Recently while on the AMSOIL Great American Circle Track Tour, Bob Bolles and I stopped by Tour sponsor Holley's corporate headquarters in Bowling Green, Kentucky. While there, we got a special treat as we were given unrestricted access to its brand-new all aluminum oval track 4150 HP carb. We got to sit down with the engineers who designed it, Laura Shehan and Jay McFarland, for several hours. They let us take pictures, video, and answered all of our questions. The opportunity was significant because this is the first redesign of the HP carb since its introduction almost 20 years ago. It's so new in fact that you'll see in the pictures that some parts are still just prototype castings. As of press time, certain aluminum components were not even completed. To make things even better, Bolles and I were the very first media guys to see this carb and we've got the complete expose right here for you.
The new HP's main body is a radical departure from the old design. As you can se,e the por
Laura, Jay, and the other engineers at Holley took a clean slate approach to redesigning the HP. They left no stone unturned and in Jay's words, "a lot of the changes are subtle but when you add them all together they are really significant. And many of these changes were ideas that we had developed over time from just working on these carburetors for so many years."
Significant is an understatement. One of the main goals in the redesign is to make the carburetor more efficient, from airflow to fuel consumption. Another is a increase in fuel capacity, the new HP has a 13 percent greater capacity over the old design. That extra capacity will require you to run different fuel lines, but Holley can help with that through its Earl's brand. Finally, and the most obvious one, is the fact that this HP is all aluminum, a design that has shed nearly 4 pounds off the original HP. That's a substantial weight savings and weight that, given its placement on top of the engine, is high above the center of gravity of the car. Let's get right to it and take a look at the new HP part by part.
Starting with the main body, the first thing you'll notice is the newly designed aero boosters. This is one of the changes that affect the airflow in a positive way. Gone is the trademark bump on the arm of the booster. It is now one smooth transition from the side of the body to the end of the booster. Check out photos 1 and 2 for a side-by-side comparison.
Photo 1 The old Booster design had a knuckle where it connected to the main body.
In addition, the air bleeds have been moved farther out of the main airstream of the carburetor. Photo 3 on the following page shows the side-by-side comparison. Like the booster change, this allows the air to flow into the carburetor uninterrupted.
Also in the main body you'll find the idle bypass circuit-reference photo 4. "We still have some development to do with this system. It's not rocket science machining," said Shehan. "What we've done is provided a needle of sorts coming up through the bottom of that air cleaner stud, it's spring loaded so it holds tension and won't self adjust. You pull out your air cleaner stud and make your adjustment. It's all contained within your unit and you don't have to redrill anything." Shehan says the system works by opening up or closing off the airflow passages that you can see in photo 4.
Development of the idle bypass circuit is a microcosm of the entire redesign of the HP. "We had three different designs (of the idle bypass circuit), says Shehan. "The first one we came up with was one where you never took the air cleaner off, but there was good and bad with that design, such as how do you adjust the stud length for people who run different sized air cleaners? Then we went to the next generation design where the spring-loaded mechanism was on the top, but we had to consider that design's affect on the manufacturing line.
Photo 2 In the new HP the knuckle is gone, improving airflow in the carburetor.
Photo 3 Another major improvement in airflow can be attributed to the fact that engineers
Photo 4 The arrow points to the Idle Bypass Circuit, a new feature not previously availab
Photo 5 You can see the dramatic difference in the internal baffling added to the fuel bo
Throughout this redesign that was another whole set of hurdles that we had to consider. And the way we manufacture these units actually gave us the idea of going through the bottom. If the carburetor needs any adjusting during the flow stand testing (one of the final quality control steps) the techs could adjust it right on the flow stand with this idle bypass circuit.
The idle bypass circuit is a really cool addition that you won't find on an old HP. "This was really designed for that guy who is running a big camshaft with a lot of overlap. He has no idle at vacuum so the first thing he does is crank his idle up," says Jay McFarland. "You know the drill, it gets out of the transfer slots, gets to the top and it starts pulling fuel into the main circuits and ends up getting a rich idle. He can't adjust his idle and then his carburetor is a piece of junk and he throws it out, even though it's not his carburetor's fault. The idle bypass circuit allows him to fine tune it so he can get his primary throttle plates back down into the proper spot on the slot so that he has idle control. It's more of a fine tuning tool."
Here is a close-up of the shelf that is designed to reduce aeration and foaming of the fue
Both McFarland and Shehan are excited about the fact that Holley will be machining the billet baseplate and metering blocks in house. "One of the wonderful things about machining in house is that it allows us to make changes easier," says Shehan.
In the past, Holley technicians had to drill and press in plugs since this carb has an external accelerator pump. In the new HP however, Holley is machining in the gasket locating pins as opposed to machining the holes and pressing the pins in place.
"This alleviates the condition where the pins are put in crooked, fall out or are omitted all together as in some billet designs," explains McFarland. "We have also changed the machining of the accelerator pump channel to have the metering block plugged internally rather than the outside to eliminate a potential leak path that exists with external plugging.
This beefed up sealing lip allows racers to mill out or "lower" the floor of the bowl to e
"Further, we redesigned the vent baffle such that it is press in place and round, instead of the operation where these are drilled and you put the little plastic baffle in place and then on the line you drill through that plastic and install the pin. It's already there. There's no debris. It's less costly to manufacture the oval slot versus all of the drillings. That's another benefit to doing things in house."
A few other significant changes in the metering block is that they've lowered the power channel restrictors to keep them lower in the fuel and they've hollowed the block out to gain fuel capacity but not so much that it cost twice as much to manufacture. Engineers are also working on designing a recess that racers can easily use to pry the block from the main body when working on the carb. Another benefit is that since the metering blocks are billet Holley will be able to manufacture them in different colors. Want a pink block? Go for it!
Inside the fuel bowls, Holley designed internal baffling all the way around, something the previous fuel bowls did not have (see picture 5 for a comparison) along with a larger diameter needle and seat boss.
Through testing and research, the team discovered that fuel entering from the needle and seat goes down and hits the float tab. That contact sprays the fuel onto the flat bottom, causing a lot of aeration and foaming. With the additional baffling and the center shelf design however, when the fuel enters it hits the shelf and has a tendency to dribble down the side, almost like a slow waterfall. The net result is less aeration and less foaming of the fuel. Some foaming does still occur but it does so up on the shelf and not down in the main fuel level.
The increased fuel capacity necessitated an increase in bowl size as you can see from this
The shelf and its accompanying baffles has a secondary benefit. In the old design, under heavy acceleration the fuel runs to the back of the bowl and up that side and sloshes around, coming back up on top of the needle and float-this can cause problems with the needle and seat closing when you don't want them to. The shelf blocks this whole process from taking place. Instead in the new HP when the fuel comes back under heavy acceleration it hits the shelf and breaks apart instead of rolling all the way to the top like a wave on top of the float. The shelf in addition to all of the different baffles is designed to trap the fuel around the jets and prevent it from sloshing around. And it does so quite nicely.
In a circle track environment, you need to keep the fuel as close to the jets as possible. Conventional thinking says that when you're on a big track running high rpm over a long period of time, you begin to exhaust the fuel out of the bowl so if you lower the jets you will prevent fuel starvation. A common practice to achieve this is to mill out the floor of the bowl and then lower the jets farther into the fuel supply. However, this can cause leakage especially if it's done by somebody not familiar with carburetors. On the new HP, Holley beefed up the sealing lip of the floor so that if someone does decide to mill out the bottom to lower the jets themselves, there is still enough wall to have a solid seal. In addition, engineers created a trough to try and keep fuel on the jets as much as possible.
Here is another angle of the bowl comparison.
Typically when Holley puts the floats in it cores the section and uses self tapping screws. But that has changed. "We got enough feedback from users that they really prefer that we drill and tap those areas. That's one additional feature that we've added," says McFarland.
Holley is also offering the option to have either a plug or a pyrex sight glass on the new HP. This will allow racers to choose the option that best fits their needs and rules. Speaking of rules, one upgrade that caught our eye, small as it may be, is the fuel bowl drain. It is, for want of a better explanation a brake bleeder screw that allows you to drain fuel out of the bowls before disassembling the carburetor. We thought this was not only a great addition to tech officials who need to take fuel samples, but it's great for the racer too. Just answer this question:
How many times have you changed jets on the carb while it was still on the motor? We had to do that very swap during our recent Project G.R.E.E.N. test. It was dicey at best with the motor and headers hot from running laps on the track and time running short because of approaching bad weather. We got through it fine and none of the spilled fuel ignited but it sure would have been nice to have that fuel bowl drain on our carb. Hey, you can even put a vacuum pump on it a suck all the fuel out.
The metering blocks were designed using CAD, in fact, Holley engineers designed the entire
In addition to moving the machining of the billet metering blocks and billet baseplate in house, Holley has made a number of other improvements to the HP. The carb now features welded Teflon-coated shafts, with the primary shaft capped on the one end of the throttle shaft. This does a great job of keeping dust and dirt out of the shaft. The primary and secondary links are progressively adjustable so that you can adjust them without replacing links.
Holley has also developed a hand-adjustable speed (idle set) screw. "How many times have you been working on the car and you've tried to bump the speed up but you had to yell to a guy, 'Hey, go get a screwdriver,'" asks McFarland. "With this design you still can use a screwdriver but you have the option to use your fingers. We made it just big enough where you can get a grip on it and its got a nice chamfer on the end of it."
The new metering blocks and baseplate will be CNC machined out of billet aluminum in house
Another great improvement can be found on the screw for the squirter. That screw has always been either a slotted screw or a Phillips head. Granted, this is a bigger improvement on a street carburetor where you have to fish down deep into the choke plate, but since they've gone to a rounded allen head instead of a big flat head you've again achieved better airflow than the previous design. While McFarland insists that particular improvement is nothing major, perhaps a 0.5 percent gain, it shows the depth that Holley engineers have thought through the overall design of the new HP carb.
The HP design has been around since 1993 while the original 4150 was brought out in 1958. This is the first time any design changes have been undertaken on the HP in almost 20 years. A lot of these changes are subtle but when taken together, it's a carb for the 21st century.
The team took a very methodical approach to designing the new carb. At every step it balanced the new designs it was considering with how they would impact the manufacturing process together. In addition, the team staggered the parts coming in so it could do the development work on the main bodies before it started dealing with fuel bowls and metering blocks.
The testing protocols were equally as methodical. The team documented the flow curves on the current 4150. It replaced just the main body with the new design, leaving everything else (metering blocks, boosters, and so on) exactly the same. What did the team find out? The results were very consistent on the main bodies with the new main body 3 percent richer than the current main body.
Carbs are lined up for final quality control inspection in Holley's production facility. T
When it started working on the boosters, the team again got very consistent results, only this time the flow curves laid over top one another perfectly. At that point, it knew it could put this booster in the main body and not have any unknowns and continue development work. The engineering team took that same approach with the throttle bodies and the metering blocks. While the throttle bodies had similar flow curves from new to old, the new metering blocks ran richer as well. "I attributed that to better fuel flowing passages versus the old design, the changes that we made allowed the carb to pump more fuel," explains Shehan.
Both McFarland and Shehan are itching to get this carb out into the real world and test it. After all, it has been a long road to this point. "Right before Christmas we released the models to the casting foundry," says Shehan. "Something like this HP is not an overnight development. This is an 18-month, almost 2-year project to get where we are at right now."
They estimate that the carbs will be on the track atop test cars within the next several months and they've invited us back toward the end of the year to go play with one in a real world test. You can bet we'll be there! Stay tuned!