is backpressure hurting your combo?

" GRUMPY? I recently swapped from 1 .625" shorty headers and a 2.75" dual exhaust to 1.75" full length headers and 3" exhaust with an (X) pipe to reduce the cars effective back pressure, the car runs noticeably better once the rpms exceed about 5000rpm but my 60 foot times got slower and I seem to have lost some lower rpm torque, do I need to make a change to increase my exhaust back pressure?"


first lets point out a few things, headers are designed to increase cylinder scavenging, increasing cylinder scavenging in theory reduces the level of previously burnt exhaust gases , and increases the percentage of fresh fuel/air entering the combustion chamber and being compressed , thus increasing the potential power the engine can make. but in most cases this also means some of the new fuel/air charger is drawn out thru the exhaust before the valves seat on the compression stroke. the diameter of the primary pipes and the length of those pipes plus the collector they feed all effect the exhaust gas velocity as does the cars displacement, compression ratio and cam timing, its also common that changes to the cylinder scavenging will require changes to the jetting, or intake design to maximize the cylinder fill efficiency. and the intake design will also effect how efficiently the cylinders get fed and scavenged by the exhaust gas inertial drawing in the next intake runner charge of fuel/air mix.
back pressure is always bad, but if you increased the header size too much you reduced the exhaust gas velocity thus reducing the cylinder scavenging, this has zero to do with back pressure, but a good deal to do with effectively using the velocity of the gases flowing out of the cylinder to draw in the next fuel fuel air charge.
in many cases adding an extended collector to the headers will get you back the previous headers efficiency, in many case shorty headers use the exhaust system they are matched with as an extended header collector, swapping to a full length header and a larger exhaust effectively reduces the working length of the exhaust.
BACK PRESSURE will ALWAYS restrict the engines power levels IF the engines properly tuned, the cam timings correct and youve got the car geared correctly, if you do some careful investigation Im reasonably sure youll find the increased exhaust header primaries and lower restriction exhaust slowed the exhaust gasses and may have EITHER reduced or increased cylinder scavenging , as youve changed the pressure levels in the cylinder and/or the change in scavenging changed the effective fuel air ratio, in the engine, this is VERY COMMON, and in many cases just using a vacuum/pressure gauge on the intake and header collectors and reading the plugs condition or ideally using a fuel/air ratio meter and an infrared temp gun on the headers where they exit the cylinder heads,will show you that change. in many cases a tighter LSA cam will help increase the power levels as the slightly change overlap tends to offset the increased scavenging and increase torque due to the valves closing a bit earlier.
in some cases retarding the cam will allow the engine to breath at the slightly increased rpm levels the better scavenging allows.
in most cases youll need to make changes to the tuning to try to get as close to 12.5:1 -13:1 on the fuel air ratio as you can, as thats where most engines make the best power levels, in some cases a cam change to more closely match the ideal cylinder scavenging will be required or a rear gear change to match a change in the effective rpm range where your now making power.
Ive seen several cars where cheaper "LONG TUBE HEADERS" like these in this picture, failed to provide the intended power until an extended section like the one posted just below was added, which markedly increased the effectiveness of the header scavenging.
keep in mind most of the less expensive headers are designed with low cost and ease of installation as far more important goals in the fabrication process than maximizing power levels, plus the header manufacturer has zero idea as to the cam timing compression, displacement or other factors in the engine they will be matched with.
you might also consider that longer and larger diameter headers will more effectively handle increased exhaust gas flow, if you feed the flow into a common (X) pipe that smooths and blends the exhaust flow from both sides of the engine

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READ THE THREAD THRU TO THE END AND BE AWARE IT, AND ALL OTHER THREADS ARE CONSTANTLY UPDATED WITH NEW LINKS AND INFO

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http://www.summitracing.com/parts/FLO-C ... /?rtype=10
This is not, or at least should not be, random guess work, its a process of, testing, observing and correcting how the engine runs from observed test results. while most guys rely on a system of trail and error, the results can be calculated, the physics involved are well known, so you,ll have too spend far less time swapping random parts to find the ideal component config. if you read the links and do some calculations and testing
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related info, the factors effecting your engines current power level can be tested, observed and if necessary corrected,

http://maxracesoftware.com/pipemax36xp2.htm


http://www.circletrack.com/enginetech/c ... ewall.html

http://www.bing.com/search?q=how%20head ... owAppsUI=1

http://perfweldheaders.com/headerscavengetech.html


http://blog.racingarticles.com/2008/02/ ... aders.html

http://www.hondatuningmagazine.com/tech ... ewall.html




http://www.carcraft.com/techarticles/he ... ewall.html
 
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http://www.popularhotrodding.com/engine ... ewall.html
quote
"For me the first really serious look at how to muffle a high-performance race engine without loosing a significant amount of power started in 1980 when I built a 400lb-ft, 404hp 350 to replace the very lame 158hp 305 in my California-spec Pontiac Trans Am. Having worked very hard to build a pump gas fueled engine (gas was really bad in those days), that would cross the 400 hp barrier, I was very disappointed to find that, regardless of what mufflers were used, the output dropped by some 20 lb-ft and 25 hp. Having had some experience designing a no-loss system for the original style British Mini Coopers, I felt confident I could pull off the same stunt for significantly bigger V-8 engines. The result, aided by an acoustics expert friend, was the Sonic Turbo. This design went on to be manufactured by Cyclone (now a division of Walker/ Dynomax). After the smoke cleared from a big muffler shootout (done at Gale Banks facility and published by Hot Rod magazine), a pair of 2.25-inch Sonic Turbos (the 2.5-inch ones were still a couple of months off) sunk everybody else's 2.5-inch items. This, it seemed, was just what the hot rod fraternity wanted and they sold by the hundreds of thousandths. That was good, but more importantly, it appeared to spark the industry into aggressively pursuing significantly more functional mufflers and exhaust systems. The result is that 20-some-years later, all the necessary components to build a highly effective, no-loss system are at hand, and not necessarily that much money either. All that appears to be lacking is widespread know-how as to what is needed to achieve this happy state of affairs. As of now, we are going to make a start on putting that right.

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.

Muffler Flow Basics
We select carbs based on flow capacity rather than size because engines are flow sensitive, not size sensitive. This being so, why should the same not apply to the selection of mufflers? The answer (and here I'd like muffler manufactures to please note) is that it should, as the engine's output is influenced minimally by size but dramatically by flow capability. Buying a muffler based on pipe diameter has no performance merit. The only reason you need to know the muffler pipe size is for fitment purposes. The engine cares little what size the muffler pipe diameters are but it certainly does care what the muffler flows and muffler flow is largely dictated by the design of the innards. What this means is that the informed hot rodder/engine builder should select mufflers based on flow, not pipe size.

A study of Fig. 6 will help to give a better understanding as to how the design of the muffler's core, not the pipe size, dictates flow.

Let's start by viewing a muffler installation as three distinct parts. In Fig. 6, drawing number 1, these are the in-going pipe, the muffler core and the exit pipe. Drawing number 2 shows a typical muffler which has, due to a design process apparently unaided by a flow bench, core flow significantly less than an equivalent length of pipe the size of the entry and exit pipe. Because the core flow is less than the entry and exit pipe then the engine "sees" the muffler as if it were a smaller and consequently more restrictive pipe as per drawing number 4. If the core has more flow than the equivalent pipe size, as in drawing number 5, it appears larger than the entry and exit pipe. Result: the muffler is seen by the engine as a near zero restriction. A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (10.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe. If we have a 2.5-inch muffler that flows 400 cfm, the engine reacts to this just the same as it would a piece of straight pipe flowing 400 cfm.

At 115 cfm per square inch, that's the equivalent to a pipe only 2.1 inches in diameter. This is an important concept to appreciate. Why? Because so many racers worry about having a large-diameter pipe in and out of the muffler. This concern is totally misplaced, as in almost all but a few cases, the muffler is the point of restriction, not the pipe. The fact that muffler core flow is normally lower than the connecting pipe can be off set by installing something with higher flow, such as a 4-inch muffler into an otherwise 2.75-inch system"
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A CORRECTLY TUNED SET OF HEADERS , MATCHED TO A CORRECTLY DESIGNED CAM TIMING HAS A SIGNIFICANT EFFECT ON INTAKE FLOW AND CYLINDER SCAVENGING EFFICIENCY, EXHAUST SCAVENGING CAN BE 5 TIMES STRONGER THAN THE PISTON, MOVEMENT INDUCED NEGATIVE PRESSURE (VACUUM) IN THE INTAKE RUNNERS
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http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html
 
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I've replaced the exhaust systems, and swapped out exhaust manifolds for headers of various muscle car designs, and engines, on a great many cars and I've occasionally had discussions, with the cars owners who almost universally seemed a bit disappointed after installing headers , Especially if they kept the stock exhaust system,or if the engines they installed those headers on were basically stock, as most of the guys, never seem to understand that a set of headers , is only one component in the system, will only work correctly , or up to near its full potential, if they have nearly ZERO exhaust back pressure, if the exhaust system behind the headers are designed to have very low flow restriction at the upper rpm ranges and will only work at neat the full potential if the cam used is designed to fully exploit the headers scavenging.
and headers work far better if the engine has both high compression, and a cam timing that takes full advantage of those headers potential to scavenge the cylinders.
theres a good deal of math, CHARTS DIAGRAMS,AND CALCULATOR"S in the links posted below, but what it comes down to is the fact that a great deal of the engines potential power will be either restricted or noticeably enhanced through the careful design, and use of a carefully matched cam timing and exhaust system design.
you can,t reasonably expect a set of headers flowing into a restrictive exhaust to effectively scavenge the cylinders. and if you select a cam timing that will not enhance the scavenging effect that the tuned headers, youll loose a great deal of the total exhaust systems potential, to effectively scavenge the cylinders as effective scavenging has a very pronounced effect on the intake flow rates. now that might not sound all that important but get it correct and Ive seen even mildly modified engines jump 40 plus ft lbs in low speed torque, and jump 50 plus hp over the stock exhaust manifolds and exhaust,on even mildly modified cars. so its easily the difference between a competitive car and a car far down the list in finishers on any list.
one other factor thats also frequently ignored is that your fuel/air ratio and ignition advance curve is absolutely going to need to be changed to match the new flow rates.
Id also point out that the transmission shift point and gearing will effect the results, if your cam and header design is set up to maximize power in lets say the 5000-rpm-6500-rpm power band, and your transmission shifts at 5500-rpm your not going to realize the headers full potential.


READ THE LINKS, AND SUB LINKS
http://garage.grumpysperformance.com/index.php?threads/calculating-header-design.185/

http://garage.grumpysperformance.com/index.php?threads/x-or-h-pipe.1503/

http://garage.grumpysperformance.co...ers-and-yeah-thinking-it-through-helps.15137/

http://garage.grumpysperformance.co...ful-exhaust-valve-size-and-header-info.11265/

http://garage.grumpysperformance.co...-guys-that-just-slap-on-factory-headers.3155/

http://garage.grumpysperformance.com/index.php?threads/building-an-exhaust-system-for-your-car.1166/

http://garage.grumpysperformance.com/index.php?threads/dyno-testing-headers.3529/
 
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you need to read through the linked info

http://www.veryuseful.com/mustang/tech/engine/exhaustScavenging.pdf

[quote ]any increase in back pressure proved to decrease torque on a properly tuned engine. What increasing the backpressure does do is dramatically quiet the exhaust. One of the engine dyno tests carried out by Kevin was on a modified 351-4V Cleveland V8. Following the extractors he fitted a huge exhaust that gave a measured zero backpressure. Torque peaked at 423 ft-lbs at 4700 rpm, with power a rousing 441 hp at 6300 rpm. He then dialed-in 1.5-psi (10.4 kpa) backpressure. As you'll see later, very few exhausts are capable of delivering such a low backpressure on a road car. Even with this small amount of backpressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the exhaust to give 2.5-psi backpressure. Torque and power decreased again, both dropping by 7 per cent over having zero backpressure. These results were achieved on a large engine with a large overlap cam - one of the types some people suggest is 'supposed' to like backpressure. If, in fact, power does increase with increased exhaust back pressure, it is most likely the air/fuel ratio and/or ignition timing that are no longer optimal for the altered state of engine tune." Larry Widmer comments on the above textbook quote: At less than WOT and peak power rpm, the diameter of the tubing should change in ID. Just as with intake ports (unless we're just running off port volume), cross sectional area should be only sufficient to supply the flow rate necessary to feed the engine. High velocities, that don't incur pumping losses are the rule. The exhaust system is much the same. Just changing backpressure is a bogus way of trying to create the "ideal" pressure in the system. The exhaust system should work like a correctly conceived header. It should extract the exhaust from the header, to minimize pumping pressures. The only way to create a system that will serve as an extractor is to properly size the tubing to allow the flow velocity to create a sort of "vacuum" behind it. Just as with headers, creating a system that will provide the best of all worlds at all throttle positions and rpm ranges is impossible. It's all going to be a trade-off. You can tune for the throttle positions and rpm ranges where you desire the greatest performance, but you'll sacrifice performance at the other end of the rpm range. Building a system to divert the flow into a smaller system can help bolster lower rpm power, just as with today dual runner intake manifolds, but you'll never find a dual runner intake on any engine that's targeting the greatest performance potential possible. I should also add that such systems are inefficient from a standpoint of weight and surface area. For mid-performance applications, these type systems will be as popular as their costs will allow. In our quest for "more", we seldom work to achieve mid-level (mid rpm range) performance, so just as the gentleman who wrote the book in the post from above, we prefer to tune with the least amount of backpressure possible. We do have to observe rules and regulations (noise levels and EPA regulated emissions) and the systems must fit the vehicle in question without dragging the ground, so there will always be compromises. I suppose that I should mention that cost is another consideration. If it wasn't, a lot of our street systems would have greater area and they wouldn't necessarily be circular in configuration either. In the stock ITR, backpressure becomes a power "liability" by the time the engine's making 210 flywheel HP. Relative to wheel HP, if you're making more than about 11 HP more than "stock", the system's costing you....and yes, detonation can be caused by excessive backpressure.

The other problem you face with excessive backpressure is one of reversion. The higher the backpressure, the more inert exhaust components re-enter the cylinder. A few of these bad-guys can really steal big hunks of power in a hurry. If you don't believe me, just run a pipe from your exhaust tip up near the air cleaner on your next trip to the dyno. A little sniff of the exhaust will absolutely kill your power. 2. Calculations and Comments by Dave Stadulis of SMSP Exhausts Relating Flywheel HP to Exhaust CrossSectional Area (Diameter): quote:
Here are the numbers for 16g tubing: OD (in.)....ID (in.)...Area....%Increase......HP.......HP/in^
2 2.25........2.120.......3.53.....0%...............200.......56.66
2.50........2.370.......4.41.....25%..............275.......62.34
2.75........2.620.......5.39.....22%..............318.......59.00
3.00........2.870.......6.47.....20%..............400.......61.83 OD
is exhaust outer diameter, ID is inner diameter, Area is tube cross-sectional area,
% Increase is increase from the prior OD,
HP is Flywheel hp, and HP/in^2 is hp per square inch cross-sectional area.
For the 2.75 in. tube, I assumed 59 HP per square inch of flow area,
I used Larry's numbers for the others....you are talking HP at the crank : 2-1/4" for up to 200HP
@ the crank, 2-1/2" for 275HP,
2-3/4 for 320HP... or 60HP (at the crank) per square inch of (cross-sectional) flow area.
This 60HP/in^2 is to get you in the general vicinity.
It also is based on the inside diameter of the tubing not the OD (i.e. 2" in your example).
The ID for 2' 16g tubing is 1.87" and this will yield a limit of 165 crank HP.
2-1/4" 16g (212 HP), 2-1/2" (265 HP).
Now you can get different sized tubing such as 2- 1/8" and 2-3/8" to fine-tune a vehicle but you can't get cats and mufflers in those sizes so you should go up a size when building an exhaust in those cases.
 
Big open and Loud pipes I do like Grumpy.
Max Torque on Hand then.
 
early in this thread I suggested you get a vacuum/pressure gauge and test the exhaust back pressure, I find for some reason only the more dedicated engine tuners bother, so here's a bit of info that may help is your lazy or don,t think its required, you can use the plenum vacuum readings to point to the problem and they can be a good reason for justifying further in depth testing,if theres no easily accessed , low restriction escape path for the exhaust gasses to use, you don,t get effective cylinder scavenging and thus you restrict the intake flow rates significantly, a really restrictive exhaust forced burnt gasses from the previous cycle back into the cylinder to mix with the following intake charge further diluting the power potential even further.
I'm always rather amazed at the number of guys who don,t recognize clear indications that they may have a problem or the obvious symptoms, that generally point toward the problem. if you hook a vacuum gauge to your carb or intake plenum on your EFI at idle it should show a significant vacuum reading and the needle should move consistently a bit, obviously a longer duration cam will cause the reading to be lower and the needle fluctuation's to be greater
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http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html
but if you bring the rpms up to 3000rpm the vacuum should drop noticeably, as you first open the throttle , but then rise a bit and level off at a new level as you hold it at 3000rpm,, if the vacuum reading starts to drop off , its a rather clear indication the exhaust is restrictive (partly plugged catalytic converter?) or your exhaust , mufflers and pipes extra , are restrictive, jump it to 5000rpm, and hold it, and you should see the similar result, it should drop, level off, and stay steady at the new reading, if it starts falling even faster at the higher flow rate its a sure indication of a restrictive exhaust, keep in mind any significant exhaust back pressure will rapidly reduce or even eliminate and effective header scavenging of the cylinders thus rendering the headers rather ineffective or nearly useless
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Many studies have show that flame propagation in the combustion chamber under combustion pressures and due to air turbulence (swirl and tumble) is actually faster and non linear compared to fuel being ignited in free air conditions.

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http://www.aa1car.com/library/exhaust_backpressure.htm

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jbranne said:
Since many people get confused about backpressure, scavenging, exhaust sizing, etc., I wrote this up for another board. I just did a quick cut and paste here, so enjoy...





There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.
 
Would be better if they used an x or h pipe in there comparison and wish it was a sbc so I could have better results to compare for my build.
 
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while thats a good point I don,t remember a single instance in 45 plus years that had a performance engine combo fail,
to run at least marginally better with a full length 3" exhaust with an (X) pipe mounted as close,
to the collectors as the physical space and clearance restrictions under the car allowed!
(once the car was retuned to take full advantage of the reduced back-pressure and increase in intake port flow rates that resulted)
headers only function at near full efficiency with no, or at least minimal back pressure

http://garage.grumpysperformance.com/index.php?threads/calculating-required-exhaust-pipe-size.11552/

http://garage.grumpysperformance.com/index.php?threads/building-an-exhaust-system-for-your-car.1166/

http://garage.grumpysperformance.com/index.php?threads/dyno-testing-headers.3529/
 
I agree but again this is a clear push the product test video. I am planning 1 5/8 full length headers into a 3" x-pipe with electronic opened cutouts then funneling down into my current 2.5" exhaust might even put the cut outs on an arm switch then have my ecu open them at a certain rpm. 3 way switch open all the time cpu opened and always off.
 
while thats an interesting concept (having the cpu control exhaust cut out)
I think keeping the control limited to a full manual control would put you in a much better position ,
you darn sure don,t want to forget, and have the exhaust cut-out pop open under full acceleration, under load for instance,
in an inclosed tunnel with the local highway patrol behind your car!
 
Yes True we will see when the time comes but think it might happen, that and the lockup converter would be nice if I can do similar control style. Not to many tunnels in my travels. And would be pretty cool as most of the time would be off anyways.
 
I was looking at the graph above and was wondering your opinion or am I reading into this wrong my heads are advertised for flowing these exhaust numbers from the AFR website
AFR 195 Street Eliminator
http://www.airflowresearch.com/195cc-sbc-street-cylinder-head/
Exh .200-119 .300-166 .400-197 .500-213 .550-218
Test conducted at 28" of water (pressure) on Superflow 600
Bore Size: 4.060", 3/4" radius plate exhaust, 1 3/4" curved pipe

My cam max lift currently is .535 even if I replace my cam it would be bigger but as far as primary size goes. I know more people over due header size getting 1 3/4 or 1 7/8 headers instead of 1 5/8 now I actually am moving allot of air with my heads would my engine I am building actually fall into that category where I would benefit from a 1 3/4 over a 1 5/8 header? I have realized my 2.5 dual is restrictive and will be replaced with 3" down the road but I am starting from the headers and working my way back.
 
HEADS ARE FLOW TESTED ON A FLOW BENCH
where they get the posted flow rate that has zero to do with real flow conditions!
(at a constant 28 inch of vacuum)
a heads exhaust port operate's at hundreds of psi when first opening, as the piston forced the burnt gasses out, and the pressure drops off very rapidly to less than zero, as the inertial of the gasses traveling away from the cylinder trys to drag following exhaust with them, if you run the correct header, remember at 6000 rpm, theres 50 exhaust pulses PER SECOND, so theres 1/50th of a second maximum between pulses
 
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I was speaking in reference to this chartEXFLOWZ1.jpg
Do you feel I would be sacrificing my low midrange for high end or is my flow high enough that I would be at a loss with the smaller header?
 
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I definitely agree with that Grumpy I will read through the links I don't want to lose more then is gained that is my fear. I am definitely switching to a long tube from a shorty just trying to get the most over the largest area I can.
 
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