I have spent a lot of time doing searches trying to find an answer. Guess what, I didn't find one. I understand how no backpressure affects the power range. What I don't understand is how having no back pressure can hurt the engine in the long or short term. Just wanting to gain some knowledge on the subject. FYI I have a 97 with a 4.6. Also I have seen some of your x pipes and was wondering the + and - about the 2.5" pipes w/ 2.5" core versus the one with a 3" core.

Thanks

 

i'd be interested to hear what mike thinks of it too, but here's what i understand and it makes sense to me. everything i've always heard has pointed at exhaust valve damage when there is too little backpressure. there is some amount of valve overlap in an engine which means both intake & exhast valves are open at the same time. when too little backpressure is present, the air rushes into the combustion chamber and right back out before it has a chance to burn completely (b/c of the low pressure on exhaust side). so you've got some unburnt fuel hitting the O2 sensors indicating a rich mixture which in turn causes the computer to lean the mixture out which heats everything up. thus burning/warping the valves. i've also heard this rush of cool air across the hot exhaust vales causes improper cooling cycles and leads to warpage.

 

Hi.

If you did indeed do extensive searching, you'd discover that "backpressure" is largely a myth and quite misleading as far as terminology goes.... right up there along with "unthawing" and "irregardless" ;-))

Try " scavenging", or "exhaust gas velocity" ...

Oh, and this probably belongs in the "Exhaust" forums ;-))

Cheers

 

The problem with backpressure is there are many factors that can contribute. Also depends on which of these factors would come into play in your case. Valve size, cam profile, compression, runner size, head flow, chamber design, along with tons of others. On a mostly stock vehical I'd just worry most about heating up the exhaust vavles to much & maybe loosing a little Q.

 

Read this and you will know everything there is to know about headers.
http://www.ssheaders.com/header.htm

JMC

 

I have posted this in the exhaust forums and someone refered me to ask Mike(this was 3 months ago, its been a hard semester). I did the search topics you mentioned but all I get is a bunch of stuff that contridicts what I read previously. That is also why I am asking Mike because of what he does and the fact that he nows more about these engines than anything else. Just to let you know why I have posted an exhaust question in the chips and programming forum.

 

Hi CEE21,

Backpressure, in and off itself, is really not the concern IMHO - as 04Triton mentioned in his post, backpressure is simply one of *many * factors that have an effect on power production, and it's a by-product at that - not the be-all & end-all.

Cylinder fill & scavenging characteristics is really the more important issue, and is affected by the specific cylinder head's combustion chamber design, everything from quench characteristics, piston top design, valve geometry & valve event timing (camshaft profile), & numerous other aspects.

Getting the basic exhaust gas velocity, exhaust system flow engineering, cylinder fill & scavenging, valve event timing, etc. matters configured in a manner appropriate for the application are the really important matters, and you're going to be hard put in practical terms to actually get to true zero backpressure. Remember, backpressure in an of itself is not particularly desirable, nor the main point - it's airflow *velocity* when we're talking about torque production in these vehicles. Now there will be plenty of people who disagree with that, but that's them & their problem - I focus on airflow velocity and make maximum torque at any rpm, load & throttle opening, and then think of backpressure primarily in terms of additional resistance to airflow velocity. I don't worry about attempting to achieve zero backpressure - if you have an exhaust system of just about any kind on these vehicles, you're going to have *some* backpressure.

This really gets into far more complexity than can ever be covered here properly. The important thing to focus on is what you are actually trying to accomplish.

If you want to learn engine theory, you need to buckle down with good books & read, then experiment by building different configurations & testing - & study, study, study. Exhaust flow engineering is a subject where you will never "learn it all," there is always something new & different to think about, experiment on, etc. But getting a good grounding in basic engine theory with specific emphasis on exhaust flow engineering as it relates to cylinder filling/scavenging is probably what will do you the most good in trying to understand this. Your local library can get you a decent start in this, and one of the people whose thoughts & work I respect is Smokey Yunick (sp?). His thinking makes for excellent study material to get a good grounding in many aspects of engine design & exhaust work. I don't agree with everything I've seen attributed to him as having said, but he was one of the sharpest cats to ever come down the pike, and set NASCAR on it's heels numerous times thinking outside the box & getting real results. For example, the basic process of extrude-honing, as I understand it was in no small part an offshoot of some of Smokey's thinking - at the time (the 50's), the NASCAR rules prohibited machine work to increase cylinder head/manifold flow - so Smokey thought about it and reportedly said hmmmm........well, they didn't say anything about NOT porting by forcing a high-pressure abrasive slurry mixture thru those parts, so he did just that - and kicked butt with the additional power he made via the increased raw flow capacity.

So I'd suggest spending time reading some good books on this, and start off the cheap & easy way, by going to a good-sized local public library, search titles on the Internet, etc.

 

Thanks for the reply but I think you either missed my question or it was hard to understand. What I am wanting to know is how does having almost no backpressure (headers, no cats or mufflers) hurt a stock 4.6L if it does hurt it. I have heard every thing from warping the valves to blowing the engine. FYI all I really plan on doing is replacing the Y pipe w/ a X.

 

I understand - but I just don't have time to be a "teacher," so to speak. Additionally, this isn't the place to discuss exhaust issues, of course. For any questions as to the effects of any of our products on your vehicle, please give us a call at our number listed below.

Just briefly - the time you install an X-pipe is if you do a true dual exhaust system - generally speaking, an X-pipe has no place in a single exhaust. So if you are going to separate the system at the Y and then run true duals, yes, absolutely - put an x-pipe right there where the factory Y originally was. You want it as close to the cats as possible for maximum effect (heat aiding velocity thru the crossover).

Regarding using a 3" core over the 2.5" core, we have always felt that if the motor made power within a certain range, it's beneficial to use the 3" core - that is what I would do if the engine were making say, about 400HP to 500 HP. If the engine was making 550 HP or more, I'd go 3" on the entire X. But for the "typical" 250-375 HP F-150 motor, I would just use the 2.5" X-pipe including the 2.5" core.

I hope that helps you for now, & please feel free to call us to go over any of this in more detail, OK?