski, Libau, Manitoba

We assume you’re referring to our Nov. 2009 issue (pg. 20)? Here we stated that we put 1.075-inch Snow Studs on a 2009 Polaris Dragon 800 SP (not the 600) which did come with a 1.25-inch lugged RipSaw track. But yes, we indeed had a typo of our own six seasons ago. It should have read that we studded it with the Snow Studs-recommended 1.325-inch studs. Snow Studs was our traction sponsor that year and has stud recommendations dating back to 1996 on their website. Sorry for any confusion. Thanks for the blast from the past, and for keeping us honest! – Experts

Got Good Gas?

Q: I purchased a five-gallon can of VP C14 Plus last weekend. After I got the can home, I noticed it had a date of 2011 stamped on the can. It is a sealed unopened can. Will this gas be fine to run or should I contact the seller and see if I can get a fresher can? – Riley

A: I would be terrified to use fuel that old in a well-tuned race engine, even if it’s in a sealed pail. You must return it and ask for a more recently filled pail (there should be a date on the bottom).

VP’s chief chemist, Duane Minazzi, explained to me that the fuel’s “front ends” (i.e., the easiest-to-vaporize components, like butane and isopentane) can escape out of the flexible plastic spouts on sealed pails, causing a reduction in volatility (indicated by lower-than-spec Reid Vapor Pressure). The “front ends” are vital. They’re the first to transition from non-flammable liquid/globules to flammable vapor in the intake tract. One study indicates that by the time the fuel reaches the combustion chambers, 80% of the front ends are vaporized, but only 20% of the heavier ends of the fuel are ready to burn! So you can see, when the front ends are lost, you can have an extremely lean, HP-robbing, piston-sticking net mixture in the combustion chambers, even with safe-looking EGT’s (the unburned globs cooling off the probes), and dandy wideband A/F readings (the globs finally vaporize in the heat of the exhaust systems, and finally burn there too late, fooling the A/F gauge). Watch those piston domes!

It’s quite practical to do your own fuel testing. RVP is a sure indicator of present or lost front ends. I have several informative YouTube videos showing RVP testing on my DYNOTECH Research channel. You can also check out the DTR blog that explains how you can create a Home [Reid] Vapor test unit for $30 that works nearly as well as systems that cost thousands. Watch a video here.

Never assume that any pail, even one filled yesterday, is fresh. It may have been trans-filled from nine-month-old bulk or an old opened drum, then sealed. RVP testing is cheap and easy, and if you get substandard stuff, then you should complain loudly! – Jim Czekala, DynoTech
Dayco Ultimax snowmobile belts
Belt Break In
Q: Your Nov. 2015 tips on changing a snowmobile drive belt stated that after changing the belt you should drive your butt off to catch your buddies. I was at the Toronto International Snowmobile show on the weekend and stopped at the Dayco booth to inquire about a proper break-in distance. I thought the answer would be to drive gingerly for a couple miles. They recommend different maximum speeds until fully broken-in at 50 miles/80kms. (It’s possible they said 80miles). I don’t find the actual numbers on their website, but they were on the sleeve for new belts. – Norman Schwegel, Kitchener, Ontario

A: Thanks for reading American Snowmobiler! We were having a little “tongue-in-cheek” fun in the article when we said that you should "ride fast to catch up with your buddies." We don't often ride "gingerly," and our own testing over several decades has led us to believe that varying your RPMs, speed, acceleration and braking is the best way to get a belt broken in. Each manufacturer has their own set of “break-in” instructions that include maximum operating speeds, distance, etc.

For example, Bill Hay of Dayco Canada states, “For maximum drive belt life, do not exceed 80 km/h (50 mph) during the first 30 kilometers (20 miles) of use. Exceeding 80 km/h (50 mph) during the break-in period will greatly shorten the life of this product. Washing a belt with soapy water (as many people suggest) also only applies to specific belt manufacturers (not Dayco). We don’t use a silicon on the outside of the belt in the manufacturing process. The idea of the soapy water is to remove this silicone so as to promote better gripping of belt by clutch.” – Experts

Reader Response: Weak Spark
I fired up my pro stocker yesterday, and it ran barely 20 seconds, quit and lost spark somehow. I checked grounds, cleaned all plug-ins and still have spark that’s weak every once in a while. My batteries are fully charged. The red LED light comes on when I pull it over, but there’s not enough spark to start it. I hope the MSD isn’t fried! – lastminute1200

A: Pull the plug apart going to the triggers. You can use a jumper wire going back to the box to test for spark. – bman
Ski-Doo 700 triple compression ratio
Compression ratio charts formulas
Checking Compression Ratio
How do I check comp ratio on a 700 Doo triple? I know you add oil to the cylinder, but what formula do you use after you fill it? – mach69x

A: Checking compression ratio on the 700 Ski-Doo triple would be the same as most 2-stroke engines: 
  • Put the piston that you are going to check at “true” top dead center.
  • Be sure to seal around the edge to avoid any fluid leaking past the ring.
  • Install the head and bolt it down.
  • Fill with your test liquid to the bottom of the spark plug with a burette. This will give you the installed cc’s.
  • Next, you must know your bore and stroke. There are two ways to calculate the compression ratio: standard (takes into account the entire stroke) and corrected (accounts for when the exhaust port is completely closed).
The charts on the right are an example of an 800 Ski-Doo that measures 25.5cc installed. This will give you 16.7:1 standard compression ratio, and 8.4:1 corrected compression ratio. - Jason Houle, Straightline Performance

How Much Timing?
How much timing should you give for a pro stock 700 for grass drags? – mach69x

A: Ignition timing will vary depending on compression ratio, gas used, and distance running. A good starting point for grass drags will be around 15 degrees. – Jason Houle, Straightline Performance

ISR Pro Stock Drag Rule
When I read the 2015-16 ISR rules, they state a chaincase must be used. It doesn’t matter if it is rolled forward, backwards, up or down as long as the jackshaft remains on top of the tunnel. Can someone please verify that I am understanding this wording correctly? – catman289

A: ISR Pro Stock Drive Rule 3,C states, “Crankshaft to jackshaft center to center is allowed +/- 1” (one inch) tolerance from OEM specification for the model.” (2015-16 ISR Rule Book, pg. 21) Hopefully this helps! – Experts
Did you know ...
There’s oil in them there shocks! And just like the oil used by your car or 4-stroke sled engine, your shock oil should be changed. We detailed servicing your FOX shocks in an earlier issue of AmSnow (Nov. 2015, p. 48), but all makes of snowmobile shocks should be freshened up annually. Just like engine oils, the oil you use in your shocks really matters too! It’s important to consult with the manufacturer of your shocks to make sure you’re using the proper type of oil for a shock rebuild. The oil each company uses has specific performance characteristics designed to work with all the moving parts and components of their shocks. So do yourself and your sled a favor, and take the time to learn what oil should be used for your shocks! It may cost a few more dollars for the right oil, but you’ll be rewarded with better performance and longer shock life.

If you ask it, they will answer!
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