Carburetors are relatively...
Carburetors are relatively simple devices, but if you can negotiate your way around these Holley circuits, there are many little tricks you can use to improve power without sacrificing mileage or driveability.
It's an interesting fun fact that once electronic fuel injection appeared on the scene, carburetors enjoyed significant improvements in technology. Perhaps this is mere coincidence, but the basic Holley carburetor has come further in the last 10 years than it did in previous decades. Despite several premature predictions of this fuel mixer's death, carburetors are more alive than ever. To emphasize how easy they are to tweak, we've run down a few simple tricks that are affordable and fun to do that will pay off in terms of more airflow, a little more horsepower, and perhaps even greater driveability. This is actually several stories rolled into one handy package, so check out our Holley carb mod extravaganza.
| | Peak TQ | Peak HP | Avg. TQ | Avg. HP |
| Carburetor |
| Holley 650 | 455 | 476 | 428.5 | 380.4 |
| 750 HP Body | 456 | 477 | 429.8 | 381.6 |
Simple Swap
The Holley HP carburetors have become some of the best street fuel mixers for car crafters wanting to step up to a true performance four-barrel. One way you can do this is by converting the main body rather than buying a whole new carburetor. We thought we'd combine that idea with another slick little trick we've mentioned before. In looking through the Holley spec book (we know, it's a sickness), we noticed that the 600- and 650-cfm Holley mechanical-secondary carburetors use the same throttle plate as the 750. That means by purchasing a 750-cfm Holley HP main body, you can upgrade a 600 or 650 double-pumper to a 750 for less than $150.00. You can also apply this same HP upgrade idea to a 0-4779 Holley 750 carburetor. Of course, if you want to just step up, Holley offers a Street HP carburetor that is a great way to combine all these features in a carb we really like.
We first baselined the 650...
We first baselined the 650 mechanical-secondary carb and were surprised at how much power it made after we increased the jetting for best power. Then it took mere minutes to strip the 650 carb and convert it to the 750 HP main body.
We wanted to put this remake to the test on the dyno. The K&N filter people offered their SuperFlow 902 dyno, so we bolted our 383 small-block to the pump to evaluate exactly what this swap would be worth. This is the engine we tested back in the Sept. '08 issue, built with 9.8:1 compression and a Jay Allen-designed Camshaft Innovations flat-tappet mechanical lifter cam. The top end is a set of 195cc Canfield aluminum heads and an Edelbrock Performer RPM Air-Gap intake, which makes a great near-500hp test engine for carb flogging.
The first step was to test the 650-cfm carb to create a baseline. On the first pull, the power was way down with the little carb, but after a quick jet exercise, the motor cranked off an excellent 476 hp at 6,100, which was right where we left off when the engine ran the last time at Westech. The next step was easy, since all we had to do was swap the small 650's main body for the larger Holley HP. Once the swap was completed and back on the engine, we ran through a couple of jet changes to make sure the engine had all the fuel it wanted. Curiously, the power didn't really change much and neither did the amount of vacuum in the manifold at peak power, indicating that even with this larger carburetor, the measured airflow through the engine only increased about 5 cfm. Since airflow didn't increase, the power remained basically the same. While this move to an HP main body didn't help on our engine, it's clear the restriction to making more power is somewhere else. If we were looking for more peak horsepower, we'd swap to a single-plane intake, but this would probably also diminish midrange torque. Even though we didn't make a ton of power with this conversion, it is still a great way to upgrade a budget-priced 600- or 650-cfm Holley to 750 HP status easily and without spending a ton of money. It doesn't get much easier than this.
The HP main body comes with replaceable idle and high-speed air bleeds in the top of the carb. We first tried using the original 650 air bleed sizes, but that didn't work. Then we tried the 750 HP high-speed air bleeds and had to reduce them in size from 0.030 to 0.026 inch to get the air/fuel ratio where we wanted it in combination with more jet changes. RapidJet sells a tool called an air bleed fish wire, which is a tiny wire that slips through the bleeds when changing them to prevent them from dropping into the engine.