VP RACING U4.2 RACING FUEL

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WHAT IS IT? A new version of VP’s extremely popular U4, but without its two worst traits.

WHAT’S IT COST? $13.90 (per gallon, in five-gallon containers)?(951) 674-9167.

WHAT’S IT DO? If you’ve been around racing for very long, you know what VP Racing Fuels’ U4 is famous for. For those not initiated into the world of hopped-up fuel, here is a quick rundown. U4 was released in June of 2002. It was an overnight hit, but not because it was a high-octane fuel?it wasn’t. U4, originally called Ultimate 4, was a high-oxygen fuel. By increasing the oxygen content from the then AMA mandated 2.8 percent to around eight percent, U4 was good for three horsepower in a bone stock 450 (and around 1-1/3 horses on a 250F).
The three problems with U4, which MXA documented many times, were: (1) It had a pungent sweet smell that could waft out of your bike’s gas tank and end up offending your significant other’s nose (even if she was far from the garage). (2) If U4 was allowed to sit unused in your bike’s carburetor for two or three weeks, it could leave a gummy residue (often requiring total carb cleaning). (3) U4 was not AMA Pro Racing legal. It was, however, legal in every organization below the AMA National Championships.
To address some of U4’s shortcomings, VP developed U4.1 and U4.2. The MXA wrecking crew set out to test both of the new breed of oxygenated gasolines.

WHAT STANDS OUT? Here’s a list of things that stand out with VP’s U4.1 and U4.2 fuel.
(1) Choice. MXA tried to test VP 4.1, the earlier replacement for U4, but found it to be a jetting nightmare. To get our stock 2007 RM-Z250 to run we had to up the mainjet by ten sizes. This was too much jetting for the average consumer to handle. Luckily, U4.2 was a hands-off fuel. It required little more than a fuel screw adjustment to work (and only minor jetting for temperature or altitude changes). We’d pass on 4.1, but embrace 4.2.
(2) Power. Thanks to abundant oxygen, U4 delivered tremendous energy potential. Peak horsepower on our KX250F jumped a full horsepower from 7000 rpm all the way to sign-off. More significantly, with pump gas our well-used KX250F never broke 36-horsepower, but with U4.2 it not only cracked the 36 horse barrier but stayed above it from 10,300 from all the way to 11,600 rpm. That is a usable power gain.
(3) Spec. If you are a tech weenie, you will appreciate that U4.2 has a motor octane of 108, specific gravity of 0.718, and an oxygenate rating double that of the current four percent AMA limit.
(3) Problem solving. U4.2 doesn’t have the sickly sweet smell of U4; instead it has an acetone smell. It also doesn’t gum up your carburetor as much as U4 (although we still recommend running the float bowl dry after every race day). There were no significant jetting changes required to switch from pump gas to U4.2 (which can’t be said about U4.1).

WHAT’S THE SQUAWK? Unlike an exhaust pipe that might produce a three horsepower increase for a one-time charge, U4.2 requires you to keep paying?or the horsepower will vanish.

MXA RATING

This is instant horsepower (on a pay-as-you-play basis)