the original post didn't really ask about the ideas that flew around in this thread (although the info is complementary and interesting). he asked what would happen if "too little" backpressure existed. i don't know what it takes to create the conditions of having "too little", but i did post the negative consequences of it. this assumes stock conditions, particularly a stock cam in a C4.

 

Funny how things turn sometimes. Interesting read though.

 

This topic in general amazes me how many times people get this wrong or use the wrong logic when trying to explain this.

If I can put in my .02:

First of all a motor is an air pump, nothing more, nothing less. Add some fuel, and spark to a closed chamber is and ignite the spark you get an explosion that move something (hopefully).

Thinking purely in terms of exhaust there are several elements necessary that people confuse with one another.

(1) is adequate size to carry the combustion gasses out of the motor (volume)
(2) adequate velocity (velocity)
(3) savenger effect generated by (length)

The term backpressure in itself is when broken down in merely the pressure or RESISTANCE (restriction) the gasses must overcome to travel down the exhaust.

When looking at a motor, any time restiction is used in conjuction with a motor it means "power loss". Don't care if your talking topend or bottom end power, its a loss. Restriction in terms of a motor is similiar to a choke-hold on a person. You can't intake or exhaust air. NOT GOOD.

Now that you have the basics of the system we can apply them. Again remember that the motor is a combination of elements that must match one another.

The balancing act is getting enough volume to carry the ENTIRE amount of exhaust charge out of the chamber. So one might think, ok- then put on 3" primary tubes on the headers and be done with it. Nope that will not work because of element (2) above. Again its a balancing act. Element 2 above is velocity. You need to maintain velocity and to maintain velocity you need to have a optimal cross-sectional area. If you get too large, you will slow down the velocity, get too small, it will have a greater velocity, but the volume will suffer.

Lastly the element that people confuse the most with backpressure is probably length. Length is VERY important in achieving the proper amount of scavenging effect. Scavenging is produced by the hot gasses moving down the exhaust pipe. This creates a "suction" behind the charge that helps pull the remaining burt gasses from the chamber and also with larger cams that have more overlap, pull in the fresh air charge. You will often see guys that have headers changing their collector length to fine tune this effect. But people will often confuse this added length with adding backpressure...not the case. They are just fine tuning the scavenging effects. This is why good headers will ALWAYS have equal length primary tubes. If some were longer or shorter you would have an inbalance.

So in summary you need (1) adequate volume to carry the charge (2) minimal cross-sectional area to keep Velocity up, (3) adequate length to maintain the scavenging effects

Again its a balancing act. Get too much or too little of one or the other, and you can hurt performance.

But backpressure in the sense of "restriction" is NOT a good thing and will hurt performance.

Hope that clears things up here. There are some other elements that also enter into the scenerio like heat retention and how quickly the charge cools, but for the most part the above should explain things pretty well.

Lastly, heads are the same thing, only on the other side of the combustion process. Seems people have a better grasp on that though. Just remember you wouldn't want a restriction in the intake port now would you? So its stands to reason you wouldn't want one in the exhaust either.

 

Jesse, were my points invalid? I'm referring to the effects of an operationally degraded egr system (valve), and the effects thereafter (notably increased propensity toward detonation), assuming full stock trim (cam), except the exhaust system is revamped completely with LT headers, no cats, and true duals. This is for an L98. And this applies directly to what the original post is asking.

 

As long as a proper A/F mixture can be maintained, there is no such thing as "too little" back pressure.

RACE ON!!!

 

Jesse, Equal tuned length headers is sort of a non-issue on the smallblock, because of the imbalanced firing order. Primary tube size and collector size is more important.

 

Gases are fluids.

Liquids are compressible.

The main difference here, is sound waves (pressure waves) pass way faster through liquids, because of their density. Compressibility has nothing to do with it.

Liquids also change volume considerable with temperature change, which is why your car has an overflow tank.

 

No, he's very correct on that. Maintain velocity is important. Decreasing pipe diameter gradually is ideal to maintain velocity where the gases are contracting from cooling. If the pipes increase in diameter, velocity pressure converts to static pressure, this is called static regain, and is restrictive to flow.

 

Will the PATATO still work on the Crossfires?

Is there a difference in Potato Type?
Like 12 HP gain for Irish Taters and +15 for Idaho Taters?
Doesn't it make a difference how far up the exhausr pipe the tater is installed?
You know kind of like a crossover pipe, isn't there an optimal position to install the potato?
How often do the spuds have to be replaced.
What about engines with carburetors? Would apples or bananas work better for them?
THis 'BackPressure' Theory id facinating. Won't it also increase the gas mileage because any unburned air/fuel molecules wouldn't be able to escape out the exhaust?
What about just welding up the exhaust pipe?? That'll give some good backpressure won't it?

I'm very interrested in this 'new technology'. Maybe it will be called "TT" for 'Tater Technology'.
I' m checking the kitchen right now for some additional technology.
THis may be a breakthru!!

 

ski,

You've got me confused. There's a lot of technical info in your post but no mention of the 'Spud Device'.
Did you intentionally leave it's use out of your facts or was it just by accident?
Do you guys think it could work to cram a couple of 'Spud Devices' into the throttlebody openings as well?
If BACKPRESSURE is good then wouldn't a DECREASE in inlet pressure be good too?
Seems a couple of spuds in the inlet would decrease inlet pressure to about ZERO.

 

Gases are NOT fluids. For the purposes of computational physics / fluid dynamics they are treated as fluids. They are a distinct phase of matter. Liquids are essentially incompressible, especially at the level of forces taking place inside your engine. There is a distinction between being able to place something under pressure (i.e. drive water through a pipe) and being able to compress a substance.

Which overflow tank are you talking about? Cooling? That is in place to relieve pressure caused by boiled off gas. What liquids change volume? I can't think of any. In a heated liquid system you may perceive a volume change, but this is caused by microscopic boiling of gas which is dissolved within a liquid, not the liquid itself. This pushes the liquid molecules further apart, and it may appear to have changed volume.

What does the speed of sound through media have to do with my earlier post?

I liked Jesse's post above, it's always a balancing act in an engine and matching components together. Jesse's perspective is from a gearhead with lot's of experience. I was adding my .02 as a chemist with a gearhead hobby.

 

You're missing the point. I never said velocity wasn't important. You're talking about systems that continuously "flow". Static regain is much more relevant to HVAC systems where you're trying to equalize velocity and flow all the way from your furnace through the ducting to say your bedroom. That's why ducting gets smaller as you branch away from main lines. In this type of system there is a continuous column of gas all the way from the inlet to the outlet and the gas can be treated as a classical fluid.

Your engine exhaust "pseudo-flows". Exhaust gas does not come out of the engine in one continuous stream. Since exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run.

Keep in mind that for a "pulse" to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. It is so low, in fact, that it is almost a complete vacuum! The pressure differential is what keeps a pulse moving.

A good Mr. Wizard experiment to illustrate this is a coffee can with the metal ends cut out and replaced with the plastic lids. Cut a hole in one of the lids, point it toward a lit candle and thump on the other plastic lid. What happens? The candle flame jumps, then blows out! The "jump" is caused by the high-pressure bow of the pulse we just created, and the candle goes out because the trailing portion of the pulse doesn't have enough oxygen-containing air to support combustion. Neat, huh?

Ok, now that we know that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in to play here.

Just as Paula Abdul will tell you that opposites attract, the low pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively "sucking" it along. This is what's so cool about a header. The runners on a header are specifically tuned to allow our exhaust pulses to "line up" and "suck" each other along! This brings up a few more issues, since engines rev at various speeds, the exhaust pulses don't always exactly line up. Thus, the reason for the Try-Y header, a 4-into-1 header, etc. Most headers are tuned to make the most horsepower in high RPM ranges; usually 4,500 to 6,500 RPM.

But wait let's get back to low end torque. What are exhaust manifolds and stock exhaust systems good for? Besides a really cheap boat anchor? If you think about it, you'll realize that since stock exhausts are so good at restricting that they'll actually ram the exhaust pulses together and actually make pretty darn good low-end torque!

Something to keep in mind, though, is that even though an OEM exhaust may make gobs of low-end torque, they are not the most efficient setup overall, since your engine has to work so hard to expel those exhaust gasses (pumping losses). Also, a header does a pretty good job of additionally "sucking" more exhaust from your combustion chamber, so on the next intake stroke there's lots more fresh air to burn. Think of it this way: At 6,000 RPM, your engine is making 400 pulses per second. There's a lot more to be gained by minimizing pumping losses at this busy time than optimizing torque production during the slow season.

With all that said there is a point of diminishing returns. I fully agree that bigger is not better. It's a balancing act and we need to match all our components. We shouldn't all run out and bolt up a 5 inch exhaust (unless you have a turbo then you can't hardly make it big enough!). Air flow is not just influenced by the size (area) of the paths it takes into and out of the engine. It is also influenced by the speed at which it moves.

Specific Port Flow (cubic meter/sec) = Flow Velocity (m/s) x Average Path Area (m2)

Whenever an engine modification increases the average area of the airflow paths into and out of an engine, there is a chance the velocity of the flow will decrease. This is exactly what Vader86 and CentralCoaster are talking about and I agree, but remember we have pseudo-pulses in our exhaust not a continuous column of flow. Most of the time the factor of velocity decrease is very small compared to the area increased, so flow is generally increased. If modifications are taken too far, the velocity will decrease more than the area increases, so flow is adversely affected

If you want an example of going to bigger tubes to enhance flow check out stepped tube headers. NASCAR has been using these for years. About halfway to two thirds down the primary the tube steps up from say 1 1/2 inches to 1 and 5/8. This causes a small scavenging wave to be reflected back to the valve from the location of the step. This increases the pressure differential across the valve during the exhaust stroke, which effectively increases the exhaust port flow potential. The step also causes the scavenging wave, reflected back from the collector, to be stronger but narrower. Large displacement racing engines, and high RPM engines benefit the most from stepped-tube header designs.

If anything I think we all can agree that there is a lot going on within the exhaust and trying to perfectly optimize things gets complicated.

 

And don't forget the Mechanical Engineering Degree, with about 15 yrs working experience with designing and manufacturing automotive components/systems in there.

K99jao4, has it dead on - coupled with my explination 95% all the information is there.

Centralcoaster, I 100% disagree with your primary length statement not being important. I do on several fronts: (1) what motor besides 1 cylinder motors doesn't have an imbalanced firing order? Maybe I do not understand what your saying by inbalanced. (2) The length or should I say correct length becomes more critical as RPMs increase. The time in which the cylinder has to fire becomes ever shorter, and hence the scavenging effect to help draw in the fresh charge more important. If the length is not long enough for a given application the full benefit will not be realized. Or if one is shorter, that particular cylinder will suffer.

Again this depends on the particular setup and how much your willing to experiment to find the optimum length. Chances are nnne of us are going to go to these extents, and we settle for what the manufacturers determined a middle ground for most sbc's. This however does not mean that I would be confortable with shorting a primary tube considerably though.

The key to all this motor stuff and building a good setup is BALANCE. Balance of component to component. The better you balance the better your performance will be. That is why you see "stock" world records in the low to high 9s. These guys go the extra 500 yards to make sure EVERYTHING is proper and works PERFECTLY. Its like comparing us launching a model rocket from our back yard with our kids, to NASA launching the challenger into orbit. They both go up, one just a WHOLE lot higher.

 

Actually you derive the Mach number of a fluid (or gas) specifically from its compressiblilty. I can scrounge up the derivation if you'd like.. Its halfway interesting.

The name of the game with exhaust is not backpressure but simply creating little pulses that daisy chain and makes a pull on the succeeding pulse

 

OK now you're taking things WAY out of context for an automotive discussion and getting into semantics. I'll bite. And I agree it is interesting, but I don't want anyone to get the wrong idea.

I agree technically you are correct. But we need to make a very important distinction between liquid as a phase of matter and liquid/fluid within fluid dynamics.

The Mach Number is a dimensionless value useful for analyzing fluid flow dynamics problems where compressibility is a significant (keyword here!) factor.

A fluid flow is compressible if its density changes appreciably (typically by a few percent) within the domain of interest. Typically, this will occur when the fluid velocity exceeds Mach 0.3. Hence, low velocity flows (both gas and liquids) behave incompressibly.

An incompressible fluid is one whose density is constant everywhere. All fluids behave incompressibly (to within 5%) when their maximum velocities are below Mach 0.3.

Now the really important part is that fluid dynamics is essentially theorhetical. In this case compressibility is simply a term in an equation to make things balance. It's just made up on a sheet of paper and doesn't exist in the real world. However a liquid phase of matter (think glass of water) actually exists and is incompressible. Otherwise it wouldn't be possibly to get a hydraulic lock when your cylinders fill up with gas. (is Nathan Plemons reading this?)

So within an automotive engine none of this really applies. Mach number is meant for airplanes and really really high velocity liquids. Like those wicked sweet machines that use water to cut metal which actually have a supersonic flow. Now show me a submarine that can travel at Mach speeds.

 

Look at one bank of cylinders, and figure out the timing between each exhaust valve opening, that makes it uneven. 1, wait, wait, 3, wait, 5, 7, wait, 1

An inline 4-cyl however, has equally spaced exhaust pulses, which works great for making torque peaks with tuned 4 into 1 or 4-2-1 headers.

Now if you want to make a trick header where the #7 exhaust primary is longer than the #5 so that it doesn't hit the collector so soon after, then you're on your way to compensating for the imbalanced timing. The other possibility, is if you're not dealing with the first few wave reflections, you could feasibly line up, for example, the 1st reflection of #5, then the 4th reflection of #7, the 2nd reflection of #1, etc, but I don't want to speculate beyond that.

Now, I think the firing order used by most nascar teams is balanced on each side, but I'm not sure on that either.

 

So you're saying that the thermal energy stored in the molecules in a liquid doesn't force them further apart? That's news to me. Does metal expand by boiling internally too? And how does any appreciable amount of liquid boil into gas in a pressurized cooling water system at 40 degrees fahrenheit, enough to warrant an expansion tank for 40 degrees of temperature change? If you can find any helpful links on this, I'd sure be interested in learning about it.

 

Sure the thermal energy in molecules does force them apart, but not enough to appreciably change the density (by a few percent of total volume) which is the definition of compressibility. And surely not enough to warrant an expansion tank. I contend that dissolved gas molecules would change the volume more appreciably than excited liquid molecules.

If you don't believe me that a gas can be dissolved within a liquid what happens when you open a plastic bottle of Coca Cola? It immediately fizzes since the dissolved gas molecules are exposed to lesser atmospheric pressure and escape.

Of course metal doesn't expand by boiling internally. Recall that all materials are made up of atoms. At any temperature above absolute zero (-273 degrees celsius, 0 kelvin) the atoms will be moving. In a solid they will be vibrating in relatively fixed positions, in a liquid thy will be jostling past each other and in a gas they will be whizzing past each other at very high speeds. When a material is heated, the kinetic energy of that material increases and its atoms and molecules move about more. This means that each atom will take up more space due to its movement so the material will expand. When it is cold, the kinetic energy decreases, so the atoms take up less space and the material contracts.

Some metals expand more than others due to differences in the forces between the atoms / molecules. In metals such as iron, the forces between the atoms are stronger so it is more difficult for the atoms to move around. In brass, the forces are a little weaker so the atoms are free to move about more. For example, these differences in contraction are used in a bimetallic strip, which is composed of a strip of brass laid along side strip of Iron. When the strip is heated the brass expands more than the iron so the strip bends. It is used in devices such as fire alarms and circuit breakers to either make or break contacts in an electric circuit.

So you ask, "How does any appreciable amount of liquid boil into gas in a pressurized cooling water system at 40 degrees fahrenheit, enough to warrant an expansion tank for 40 degrees of temperature change?" The answer is it doesn't. At 40 degrees fahrenheit you won't need an expansion tank for the cooling system. At operating temps your cooling system will be closer to 200+ degrees fahrenheit and it is at those temperatures you will need an expansion tank.

What do you mean 40 degrees of temperature change? Are we starting at zero? Please clarify.

 

I mean a hydronic chilled water system, like one used on a college campus, or a large commercial building.

Expansion tanks are installed to handle temperature driven changes in volume. Cooling systems have smaller tanks due to smaller temperature changes, and less change to ambient. The largest temp change a chilled water will see is from it's operating temp to ambient, if say, the chillers break down. If no expansion tank is in place, it'll suck in water when it cools, and explode when it warms up.

Anyways, what does this have to do with backpressure?