for V8 - dual vs single exhaust ? headers ? high flow cats ?

[02V8Limited]

Regarding the whole air/fuel mixture thing, and having to tune after installing a dual exhaust... I had a 1990 5.0L Mustang, with duals, H-pipe, flowmasters, high-flow cats. I had a GT-40 intake, 70mm t/b, MAF, and all that typical stuff. The old EEC-IV computer was able to adjust for all those mods & keep the mixture allright. Is the computer in our X's not as good as the EEC-IV was at doing this? It seems strange to me that the newer ODB-II wouldn't be as good at compensating for changes in airflow in/out of the engine.

 

[CDW6212R]

The OBDII is far better than the EECIV, stock or "tuned." Describing your vehicle as "...mixture allright..." is not accurate. You may have liked it, and it may or may not have been actually close. It was however in fact, not right, not close, not accurate. How a vehicle feels to a driver has nothing to do with the actual A/F ratio. That is not accurate.

The only ideal test or measurement of A/F ratio is by wide band O2's taking readings while the vehicle is driven on road. Everything else is a compromise, less than optimum.

I'm interested in optimum performance, not "allright" performance. That is achieved by data logging the O2 signals from a wide band O2 sensor, while driving. Not doing that results in less HP than optimum.

 

[NotATruckGuy]

Myth Confirmed

Hi! I'm new here and would like to add my bit to some earlier stuff.

The "myth" about matching header and exhaust tube diameter to the motor to tune for better low-end torque may be a myth, but it sure works on the dyno! Remember that a reciprocating car motor produces pulses, not a continuous stream of gas. Unless you're running a well motor or a top fuel drag car, dumping your exhaust ports right out into the wild blue yonder is not efficient or practical. A set of tubular headers works a lot better.

Again, remember that you don't have all cylinders flowing a constant stream into every tube of a header simultaneously. MOF they're not even flowing at regular intervals. Depending on the firing order, one exhaust valve might open a fraction of a crankshaft revolution, while the next might open more than a revolution later. What backpressure does in this situation is to use the capacity of header tubes that aren't being used to expel exhaust to take up some of another's exhaust pulse, and like a spring, bounce it back into the collector in a way that causes negative pressure when that exhaust port does open. This helps scavenging, and can be tuned to produce more low-RPM power.

This is very complex, and I don't want to write a book here, but suffice it to say that (at least for normally aspirated applications) exhaust tuning really does work, and bigger is not always better.

When it comes to mufflers and catalytic converters, large volume-low price auto makers like Ford do tend to leave room for improvement. This is especially true for the Sport Trac, which has its breathing cut off by lots of stuff that's engineered to make it luxury car quiet at the expense of engine performance. Since the Sport Trac is more truck than car, I might skip installing headers for practical purposes. What I would do is lose the stock exhaust system and replace it with a more even flowing system. I'll be converting to dual exhaust, space permitting. If space doesn't permit, I'll try to at least make the pipes that lead to the single pipe from the two banks as equal as possible. I will choose pipes that flow well for the amount of exhaust that my engine produces. Any more would be wasteful, and can actually hurt performance.

Another thing: EEC is completely different from OBD. EEC is Ford's name for their Electronic Engine Control system. OBD is an international standard for On-Board vehicle Diagnostics. Ford's EEC V systems have OBD version 2 (OBDII) access, but so does every other car made since 1996.

Speaking of engine controls, it's important to know the difference between "open loop" and "closed loop". With the current technology that's cost-effective for large car manufacturers, being able to run in closed loop mode, where the oxygen sensors tune the fuel delivery to the best possible air/fuel ratio, is something that doesn't happen all the time. If you're racing, or doing other things to flog as much horsepower out of your motor, then you'll spend virtually no time in closed loop mode. That doesn't mean that it's wrong or useless however.

 

[CDW6212R]

The "bigger is not always better" is a highly abused phrase.

That almost absolutely always precludes words that suggest smaller is better. That is a huge mistake to buy into that line of thinking. It leads everyone to believing it's better to always select the smaller parts, of which there are many.

Please do not waste tons of words to steer people to various vague size recommendations. Be clear.


I am clear. Buy the best sized header(exhaust manifold) for the engine. That means for the given vehicle and rpm range, find primary pipe sizes and collector sizes best for the power range. That is the most important exhaust part and the only thing that takes a good bit of time to research. Some vehicles have zero choices, while others(Mustangs) will have dozens of choices. Simply find out what the best pipe and collector sizes are for your usage, and buy it.

The entire rest of the exhaust is a restriction to airflow, note that no race cars use an exhaust unless required to. Everything you add restricts airflow, and airflow is power.

For the typical car, not a highly tuned performance vehicle, don't waste time worrying about equal length pipes etc. That only is important for the headers. Equal length or flow is about 10-50 times more important for the headers. Don't be cheap or casual in choosing headers, and then waste time trying to equalize any other parts of the exhaust. You might gain 1-2hp with a lot of trouble making the down stream pipes even or equal, but the headers might gain you 20+hp if you buy the best for the vehicle.

Past the collectors, which means everything after the header, make it all as large as possible, and re-tune the engine for the extra airflow.

Having larger pipes after the headers(make the headers perfect), helps all of the time, not on a dyno but while driving. It helps airflow, which is power. The more air that flows through an engine, the more gas can be burned, and that is power.

There are two exhaust arguments, the sizes and the tuning. Way too many people actually believe that nothing you do to an exhaust affects the tune. That is very wrong, and if you can't get past that, you are ignorant and there's no reason to continue discussing it.

Power should be the goal, always, and maximizing that means increasing airflow. Playing with smaller sizes of pipe that will fit, that only makes it harder to get the highest airflow.

Fuel economy comes from the best A/F ratio while in closed loop(cruising), and with enough power to pull the car. Making big power doesn't result in the best economy, as airflow goes up along with fuel consumption. But the key is the tuning, if you do the proper tuning, that will include the A/F ratio in open loop and closed loop. That tuning of the closed loop A/F is what increases the fuel economy in almost all cars. Note that you never hear of a car losing fuel mileage with a tune, that is because the factory tune is not ideal. If you comprehend how bad the stock tune is, then maybe you can begin to understand that the open loop tune is also way off, and that's the power.

Doing no tuning is the reason most people perpetuate these myths. They alter the exhaust and the results are random based on what the A/F was and what it becomes. That's all random, thus the random results, some people gain with one big exhaust and others lose doing what appears to be the same thing. The tune of the engine is critical. Do it, don't argue about what happens when you didn't do it.

No one drives a dyno, so don't waste words on discussing dyno numbers. A dyno is only useful to get a general tune on an engine. The best tune needs to be done while driving.

 

[NotATruckGuy]

Speak for Yourself, Please

The "bigger is not always better" is a highly abused phrase.

That almost absolutely always precludes words that suggest smaller is better. That is a huge mistake to buy into that line of thinking. It leads everyone to believing it's better to always select the smaller parts, of which there are many.

Please do not waste tons of words to steer people to various vague size recommendations. Be clear.

Gee...thanks...

I looked over my post, and saw that I was very clear about what are my plans, made no recommendations, and didn't ever say "smaller is always better".

For the record, my very small Motorola RAZR phone is a lot better than my first 30# totable cellphone.

 

[CDW6212R]

Welcome to the site, and please understand I wasn't aiming at you, but thousands of people who contribute to the myths.

I own a SOHC 99 Limited, I know that exhaust well. The headers are very good as far as OEM goes. I would buy better headers for sure if my goal was more power. They are okay for economy.

These trucks have little to no room along one side to run equal pipes, that's why I remarked about the equal/even thing. It's best to simply replace the cat pipes with at least 2.5" piping, the cats I'd make 3" internally(universal cats). There is tons of room to run any sizes past the cats, so don't be hesitant about using at least 2.5" dual pipes after the cats.

Start by acquiring a flasher to be able to change the PCM program. Get with a tuner, we suggest James Henson here, and begin working towards a custom tune. The SOHC has hit 200RWHP many times by example. I think with a bigger exhaust(past header) and better headers, 225hp is possible. That's 25hp more than basically everyone has done. If the intake airflow is good enough it can do it.

 

[NotATruckGuy]

Reality Check Time

Welcome to the site, and please understand I wasn't aiming at you, but thousands of people who contribute to the myths.

Thanks for the welcome.

I still must reiterate that what you call a myth is based on a solid engineering discipline known as "fluid dynamics". So far the only argument that you've given for your "bigger is always better" myth is a misquote that equates flow rate with tube size although that's not what was said, name-calling and ad hominem attacks against people who you don't agree with. You may think that's a winning argument, but I don't. Quite the opposite in fact.

Your claim "no race cars use an exhaust unless required to" is false. I challenge you to show me any racing class that has nothing at all bolted to the ehxaust ports. There are many racing classes (Indy and F1 for example) that use very complex "bundle of snakes" exhaust systems not because the rules require it, but because the carefully tuned pipes really do increase engine efficiency and power. Whether you like it or not, using pipes that flow far more than the engine can produce will gain you nothing but dead weight. And the last time I checked, dead weight makes a car slower.

In fact, equal length headers are an old and crude attempt to equalize exhaust flow that doesn't work because, as I explained before, exhaust flows in irregular pulses, not in continuous streams. (Tri-Y headers do address the pulse timing issue, but are rarely seen on road cars.) The thing that makes long tube headers work is the length of their length, not the equality of their length.

Today however, thanks to computer aided design programs, engineers can design more compact exhaust systems that can fit into increasingly cramped engine bays, and still deliver more performance gains than older, non-engineered designs. That's not a myth, that's reality.

Another one of your myths that I'd like to bust is your notion that when the ECU is in open loop mode, it's deaf dumb and blind. Not true! There are things called mass air flow (MAF) sensors, throttle position sensors (TPS) and other input devices that give the ECU plenty of information to choose optimum fuel delivery, spark and other settings (e.g. cam phasing), depending on the engine. It has been a very long time since primitive ECUs had to rely on static maps when the oxygen sensor was unavailable.

BTW, one big advantage of systems that use a MAF sensor is that they automatically tune the ECU for optimal performance if changes are made that affect air flow. Of course there limits to this, and radical changes need custom tuning. But my point is that your latest claim that "people perpetuate these myths" is not factual.

As for your claim that chassis dynos are only useful for a base tune, I must say that you have it backwards again. Are you unaware that a good chassis dyno (I'm not talking about the simple ones that's all drag racers need) essentially duplicates real world driving conditions? How do you propose to gather data on the road without a dyno? Do you think that a $300 box with a cheap accelerometer and a rudimentary algorithm to guesstimate 0-60 and 1/4 mile times is a finer instrument than a $50,000 machine full of finely calibrated sensors that don't have to guess?

 

[CDW6212R]

Wow, I try to be nice and get beaten over the head with ignorance and assumptions.

Fool, stating that race cars don't run an exhaust does not mean that they don't have headers. By posting on this forum we assume at the very least that you comprehend a little about common sense and car knowledge.

The headers are the absolute one part all engines have and need(don't mention top fuel, use some common sense). Those F1 cars have fully tuned headers, that big can of worms you described are the headers, they are tuned that way to maximize flow. Ford Panteras have those stock, they are called 360 degree headers.

Lastly all that rubbish you posted "again" about the PCM(Powertrain Control Module(applies to all manufacturers)), is not correct.

The PCM in every vehicle made does not keep up with A/F ratios accurately(the ideal A/F ratios) when it is stock. The ideal A/F ratios are not programmed into them to start with, therefore they clearly cannot produce the ideal A/F ratios without new programming. Think about that for a second before falling back on your past incorrect logic.

Every stock vehicle made will produce more power by simply reprogramming the stock PCM. That alone proves that reprogramming is needed when altering the airflow. You will not magically change the exhaust and luck up on the ideal A/F ratios.

A dyno is a POS that simply creates numbers for people to brag about, not acceleration times. Races are won by elapsed times on track, and that is not produced by any dyno. The one good item that a dyno uses is a wide band oxygen sensor, and it's required for PCM tuning.

That wide band O2 sensor is what is needed to tune a PCM properly, and that does not have to be done with a dyno of any kind. The best testing and tuning can be done at a track, or if possible on the street. Real world loading cannot be duplicated with a vehicle setting still. Ask Nascar if they just never touch a carburetor after it was tuned on a dyno. Of course they adjust it after track time, that is real world testing.

Congratulations, you have succeeded in confusing more people by repeating the myths I keep trying to kill. You dragged out wrong assumptions again that have been rehashed before, and thus again confused people. If you wanted to really help, you would go back and edit your post, deleting the bad assumptions that I point out again and again. Regards,

 

[NotATruckGuy]

Stop, Please!

CDW6212R, now you're just being a jerk.

You may think that because you have a 5-digit post number and I only have a two-digit number, that it somehow makes your opinions more worthy than mine. Because you've been around for a while and I haven't, you probably will get deference from this site's operator and I won't. Fine, I accept that.

I came here to join a CIVILIZED discussion, and have zero tolerance for getting flamed, ad hominem and other personal attacks or any other antisocial behavior. I will not tolerate it from you. Cut it out NOW!

 

[CDW6212R]

If you are wrong, then someone must point that out.

You are wrong, and I had to be the one to do it. Anyone can tell you I'm not the most funny or sociable or light hearted member here. But the many veterans here will also tell you I am straight to the point, I don't casually "flame" people or threads, I try to address the subject directly and with reason.

You have beliefs and knowledge about vehicle computer systems which is wrong. What you think you know about dynos is not exactly accurate either.

The point is that I did know more accurate facts about those and thus my posts/opinions differed from yours.

If you really want to learn a lot more about PCM and dyno facts, and how they relate to power, I suggest that you do reading on the SBFtech site;
http://sbftech.com/index.php

That is a Ford power forum which does not mess around in myths and opinion threads. They actively discuss facts related to performance and achieving maximum acceleration. Big power numbers are just that, numbers. Over there they relate everything to the result, which is how fast the car gets to the end, or how fast the engine reaches maximum rpm.

Example, on a dyno one piece of information which could be very useful but is not used, is the rate of acceleration. You can dyno two engines that happen to make the same power, but one might reach max rpm a second faster than the other one. Are those two engines not identical, based on the only thing people pay attention to, the HP numbers? Of course not, but that exact same principal is why a dyno means nothing to compare cars or engines. There are hundreds of parts in the vehicles/engines, the dyno doesn't measure all of them. But the goal of being fastest at a track does have a way to measure them. What device does that, and has for decades? The timing device does exactly that, and it takes a piece of track or road to run the vehicles on. No one yet has duplicated the results of a real timed race. So concentrate on what does work now, save your money from buying dyno time, and invest in the devices(and people) which alter PCM programming. That may include a dyno, but don't rely solely on that one tool.

Please get beyond more poor attitude about your inaccurate posts. I try hard to deal in facts, and I become frustrated and irritated when people ignore the facts or distort them. Regards,

 

[02V8Limited]

CDW621R, what exhaust set up do you have on your rig?

 

[NotATruckGuy]

Pot, Kettle, Black

If you are wrong, then someone must point that out.

That is precisely what I have been doing when you wrote something that was wrong. You seem to have a problem accepting that you're not God; that you can make mistakes too.

It's one thing to be less than friendly, and something completely different to be an aggressive jerk (you know the word I mean). IMHO you crossed the line. You don't have to get along with society. But by the same token, you don't have any right to have the benefits of society when you act badly.

You are free and clear to try to prove any of your assertions. Bashing others is not tantamount to proof.

BTW, a "principal" is the person who runs a school.

 

[CDW6212R]

That is precisely what I have been doing when you wrote something that was wrong. You seem to have a problem accepting that you're not God; that you can make mistakes too.

It's one thing to be less than friendly, and something completely different to be an aggressive jerk (you know the word I mean). IMHO you crossed the line. You don't have to get along with society. But by the same token, you don't have any right to have the benefits of society when you act badly.

You are free and clear to try to prove any of your assertions. Bashing others is not tantamount to proof.

BTW, a "principal" is the person who runs a school.

You see, now you are being a jerk. You did not point out anything that I said that was wrong. I was not wrong in what I said. Therefore you are being vague and that contributes to the myths. People who don't know any better are assuming some of your contradictions are right, and wondering about what I said that may be wrong.

So, get to the point. If you disagree with the facts that I have laid out here, and in other threads, spit it out.

 

[CDW6212R]

CDW621R, what exhaust set up do you have on your rig?

Welcome. I have not changed my engine yet. I have collected a ton of parts, including new transmission stuff, and transfer case. I put it off to do body work first, and then the money wasn't there.

You asked about exhaust, I have mentioned my plans before elsewhere. I have two 3" cats to use, two mufflers with 3" outlets and twin 2.5" inlets. I'll swap the 347 in first, after the trans and TC. Then the stock exhaust will go, back to front. Twin 3" tailpipes to 3" cat pipes, and later fabricated 1.75" headers with 3" collectors.

Stock the exhaust has 1.5" exhaust manifolds, 2.125" collectors, 2.25" cat pipes, 2.25" pipes to single muffler, 2.125" restriction before the 2.25" resonator and tailpipe.

Stock 302 Mustangs had 2.25" dual pipes end to end. Any decent high performance V8 needs 3" dual pipes if they can fit, 2.5" is fine for a stock V8.

 

[IZwack]

Past the collectors, which means everything after the header, make it all as large as possible

Your statement above is wrong because: If you made everything (tube diameters) as large as possible everything after the header, the volume of exhaust gas slows down (per fluid dynamics's conservation of mass). A larger exhaust tube diameters then means that there is a greater volume (and mass) of exhaust gas inside the exhaust tubes that has yet to be expelled out and into the atmosphere.

Don't take this wrong but learn fluid dynamics.

 

[CDW6212R]

Your statement above is wrong because: If you made everything (tube diameters) as large as possible everything after the header, the volume of exhaust gas slows down (per fluid dynamics's conservation of mass). A larger exhaust tube diameters then means that there is a greater volume (and mass) of exhaust gas inside the exhaust tubes that has yet to be expelled out and into the atmosphere.

Don't take this wrong but learn fluid dynamics.

I guess you also have not read and understood my posts either. I've taken and passed college engineering classes, including fluid dynamics. This is not so complicated of a subject that it needs that high level of understanding.

What is wrong with letting the exhaust slow down in the pipes if it can or tends to?

The only point of the exhaust is to rid the engine of the spent gases, to get them away from the engine.

The only gains that can improve airflow, and I mean mass airflow, not air velocity(without considering mass), are done and finished in the headers. After the headers(the right header with the right collector size and length), the rest is 100% restriction, it does nothing except to slow down the airflow(mass) trying to get out of the engine(header collector) following that exhaust.

The bottom line is this simple; the ideal exhaust is just what drag cars have on them after they have tuned them properly. There will be a header, and a length of collector pipe that has been determined by testing. That produces the fastest times. Any exhaust added after that slows it down, the car. Are we trying to accelerate the car or brag about air velocities in our exhausts?

We are all dealing with street vehicles here or otherwise need some kind of exhaust after the collectors. Anything that you can name is not desirable for maximum performance(after the collector). As you work towards smaller parts, pipes/cats/mufflers/tips, that all slows down the vehicle acceleration. All of it is restricting the maximum flow of air(mass) coming out of the collectors.

The best solution is to install the largest exhaust possible in the given vehicle, and then retune the engine because of the resultant higher airflow(mass). The engine is running lean.

 

[CDW6212R]

Your statement above is wrong because: If you made everything (tube diameters) as large as possible everything after the header, the volume of exhaust gas slows down (per fluid dynamics's conservation of mass). A larger exhaust tube diameters then means that there is a greater volume(and mass) of exhaust gas inside the exhaust tubes that has yet to be expelled out and into the atmosphere.

Don't take this wrong but learn fluid dynamics.

Above that I highlighted in red is wrong, the mass is the same for a given volume. The mass stays the same, as pressure goes up or down with pipe sizes.

Allow me use your own words and the fluid dynamics to prove that you and so many others do not comprehend this.

Using much larger pipes does indeed result in slower moving exhaust. What you don't get and are ignoring is that the pressure is also dropping. That results in basically the same mass inside a given volume of pipe.

Using much smaller pipes results in much faster moving exhaust. Now try to follow this, the pressure is most definitely increasing. That results in basically the same mass inside a given volume of pipe.

The real difference is the smaller pipes are creating the high pressure at the outlet to the collector also. That pressure is what slows the exiting of the gases which the engine is constantly producing.

In each case or any case the mass coming out of the tailpipe should be the same. But the velocity is not important, what is important is the pressure. Specifically the pressure located at the collectors outlet is critical. Smaller pipes and all exhaust parts simply increase the pressure at the collector. That is the restriction, that is what hurts performance. That is the reason race cars do not have exhausts if they don't have to. No exhaust provides no restriction, atmospheric pressure is all that is at the collector.

An exhaust produces a pressure at the collectors, that slows the airflow coming out of the collectors. The only airflow velocity that matters is the flow through the engine. Any restriction you create with the exhaust(or a stock air cleaner), that all slows down the engine's pumping ability. Try to see the reality of an engine as simply an air pump. Don't choke it off with a poor intake or exhaust, open it up. Regards,

 

[NotATruckGuy]

Bravo!

Your statement above is wrong because: If you made everything (tube diameters) as large as possible everything after the header, the volume of exhaust gas slows down (per fluid dynamics's conservation of mass). A larger exhaust tube diameters then means that there is a greater volume (and mass) of exhaust gas inside the exhaust tubes that has yet to be expelled out and into the atmosphere.

I took EE myself, but the first two years for every undergrad engineering student is the same. Ergo every trained engineer knows at least a little Fluids.

In my case, I had to know some fluid dynamics since I was building PA systems. Many of our loudspeaker designs used lenses, horns and other devices to match the mechanical impedance of the driver to that of the open air. "Mechanical impedance" is one of many analogies that are used to promote understanding of the similarities of the two disciplines, for those who aren't engineers.

I'm guessing that you meant "conservation of energy", even though Einstein's famous equation says they're directly proportional. And according to Bernoulli, the smaller diameter of tubular header pipes reduce pressure and/or increase velocity (within the limits of compressible gasses, of course), which helps with scavenging. Of course the exact dimensions depend on how much the motor can pump out, and what RPM range you want to enhance. AFAIK nobody has figured out how to make variable geometry headers, so they're still used to favor one part of the power curve over the rest.

In the days of carburetors and intake manifolds with short runners, long tube headers were used to compensate for the limitations of the intake. Today we have long tube intakes (some with short runners added for high RPM operation). The '06-'08 Cobra Mustang and the new 5.0 mod motor are good examples of this. Because of this it's no longer true that the cheapest power improvement comes from replacing a cast iron manifold with long tubular headers.

Nice to see some informed and rational input, BTW. :biggthump:

 

[IZwack]

Above that I highlighted in red is wrong, the mass is the same for a given volume. The mass stays the same, as pressure goes up or down with pipe sizes.

Allow me use your own words and the fluid dynamics to prove that you and so many others do not comprehend this.

Using much larger pipes does indeed result in slower moving exhaust. What you don't get and are ignoring is that the pressure is also dropping. That results in basically the same mass inside a given volume of pipe].

Ah but what you dont get is that this is fluid dynamics, not static (Bernoulin's principle vs the ideal gas law). Pressure doesnt decrease if you enlarge the pipe diameter for a volume of flowing gas in fluid dynamics - in fact it is the opposite (per Bernouli's principle - makes airplanes fly).

In the image above, there is a greater pressure in the larger diameter sections of tubes (A and C) because:

This buildup of pressure in section A of the tube is what accelerates the fluid through section B - and is what slows down the fluid as it goes from section B to section C.

As such, because velocity has decreased in section C, there is infact a greater amount of mass inside the larger diameter of pipe because it takes longer for a volume unit of gas to travel the length of pipe (classical mechanics).

Dunno how else to tell you this CDW6212R.. but you are wrong.

 

[CDW6212R]

I am tired of arguing with people who are too ignorant or stupid to comprehend logic. I have tried to help everyone I can to improve performance of their vehicle.

You are doing nothing but confusing the issues or ignoring the priorities. That results in readers not learning what is most important, and thus you perpetuate the bad logic which is what a myth is.

The goal is maximum acceleration of a vehicle. That is not accomplished by purposely restricting the engine's ability to expel the exhaust. Installing any exhaust smaller than what is possible to fit is just dumb and/or ignorant. That's why some people win the race at the track, and the losers scratch their heads and don't know why they lost. Goodbye, I'm out of this waste of time.