Holley’s Ultra XP Series is the New Version of Carb Loading
When triathlon runners get ready for a race, they do something called carb loading, eating lots of carbohydrates to build an energy reserve for the race ahead. What we’re really talking about here is Ultra XP carb loading. But instead of carbohydrates, we’re referring to Holley’s latest series of Ultra XP circle track carburetors – the Ultra XP 650 and 750 cfm carburetors.
The Ultra XP is the top of the line for 4150 Holleys and a step up from the very popular HP line. So why should the Ultra XP become your new favorite carburetor?
Let’s start with losing weight. While we all could probably stand to lose a few pounds, the Ultra XP line has already done that by trimming 2.8 pounds off the girth of the zinc-based HP line using cast aluminum components combined with billet aluminum metering blocks and base plate.
You might even notice that air bleeds in the top of the carburetor have been moved ever-so-slightly outboard to create both a smoother transition into the main body and to improve their function. If you look closely, Holley has also added a radiused Allen head fastener to retain the accelerator pump nozzles and fitted the main body with anti-siphon valves to prevent fuel pull-over from the nozzles.
The fuel bowls also feature several changes, including -8 male, O-ring boss (ORB) fuel line fittings that seal to the bowls with O-rings rather than those thin gaskets. Fuel bowl capacity has also been increased by a full 20 percent, which might also encourage slightly lower fuel pressures to minimize aeration of the fuel. You might also have picked up on the handy fuel bowl drain located in the center of each fuel bowl.
Billet metering blocks means all the passages are precisely drilled, making all the main and idle circuits consistent and responsive to changes. You can also more easily dial in the idle circuit with screw-in idle feed restrictors that will control the four corner idle mixture adjusters.
There’s even the very handy idle air bypass featured on all Ultra XP carburetors. While setting the idle speed might not seem like a big deal for a race engine, it’s still an important setting to prevent excessively rich mixtures at idle. The idle bypass adjustment fitting is located underneath the air cleaner stud. Carburetors with this feature are easily spotted as they all feature four small bypass holes surrounding the center area near the air cleaner stud.
The bypass allows the tuner to adjust the idle speed with the bypass adjuster instead of the normal curb idle adjuster. It’s best to leave the throttle plates in their original position so they remain in the correct relation to the idle transition slot. This prevents pulling excess fuel out of the transition slot at curb idle and also will drastically improve off-idle throttle response.
Moving to the base plate, the secondary side now has its own, easily-adjustable stop located near the secondary accelerator pump instead of hidden underneath the baseplate. Both throttle shafts are now PTFE-coated and use oil-impregnated bronze bushings. As an added bonus, the ends of the throttle shafts are capped with bronze cups to prevent dirt from entering the shafts, which is a real plus for oval dirt racing.
Overall, the Ultra XP offers many additional tuning avenues that previously required sending your carb out to a specialized carb tuner. This kind of adjustability is a big reason carburetors have continued to deliver excellent performance for circle track classes all over the country. Carb loading can be a good thing as long as you choose the right one!
|Holley Ultra XP 650 Circle Track, polished||0-80812BKX||Summit Racing||$728.95|
|Holley UItra XP 650 circle track, grey||0-80812HBX||Summit Racing||$779.95|
|Holley Ultra XP 750 Circle Track, polished||0-80813BKX||Summit Racing||$758.95|
|Holley UItra XP 750 circle track, grey||0-80813HBX||Summit Racing||$809.95|
|Holley fuel line, -8 black fittings||34-43||Summit Racing||$124.95|