There are a few universal truths in the automotive world. Primary among them, boost rules. Among the other few: supercharging an engine is the most cost-effective means to produce more power.
Superchargers are the ultimate example of the axiom, “There’s no substitute for cubic inches.” To use the engine-as-air-pump metaphor, the primary way to make more power is to pump more air. An engine 30 percent larger than another can pump 30 percent more air at a given speed. An engine spun 30 percent faster than another can theoretically pump 30 percent more air. But increasing the size of an engine without increasing its package size costs wheelbarrows full of money. The same goes for making an engine spin faster. Worse yet, building an engine to spin faster also takes away drive quality at slower speeds.
Whether driven by belt, exhaust, or gear, a supercharger can make an engine pump more air with the same displacement and at the same speed. How much more? Well, consider the atmosphere at sea level; it works out to 14.7 psi. If we cram another 14.7 psi into an engine we theoretically double that engine’s displacement. So your 350 suddenly works out to about 700 cubic inches (well, a little less due to pumping losses, of course). And it gets better. Because the internal dimensions of the engine remain the same, the extra air crammed in increases the cylinder pressure dramatically. That shoots the efficiency right through the roof, and efficiency is power. And because a supercharged engine makes its power at conventional engine speeds, we don’t have to sacrifice good driving manners to the gods of overlap and reversion, either. It’s like the best of both worlds: great manners and even better performance. Stupid-crazy performance.
While modern electronic fuel injection made tuning a supercharged engine a lot easier of late, the simplest way to feed fuel to a supercharged engine is still with a good ol’ carburetor. But a carburetor for supercharged applications differs from a carburetor for natural aspiration. In fact, every component in the fuel system needs attention in a boosted/carburetor application.
To find out what it takes, we consulted one of the leaders in the carburetor world: Quick Fuel Technology (QFT). The company is one of the few that manufactures carburetors specifically for supercharged applications and possibly the only carburetor manufacturer that produces a model for blow-through operation (yes, there are differences).
There we got the benefit of QFT’s technical manager Zach Baker. Baker isn’t a desk jockey; he’s one of the guys with hands on the products. Case in point, in 2012 he and production and custom specialist Phil Freeland won the second-annual BLP Racing carb-shootout, a competition that judges entrants on how much power they can make with an unmodified production carburetor. These guys know their stuff.
The first thing we discussed was the idea that the carburetor is merely one component at the end of a greater fuel delivery system. At the other end of that system is something we all take for granted: the fuel tank.
“Every tank needs an adequate breather,” Baker begins. Breathers exist to replace the volume of fuel consumed by the engine. Without it, the tank will develop negative pressure and overcome the pump’s ability to deliver fuel. And that fries pistons. “I like 3/8-inch,” Baker says. “It doesn’t need to be that big, but then I know that the breather is never the issue.”
But capacity is only one component to a worthy breather. “You want some sort of filter at the end because 9 times out of 10 the tank vents underneath the car where it’ll pick up road grime,” he says. Though a rollover valve is a good idea, he maintains that a simple loop in the line covers the bases.
Go with the flow
“At the tank you’re going to want to pick up the siphon-tube size,” Baker says. “Most of the time they’re 5/16- or, if you’re lucky, they’re 3/8-inch.” He says that a 3/8-inch line has capacity sufficient to support 600 horsepower but recommends going larger for a greater margin of safety. “I like to see at least 1/2-inch line going forward,” he reveals. “That’s number 8 in AN, and that’s the minimum as far as I’m concerned.” For systems that use a bypass regulator, he recommends a return line at least the same diameter as the feed line. “You don’t want to build any pressure in the return line,” he cautions. “It can increase the pressure at the carburetor and that can cause problems.
“You’re also going to want to use a good, high-flow filter,” he continues. “Obviously we know the reason why you run (a fuel filter), but the reason you run a good one is flow capacity; a cheap, paper-element filter won’t flow the numbers we need. A filter that doesn’t flow will burn up the pump motor.”
Baker recommends an assembly with a cleanable stainless element that filters down to 40 microns. “They’re expensive up front, but you don’t need to buy another one.” And if you need any more justification for a premium filter, consider this: “The reality is, the most expensive fuel filter is cheap compared to pistons.”
Baker also recommends retrofit sump kits. “When you’re on it, the fuel’s all going to go to the back of the tank,” he explains. “Well, if the fuel runs away from the pickup tube, you’re suckin’ air. That can cause a lean condition and those destroy parts.
01. Because carburetors have larger passages than injectors, Baker says a 40-micron filter is sufficient. He couldn’t stress capacity enough, though, observing that a good filter costs far less than a burned-up piston. This is the element detail from Quick Fuel’s 30-7308 filter.
02. The Competition Engineering C4041 sump kit is about as simple as it gets; it welds to the bottom of pretty much any flat-bottom tank. The small pocket catches and holds an amount of fuel usually sufficient for most hard accelerations.
Suck or blow?
We can mount the supercharger in one of two locations relative to the carburetor. It’s a decision with implications; location dictates everything beyond the fuel filter.
We refer to these two mounting options as draw-through and blow-through. The most common, draw-through, is the simplest: it mounts the supercharger on the outlet side of the carburetor as to make the supercharger draw through the carburetor. Without a doubt the most recognizable version in the domestic world is the classic roots-type supercharger feeding off of a carburetor (or two or three or four). Early OEM turbocharger systems employed the draw-through design as well.
The blow-through design mounts the supercharger to the inlet side of the carburetor to make it, appropriately enough, blow through the carburetor. The distinction is critical because it determines most components in the fuel system.
To select those components we have to understand how those two configurations influence the dynamics in a fuel system. And to do that we have to jump way ahead of ourselves and do a Carburetor 101 thing.
If you learn only one thing today let it be this: a carburetor is a pressure-differential device, basically a very sophisticated leak. To understand it fully it’s best to dispense with the idea that vacuum sucks fuel out of the jets. In a roundabout sort of way that’s kind of true. But as we’ll explain, it really confuses things.
Here’s a better way to think about the way a carburetor works. At sea level on a dry, 60-degree day the atmospheric pressure is 14.7 psi. That means there’s 14.7 pounds pushing on everything, including the carburetor inlet and float bowl.
As the piston descends during the intake stroke it creates a low-pressure atmosphere in the cylinder and manifold. You probably called that low-pressure area a vacuum your whole life, but it’s misleading. First, it’s only a partial vacuum. Second, it doesn’t relate directly to the atmospheric pressure. It’s expressed as the inverse of atmospheric pressure and in a different format at that (inches of mercury).
Here’s why it’s easier to think of pressure pushing fuel through jets. Say the descending piston reduces the area in the manifold to about 10 psi. That represents a 4.7-psi imbalance between the atmosphere and the inside of the manifold (14.7-10=4.7). More specifically, that’s a 4.7-psi imbalance between the pressure exerted on the carb inlet and float bowl and the pressure within the manifold itself.
Well, nature hates an imbalance and it will use its resources to restore equilibrium. The greater atmospheric pressure in the inlet and fuel bowl pushes air and fuel through the carburetor as a means to restore the balance between the atmosphere and the manifold. Get that idea into your head, and carburetor tuning will get so much easier. Trust us on that one.
03. They look the same but these two carburetors could hardly be any more different. The differences are almost entirely skin deep, among them, floats, jets, and fuel discharges. Keep reading to find out how they differ.
Draw-through fuel systems
Now here’s where the rubber hits the road, so to speak. “In a draw-through system everything works pretty much the way it does on a naturally aspirated system,” Baker says. In other words, the carb inlet and float bowl operate at the same pressure as the atmosphere around us, and the area below the carburetor operates at a pressure less than atmospheric. “For draw-through, you need something that will keep up with the volume for the horsepower.”
He continues, “It takes about half a pound of fuel per hour per horsepower. With that formula, that’s what needs to be in the cylinder functioning. But you’re losing some in the intake. You’re losing pressure here, there, and yonder. You’re fighting g-forces and other things in the car. But the reality is that’s what needs to be in the cylinder to make it happen.” He recommends 20-30 percent over what’s necessary for safety’s sake.
Naturally, he recommends his company’s wares. “If you’re at 500-600 horsepower, the 30-175 pump we have is fine. If you’re going to get 800-900 then the 30-260 that’s coming back out would make a really nice piece.”
High-volume fuel pumps for carbureted applications invariably produce pressure greater than what a carburetor can use. Case in point, those pumps produce between 15-21 psi whereas the typical carburetor can’t tolerate more than about 6 psi greater than atmospheric pressure. We need something to regulate that 15-21 psi to about 6 psi greater than atmospheric pressure.
Fuel-pressure regulators come in two flavors: deadhead and bypass. Deadhead regulators have a valve that simply clamps down on the line to reduce pressure. “The reason people like the deadhead style is ease of plumbing,” Baker says. He maintains that a deadhead regulator works perfectly well for a fuel pump that develops less than about 22 psi. “Any more than that and the pump can overheat,” he says.
The bypass regulator, on the other hand, diverts excess fuel to a secondary line that dumps back at the tank. “The bypass is really nice just because it lets fuel keep flowing,” Baker says. “It’s not beating on the pump so you’re also going to generate less heat in the line.” For what it’s worth, he recommends the 30-1803 deadhead-style regulator and 30-1900 bypass-style regulator.
04. A carburetor or two perched atop a Jimmy blower is the classic model of the draw-through design. It’s about the simplest way to supercharge an engine, but it has a definitive style and imposes packaging limitations. Draw-through is limited in the sense that it forbids the use of intercoolers.
05. Without a doubt the most critical distinction of a draw-through carburetor is the boost-referenced power valve. The modification requires blocking the passage that leads to the baseplate and drilling the body for a tube. A hose connects that tube to the area after the blower, aka, the engine manifold. It’s a simple—albeit critical—modification. Drill a little bit off and you have a junk carb body.
06. Universal floats like tops that are symmetrical because the fuel moves back-and-forth in a standard installation. But in most dual-quad applications the carburetors must mount sideways to fit. That means the fuel sloshes to what’s usually the side of the carburetor. Fuel piling up to one side of the carburetor forces the float up, which shuts off the needle valve.
07. The bases of wedge floats rise up on the side where the fuel sloshes under hard acceleration. They ensure adequate flow under the most demanding conditions: heavy acceleration under lots of boost.
08. Mounting carburetors sideways can play havoc with things like linkages. Trimming the offending nubs off is perfectly adequate but there’s merit in parts made for the specific application—like no exposed steel to rust. They’re standard on most carburetors intended for draw-through applications like Quick Fuel’s.
09. A deadhead-style regulator like Quick Fuel’s 30-1803 controls fuel pressure by basically restricting the fuel to the carburetor inlet. Most draw-through supercharged systems can use a deadhead-style regulator because they don’t require a bunch of pressure headroom the way blow-through systems do.
Blow-through fuel systems
Things change in a blow-through system. The descending piston still creates a manifold pressure less than the pressure exerted on the air in the inlet and fuel in the bowl. However—and this is a biggie—the inlet and float bowl operate at a pressure far greater than atmospheric pressure. Say your supercharger generates 14.7 psi of boost at the carburetor’s inlet. That’s 14.7 psi greater than atmospheric pressure.
Let’s say a manufacturer recommends a fuel pressure 6 psi greater than atmospheric pressure. Now let’s make our 14.7 psi boost. That’s 8.7 psi greater than the fuel pressure. Remember that thing about things moving from high-pressure areas to low-pressure areas? Because the pressure in the bowl is greater than the pressure exerted on the fuel in the line, the fuel will not flow into the bowl. And without a constant supply of fuel the carburetor will run out of fuel. And when the carburetor runs out of fuel, the engine goes boom.
What we need is a fuel system that constantly maintains fuel pressure 6 psi greater than atmospheric pressure. That’s possible with what’s referred to as a boost-reference fuel-pressure regulator.
As its name implies, it references the pressure in a given area. The area in our case is the area inside the float bowl. Because the float bowl operates at the same pressure as the much more convenient carb inlet, we can use that area instead. If we set our operating pressure at 6 psi then the regulator will always maintain 6 psi greater pressure than the pressure in that area. So when the supercharger makes 14.7 psi of boost, the fuel system will generate 20.7 psi.
Now, we know some of you just spit up your coffee. “But 20.7 psi is way too much for a carburetor,” you say. Well, understand that the fuel-pressure setting dictated by carburetor manufacturers represents the differential between atmospheric pressure and line pressure. Truth be told, regulators and mechanical fuel pumps are reference regulated. It’s just that they reference atmospheric pressure. Otherwise, thy carburetor would runneth over when you drove to the top of Pikes Peak where the pressure drops from 14.7 to 8.7. At that elevation your fuel pressure would still be 6 psi; it would just be 6 psi greater than 8.7 instead of 6 psi greater than 14.7. A boost-reference regulator merely references any point at the end of a hose, in this case the area in the carburetor inlet.
Now here’s the thing: the 30-1803 deadhead-style regulator and 30-1900 bypass-style regulator that Baker recommended for draw-through applications are boost-referenced regulators. They just reference the atmosphere if they’re not plumbed to the carburetor inlet.
The only thing that really changes in the fuel-delivery system is the pump, and even then that’s not always the case. Take the 30-175 and 30-260 pumps from earlier. The former produces 15-18 psi. So if the carburetor requires 6-psi inlet pressure, that pump would support about 9-12 psi of boost. The latter makes 18-21 psi. Using the same 6-psi line pressure, that pump would be good for 12-15 psi. Beyond that requires a pump with greater capacity than the ones Quick Fuel makes. Baker recommends consulting Aeromotive or Fuelab for an appropriate version.
10. Even blow-through systems can use a deadhead-style regulator like the 1803 but only for relatively low-boost applications as Baker noted. The bypass-style pump like the 30-1900 shown here regulates fuel pressure by diverting the excess pressure back to the tank by way of the line under the regulator. That way fuel keeps moving rather than churning in the pump when the line achieves the pressure limit.
11. Pumps like the 30-260 produce about 21 psi, technically sufficient to feed a blow-through system to about 17 psi. We’re not going to call that the threshold for a system without an intercooler, water/methanol injection, or on E85 fuel, but it’s certainly right at it for most applications.
12. Adding an intercooler, water/methanol injection, or simply by running E85 fuel opens the door to boost pressure far greater than 17 psi. Baker recommends a unit from Fuelab or Aeromotive. Aeromotive designed its 340-series pumps to produce 60 psi, making it appropriate for not just injection systems but for high-boost carbureted applications like these. The company rates it for 900 hp in blown-carbureted applications.
A story about carburetors in supercharged applications wouldn’t be complete without talking carburetors. At face value, a carburetor for boost applications really isn’t all that different from a conventional carburetor. They all look the same. In fact, they all have the same parts. But in operation, a boost-ready carburetor is a very different beast. What’s more, a boost-ready carburetor for a draw-through application is considerably different than one for a blow-through. In fact, the two aren’t interchangeable.
Baker explains. “A carburetor for a draw-through application is the closest to a carburetor for a naturally aspirated carburetor,” he says. “The biggest difference is the boost-referenced power valve.”
Remember the boost-referenced fuel-pressure regulator? Well, this is the same deal. A supercharged engine has a split personality in the sense that it has an incredibly broad operating range. Just loafing along, they consume the same amount of air and fuel as any other similarly built naturally aspirated engine. But pour on the sauce and they consume the same amount of air and fuel as an ultra-high-performance engine does. That’s a lot to ask of a carburetor.
“As you crowd the throttle and go under boost, the blower can begin to out-flow the carburetor,” Baker explains. When that happens the manifold pressure drops to the point where the power valve can shut. “If that happens it’s going to lean out eight to 10 jet numbers,” he says. “Then you’re fixin’ to go into nuclear meltdown. At idle and cruise we still want it shut, but we want to make sure it’s open for wide-open throttle.”
Boost-referencing the power valve is a fairly straightforward deal: plug the existing reference hole that reads the pressure at the carburetor base and drill another one for an external line. “That line can go anywhere below the centerline of the rotors,” Baker instructs. The same holds true for any other supercharger design that doesn’t use rotors—say a turbo or centrifugal or screw-type supercharger. Just plumb the line to any point after the supercharger outlet.
Draw-through carburetors fall under two subcategories depending on their orientation. Small-displacement superchargers—say a small Roots-type or a turbo—can get away with a single carburetor mounted longitudinally. But on top of larger superchargers—say 6-71-and-up Jimmy blowers—Baker recommends doubling up. Rather than for total flow (which does have its advantages), it’s for distribution. “Two carburetors are going to distribute fuel a lot more evenly over a long blower,” he reveals. “Also, it reduces heat.”
Of course, doubling up imposes its own requirements. For one, it means mounting carburetors sideways. “The problem is that carburetors weren’t designed to be mounted sideways,” Baker reveals. “With the 2x4 stuff the fuel will run away from the jets (under acceleration) so we equip those carburetors with jet extensions.” What’s more, the fuel runs to the side of the bowl where it lifts the floats artificially high. “That shuts off the fuel and causes the engine to lean out. So we run a wedge float so the fuel has someplace to go and the float can still operate properly.” Naturally, it has notches for the jet extensions. Finally, mounting carburetors sideways makes the levers from one carb interfere with the other. “We cut the levers for 2x4 applications. All 2x4 carburetors come with jet extensions and wedge floats with notches.
Interestingly enough, Quick Fuel recommends simpler down-leg boosters for fuel delivery for most draw-through systems. “The fuel’s going to be running through a pair of rotors,” Baker says. “So it’s gonna be atomized by the time it gets to the cylinders.”
Blow-through carburetors require entirely different modifications. For one, they can’t use brass floats. “The pressure will crush it like a beer can,” Baker says. “Nitrophyl only.”
They also require sealed throttle shafts to prevent fuel from leaking. Quick Fuel does it by using a Teflon seal and modified throttle body. The seal works much like any other shaft seal but it alone isn’t enough to prevent a heavy boost charge from pushing fuel past. The throttle body has another channel that diverts pressure to the outside of that seal. Without the pressure differential nothing can leak.
Designing a blow-through system presents a unique tuning component: smaller carburetors offer better performance, at least to a point. It’s a matter of fluid dynamics; boost condenses air. In other words, it mashes a given volume of air into a smaller space. That means under boost, more air can pass through the carburetor in a given amount of time.
“You’re going to choose a draw-through carburetor the same way you would a carburetor for a naturally aspirated engine: on displacement. But you’re going to choose a blow-through carburetor on just horsepower. You could tell me every piece of the engine and we could talk for hours about it. But in the end, I need to know how many cubic inches and how much power you want to make with it.”
Running a smaller carburetor to make bigger numbers raises an issue: smaller carburetors usually have smaller needle valves. “A standard 750 comes with a 100 or 110 needle valve,” Baker begins. “Well, on a naturally aspirated engine a 750 might make 500 horsepower. Well, flow-wise that carburetor will make almost twice that on a blow-through system. But it’ll still have a needle and seat rated to make 500 hp. So we’ll need to step up from say a 100 to a 130 because we need to cover 850 hp.”
Quick Fuel also equips blow-through carburetors with annular discharges. It’s for the same reason it recommends them for naturally aspirated engines: “They pick up signal faster and atomize fuel better,” Baker says.
He also says that a blow-through carburetor requires a slightly richer metering curve. Altering the metering curve requires little more than just changing the main circuit’s fuel jets and air bleeds. Changing mains and bleeds changes the air/fuel ratio in a global sense, meaning across the entire range. Changing the curve means changing the actual emulsion system.
The emulsion system is actually what the air bleeds feed. That system is a series of brakes that reduce the flow of fuel. Without it, the air/fuel ratio would get progressively richer until the engine died.
In ultra-simple terms, the emulsion system feeds air to the well that supplies fuel to the discharge. The discharge is a tiny venturi in the throttle bore that develops ultra-low pressure. The pressure exerted on the float bowl; be it from the atmosphere or from the supercharger, pushes fuel through the main jet, into that well, and into that ultra-low pressure in the discharge. That’s what feeds fuel to the engine.
Essentially, the emulsion system is a series of controlled air leaks. The pressure in the float bowl comes from the pressure in the carburetor inlet. That pressure also pushes air into the bleeds; that air leaks through the emulsion holes into the main well that feeds the discharge.
The emulsion holes work progressively relative to the engine speed. The top ones work at slower engine speeds. Increasing the engine speed causes the next lower in the system to start flowing air. And things progress that way until all of the emulsion jets work by the time the engine reaches redline.
The diameter of these emulsion holes governs the amount of air they leak. Smaller ones administer less air, which barely leans the mixture at that point whereas bigger ones administer more air, which leans the mixture to a greater degree at that point. By manipulating the size of the holes in this system we can control the mixture at any given engine speed.
What Quick Fuel does for blow-through carburetors is reduce the diameter of the lower emulsion holes in the system. That causes the mixture to go richer at faster engine speeds.
Now, in a conventional production carburetor, the emulsion circuit is drilled in a small tube or in the metering block. Quick Fuel drills its metering blocks but then it taps the holes to take replaceable emulsion jets. Therein lies the beauty of a Quick Fuel carburetor: you can transform a conventional QFT carburetor to a blow-through model by swapping the needle valve, float, and these jets. It’s the ultimate in convertible designs.
Naturally, there are more variables. For example, rather than an air filter, a carburetor in a blow-through system wears a pressure hat. And Baker says even the angle of this hat can make a difference. “We’re talking 60 and 70hp worth of difference,” he says. “To be honest, it’s a black-magic science. I can’t tell you why, but it’s happened to me three times. But guys will call saying the car just lays over. Well, we can rotate the hat like 20 degrees and then you can’t keep the tires under it.”
This, by means, doesn’t reveal every component of a boosted-carburetor system. That generally differs on a case-by-case basis. But it certainly reveals the high points that most people don’t understand.
Though supercharging is considered exotic and expensive, it really is the simplest and most cost-effective way to make more power. In fact, it’s a method that makes more sense for most enthusiasts than building a large or high-strung engine. After reading this, hopefully more will join the revolution. Because really, boost rules.
13. Blowing through a carburetor can actually push fuel through conventional throttle-shaft bearings. They need positive seals to prevent any leakage, something very difficult for laypeople to pull off. Quick Fuel machines a groove in the shaft to accommodate a ribbon bearing. They’re actually Teflon strips that wrap around a groove machined into the throttle shaft.
14. While not exclusive to supercharged applications, jet extensions are pretty much necessary in blown applications where the carburetor mounts longitudinally. They keep the secondary main-fuel jets immersed in fuel even when that fuel sloshes up against the back of the bowl.
15. The float bowl sees boost pressure in a blow-through system. A brass float can’t withstand even moderate boost in a blow-through system without collapsing, so nitrophyl is the rule. The notches in this are to clear the jet extensions just shown.
16. Supercharging condenses a given amount of air into a smaller volume so a carburetor set up to operate blow-through can actually flow more air than its rating indicates. But smaller carburetors usually come with appropriately smaller needle valves that can’t keep up with the improved flow capacity. So consider a larger needle valve a necessary component—they’re part of every Quick Fuel carburetor for blow-through applications.
17. The emulsion circuit determines the fuel curve to achieve the engine’s ideal air/fuel ratio over the entire wide-open throttle range. The emulsion circuit in most production carburetors is set in stone in the sense that it’s just drilled into the metering block. But Quick Fuel uses replaceable jets in its circuit, meaning you could buy a carburetor in anticipation for a blow-through application but run it with a naturally aspirated emulsion circuit while you set up the supercharger system.
18. Annular dischargers atomize fuel far better than down-leg boosters can, making them ideal for all installations where the carburetor feeds the cylinders directly and by a short path. Because blow-through carburetors usually mount the same way as carburetors do in naturally aspirated installation, they’re a good idea. In fact, Quick Fuel outfits its blow-through carburetors thusly.
19. Baker recommends thinking of the hat or bonnet as a tuning tool; rotating it could cost or make power. Vortech’s PowerHat is one of the better-looking options, and the company makes them in a variety of single- and dual-inlet configurations. This one measures only 3.2 inches tall, has a 3-inch inlet, and features a diffuser to reduce mixture issues.
Naturally, there’s a debate about whether draw-through or blow-through is best. Typical for debates, there’s no definitive answer. But there certainly are reasons why one may be better than the other for a particular application. Here are a few.
The primary benefit of a draw-through system is simplicity. They don’t require any special fuel-system modifications like boost-referenced fuel-pressure regulators or return lines for most applications. They package easier on Roots- and screw-type superchargers. In fact, those applications are pretty much exclusive to draw-through systems. For the most part all a carburetor needs for draw-through is a boost-referenced power valve, a modification that most enthusiasts are capable of doing.
The primary shortcoming of draw-through systems is versatility and output on pump gas. Intercooling is off the table; the intercooler is a giant condenser that would transform the burnable atomized fuel into a stream that won’t burn. So that limits boost potential. Mounting the carburetor on the yonder side of the supercharger also tends to diminish throttle response. Also, the longer intake tracts tend to disrupt fuel distribution. Almost no turbochargers use positive seals; rather they employ a pressure differential to keep oil in the bearings. A restriction on the inlet side (like a throttle butterfly) disrupts that pressure differential and that forces oil into the compressor and ultimately the intake tract. So most turbochargers require a positive seal like a carbon seal.
The primary benefit of a blow-through system is efficiency. Engines typically enjoy far better throttle response, atomization, and fuel distribution because the carburetor mounts close to the cylinders. Blow-through also opens the door to intercoolers since the supercharger blows only dry air. Blow-through systems also tend to package more neatly as the carburetor stays in the conventional location.
The primary shortcomings of blow-through systems are the complexity of the fuel and emulsion systems. They require a boost-referenced fuel-pressure regulator, but seeing that many regulators have that feature nowadays it’s not such a problem. High-boost systems also require a fuel pump capable of delivering more pressure.
In general terms, blow-through is the superior design: it can produce more boost without detonation, it packages easier, and maintains better throttle response and atomization. But it’s not always the best choice, especially if running a Roots- or screw-type supercharger.
So at the end of the day, the best choice is really governed by what best fits the application.