N/A UPGRADE BIBLE.

[jbanks20]

Originally Posted by 93 Green RT

I'd have to disagree with this as well. As long as your exhaust mods keep opening things up more WITHOUT creating addition turbulence, the NA will continue to gain hp simply by being able to breath better each time. The only loss will be a little low end torque, but from midrange through top end the engine comes alive like a scalded ass ape!

i dont know who told anybody that freeing up youre exhaust helps you're horsepower or torque, but they were wrong if you put larger pipes and take off you're muffler and you're cat you're going to lose alot more than you gain. engines run of off combustion which is caused by compression and taking all that backpressure out of you're system you lose alot of power. So yes you can free up some but you need to restrict you're exhaust to keep the hp you want to gain.

[SOHC3000GT]

Originally Posted by jbanks20

i dont know who told anybody that freeing up youre exhaust helps you're horsepower or torque, but they were wrong if you put larger pipes and take off you're muffler and you're cat you're going to lose alot more than you gain. engines run of off combustion which is caused by compression and taking all that backpressure out of you're system you lose alot of power. So yes you can free up some but you need to restrict you're exhaust to keep the hp you want to gain.

lol, you hace no idea what you are talking about. The exhaust has nothing to do with compression since the valves are closed(usually) during the power stroke. Having a totally free flowing exhaust is a great way to unleash your engines power potential. The full exhaust on my SOHC made my car much faster, you would have to be dumb to think that I would spend so much money to make my car lose power.


Don't post about a topic you obviously know nothing about.

[TheRookie]

Exhaust
I have gone to months of courses on this and used to bend custom mandrell exhaust for a living. I have bent headers and full exhausts for anything from motorcycles to some sick drag cars. Now since this is in a section for an NA car I am going to leave the changes that a turbo creates alone. That is a whole different can of worms than it is for NA engines.

The rumor that backpressure is needed, TOTAL MYTH!. I cant make this clear enough. Many believe that some backpressure is needed, this isnt anywhere close to true. When we get into where that myth comes from it will be more clear why people think this.
(side note, thinking backpressure has ANYTHING to do with compression is totaly absurd and explained above my post)
See when it comes to exhaust you need FLOW. Lanear flow to be exact (yes this is the right spelling it isnt linear) Lanear flow creates a scavenging effect. This effect actualy pulls exhaust gasses out of the combustion chamber. When you lose this flow you dont get as much of the exhaust gasses out. This is when you have the loss in torque. If you just throw huge pipes on an engine your exhaust gas velocity is quite slow. This intern hurts flow and takes away from the scavenging effect. Backpressure has nothing to do with flow, its often mistaken for it though. Remember big straight pipes = slow flow. The cure is to find the perfect size. This size is different drasticaly not only from one vehicle to another but varries by number of bends, bend radius, even climate and conditions.

Now backpressure is the number one enemy of engine power(as far as exhaust in concerned) , Turbulence also ruins flow. This is why just gutting a cat rarely actualy helps any. People say it does from the placebo effect. Try it at a dyno. It does help on occasion but that occasion is usualy when that cat was partly clogged or in poor shape to begin with. When you gut a cat you make a big open space with a smaller in and outlet. So you get a big turbulent mass of exhaust. This is more of a restriction than a cat in good shape usualy. It is also why test pipes exhist. They eliminate this problem by keeping the air flowing smoothly.


*disclaimer, This has verry little to do with the cases of a forced induction engine. Specificaly has little to do with a turbocharged engine. but this is about NA mods.

[Barefoot]

Ug, how many times will this conversation come up?!?

Read this if you don't understand backpressure, or rather the misconception of it..

Well, here I go again.
This is not my work I just saved it from some site, some time ago, but it makes for great reading, Enjoy.

…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most important ones you need to be aware of to understand the things I'm about to say:

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is its pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth; just that it makes sense in my eyes.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to achieve optimal gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; because is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhaust gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburned mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. If the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.

It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace. "

Here is a reply to the above post-

"I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks.

There is one thing I'd like to add to Texan's work:
Exhaust Scavenging
In essence, this is the opposite of the exhaust reversion that Texan describes.

Reversion: at the beginning of the intake stroke during cam overlap, exhaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.

Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.

Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.

Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.

Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.

There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall efficiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
"You want low backpressure and high positive delta pressure".

[TheRookie]

Wow, that is really well written. It just agrivates me when people fight that backpressure is needed. The author of that post really understands how it all works. And did a far better job of describing it than I ever could

[SOHC3000GT]

I thought you described it very well.

[jbanks20]

oh okay so my two years of automotive engineering and performance were all wrong and a waste of money. Thanks i wish i had met you two years ago so i could have saved my time and energy.

[SOHC3000GT]

You obviously didn't learn anything, you have no idea what you're talking about.

[Barefoot]

Originally Posted by jbanks20

i dont know who told anybody that freeing up youre exhaust helps you're horsepower or torque, but they were wrong if you put larger pipes and take off you're muffler and you're cat you're going to lose alot more than you gain. engines run of off combustion which is caused by compression and taking all that backpressure out of you're system you lose alot of power. So yes you can free up some but you need to restrict you're exhaust to keep the hp you want to gain.

Originally Posted by jbanks20

oh okay so my two years of automotive engineering and performance were all wrong and a waste of money. Thanks i wish i had met you two years ago so i could have saved my time and energy.

Firstly, Please read what I previously posted MANY times, as you seem to need the help.

If freeing up your exhaust only makes you lose power, why dont you throw 5 more mufflers on the car? Backpressure is simply resistance to flow, which is undersirable. Im not going to explain it as you would know if you read/understood that article.

No one said your classes were wrong, and I'm sure they were right for the most part, but that does not mean everything you learned in a general automotive class directly relates to high performance vehicles. Cut the sarcasm and do some research.

[ProwlerGT]

For something labeled "BIBLE" such as this thread is, it serves to mind that the last thing that should be present herein is arguments and knocking heads. Frankly: End it and go in peace. If people want to believe there is a need for back pressure then let them. If people want to believe there isn't a need for back pressure then let them. The truth (who I'm friends with) will be revealed to the person who is wrong if they are serious & honest with themself enough to do the testing to find out.

Besides that I will offer up my own thoughts by way of deducive logic removing all ego and machoism or thought of who is right/wrong.
Ask yourself these questions:

What is the stock exhaust system comprised of?
What is it comprised of on MOST vehicles?

Answer:
1. Pipes to channel exhaust to the rear of the vehicle.
2. Why do the pipes lead to the back? Why o why? Likely because the rear of the vehicle is the best place to put the exhuast for reasons of noise for one.
3. There is a Catalytic Converter - What is its purpose? TO FILTER OUT PHYSICAL AIR POLLUTANTS.
4. There is a MUFFLER - What is its purpose? TO FILTER OUT NOISE POLLUTION.
5. Using common sense and thinking since the Cat's purpose is for filtering out pollution and the Muffler (as the name implies) muffles noise then what do both objects cause as exhaust air passes through them? Answer: Restriction. Restriction of air flow speed is a bi-product of the Cat and Muffler which were created for different purposes, none of which were engine power.
6. Most people buying a passenger vehicle whether its a sports car or not are not concerned with even 1/4 of the issues discussed in ALL of 3SI's forums, therefore to make most people happy the factories make the cars quieter and less pollution prone.

Engine power has nothing to do with why a cat and muffler were put on vehicles to begin with and the removal or upgrade of such parts only comes into play with a Performance Based Sports car mindset (IE: A gear-head) as most of us on here are. Deducing said facts with common sense will (should) show you that removing the cat and muffler will relieve exhaust air restriction allowing the engine to "exhale" faster thus allowing it to run better thus allowing it to gain more power (Yet GAINING more power is an illusion that we often speak of. The truth here is that we're just unlocking engine power that was already there, but held back due to stock setup).

Example: Think of your MOUTH. Put a heavy duty Brawny Paper Towel over your mouth and BLOW through it. What happens? Is it easier to blow air out of your mouth with or without the paper towel in the path of the air flow? Do our mouths need back pressure to blow air better? No of course not. Don't flame me for my example, but just listen. This is just fundamental basics on air flow (whatever the source of the air). The paper towel is just as much a restriction of us exhaling as is the cat and muffler to the engine.

The only reason cats and mufflers exist is for reasons of pollution of NOISE and PHYSICAL air type. If you took off the mufflers and cats for most or all passenger cars IN THE WHOLE WORLD you would have noise bombarding residential homes as the cars passed by as well as a much higher pollution rate causing air quality where we all live to become horrible not to mention global warming if you believe in that theory.

Conclusion: Arguments if need be should be over good points. This argument ya'll have been having is an argument about NOTHING and common sense shows that. This is no knock at anyone's intelligence, but simply a review of facts. I hope this will help people and I hope this will put this unnecessary argument to rest.

Side note: I personally hold the belief that installing exhaust pipes larger in width than the diameter of the throttle body air intake orifice is a waste of time and just for show. I believe this idea goes along with what I've said above.