Site
Videos

Ultra Carb Loading

Holley’s latest circle track-specific units are the Ultra XP 650 and 750 cfm carburetors offering a raft of new features that can make performance tuning quicker and easier.

Holley’s latest circle track-specific units are the Ultra XP 650 and 750 cfm carburetors offering a raft of new features that can make performance tuning quicker and easier.

Holley’s Ultra XP Series is the New Version of Carb Loading

Words: Jeff Smith; Photos: Holley and Jeff Smith

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?

Looking straight down through the venturis, there are screw-in high speed and idle air bleeds for each venture. Also note the drop-leg boosters and the radiused Allen bolts now used to retain the accelerator pump nozzles. If you look closely, you can also see the more aerodynamic button-head Allen screws used to retain the throttle plates.

Looking straight down through the venturis, there are screw-in high speed and idle air bleeds for each venture. Also note the drop-leg boosters and the radiused Allen bolts now used to retain the accelerator pump nozzles. If you look closely, you can also see the more aerodynamic button-head Allen screws used to retain the throttle plates.

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.

This tight view of the fuel bowl reveals the drain plug in the bottom of the bowl along with a good look at the new -8 male fuel bowl O-ring fitting. Also note the fuel level sight glass is now retained with retaining rings.

This tight view of the fuel bowl reveals the drain plug in the bottom of the bowl along with a good look at the new -8 male fuel bowl O-ring fitting. Also note the fuel level sight glass is now retained with retaining rings.

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.

The billet metering blocks feature easily removable emulsion, power valve channel restrictor, and idle feed restrictor bleeds. This gives the knowledgeable tuner a tremendous range of tuning options.

The billet metering blocks feature easily removable emulsion, power valve channel restrictor, and idle feed restrictor bleeds. This gives the knowledgeable tuner a tremendous range of tuning options.

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’s idle bypass feature allows setting idle speed with this adjustor located underneath the air cleaner stud. The four surrounding holes feed bypass air into the manifold to set idle speed without having to disturb the ideal setting of the throttle blades relative to the idle transfer slot. This eliminates having to drill holes in the throttle blades.

Holley’s idle bypass feature allows setting idle speed with this adjustor located underneath the air cleaner stud. The four surrounding holes feed bypass air into the manifold to set idle speed without having to disturb the ideal setting of the throttle blades relative to the idle transfer slot. This eliminates having to drill holes in the throttle blades.

The underneath view shows the billet aluminum base plate. The idle bypass air enters the manifold through that hole in the center of the base plate.

The underneath view shows the billet aluminum base plate. The idle bypass air enters the manifold through that hole in the center of the base plate.

These circle track version Holley come with wedge design floats that allow the needle and seat to remain open during high-g corner loading. Traditional flat floats can sometimes close the inlet when fuel climbs the outside portion of the bowl, raising the float.

These circle track version Holley come with wedge design floats that allow the needle and seat to remain open during high-g corner loading. Traditional flat floats can sometimes close the inlet when fuel climbs the outside portion of the bowl, raising the float.

There’s even an adjustable secondary throttle stop placed in this convenient position. This knurled adjuster can be turned either with a screwdriver or by hand.

There’s even an adjustable secondary throttle stop placed in this convenient position. This knurled adjuster can be turned either with a screwdriver or by hand.

Parts List

Description PN Source Price
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

Source

Holley Performance Products
holley.com


Post A Comment

Post A Comment

OneDirt Newsletter Signup