well you could always accidentaly install you cherrybombs backwards and get the glass blowen out but its possible my mazda b2600 was a single cherry bomb coming out of a header and its loud as hell

 

I agree 100% with this.

Not true. You need velocity and as little backpressure as you are capable of producing. Every bit of backpressure put on an engine is taking work from the engine... work that the engine is doing to push the exhaust out that it could be using to propel the vehicle forward. The only thing you may need backpressure for is if you have a fuel injection system that cannot compensate for the change in air/fuel ratio. The more backpressure you lose, the more air will get sucked in through the intake... this is because of the overlap in the intake and exhaust valves. This increase in air not compensated for will cause a lean mixture, producing less power, and if ran long enough, burn up the exhaust valves.

I agree that the engineers were paid a lot of money and spent a lot of time coming up with an exhaust system that works well with the powerband they were trying to accomplish while meeting emissions and noise standards and keeping costs low. But that is just it, high performance exhaust systems cost a lot of money and they need to consider profit margins. Also, not everybody wants the factory powerband and most people interested in producing more power could care less about emissions and noise. I know the 5.2L peaks torque at 3200 rpm and peaks hp at around 4500-4800 rpm. If you use your truck to tow on the highway, you won't be turning that high of rpms, so you may want to lower your powerband. If you want to race your truck, you probably want to turn higher rpms, so you want to raise your powerband. To properly do this, the three biggest things you need to look at are the intake (tb, manifold and heads), cam and exhaust. Any intake and cam changes need to be compensated for in the exhaust cause you are now burning a different amount of fuel, and any changes in the exhaust will effect the amount of air sucked in and must be compensated for in the fuel system. But all that aside, if your looking for a lower than factory powerband on a mostly stock vehicle, you generally need to decrease the size of the exhaust. If you want to raise it on a mostly stock vehicle, you generally need to increase the size of the exhaust. These are just generalizations and may not be true for every application. But as stated above, don't overdue this in either direction.

I believe it that you can do full exhaust and not see any gains, but were the other systems compensated for? You can't change one system of the engine and leave the rest alone and expect to see much gain. At the same time, you also can't take the lack of gains to mean that high flow velocity doesn't help unless you have actually measured that the velocity increased and are sure that the other systems were correctly compensated for.

Define better! The engineers who designed the factory equipment have their limitations and need to meet specific requirements while keeping costs low. Everything they must incorporate to pass noise and emissions requirements (catalytic converters, resonators, and mufflers) produce backpressure and take power, which robs milage. If by better, you mean produce more power, you better like more noise and greater emissions... but not everybody considers a louder exhaust to be better, even if it does produce more power. Some people out there do consider quieter to be better. Also, maybe someone is more interested in different than just simply better. The factory chose the best setup they could for a combination of low and high end, but that doesn't mean you can't make either the low end better or the high end better. Just remember, you can't simulaneously have the best of both.

I can't comment on this cause I have no idea what setup was trying to be accomplished by these headers. I'm sure it goes back to the idea that the pipes used on the headers were too big and caused too much turbulance for the limited amount of exhaust gas in the low rpm range.


I sure would like to know how you measured this! Sorry, but a typical factory exhaust system with catalyic converters, a resonator, and a muffler produces anywhere from 16 to 30 inches of Hg... anywhere from 7 or 8 up to 14 or 15 psi, so try again on the 3 psi factory backpressure!! Also, keep in mind that any instrument that takes these readings has a particular error present due to the tool itself causing turbulance in the flow. Now, while you were changing the backpressure of your exhaust system, were you also measuring the intake velocity and the a/f ratio to make sure as you changed the backpressure that the intake velocity didn't change for the worse and that the a/f ratio stayed in its optimum range? Like I stated before, the more backpressure you lose that is not compensated for, the leaner the mixture gets. As a general rule proven by several studies I've read, for every 1 inch of Hg of backpressure present in the exhaust system, you lose 1 hp. Just some conversions: 1 inch of Hg = .491 psi and HP = (torque * rpm)/5252. So basically, what you're saying, is you think you managed to drop the backpressure down half a psi so you gained about 1 horsepower through the exhaust?! Thats not much to become a towing machine just from the gains in exhaust! There were obviously many other factors you didn't take into account in the intake and fuel systems. And this brings me to my main point... all the misconceptions out there about exhaust systems are from people who think that performance changes (good or bad) that resulted when they changed the exhaust were soley because of the exhaust changes.

Anyone who doesn't want to take my word on these numbers, I invite you to do a google search on this subject!

Now I agree that just throwing on headers and exhaust on an otherwise factory vehicle won't be a night and day difference by any means, but when I put the headers on my truck and reset the computer (and I made no other changes at that time), it really brought out the low end and gave a little more high end as well. I don't have a stock truck bu any means, but my point is, throwing headers on certainly gave me a noticable amount of low end, so I don't know where you get off saying you loose torque!

If all you are looking for is milage and a power gain in the factory powerband, then removing the cat, or putting in a high flow cat and losing the resonator and muffler will give you this. Any changes to the size will change the powerband. I personally like magnaflow, or if you can afford them, the borla xr-1 mufflers.

 

I see a lot of ripping into single sections of my posts without understanding what it means?????? If you read through the whole post as whole it answers it's self. First re-read my entire post and understand I am bringing a difference of opinion. People can throw numbers all over the place but lets get down to reality, if you thing you can bolt on a lil intake work and exhaust work and gain 50 hp you got another thing coming. You are talking about 1 hp at a time like it's some kinda master of itself or something.......LOL


If you have questions on my post lets talk about it but to answer by ripping into a post in it's entirety isn't the best way to get the information you would like? Lets first look at each statment one at a time?


For instance, #1 you totally agree 100% with a tuned exhaust is the best but in the #2 you say not true?????? I am talking about the same thing dude?



My first question for you is this, how many miles on average can one get out of a completely OEM engine with good overall maintenance on scheduled intervals?

Then how many miles in the same scenario with a good modified engine/exhaust with all the same intervals?

 

I did read your post in its entirety and do understand what you were trying to say. I ripped into the thread cause you're not completely wrong, just wrong about the idea you have about backpressure. It's not a matter of difference of opinion... it's not a what color do you like best type of question! I would never argue with people's opinions. This is a matter of facts. Engineers don't go by their opinon, they go by facts from what they or others have tested and proven. The fact is, backpressure that can be compensated for robs power. Also, I didn't rip into your thread because I have questions about it. The only question I did have is how did you measure the backpressure in your exhaust? I was just correcting what I know to be wrong. If you or anyone else who reads this doesn't believe me, thats fine, it won't hurt my feelings, but I've spent a lot of time and money studying these subjects in school as I'm going for a degree in mechanical engineering.

For the #1 vs #2 comment, I guess it depends on your definition of a tuned exhaust! A tuned exhaust to me on a 4 cycle engine has always been an exhaust system that creates a vacuum in the collector in the desired powerband. So yes, I agree 100% that a tuned exhaust is the best idea, but that doesn't include any backpressure like you stated in #2, so they are not the same thing.

Also, where did the 50 hp come from? I never gave any kind of numbers close to that. And I agree you will never see gains like that from exhaust on an otherwise stock setup! But that being said, those numbers will come out on a modified engine by doing exhaust work. An example, I know someone who put a supercharger on his mustang and did a bunch of intake and head work, and gained 60 or 70 hp. I forget how much boost he was pushing but the gains just weren't up to where they should be. He then went back through his exhaust and took his time coming up with a performance setup and saw an extra 130-140 hp out of that supercharger. That 1 hp per half a psi I stated before... that is based on just doing exhaust work on an otherwise factory engine and nothing else.

And I agree that the more performance you get out of an engine and use it, the shorter it's life is going to be, I never said anything to contradict that.

 

With that said,

The modified engine with exhaust will not last as long because a user is building a machine that isn't the best for longevity. Therefore some back pressure MUST be good to some degree. By saying that the back pressure will lean out the mixture to somewhat is correct but incorrect because IF that is happening the valves on all OEM motors would burn out! Guess what, they don't!

With that said SOME back pressure is good. Now I'm not saying a lot is good, but some is good especially if you are causing a vacuum with a larger piping after the "Y". If there is no pressure then the motor must work harder to dump the motors exhaust to build it up to a vortex and therefore it stumbles on low end torque when you need it.

Now to answer your question, I had my back pressure measured from the difference between the intake and exhaust sides of the CAT not the psi from the motor. The psi difference was 2.5 lbs.


As for the 50 hp I was referring to, most people fall into the manufactures arms with promises and mis-information. I wasn't referring to anyone in particular. I realize that back pressure and tune are different but you need both back pressure and vacuum to create a tuned exhaust and to do this somewhere in the power band something is going to be affected. To me I would rather give up the Hi end then the low to middle end.

The stocker 2.5" CAT's run very well in our trucks and even with mild modification will work just fine. There is no reason if you are running mildly modified that you should run out and by a Hi flow CAT and exhaust unless you have done enough intake work to warrant doing so. Most people reading this thread will NEVER go into this realm of building and therefore this type of information should go into a well established exhaust building thread. The fact that coming off a small question on a 4.7L into a HUGE exhaust thread obviously is WELL over kill. To answer his question I think a simple couple of questions could have helped him in building what he needs and not blowing out the brains of most relatively unpronounced users. (Although I like it.....LOL)

One last thing you refereed to..... You said that nobody should be running in the 3000 rpm range during towing, well if you are towing the correct way you should be locked out of overdrive so you WILL be running in this range on the freeway. I run at 68 mph in 3rd at 3000 rpm.

Headers go into "tuned" exhaust realm because that is what their main job is. To tune the exhaust to dump at the appropriate timing.


Now I tow 7,300 lbs and cruise just fine on the freeway but I surely would NOT do in in OD unless down hill! In comparison, when I take off I can roast the 31 X 10.5 R15 BFG KO's with 750 lbs tongue weight tow my trailer. Not many can say that so I must have something right?

 

What is you guys thoughts on exhaust cutouts?

 

In my opinion, a good exhaust has no need for a cutout. A good muffler sounds better than open exhaust, flows plenty, doesn't drone and is loud when it needs to be. Seriously, how loud does it really need to be?

 

Sorry for tearing down your post again... nothing personal, it's just easier for me to collect my thoughts in sections...

You just made another point toward my case!! In your previous post, you said that the engineer's setup cannot possibly be made better, but longevity may not be a performance builder's idea of better!! Most people who build high performance machines understand that they will wear out quicker. And the valve burning issue.. most modern day fuel injection systems can compensate for the lean mixture so the valve burning isn't such a big deal. Back in the days of carburators, they actually did need some backpressure to function properly or the mixture would lean it out to a point where it cannot be compensated for based on the flow design of a carburetor. Back in those days, valve burning was actually a real issue.

Not true. The vaccum produced at the collector will literally help to suck the exhaust out while the piston is also pushing it out. This creates less work that the piston has to do to move the exhaust... work that is therefore put into moving the vehicle forward.

Now that makes a lot more sense!! I can definately belive that the cat is producing those kinds of backpressure numbers, but you also have to take into account the resonator and muffler (if you still have them) to get the entire backpressure of the system.

Lol, yeah, you can never trust the people trying to sell you the products! But I still disagree with the backpressure part of the statement. The trade off part is absolutly true and what I've been saying all along... you can't have both. I still believe it is the the turbulance based on the size of the pipe that determines this. In the low rpm range, you have less exhaust that is already moving slower than in the high rpm range. If you have a large diameter pipe (a size that needs to be determined by a series of tests... not a size that can be named out of thin air) that flows best in the upper rpm range, it is naturally going to cause a lot of turbulance in the low rpm range and slow the velocity down, causing higher backpressure and reducing low end power. If you have a smaller diamater pipe that can keep the velocity up in the low rpm range, it will create huge amounts of backpressure when you try to stuff the extra exhaust of the upper rpm range through it and drop off your high end power.

I agree with this. I believe I even stated in an earlier post to the OP that he should keep his 2.5" exhaust setup if he was happy with it's performance and just simply replace his broken muffler with a lower restriction one and be set. My first two posts in this tread were simply to end an age old myth that backpressure helps... which is something the OP had mentioned in post #1. At this point, that is all we are arguing about!! I agree with everything you have said about tuning exhaust and matching it with the intake and making sure everything compliments each other... but I don't agree that backpressure is good. I will go so far as to say it may be a neccessary evil in some instances (instances where like stated before, it changes the a/f ratio to something that cannot be compensated for), but it is not good. I stand by my point that any backpressure in the exhaust takes power from the engine, and an all over performance setup won't have to rely on backpressure to help the other systems because they will be changed accordingly.

Well... yeah... I guess that is just me having a 5 speed manual, not the automatic transmission. I suppose you are right about that, but I do tow in 5th gear on flat ground, so when I said that it is because I'm not in that range.

I agree with this, but the flipside of the dumping it at the appropriate time is to collect at the appropriate time if you're exhaust system has a collector... and most people don't do a 1 pipe per cylinder all the way back to the tip kind of exhaust setup. Thats why there is a difference in shorties vs midlength vs longtubes and the best thing you can do is build them to a length that suits your exact engine setup rather than buying someone elses and assuming it will work.

Lol, I will agree you definately have something right! And I never said you don't know how to set up a performance machine... all I said was you are wrong in saying backpressure helps make power.

It may be neccessary for the other systems to function properly to have some backpressure, but that backpressure itself is taking power from the engine. I know thats a little twisted and backwards, but an oll over performance machine does not have to rely on any backpressure because all other systems would be changed accordingly.

 

Now we agree!.......


You engineer's love to take things out of the realm of regular back yard mechanics................But if it weren't for you crazy guy's we wouldn't have trucks to work on!

If you get into top car racing then I will take a whole different approach but for the vehicles we all run, best left to not go over the edge.......LOL

 

Wow you two know how to have a debate...