October 2016 Ask the Experts

Drivers, fuel ratios, 4-arm weights and more!
2017 Arctic Cat XF 9000 CrossTrek snowmobile FOX FXR
Ryan Thompson - RLT Photos
In our October 2016 issue, our experts answered our readers' questions about anti-ratchet vs. standard drivers, VDO Extreme troubleshooting, Klotz fuel ratios, and Polar Atac 4-arm weights. Don't miss these other topics as well:

Number of drivers

Q: Just wondering if there is a difference in running two drivers over the windows of the track or running four of them? This will be on an Indy 650 speed sled. – monsteredge

A: I’m assuming you are asking about anti-ratchet over the window for speed running, vs. standard drivers. The standard drivers will be smoother, more precise, and keep the track in better alignment. This will produce a more consistent and higher mph compared to the anti-ratchet over the window drivers. – Jason Houle, Straightline Performance

VDO help
Q: I’ve got a VDO Extreme and none of the buttons seem to work. When powered up, it just reads 9.85 on the screen. Any thoughts? – dbartz47c

A: Have you tried holding the top and bottom buttons while turning the unit on to clear all the recorded runs? Also, if battery voltage is under 10V DC, then it will not work properly. – Todd Guthrie, Dyna-Tek Performance

Klotz mix ratio

Q: I am wondering if anyone has experience playing around with Klotz r50 fuel ratios. I run a 700 improved triple. I am currently at a 38-40:1 ratio with VP c14. There seems to be quite a bit of oil coming out of the exhaust pipes and the carbs seem to always have a film of oil around them. I have opened up the top end a few times since the motor has been built and everything always has looked great. Can I get away with running a little leaner on the oil, and would the benefit outweigh the risk? Any extra power always helps. – bnstock

A: I haven’t seen much in terms of HP change in my nearly 30 years of dyno testing oils and fuel/oil ratios. 40:1 is in the ballpark for a lot of top engine builders. Full Power Performance runs TF580 at 40:1, D&D Powersports likes Spectro Platinum SX 40:1, and all the Hentges Racing Polaris SnoCross teams use Amsoil Dominator at 50:1. Other savvy engine modifiers surely use other oil brands/ mixes, but 40:1 seems to be most common.

As far as horsepower goes, reducing oil percentage without reducing jet size might cause a slight loss of HP, since reducing oil percentage effectively increases the lbs./hr of fuel going through the engine. Unlike extra unneeded fuel, which vaporizes and takes up space (displacing O2) and absorbs heat (further reducing HP), oil stays in much larger droplets that slam into, and lubricate spinning and sliding parts, taking up little space in the intake charge. Good stuff!

Why don’t those oil droplets vaporize and burn completely like fuel droplets do? Oil in the mixture is difficult to vaporize because of the oil’s greater surface tension, and some of it crawls through the running engine and out the exhaust. The oil you see outside the carbs is from reversion – high velocity air/fuel/oil mixture slams into the closed reeds during the power stroke and bounces back out through the open carbs, picking up another load of fuel/oil mix as it passes the needle jets once more. Some of the unvaporized droplets of fuel/oil mixture invariably fall out of suspension once the velocity slows outside the carbs. The fuel evaporates, leaving the oily residue.
Inside the combustion chambers, the heat of combustion vaporizes and burns just some of the oil—hence the light “oil smoke” we see.

So, the only real disadvantage of using lots of oil is (1) annoying oil film on external parts and (2) slight reduction of motor octane from the oil. But FPP, D&D, and Hentges can all make max HP with great reliability with their extremely high BMEP race engines with volatile high RVP 116 octane fuel and reasonably heavy oil mixtures. The advantage is longevity of rings, pistons and bearings, crankshafts still rotating at the end of the season, and many races won! – Jim Czekala, DynoTech Research

Polar Atac 4-arm weights

Q: Does anyone know if you can mix weights on opposing arms to tune rpm? For example, 10s on opposite sides of each other and 9s on the other arms opposite each other. – Redlined

A: It is not uncommon to have different gram weights on opposing arms of a 4- or 6-weight clutch. The arms should be the same profile and shift curve. When choosing the weights, be sure to keep the grams as close as you can. Don’t go varying large amounts of grams. – Jason Houle, Straightline Performance
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