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Is it possible to have too little backpressure?

  • yes

    Votes: 7 70.0%
  • no

    Votes: 1 10.0%
  • irrelevant

    Votes: 1 10.0%
  • just pop a couple Asprin and the backpressure will go away

    Votes: 1 10.0%
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Discussion Starter · #1 ·
I was hanging out with some Subaru guys this weekend and got on the topic of exhaust. We were discussing whether you can have too little backpressure in your exhaust, causing reduced power.

Case in point: My mostly stock '69 'vette with 350 at 11:1 compression has Hooker sidepipes on it. Headers have 1-7/8" primaries dumping into 4" sidetubes with no mufflers or cats.

I'll reserve my opinion until a few others have chimed in...
 

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Absolutely; reduction in back pressure causes a reduction in low end torque. For cars like 4 cylinder ricers that are already deficient in low end torque, a reduction in back pressure can cause the car to seem weak and stumble from a dig. However, sbc and bbc engines are not low end torque deficient (comparatively), so a reduction in back pressure will not be as noticeable.

Conversely, a reduction in back pressure allows the engine to breathe easier and thus spin at higher rpm. If your engine is designed to have a power range at 7000+ rpm, then you want as little back pressure as possible. Also, remember that torque and horsepower are inextricably related to one another.
 

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Discussion Starter · #4 ·

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why is that exactly?
A reduction in back pressure is “usually” coupled with a reduction in exhaust gas velocity, i.e. larger pipes = slower exhaust gas velocity = reduced back pressure. However, the slower exhaust gas velocity results in a decrease of the scavenging effect that the engines utilizes to get fuel into the combustion chamber between the exhaust and intake cycles. Reduced velocity = increased pressure = reduced vacuum ability (Bernoulli Principle). As a result, cars can't get as much fuel and air into the combustion chamber at low engine speeds with a loss of exhaust gas velocity (again usually coupled with reduction in back pressure). At higher engine speed, the Bernoulli Principle is reduced due to the fact that the engine is operating less cyclically (scavenging is less of an occurrence / necessity). At high speeds, it is almost like the engine is operating in a continuous flow.
 

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NO!
but its common to have the wrong size headers, exhaust system , or header collectors, to match your engines displacement, cam timing and compression, or for your engine tune to not take full advantage of the cylinder scavenging or ignition curve, increased back pressure in the exhaust is ALWAYS going to restrict flow, and its almost always going to hurt power, some engines have the cam timing or headers designed to compensate for back pressure, but in a correctly designed system with decent headers the power tends to go up as the back pressures REDUCED provided the cars FUEL/AIR mix and timing are adjusted to maximize the potential the lower restriction to flow provides.

a poorly tuned engine will frequently get leaner as back pressure reduced, and if your headers or exhaust is too large it will tend to reduce the cylinder scavenging, but its NOT back pressure, but exhaust gas velocity , or the lack of it thats the cause.

Destroying a myth. (that your engine runs better with back pressure in the exhaust)

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

Some say that "an engine needs exhaust back pressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs back pressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate back pressure with torque, and others fear that too little back pressure will lead to valve burning.

The first reason why people say "back pressure is good" is because they believe that increased back pressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that back pressure produces more torque. The increase in torque is not due to back pressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "back pressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of back pressure, but to the changes in the a/f ratio.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in back pressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetors often could not adjust because of the way that back pressure caused air to flow backwards through the carburetor (reversion pulse) after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced back pressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if back pressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing back pressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust back pressure reduction, these limits won't be reached.

- Adapted from Thomas V.
http://forum.grumpysperformance.com/viewtopic.php?f=56&t=1730

http://forum.grumpysperformance.com/viewtopic.php?f=56&t=185

whats the first thing most guys do at the track after installing their SLICKS, thats right! they open their headers, why? because open collectors on headers usually result in more horsepower than leaving the stock exhaust connected, look, if your headers are designed correctly the combo of the primaries and collectors are the correct length to efficiently scavenge the cylinders and help draw in the following intake charge as the low pressure wave reflects back to the exhaust port., this only works over a limited rpm band but it helps fill the cylinders, by tending to allow the fast exiting exhaust to drag in the following intake runner volume PROVIDED the cam timing matches the header design.
ANY BACK PRESSURE tends to reduce cylinder scavenging

read thru the linked info

http://forum.grumpysperformance.com/viewtopic.php?f=56&t=495
 

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A reduction in back pressure is “usually” coupled with a reduction in exhaust gas velocity, i.e. larger pipes = slower exhaust gas velocity = reduced back pressure. However, the slower exhaust gas velocity results in a decrease of the scavenging effect that the engines utilizes to get fuel into the combustion chamber between the exhaust and intake cycles. Reduced velocity = increased pressure = reduced vacuum ability (Bernoulli Principle). As a result, cars can't get as much fuel and air into the combustion chamber at low engine speeds with a loss of exhaust gas velocity (again usually coupled with reduction in back pressure). At higher engine speed, the Bernoulli Principle is reduced due to the fact that the engine is operating less cyclically (scavenging is less of an occurrence / necessity). At high speeds, it is almost like the engine is operating in a continuous flow.
This may have been true in carb'd engines but with injection the fuel is pushed and shouldn't rely on scavenging.. or am I missing something? Plus throw in no overlap and the only reliance on low pressure is to help clear the chamber of burnt fuel/air by the push from the pistons. thereby increasing efficiency by improving air quality for the "new" air fuel ratio..
 

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This may have been true in carb'd engines but with injection the fuel is pushed and shouldn't rely on scavenging.. or am I missing something? Plus throw in no overlap and the only reliance on low pressure is to help clear the chamber of burnt fuel/air by the push from the pistons. thereby increasing efficiency by improving air quality for the "new" air fuel ratio..
Ok, the first issue would be that fuel injected engines still require engine vacuum to get the fuel AND air into the combustion chamber; thus making them “normally aspirated”. Forced induction engines, i.e. blown, supercharged and turbocharged are different because they induce a positive pressure to force the air and fuel into the combustion chamber (pressure operated systems versus vacuum operated systems ~ huge differences in operational dynamics).

With the fuel injection, the fuel is injected, usually, directly into the head via a nozzle (fuel injector) that properly atomizes the fuel so that it can mix thoroughly with the inrushing air. The engine still has to do the work of drawing that inrushing air into the head / combustion chamber to mix with the injected fuel. As such, you still require a certain degree of scavenging to optimize the amount of air and fuel you can get into the combustion chamber.

Remember that while your engine may be 454 cu, it is in no way 100% volumetrically efficient. Therefore while the engine may displace 454 cu, you don’t necessarily get 455 cu worth of air and fuel. You get a percentage of that based on total line-losses, which is due to frictional and design characteristics of the engine (port shape, valve size, valve angle ect ect.) Scavenging is just a way to overcome the total line losses and get the engine closer to 100% volumetric efficiency.

Performance cams will have a significant amount of overlap...for naturally aspirated engines. For forced induction engines, everything goes out the window, as you don’t really need scavenging to get the maximum fuel and air into the combustion chamber. However, with advancements in camshaft design and increased head, intake and exhaust system flow characteristics, you may not need as much overlap to achieve the same volumetric efficiency as you once did. It has been a while since I took a class on camshaft design, so I am sure much has changed since then.

Lastly, air to fuel ratio should never change unless you are running different fuels or at significantly different altitudes. The stoichiometric ratio of air to fuel for a complete product yield at the maximum possible kinetic and thermodynamic conditions will never change. If you have no overlap, i.e. no scavenging, you will never be able to completely clear all the spent fuel (exhaust), unless you have a compression ratio something around infinity:1. So you want to get as much of the product (exhaust) out of the chamber as possible before the next batch of reagents (air and fuel) rush in, lest you want to dilute your previously optimized air to fuel ratio (optimized air to fuel ratio assumes your carburetor is properly tuned for the engine set up, or fuel mapping is properly tuned for a fuel injected car). Scavenging is the best way to accomplish this short of forced induction; hope this helps.

One last note, since backpressure and exhaust velocity are inextricably linked (lest you want to micro-polish or Teflon line your exhaust pipes), you have to ask yourself, “where do I want my engine to perform?” Do I want it to produce max power at low end rpm, or upper range rpm? If you want low end, you will be “forced” to run a higher degree of backpressure to increase exhaust velocity in order to give you the maximum fuel and air in the combustion chamber at low engine speed. This usually results in the engine suffering at higher rpm since the engine does not breathe as well. Conversely, if you want an engine you can scream, that produces max power via increased rpm, then you want to make it as easy as possible for that engine to breathe, i.e. reduce backpressure. Do that and the engine will suffer at low engine speed due to a decrease in scavenging effects. Likewise, camshafts (as well as all other engine components) should be selected to compliment each other and allow your engine to reach peak performance at the designed speed. That is why they tell you what their power range is when you buy them. Just remember when deciding whether you want a screamer or a torquer engine…

Horsepower = (Torque [ft*lb] * Engine Speed [rpm]) / 5252

So you can either build and engine that will produce gobs of torque at low engine speed, or one that produces extreme HP at high engine speed. It is very challenging to design and engine that does well at both speeds; it’s kind of a compromise.
 

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The answer is YES and esp with the fast burn heads of the LSx engines.

Good info to read :

Headers are built for power and their design can be very specific.
Let's look at how headers work, and clear up a couple of common misconceptions.

First is the size of the primary tubes.
it's easy to assume the bigger the tube the better, but that's FALSE.

Fact is, primaries that are too large actually cost you torque and horsepower by slowing down the rate at which the exhaust travels through the system and effect exhaust pulses to pull the exhaust out of cylinders so next firing will have air/fuel and not left over exhaust

Think of your engine as an air pump. Every time the exhaust leaves the combustion chamber, it's being forced into the primary tube for that cylinder.

Smaller diameter pipes flow less volume than large ones, but the exhaust in the smaller pipe flows faster.

Until you reach the RPM where the sheer volume of exhaust gases require bigger primaries, smaller tubes scavenge far more efficiently.
If you're using the engine in the 1,500-3,500 RPM range, which is typical for a street driven car, you definitely want 1- 1/2" to 1-5/8" primary tubes for a small block and 1-3/4" to 1-7/8" for a big block.

Any bigger and you'll lose plenty of low end torque.

Beyond 3,500 RPM it's a question of where you want the power peaks.
Small tubes don't lose their edge in horsepower or torque until you get above 5,500 RPM.

Even if you're running a radical cam and blower, you're better off sizing your headers smaller rather than larger, unless you plan to do most of your driving at full throttle.

Consider the size of the primary tubes.

Bigger tubes will give better breathing for top-end power, but the low velocity at lower rpm will make for more reversion to contaminate the next charge, so larger tubes will usually cost some low-end power.
It is a common mistake to port the heads to match the headers primary pipes, but this does more harm than good.
The primary tubes should be larger than the exhaust ports, this makes a reversion dam to limit reverse flow.

This is most important on the port floor where velocity is the lowest.
If your looking to enhance power in the 1500-3500 rpm range, you'll want 1 1/2"- 1 5/8" primaries on a small-block and 1 3/4"-1 7/8" on bigger cubic inch big blocks.

Big tube race headers can help power above 5500 rpm, but may cost some low-end power.
If you don't intend to rev the motor much past 5500 rpm, go with a smaller tube header.
On a small block Chevy, going bigger than 1 3/4" will require an adapter plate that will need to be ported to the head and you must use a header with a special bolt pattern to bolt up to it.

Primary Tube Length

The shorter the primary tubes are the higher in the rpm range they will help power.
Each exhaust pulse causes a high pressure wave to travel toward the collector. When it reaches the collector it is inverted and travels back toward the cylinder as a low pressure wave.
It is this low pressure area that helps scavenge exhaust out of the cylinder during the overlap period. This happens when the low pressure area reaches the exhaust valve during the overlap period.
The low pressure area helps draw out the exhaust and draw in more fresh intake charge. All the pressure waves travel at the speed of sound (1200-1300 ft/sec in the hot exhaust).

With all the pressure waves traveling at a constant speed, you can see that the header can be only be tuned to a narrow rpm range.
On a street car that needs low-end, the tubes should be longer, in a high rpm drag car they will need to be much shorter. To get you close to the optimum primary tube length, use this formula:

Length (in inches) = (CID x 1900) ÷ (rpm x pri.OD2)

Equal Length Primaries

Equal length headers can be an advantage on a street car with a full exhaust system, but even more so an open header engine race engine.
Whether they are worth it on a street car really depends on price, they are usually much more expensive.
That money may get more bang for the buck elsewhere in the engine.
As long as the primaries differ no more than 2 inches, they will be fine for a full exhaust system street car.

Equal length headers will work the best when the banks of an engine are even firing (and most V8's are not), 180° crankshafts even out the cylinder firing and work the best with equal length headers, but the costs and the fact that they cannot be perfectly balanced makes them impractical for the street.

H-Pipes

Adding an simple connection between the pipes can boost power in a certain rpm range. Most header primaries are tuned to operate on the second set of pressure waves, to tune the crossover to the same rpm range it will need to operate off the 1st set of waves.
If your primary tube ends 30 inches. From the back of the valve and is using the second set of pressure waves, putting the crossover 60 inches from the valves will help power at the same rpm range using the 1st set of pressure waves. In order to be effective, the crossover should be at least 90% of the diameter of the pipes.

X-Pipes

This is a more expensive crossover, but works a little better. The X design allows the pressure to go form one pipe to another much easier by eliminating the 90° corner.

The gasses have smooth bend to follow. X-pipes take up much more room which makes it harder to place them in the best location when ground clearance is a problem. When figuring the placement of the X pipe, measure the point where the pipes meet.

End result is most people buy "canned" headers like a well marketed vendor high priced junk made outside the USA as a "one size fits all" that do not fit the design of engine with mods and stock LSx engines really gain little with just headers which in most cases the primary size and total length kill off performance though out the RPM band.
 

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Discussion Starter · #10 · (Edited)
NO!

...if your headers or exhaust is too large it will tend to reduce the cylinder scavenging, but its NOT back pressure, but exhaust gas velocity, or the lack of it, that's the cause...
THAT's what I'm after. Sure, backpressure and exhaust gas velocity are related, but not directly and certainly not proportionally. It seems to me that it's a wide misconception that inadequate backpressure is the cause of reduced power or reduced scavenging affect, when in actuality it's the inadequate exhaust gas velocity that's to blame.
 

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This may have been true in carb'd engines but with injection the fuel is pushed and shouldn't rely on scavenging.. or am I missing something? Plus throw in no overlap and the only reliance on low pressure is to help clear the chamber of burnt fuel/air by the push from the pistons. thereby increasing efficiency by improving air quality for the "new" air fuel ratio..
Has nothing to do with fuel being "pushed" all gas engines have pusher type fuel pumps.

In today's engine with PCMs that are programmed for countless engine conditions, torque, controls fuel flow, timing and head flow designed for this are much more complex then old carb engines with no real timing control

Many tests have been done as example of C6 Z06s that are stock and "canned" headers installed with a findings of loss of performance.
Also look at the ZR1 in 2010 GM added a X-pipe up front to help those exhaust pulses timing created from backpressure and found they should also have done this with Z06s.
If a ZR1 with no headers, having backpressure can output 640 plus HP really shows the same design on a 500 HP Z06 is not lacking due to backpressure of a stock exhaust design

You now see a stock GS also comes with a front X pipe.
 

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THAT's what I'm after. Sure, backpressure and exhaust gas velocity are related, but not directly and certainly not proportionally. It seems to me that it's a wide misconception that inadequate backpressure is the cause of reduced power or reduced scavenging affect, when in actuality it's the inadequate exhaust gas velocity that's to blame.
Actually they are directly related. If you increase velocity, you increase backpressure; decrease velocity, you decrease backpressure; unless you know of a way to change the one without changing the other, i.e. some alteration other than changing the cross-sectional flowing area. Are they proportional? Don't know for sure, that would be a factor of the design of the complete exhaust system. They're probably related exponentially; as the velocity increases, the backpressure does as well ~ only exponentially (or at least logarithmically).

It's just easier for people to say that backpressure affected their engine performance than to say exhaust gas velocity did it. For all intents and purposes, they mean the same thing, even though they are NOT.
 

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so we have a yes and a no-- with what seems to be equal amount of back up info...:huh:
 

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so we have a yes and a no-- with what seems to be equal amount of back up info...:huh:
I think we are all saying basically the same thing; it is the semantics that differ.
 

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that's what i'm after. Sure, backpressure and exhaust gas velocity are related, but not directly and certainly not proportionally. It seems to me that it's a wide misconception that inadequate backpressure is the cause of reduced power or reduced scavenging affect, when in actuality it's the inadequate exhaust gas velocity that's to blame.

bingo! You understand the concept!

Actually they are directly related. If you increase velocity, you increase backpressure; decrease velocity, you decrease backpressure; unless you know of a way to change the one without changing the other, i.e. some alteration other than changing the cross-sectional flowing area. Are they proportional? Don't know for sure, that would be a factor of the design of the complete exhaust system. They're probably related exponentially; as the velocity increases, the backpressure does as well ~ only exponentially (or at least logarithmically).

It's just easier for people to say that back-pressure affected their engine performance than to say exhaust gas velocity did it. For all intents and purposes, they mean the same thing, even though they are NOT.
your comparing apples to oranges and getting the wrong conclusions

theres THREE distinct areas of pressure/velocity
(1)
gases in the cylinders and header primary tubes as the exhaust exits the cylinders

(2)
gases in the header primary tubes and header primary tubes as the exhaust exits the header COLLECTORS

(3) the exhaust system past the collectors

(1) and (2) must be calculated to match the intended displacement ,cam timing and intended rpm range

(3) (traditional back pressure ) should be MINIMIZED


keep in mind installing an (X) almost increases the effective cross sectional area,of the dual exhaust ,to double what it had been behind a single header collector, by doubling the area that the exhaust flow sees, dropping the restriction to flow almost in half

looking thru an (X) pipe




http://www.wallaceracing.com/header_length.php

http://www.bgsoflex.com/auto.html


http://forum.grumpysperformance.com/viewtopic.php?f=56&t=185
 
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