“The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have “the fire lit” earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have “the fire lit” later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is “total timing” (i.e., the 34-36 degrees at high rpm that most SBC’s like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the “total timing” equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not “ported” vacuum – more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it’s actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you’d see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn’t need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn’t come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today’s terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don’t even HAVE a distributor any more – it’s all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was “ported spark”, which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to “lighting the fire late”) to improve the effectiveness of the “afterburning” of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle – cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these “ported spark, late-timed” engines, you’ll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees “total timing” at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems – “ported vacuum” was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet “whizbang” distributors you see in the Summit and Jeg’s catalogs? They’re JUNK on a street-driven car, but some people keep buying them because they’re “race car” parts, so they must be “good for my car” – they’re NOT. “Race cars” run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don’t need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don’t understand what vacuum advance is, how it works, and what it’s for – there are lots of long-time experienced “mechanics” who don’t understand the principles and operation of vacuum advance either, so they’re not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don’t ask Summit or Jeg’s about it – they don’t understand it, they’re on commission, and they want to sell “race car” parts.”

End.

Now, if you’ve made down this far, thank you! As this article pertains to a Weber 32/36 on a Jeep, the small primary throttle bore and single idle circuit can make it quite difficult to idle without exposing the first progression hole, making it run rich all the time.

By simply switching the distributor to manifold vacuum, the spark is initiated at a more opportune time in the combustion chamber and the additional idle speed can allow you back off of the idle speed screw which will close the primary throttle plate  to it’s correct “curb idle” position.

In addition to that, you’ll also experience cooler engine temps, much improved cold starting, improved throttle response and better mileage.

I know this one was pretty long-winded but I hope you found it worth the read.

Shawn

Booster Science
Understanding Boosters Can Enhance Top-End Output Without Impairing Bottom End.
From the February, 2009 issue of Popular Hot Rodding
By David Vizard
We have often said that there is no such thing as a carb that is too big, and we can throw in some examples to back this up. How about a 350 small-block Chevy for a working truck (big torque numbers from idle on up) with a Holley flowing 985 cfm? Or, how about a similar 350 for a Trans Am with a 1,020-cfm Holley? Yes, great power with good drivability has been accomplished successfully with large-cfm carburetors, and you can do it too.

We realize this statement appears to fly in the face of convention and what most carb manufactures preach. But, the reality of the situation is that to simplify life for the typical consumer, carb manufactures and, surprisingly, many magazines (in making that simple statement) don’t tell the entire story. Why? Because they are in business to sell–or to help sell–a carb that functions right out of the box that doesn’t teach their customer the intricacies of carb design. Given the opportunity to make a fuller statement, a carb manufacturer might say something more like, “Don’t use a carb that is too big unless you know how to select or even scratch-design a booster that will still give an appropriate signal.”

Fig. 1
Suction by the cylinder pulls air through the venturi. In doing so, it speeds up as it reaches the choke point (as depicted by the red curve on the graph above). As this happens, the pressure drops (blue line). Tapping into the minor diameter of the venturi and connecting it to a fuel supply would result in a simple carburetor.Having a working understanding of boosters can be an important asset because it will allow the use of a greater carb cfm before any negative impact on low-speed drivability. When Holley started getting into racing in a serious manner (way back when) it became apparent that making bigger, higher-flowing carbs also meant stepping up booster design research. Even to this day, much of that knowledge remains in the possession of but a few individuals working behind tightly closed doors. Compounding this is the increasing dominance of electronic fuel injection, making carburetor booster design somewhat of a lost art. Maybe it’s time to spread booster know-how, but before a coherent discussion can be had, at least a brief rundown on how they work and interact with the rest of the carb’s functions will be in order.

Venturi Action
The entire function of a booster hinges on what happens when air is drawn through a venturi. Take a look at Fig 1. What you see here is air being drawn through the venturi by the suction (partial vacuum) of the engine. As the air passes through the minor diameter of the venturi, it speeds up, as depicted by the red curve on the graph. When this happens, the pressure of the air drops in the manner shown by the blue line. As the air expands in the exit area of the venturi, it slows, and a pressure recovery takes place. The greater the airflow through the venturi, the greater the pressure drop at the minor venturi. Now we come to the key point of understanding how a booster works: the volume of air flowing through the venturi depends on the amount of suction (pressure drop) there is at its exit end. Now, let us take that same venturi shown in Fig. 1 and insert a smaller venturi with its exit end precisely at the smallest diameter of the larger venturi (Fig. 2). As you can see, the pressure drop (P2) at the smallest point of the main venturi is more than the pressure drop (P1) caused by the cylinder’s draw. By having the exit point of a smaller venturi at the point of lowest pressure of the main venturi, the amount of air going through the smaller (booster) venturi will be faster. This is because it is being sucked in not by the smaller pressure drop at P1, but by the much bigger pressure drop at P2. This means the air at P3 is going even faster than that at P2, so the pressure drop at P3 will be even higher. In other words, the booster has amplified, or as we say “boosted,” the pressure drop from P2 to P3. So, now you know why it’s called a booster. The importance of the pressure drop is that it is the signal that is used by the carb’s fuel system to not only meter the amount of fuel for a given amount of air, but also to produce sufficient atomization for the fuel to burn effectively. If either one of these factors are off by too much, power output suffers.

Selecting the right booster

Selecting the right booster can make all the difference when running a large carb on your street car. From top to bottom: annular discharge, down-leg, and straight-leg boosters. Which one’s the best for you? Read on!Booster Gain
Maximum horsepower calls for a carb with enough airflow to completely satisfy the engine’s need at peak rpm. This inevitably calls for a bigger carb than would be required if power at low and mid speed was the main criteria. The bigger the carb becomes, the more critical the booster design becomes if the carb is to operate over any acceptable rpm range. Before Holley could introduce the Dominator series of carbs with its big barrel sizes, the company had to come up with a booster design that produced high gain. In other words, it had to take a relatively small signal as generated at the minor diameter of the main venturi and amplify it into a strong, useable signal for the purposes of metering and atomization. Over the years, Holley has laid down some sterling groundwork in terms of booster design. Fig. 3 shows the characteristic form of the main variants. These are shown in approximate order of gain. For instance, at a typical wide-open throttle pressure drop, the number one booster amplifies the main venturi signal by about 1.8, while a number five booster with all the casting flash removed and a clean-up on the entry and exit delivers an amplified signal nearly four times that of the main venturi. Fig. 4 will give a good prospective of the difference in signal strengths when the five booster styles shown in Fig. 3 are tested in one barrel of an 850-cfm Holley carb.

Boosters and Carb Sizing
If the intent is to have a carb that delivers the goods over a very wide range, then the bigger the carb is for a given engine size the higher the booster gain needs to be. Only by having a very high-gain booster can we expect a large-cfm carb to produce flawless low-speed performance. If we have such a booster it is possible to run 1,000-cfm carbs on engines that would normally have 650 or so cfm considered ideal. The advantage of the bigger carb is that it does allow the engine to make more top end while the high-gain boosters still produce an adequate signal to cater to the metering and atomization of low-speed operation. About now you should be appreciating (in the normally accepted fashion) that there is no such thing as a carb that is too big, only one with an inadequate booster signal.

Fig. 2
Air flowing through the main venturi is dictated by the pressure drop caused by the engine’s suction (P1). The air flowing through the booster is dictated by the much greater pressure drop occurring at the minor diameter of the main venturi (P2). This brings about a much higher pressure drop and velocity at P3.Getting an adequate booster signal by utilizing a high-gain design is not the only way to get your four-barrel carbed engine to perform both at the top and bottom ends of the spectrum. All too often, street rodders make the mistake of assuming if the racers use it they should also do so and that usually means big cfm mechanical secondary carbs. Such a deal just won’t make it at the low end, but how about a variable-cfm carb? That would be small at low rpm and big at high rpm. If you like that idea, then the good news is it is readily available and is commonly known as a vacuum secondary carb. If you are into big-blocks, just think what a useful piece a vacuum secondary Dominator would be!To see by how much we could cheat the normally accepted “too big a carb” deal, a basic stock cam/valvetrain 350 small-block Chevy equipped with pocket-ported production line 186 head castings and dyno headers was tested with a Victor Jr. race manifold and an 850 carb. Normally, an engine with such a conservative spec would have been equipped with a 600- to 650-cfm carb. Fig. 5 shows how an 850 with stock or high-gain boosters stacks up against a stock 650. Several lessons can be learned here and the first is not to assume that bigger is better, as the stock 650 showed far better results than the stock 850. When the 850 was equipped with boosters that delivered about the same signal strength per cfm as the 650, a different pattern emerged. With the 850, the low-speed output was almost the same as the 650, but the top end was significantly better.

Fig. 3
Booster number one (straight-leg) is common to many street replacement Holleys. Booster number two (sometimes called a down-leg booster) is often used in performance-orientated carbs. Number three is a dog-leg booster with a step machined into the underside. This is a popular hop up move used by carb specialists to assist fuel atomization. Number 4 is a stepped annular discharge design while number 5 is a similar annular discharge style but without the step. The last two styles are the high-gain types most often used in big-cfm carbs.Atomization Requirements
Looking at the results so far, it would seem that the more gain a booster has, the better our carb will work. That assumption is indeed correct if we are looking at a wide operating band, but if it’s a race engine where optimizing output over, say, 2000 rpm, is the goal then things are a little different. Why? Because it is possible to have a booster that brings about a too finely atomized charge. The smaller the fuel droplets are, the more readily they evaporate into a vapor. When a droplet becomes vaporized to a gas, it takes up much more room in the intake and cuts the volumetric efficiency of the induction system. Although a charge of vaporized fuel and air will display about the best combustion characteristics, it won’t produce the best power because the amount of air in the cylinder with the vaporized fuel is less than with liquid fuel. Although a charge with fully vaporized fuel delivers the best in terms of drivability and fuel efficiency, to make maximum power a certain optimum droplet size is required.

At this point it may look like an easy task to develop a booster that delivers the required droplet size, and the job is done. Unfortunately, life in general, and high-performance engines in particular, are rarely that simple. In practice, there are many factors that influence what happens to the fuel after it leaves the booster. The most important of these is how well the fuel stays in suspension in the air and the temperature of the induction system. Ports with a high, uniform velocity and good wet-flow characteristics are a good start, but unless you are in the cylinder head business you probably don’t have much influence over that factor other than to choose your cylinder heads wisely.

Fig. 4
This graph shows the signal strength for each of the booster styles depicted in Fig. 3. Note the big difference between the lowest and highest.Where you can have a significant influence as an engine builder is on the induction temperature. What arrives at the cylinder is an air/fuel mix with a range of droplet sizes from very small to some very large ones (and a fair amount of fuel arriving in rivulets). These larger droplets will not ignite very easily and do not become part of the combustion process until the fuel that is ignitable has created enough heat to vaporize them. This means that a certain proportion of the air/fuel mix entering the cylinder needs to be in an easily ignitable vapor form. Without some vapor the ignition and subsequent burn will not be anything near effective. In simple terms, this means that the cooler the intake charge becomes, the more finely atomized the fuel delivery needs to be as finer droplets evaporate easier. Where power is the sole concern, the basic rule here is to use low-gain boosters for heated intake manifolds and high-gain boosters for cool manifolds. If you are experimenting with thermal barrier induction systems and/or artificially cooled intakes then be aware that without re-evaluating the booster design, you may not see the gains in output you hoped for.

Big-cfm carbs, such as this Dominator, are able to produce top-notch results over a wide rpm range mainly because of high-gain annular discharge boosters.What’s It Worth?
Some tests we ran a few years ago with a well-known Cup Car carb builder illustrates just how much your engine could gain or be loosing by failing to get the right booster for the job. For this test, two carbs, one with high-gain boosters and one with low-gain boosters, were used. Each was run on an engine that was first equipped with an aftermarket manifold that had crossover heat applied. The second, a Victor Jr, had no crossover heat and was air-cooled. In Fig. 6 we see a comparison between low-gain boosters delivering relatively large droplets and high-gain boosters delivering considerably finer fuel atomization. With the larger droplets, a nearer optimal percentage of the fuel is vaporized by using the low-gain boosters. The result is that they outperform the high-gain boosters, which, because of the smaller droplets delivered, have too much vaporization going on. Fig. 7 shows the same two booster styles tested, but this time the engine has a Victor Jr., which not only lacks added intake heat but also has an air passage under the runners to allow further cooling of the intake. Bear in mind that this is the same engine as Fig 6. What we see here is that the carb/booster combo that worked best on the heated intake manifold did not work very well on the Victor Jr. Note how this carb dropped a huge amount of low-end torque. In many instances this would have been accepted as a shortcoming of the Victor Jr. intake. After all, what we are looking at is a back-to-back test (Fig. 6 low-gain booster versus Fig. 7 low-gain booster). In reality, the Victor Jr. has much more low-speed potential than it is often credited with. It’s just a case of having adequate fuel atomization. The golden rule here is: “drop intake temperature, boost atomization.” If you fail to do that, your cool intake system may not show its full potential. Indeed, drop that intake temperature enough with an indifferent booster design, and you may find your efforts are rewarded with less power rather than the increase you expected!

Manifold Design
A single-plane open-plenum manifold may be the manifold to go with if top-end power is the goal, but dual planes (180-degree-style) are often short-changed when top-end output is considered. In an effort to do things right, the conscientious hot rodder may well size the carb using commonly published and accepted sizing formulas. The problem here is that a two-plane manifold divides the carb capacity in two and this is often not allowed for when computing the required carb cfm by conventional means. Modern high-flow, cool-running two-plane manifolds dictate the use of a booster that will produce the appropriate atomization. In addition to this, they need enough total carb cfm to compensate for the fact that each cylinder sees only two barrels of the carb, not four as per an open-plenum design. Getting the booster design right and having enough total cfm will endow a modern two-plane manifold with way more top-end than they are often credited with. On top of this the more appropriate atomization can also add a measurable amount of low-end into the bargain. We have successfully used as much as 1,050 cfm of carb capacity on a 383 small-block equipped with a two-plane manifold. This makes the idea of a 1,200-cfm Dominator on a good, two-plane intake very appealing for a big-block application. We wonder if any manifold manufactures will step up to that!

Booster Hop Up
Except for maybe converting a regular booster to a stepped booster, there is not much you can do to alter the basic design of a booster. That does not mean the signal gain can’t be improved. Take a close look at a booster and you will see that it has a flat-face top and bottom about 25 thousandths wide. It’s there because the casting process typically used to make boosters cannot produce a sharp edge. Removing a little metal from the booster’s inner form as per the diagram will produce an increase in signal far beyond what you may expect from such a minor mod. It does so because the sharp edge causes more air to go through the booster rather than be deflected around the outer diameter. This mod also produces a couple of extra cfm per barrel so it is definitely a win-win move.

Another popular move is to convert the so-often-used dogleg booster to a stepped-style design. The step does little to increase the booster signal, but it does bring about better fuel shearing at the discharge point, thus causing the fuel to atomize better.

End.  http://www.popularhotrodding.com/tech/0511phr_carburetor_boosters_tech/index.html

http://www.jeepforum.com/forum/f8/school-me-cams-1096204/

-Shawn

Product Review #1 AED’s 1250 Carb – too big for a 350??
Product Review #1

This is where we look at a promising product and give you a preliminary evaluation prior to any in depth evaluation we may subsequently do in our Product Spotlight Series.

AED’s 1250 Dominator

A carb for all occasions (Almost)?

by

David Vizard

“As is so often the case our quick test of this 1250 cfm AED Dominator came about by virtue of a number of ‘chance encounters’. The first step in that direction was almost insignificant. During a conversation with AED’s boss John Dickey, he happened to mention that they had got the drivability of their new series of Dominator carbs such that there was almost no such thing as a carb too big for the engine. That stuck in the back of my mind.

The next element of this series of events was that I incorrectly assembled (one of those 1 am deals) our largest dyno AED 4150 series carb (about 930 cfm) and it was sent back to AED for a rebuild. Next day the dyno becomes available to test Dusty’s new 350. This engine, even if it was near the bottom of the ladder in terms of race engines, was Dusty’s first foray into building such. Sure it had a few concessions to street use such as a 10/5/1 CR and a slightly more conservative cam than would otherwise be the case but apart from that it was the real McCoy in terms of a budget race engine build.

So we get on the dyno and as usual it’s all a bit of a rush as we have just one day to break this engine in and test several manifolds and the like. Well we get to test the essentials but, as ever, it takes a little longer than expected. Up to this point we had been testing the dual plane stuff and Dusty was hot to see if his motor could break the 560 hp number that so often cropped up as a target figure from the many hot shot engine builders he so often talks too.

So it’s now late in the day. The two plane intake comes off and the plenum prepped, port matched Super Victor goes on. Since the big 4150 series AED is at AED’s shop we have to make do with the slightly souped up 750 AED carb we have. A great carb but it is probably a little on the small side for those big numbers. Although drivability is still a factor here Dusty was most interested in the 6000 – 8000 rpm range as that, assuming the motor delivers as anticipated, is what it would use on the track.

So the ‘souped up’ AED 750 was installed. At this point the engine is also dumping through two big Flow Masters. I made three pulls on this combo and the result was 451 lbs-ft and 557 hp. At this point we have 30 minutes to do something that might net the 560 horse target.

At this point I recall John Dickey’s comment about ‘no such thing as too big a carb’ with his new Dominator. I also mentally edit out the ‘almost’ that was also in that comment. I see we have an adaptor/spacer that will allow us to bolt the big AED Dominator right on to the Super Victor. So the regular spacer and the 4150 AED were removed and replaced by our monster carb.

Since we had time for one shot and one alone at this 560 hp number Dusty elected to remove the big Flow Masters ( which in previous tests at around this power level had near zero effect on output) and install the right tuned length collectors for this setup (a previously determined length from similarly cammed motors). Unfortunately the precise length could not be had with the extensions available to us at that moment so a little compromise was forced on us here.

With about 10 minutes to go we fired up this now very raucous open exhaust 350. I played with the load and throttle at relatively low rpm to see if I could find any holes in the drivability. None appeared – so far so good. A low speed pull showed the engine made too much torque for the dyno to hold it at 2500 rpm but as it passed through the 2500 mark it did consistently show 320 -340 lbs-ft. At 3000 rpm, which was about the first point the dyno would hold it (dyno was set for high rpm, pulls) a recorded 396 lbs-ft came up. That was just 5 shy of what we saw for the souped up 750. So far so good. At this point there was literally 2 minutes left before it was time to shut shop and go home. I made a pull – just one. This 350 soared around to 8100 rpm like a 150 Decibel watch and cranked out almost 584 hp in the process. The before and after figures are as per below but please remember this is not a true before and after test of just the carb because the tuned length open exhaust also plays in here. What it does show though is that the low speed capability if this AED carb is exceptional.

Now before you all go installing one of these monster carbs on your 350 lets fill you in on John Dickey’s comments when we spoke to him about this test. Quote “Really – I knew we were on to something here but I would hesitate to recommend that much carburetion on a 350. We know that they work well on engines as small as 400 inches and drive like an electric motor at low speed”.

At this point we were outside the boundaries of what AED conceived, at that time, to be a viable deal. However this test adds to the bank on data they have to work with. What we have proved here is that this carb is very drivable even on an engine considered too small for it’s flow capacity. On this occasion we got it to work and who knows – we may get it to work equally well on our next high output 350 (and you can bet we will be looking into testing it more rigorously) What we have shown here is that this big carb at least, to a sizable degree, resets the standard when considering what the maximum flow needs to be if the drivability and low speed is not to be negatively impacted past a certain minor degree.

Here, from the data from one pull, is the before and after when the carb was changed out to the AED 1250 Dominator and the exhaust opened up with near optimal tuned length collectors.

I am now sufficiently confident that if we are doing a high effort engine of more than say 390 cubes, this is the carb we will give most serious consideration too.”

David Vizard

Simple Steps to Success
Although the mode of function of an exhaust system is complex, it is not (as so often is believed, even by many pro engine builders) a black art. To help appreciate the way to get the job done I will go through the process of selecting exhaust system components for a typical high-performance V-8 in a logical manner from header to tail pipe. Although the entire exhaust functions as a system, we can, for all practical purposes, break down many of the requirements that need to be met into single entities. Fig. 1 details the order of business. But before making a start, it is a good idea to establish just why getting the exhaust correctly spec’d out is so important. This will allow realistic goals, improved component choice, and a more functional installation.

The V-8 engines we typically modify for increased output are normally categorized as four-cycle units. Although pretty much the case for a regular street machine, this is far from being the case for a high-performance race engine. If we consider a well-developed race engine, the usual induction, compression, expansion (power stroke) and exhaust cycles have a fifth element added (Fig. 2). With a race cam and a tuned-length exhaust system, negative pressure waves traveling back from the collector will scavenge the combustion chamber during the exhaust/intake valve overlap period (angle 5 in Fig. 2). To understand the extent to which this can increase an engine’s ability to breathe, let’s consider the cylinder and chamber volumes of a typical high-performance 350 cubic-inch V-8.

Assuming for a moment no flow losses, the piston traveling down the bore will pull in one-eighth of 350 cubic inches. That’s 43.75 cubic-inch, or in metric, 717cc. If the compression ratio is say 11:1, the total combustion chamber volume above this 717cc will be 71.7cc. If a negative pressure wave sucks out the residual exhaust gases remaining in the combustion chamber at TDC, then the cylinder, when the piston reached BDC, will contain not just 717 cc but 717 + 71.7 cc = 788.7 cc. The result is that this engine now runs like a 385 cubic-inch motor instead of a 350. That scavenging process is, in effect, a fifth cycle contributing to total output.

But there are more exhaust-derived benefits than just chamber scavenging. Just as fish don’t feel the weight of water, we don’t readily appreciate the weight of air. Just to set the record straight, a cube of air 100 feet square will weigh 38 tons! If enough port velocity is put into the incoming charge by the exhaust scavenging action, it becomes possible to build a higher velocity throughout the rest of the piston-initiated induction cycle. The increased port velocity then drives the cylinder filling above atmospheric pressure just prior to the point of intake valve closure. Compared with intake, exhaust tuning is far more potent and can operate over ten times as wide an rpm band. When it comes to our discussion of exhaust pipe lengths it will be important to remember this.

At this time a few numbers will put the value of exhaust pressure wave tuning into perspective. Air flows from point A to point B by virtue of the pressure difference between those two points. The piston traveling down the bore on the intake stroke causes the pressure difference we normally associate with induction. The better the head flows the less suction it takes to fill (or nearly fill) the cylinder. For a highly developed two-valve race engine the pressure difference between the intake port and the cylinder caused by the piston motion down the bore, should not exceed about 10-12 inches of water (about 0.5 psi). Anything much higher than this indicates inadequate flowing heads. For more cost-conscious motors, such as most of us would be building, about 20-25 inches of water (about 1 psi) is about the limit if decent power (relative to the budget available) is to be achieved. From this we can say that, at most, the piston traveling down the bore exerts a suction of 1 psi on the intake port Fig. 3.

The exhaust system on a well-tuned race engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Because this occurs during the overlap period, as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore. The only conclusion we can draw from this is that the exhaust is the principal means of induction, not the piston moving down the bore. The result of these exhaust-induced pressure differences are that the intake port velocity can be as much as 100 ft./sec. (almost 70 mph) even though the piston is parked at TDC! In practice then, you can see the exhaust phenomena makes a race engine a five-cycle unit with two consecutive induction events.

With the exhaust system’s vital role toward power production established, it will be easy to see that understanding how to select and position the right combination of headers, resonators, routing pipes, crossovers and mufflers will be a winning factor. This will be especially so if mufflers are involved in the equation. I first started putting out the word on how to build no-loss systems as much as 20 years ago and I am somewhat surprised that it is still commonly believed that building power and reducing noise are mutually exclusive. Historically, this has largely been so, but building a quiet system that allows the engine to develop within 1 percent of its open exhaust power is entirely practical. Be aware that knowing what it takes in this department can easily deliver a 40-plus hp advantage over your less-informed competition.

Headers — Primary Pipe Diameters
Big pipes flow more, so is bigger better? Answer: absolutely not. Primary pipes that are too big defeat our quest for the all-important velocity-enhanced scavenging effect. Without knowledge to the contrary, the biggest fear is that the selected tube diameters could be too small, thereby constricting flow and dropping power. Sure, if they are way under what is needed, lack of flow will cause power to suffer. In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power.

At this point determining primary tube diameters is starting to look like a tight wire act only avoidable by trial and error on the dyno. Fortunately, a little insight into what it is we are attempting to achieve brings about some big-time simplification. Our goal is to size the primary pipes to produce optimum output over the rpm range of most interest. The rate exhaust is dispensed with, and consequently, the primary pipe velocity, is strongly influenced by the port’s flow capability at the peak valve lift used. From this premise it has been possible to develop a simple correlation between exhaust port-flow bench tests and dyno tests involving pipe diameter changes. This has brought about the curves shown in the graph Fig. 4 which allow primary sizing close enough to almost eliminate the need for trial-and-error dyno testing.

Primaries For Nitrous UseSince nitrous injection is so popular, it’s worth throwing in the changes needed to optimize with the nitrous on. For a typical race V-8 the area of the primary pipe needs to increase about 6-7 percent for every 50hp worth of nitrous injected. For street applications, where mileage and performance when the nitrous is not in use is the most important, pipe size should not be changed to suit the nitrous.

Headers — Primary Pipe Lengths
Misconceptions concerning exhaust pipe lengths are widespread. Take for instance the much-overworked phrase “equal-length headers.” More than the odd engine builder/racer, or two, have made a big deal about headers with the primary pipes uniform within 0.5 inch. The first point this raises is whether or not what was needed was known within 0.5 inch! If not, the system could have all the pipes equally wrong within 0.5 inch! Trying to build a race header for a two-planed crank V-8 with lengths to such precision is close to a waste of valuable time. Under ideal conditions it is entirely practical for an exhaust system to scavenge at or near maximum intensity over a 4,000 rpm bandwidth. Most race engines use an rpm bandwidth of 3,000 or less rpm. If the primary pipe scavenging effect overlaps by 3,000 rpm then it matters little that one pipe tunes as much as 1,000 rpm different to another. Since this is the case, then all other things being equal, pipe lengths varying by as much as 9 inches have little effect on performance. A positive power-increasing attribute of differing primary lengths is that it allows larger-radius, higher-flowing bends and more convenient pipe routing to the collector in often confined engine bays.

Apart from the reasons just mentioned, there is also another sound reason why we should not unduly concern ourselves about equal primary lengths. In practice, the two-plane cranks that typically equip V-8 race engines render the exhaust insensitive to quite substantial primary length changes. Experience indicates inline four-cylinder engines are more sensitive to primary pipe length, but a two-plane cranked V-8 is not two inline fours lumped together. It is two V-4s and, as such, does not have even exhaust pulses along each bank. With a conventional, as opposed to a 180-degree header, exhaust pulses are spaced 90, 180, 270, 180, 90 and so on. The two cylinders discharging only 90 degrees apart are seen, by the collector, as one larger cylinder and accounts for the typical rumble a V-8 is known for. This means the primaries act like they do on a four-cylinder engine, but the collector acts as if it were on a 3-cylinder engine having different sized cylinders turning at less revs. (Doesn’t life get complicated?) This, plus the varied spacing between the pulses appears to be the cause of the system’s reduced sensitivity to primary length.

These uneven firing pulses on each bank seem to work in our favor. Evidence to date suggests that single-plane cranked V-8s, which have the same exhaust discharge pattern as an in-line four-cylinder engine, make less horsepower and are more length sensitive. Dyno tests with headers having primary lengths adjustable in three-inch increments show that lengths between 24 and 36 inches have only a minor effect on the power curve of V-8s that you and I can typically afford, although the longer pipes do marginally favor the low end.

Secondaries — Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily and it is often best optimized at the track rather than the dyno.

As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. Except for the most highly developed engines, many collectors I see at the track are too large in diameter and either too short, or of excessive length. For a motor peaking at around 6,000-8,500 rpm, a collector length of 10-20 inches is effective.

Getting secondary lengths nearer optimal can be worth a sizable amount of extra power as Fig. 5 shows. If you want to bump up torque at the point a stock converter starts to hook up the engine, you may want a secondary as long as 50 inches but something between about 10 and 24 is more normal. The shorter of these two lengths would be appropriate for an engine peaking at about 8,500 rpm whereas the longer length would be best for an engine that peaked at about 4,800-5,000 rpm.

Mufflers — Two Golden Rules To Avoid Power Loss
Inappropriate muffler selection and installation (which appears so for better than 90 percent of cases) will, in a very effective manner, negate most of the advantages of system length/diameter tuning. The question at this point is what does it take to get it right and how much power are we likely to loose if the system is optimal? The quick and dirty answers to these questions are “not much” and “zero.” This next sentence is the key to the whole issue here, so pay attention. To achieve a zero-loss muffled high-performance race system we need to work with the two key exhaust system factors in total isolation from each other. These two factors are: the pressure wave tuning from length/diameter selection, and minimizing backpressure by selecting mufflers of suitable flow capacity for the application. If we do this then a quiet (street-legal noise levels) zero-loss system on a race car is totally achievable without a great deal of effort on anybody’s part. Ultimately, it boils down to nothing more than knowledgeable component selection and installation, so let’s look at what it takes in detail.

Part 2 coming soon…