As for outright power, a good air-gap dual-plane with the right carb still beats a 3 x 2. That said, a well set up 3 x 2 can have great drivability on the street while turning in excellent drag strip times on an otherwise street-orientated carb setup. As simple as a spacer is, its mode of operation is often not understood. The reality is that spacers work because they have increased something that the engine likes. That increase may take the form of extra flow, more velocity, greater anti-reversion properties, or additional plenum volume.
This variety of spacers should just about cover the ones you are likely to come across. Each has its virtues. The dyno and the drag strip are likely to establish which one performs best for your application.
A spacer has the ability to make the plenum volume larger and, usually, helps the air flow through the carb by up to 20 cfm. Because a stretched big-block is always hurting for air it is worthwhile finding out whether or not a spacer helps. In most cases they do. Most intake manifolds are designed with the minimum height possible for the application. This means that the plenum is often too small, so the use of an open spacer is an easy fix and it reaps the benefits of additional output.
The result is that the engine sees greater flow from the carb. If the carb is a little on the small side, this type of spacer almost always pays dividends in output. It takes little more than lifting the carb and installing the spacer and longer studs to find out what the engine likes.
This means it is a good idea to test a spacer any time the opportunity presents itself. But be aware that installing a spacer always increases the plenum volume, often making a small but relevant reduction in the sharpness of the signal at the booster.
Consequently, if the jetting was on the money before the spacer was installed, the carb may need to have a size or two larger main jet to compensate. Because successful racers use mechanical secondaries many street performance enthusiasts tend to regard vacuum secondaries as something of a necessary performance downgrade dictated solely by the need to have a streetable induction system.
In reality nothing could be further from the truth! The correct way to view a vacuum secondary carb is as a high-flow performance carb fitted with a device that allows you to use that carb in a far more effective manner on the street.
In fact, a well set up vacuum secondary carb can provide better performance and faster times on the track than a mechanical secondary carb. The reason is that, in effect, a vacuum secondary carb is like two carbs rolled into one. A small-CFM 2-barrel due to a sufficiently active venturi and booster combination can supply a well-atomized mixture to the engine at part throttle and low-speed WOT. On about 1 in 10 engines, they seem to deliver what is called for. This spacer provides the same effect as using both the four-hole and open spacers.
This spacer has tubular, sharp-edge extensions, which protrude into the plenum of the intake manifold. This provides a measure of anti-reversion properties to the flow exiting the carb.
This spacer not only has anti-reversion lips on the four exits but also fuel-shear ridges on the wall of the open part of the spacer. This type of spacer acts as a means of altering fuel distribution to a more favorable pattern and as an anti-reversion devise. The slot allows the fuel to enrichen a weak area within the plenum. This spacer seems to work best when used in conjunction with a 1- to 2-inch open spacer.
In practice, this means that the user of a vacuum secondary carb can ultimately select a slightly bigger carb CFM without any penalty at the low end. A vacuum secondary is of little or no advantage when the stall speed of the converter is above the RPM at which a vacuum secondary comes in.
If the car transitions through the first gear to RPM somewhere at or above peak torque RPM very quickly then, once again, a vacuum secondary might not be of any advantage. If you select a vacuum secondary carb for an engine that does not really need one, there is no real downside.
If you select a mechanical secondary for an engine that could really use a vacuum secondary the downside is a possible reduction in output everywhere. Sizing with Alcohol-Based Fuels or Nitrous. So far the subject of sizing the carb CFM to the engine has been discussed assuming that gasoline was the fuel being used.
With methanol and ethanol-based E85 fuels the vaporization curve is far less favorable for good combustion initiation than with a good gasoline blend. This means that any potential mixture quality or wet flow problems that might be imminent with gasoline can be greatly magnified to the point that any power gains that may have been possible with the alcohol fuels are nullified.
To combat this, make sure that the fuel is well atomized. Rule number one here is: Do not use a carb with venturis too big for the application. Rule number two is: Make sure that the booster is of high enough gain to generate good atomization. Rule number three is: It is often better to err on the smaller side for an alky carb.
Because these fuels cool the carb so much more than gasoline does, the mass flow lbs-min can increase. However, countering this is that the fuel takes up a lot more room in the intake so carb CFM is reduced. The bottom line here is that you want as high a speed in the venturi as possible along with the strongest booster signal possible.
For example, if the carb is a style unit, consider a main venturi minor diameter of 1. This, along with a big throttle bore, seems to work well. Under such circumstances, choose a carb that errs on the smaller side by about 50 cfm. The rationale here is that the nitrous produces all the extra power required within the realm of streetable mechanical reliability.
This being the case you may as well have the benefits of good street manners from your engine; selecting a carb a little smaller favors that aspect. For a race-only situation, things change a little. Here you have three goals.
First is to go as fast as possible, second is to use as little nitrous as possible, and third is to have your engine survive the rigors of a very substantial power output. When the nitrous comes into action the temperature of the charge in the intake drops considerably. This causes the air that passes through the carb and into the manifold to shrink. At first, this looks as if it should increase the airflow into the engine, but the reality is actually the reverse.
A portion of the liquid nitrous entering the induction system turns to a gas, and consequently it takes up room that would have otherwise been occupied by the air from the carb. All this might be leading you to think that using a smaller carb for the street is the best route, but in many cases, the reverse applies.
The use of a slightly larger carb usually pays off, especially if the nitrous is port injected. Brake Specific Air Consumption. Some advocate rating a carburetor by the horsepower it can readily support.
At first sizing a carb according to the horsepower it can support seems like a better method, but it makes an assumption that can throw a wrench into the works. This carb, when fully prepped, was good for about cfm although some teams were running radically modified carbs with well over 1, cfm and, for a Cup Car application, supported the needs of a hp ci engine.
Another example is the use of a platform carb on a modified ci big-block Chevy. If you do the math in terms of the required CFM, these carbs look far too small to be able to allow the production of such big horsepower numbers. But there is one factor that you should be aware of: An engine may well draw in a certain amount of air, but it is very important how efficiently it uses that air.
Each engine made virtually the same 1, hp. The first had very well sorted combustion characteristics. In fact, the induction system produced a well-prepared charge and delivered a BSFC of 0. As a result it produced 1, hp for the 96 pounds of air it consumed each minute. This works out to be a BSAC of 5.
This translates into a carb flow demand of 1, cfm. The other big-block ingested pounds of air for 1, hp. That was good but not as good as the first example. The same output was delivered with a BSFC of 0. These figures indicated a more than reasonably well sorted induction system. So at the same pressure drop across the carb, the second engine needed a carb of about cfm more. Say you are installing a carb that passes a certain amount of air at a certain pressure drop into the engine. Whatever volume of air the carb flows, it is now up to you, as the engine builder, to utilize that air as efficiently as possible.
Couple this with good head and induction system flow and you can expect output numbers that trailer your competition see my best-selling CarTech book How to Build Horsepower. If you liked this article you will LOVE the full book.
Click the button below and we will send you an exclusive deal on this book. At first I was inclined to say that most seasoned enthusiasts and professional builders are beyond the scope of this book.
But I've seen what often passes as traditional or period correct and in light of that I wholeheartedly recommend it, especially to those who've convinced themselves that they know it all. Close menu. Recently the Engine Masters show on the Motor Trend On Demand site did a comprehensive test to see just what differing sized carbs would produce. Here is what they learned. A better than average HP street performance engine. Now this series of dyno pulls was on the Westech Performance dyno.
Probably one of the best facilities around. Here is what they found. Remember these numbers were from wide open throttle. So, will a bigger carb make more power? Looks like bigger is not always better.
If this was to be a or engine street driven just for pleasure, the will give the best all-around performance, excellent throttle response, good fuel mileage, great low and mid range performance.
No one will notice the 3 or 4 HP missing. Unless you have a really souped up, hot, which I don't think you do is a little on the large size. Your carb CFM should be 1. A would be about perfect. Do carb spacers add horsepower? Carb spacers only do one thing, increase the size of the plenum area of the intake manifold.
What's the best carburetor? What size carburetor do I need for a stroker? Carburetor For Stroker recommends a cfm carburetor. A cfm carburetor will make the engine more powerful, but it is the largest that should be used unless the engine is being used for racing. What size e85 carb do I need? For to ci engines, choose the E85 carb. For to , E85 carb; to , E85 carb; and for greater than , choose the new E85 carb. Of course if your engine will rev to a much higher rpm, you will need to select a larger carb.
What size Holley carb do I have? It's like a "VIN" for your carburetor and should be hand-stamped on the front of the choke tower to the right of the vent tube on most typical Holley carbs. The list number typically is four to six digits long and may or may not have a suffix number behind it.
The date code will be right below it.
0コメント