What are the Differences in Big Holley 4150 Carburetors? Part 4

Wayne Scraba

31 Oct 2019

Over the past several weeks, we looked at three very different Holley carburetors – a Classic Double Pumper, an HP-series model and one of the newest XP-series examples. All are double pumper configurations and while each share similarities, they also have different components and features. To conclude the series, we’ll remove the bowls and metering blocks and look inside. The differences are plentiful. Both the Classic Double Pumper and the HP carbs use cast metering blocks. Meanwhile, the XP series incorporates billet aluminum blocks. The Classic double pumper and HP blocks are similar, but with a different layout. For example, the Classic block is set up with a vacuum port. This is a timed spark port (it provides no vacuum at curb idle). None exist in the HP. The XP, however, is a different story: Open up an XP carb and you’ll quickly discover they’re pretty much tunable for anything. Aside from the standard replaceable jets and power valves, the metering blocks allow for tuning the idle fed restrictions, emulsion bleed restrictors and the power valve channels. Holley determines the idle circuit calibration by the diameter of the idle feed restriction (IFR) coupled with the idle air bleed. The pair of idle feed restrictors is nothing more than a jet for the idle system while the air bleed serves as the airflow-regulating orifice. Holley also notes that by turning the idle mixture screw(s), you vary the volume of air/fuel emulsion discharged into the intake manifold, not the actual air/fuel ratio. Given the new design of the metering block, it’s possible to change the idle feed restrictor size to compensate for a large (big duration, high overlap) camshaft. With a combination such as this, the intake charge is often diluted at idle. The dilution is created because the intake charge is pulled out the header due to a late closing exhaust valve. To bring back idle quality (and sensitivity) to the idle mixture screws, the idle feed restriction size must be increased. Holley also includes power valve channel restrictions in each metering block. There are two restrictions, visible when the power valve is removed. These restrictions meter the flow of fuel into the main carburetor well. It is the diameter of these restrictions that dictates the amount of fuel that goes into the circuit. When the size of the restriction is changed, the air/fuel ratio will change at wide-open throttle (full power). When the power valve is closed at idle or at part throttle, the power valve restrictions will obviously have no effect. In total, there are ten emulsion bleeds on each metering block. Emulsion bleeds can have an effect upon power, but this is highly dependent upon the engine combination. If you wish to tune by way of emulsion bleeds, it must be done on a dynamometer where you can carefully monitor the air/fuel ratio. Holley offers a wide selection of emulsion bleed tuning components, ranging from a twenty-bleed kit to emulsion bleed two-packs. Finally, when you consider the wear and tear on a set of metering blocks (particularly in a race application), you can see the use of a billet aluminum block is a great addition. Sealing is improved, too. Holley also added a handy pry slot on each block for easier disassembly. Look inside the bowls and you’ll also find some differences. The Classic Double Pumper has a brass float bowl. The HP series carburetor comes with a nitrophyl float. The XP also uses a nitrophyl float assembly. Here, the floats are notched on both sides for jet extensions. We’ve mentioned this before, but the HP can be plumbed for fuel on either (or both) of the carburetor’s sides. Ditto with the XP. The XP has a slightly larger bowl capacity, and internally, the bowl is engineered with special fuel “trough” that directs fuel to the jets. As you can see, there are a lot of differences (and similarities) between the three Holley double pumper carbs. The Classic is still a great street carburetor. The HP balances between hard core street and strip and the XP is an outstanding race fuel mixer. For more info, check out the accompanying photo slideshow.

What are the Differences in Big Holley 4150 Carburetors? Part 4 — Over the past several weeks, we looked at three very different Holley carburetors – a Classic Double Pumper, an HP-series model and one of the newest XP-series examples. All are double pumper configurations and while each share similarities, they also have different components and features. To conclude the series, we’ll remove the bowls and metering blocks and look inside. The differences are plentiful. Both the Classic Double Pumper and the HP carbs use cast metering blocks. Meanwhile, the XP series incorporates billet aluminum blocks. The Classic double pumper and HP blocks are similar, but with a different layout. For example, the Classic block is set up with a vacuum port. This is a timed spark port (it provides no vacuum at curb idle). None exist in the HP. The XP, however, is a different story: Open up an XP carb and you’ll quickly discover they’re pretty much tunable for anything. Aside from the standard replaceable jets and power valves, the metering blocks allow for tuning the idle fed restrictions, emulsion bleed restrictors and the power valve channels. Holley determines the idle circuit calibration by the diameter of the idle feed restriction (IFR) coupled with the idle air bleed. The pair of idle feed restrictors is nothing more than a jet for the idle system while the air bleed serves as the airflow-regulating orifice. Holley also notes that by turning the idle mixture screw(s), you vary the volume of air/fuel emulsion discharged into the intake manifold, not the actual air/fuel ratio. Given the new design of the metering block, it’s possible to change the idle feed restrictor size to compensate for a large (big duration, high overlap) camshaft. With a combination such as this, the intake charge is often diluted at idle. The dilution is created because the intake charge is pulled out the header due to a late closing exhaust valve. To bring back idle quality (and sensitivity) to the idle mixture screws, the idle feed restriction size must be increased. Holley also includes power valve channel restrictions in each metering block. There are two restrictions, visible when the power valve is removed. These restrictions meter the flow of fuel into the main carburetor well. It is the diameter of these restrictions that dictates the amount of fuel that goes into the circuit. When the size of the restriction is changed, the air/fuel ratio will change at wide-open throttle (full power). When the power valve is closed at idle or at part throttle, the power valve restrictions will obviously have no effect. In total, there are ten emulsion bleeds on each metering block. Emulsion bleeds can have an effect upon power, but this is highly dependent upon the engine combination. If you wish to tune by way of emulsion bleeds, it must be done on a dynamometer where you can carefully monitor the air/fuel ratio. Holley offers a wide selection of emulsion bleed tuning components, ranging from a twenty-bleed kit to emulsion bleed two-packs. Finally, when you consider the wear and tear on a set of metering blocks (particularly in a race application), you can see the use of a billet aluminum block is a great addition. Sealing is improved, too. Holley also added a handy pry slot on each block for easier disassembly. Look inside the bowls and you’ll also find some differences. The Classic Double Pumper has a brass float bowl. The HP series carburetor comes with a nitrophyl float. The XP also uses a nitrophyl float assembly. Here, the floats are notched on both sides for jet extensions. We’ve mentioned this before, but the HP can be plumbed for fuel on either (or both) of the carburetor’s sides. Ditto with the XP. The XP has a slightly larger bowl capacity, and internally, the bowl is engineered with special fuel “trough” that directs fuel to the jets. As you can see, there are a lot of differences (and similarities) between the three Holley double pumper carbs. The Classic is still a great street carburetor. The HP balances between hard core street and strip and the XP is an outstanding race fuel mixer. For more info, check out the accompanying photo slideshow.

The metering blocks on the Classic Double Pumper and the HP series carb are similar (Classic Double Pumper on the left).

One place where they differ is the lack of a vacuum port on the HP series carb. This is a timed spark port on the Double Pumper.

Metering blocks are billet aluminum on the XP series carburetor, and they’re designed for maximum adjustment capability.

In this photo, you can see the replaceable emulsion bleeds found on the XP.

The pointer locates the replaceable idle feed restrictor on the XP.

This photo shows the pair of replaceable power valve restrictors (they’re visible once the power valve is removed). The text offers more info on the adjustment capability found within the metering blocks. It’s considerable.

Bowls used on the Classic Double Pumper and the HP are similar; however, the XP (not shown here) bowls have increased capacity.

The XP series carb also uses nitrophyl floats, and they’re notched. Holley designed a special fuel “trough” within the bowl to direct fuel to the jets.

The notch on the rear float allows you to use a set of jet extensions, which are included with the carburetor.

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