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www.team-integra.com has some really great tech articles that are very good reading on many subjects. I highly recomend joining the site for the info.
Here is some of the article on exhaust back pressure.

"Myth 3: I Need A Little Bit of Backpressure For Midrange Power

THE MIGHTY BACKPRESSURE MYTH:

You want zero backpressure not some backpressure as you may sometimes hear from a salesman or an oldtimer domestic V8 hot rodder.

Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 2-5 psi backpressure. "Bolt-ons only" engine packages, in the past, used exhausts with some backpressure, since there is this incorrect belief that having a little backpressure prevents the fresh air/fuel from shooting into the header at cam overlap (when both the opening intake valve & the closing exhaust valve are simultaneously, partially open). The backpressure supposedly "pushed" the fresh air/fuel back into the combustion chamber rather than having it go into the header. This shooting of fresh air/fuel from the intake manifold and intake port into the header cannot happen at cam overlap, since the pressure inside the header is already much higher than on the intake side , even when there is zero backpressure.

In reality, having more backpressure reduces the difference between the higher pressure in the head's exhaust port and lower pressure in the header and cat. You need this difference in pressure going from the head to the exhaust system or "pressure gradient" to keep the exhaust flow speed or energy at a high level. Having some backpressure during cam overlap and the exhaust stroke means that the exhaust gas must now push against something and therefore, this backwards force slows exhaust gas down.


This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to backwards flow or "reversion", where the exhaust gas travels backwards into the combustion chamber and dilutes the fresh intake charge at cam overlap. At the very least, it slows exhaust flow velocity or energy and prevents the creation of a vacuum for scavenging.

So please ignore the obsolete "you should have at least some backpressure" sales pitch. It's all about the creating high exhaust flow velocity/speed or energy leaving the exhaust port, in order for the header-cat-exhaust SYSTEM to do it's job properly (i.e. remove all the burnt exhaust gases and help pull in fresh intake charge by scavenging at cam overlap) and make power for you.

Regarding the backpressure issue:

Many people use backpressure to get midrange driveability at the sacrifice of lower power potential at the upper powerband rpms. Using back pressure is the wrong way to build a high performance exhaust system. The exhaust system should extract the exhaust gas from the header, to minimize parasitic pumping pressures.

The proper way to make an exhaust system that will act as an extractor is to properly size the tubing so that the the exhaust gas' flow velocity creates a "vacuum" behind the header.

Also, you have to realize that making a sytem which provides the best performance at all throttle positions and all powerband rpm ranges is next to impossible. There's always going to be a compromise and giving up some optimal power potential in one area of the rpm range.

You must tune the exhaust size/length for the throttle positions and rpm ranges where you want the most performance knowing that you'll sacrifice performance at the other end of the rpm range." witten by MichaelDelaney on team-integra.com

Here is a link to the article pages http://www.team-integra.net/sections/articles/listArticles.asp?Show=1
 

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Nice article for sure, it is funny though, I have read for years and been taught and shown in performance shops to always size the piping correctly to make sure it has some backpressure. The funny thing is, my camaro's old setup was long tube headers, off road y-pipe, straight pipe out the back with a car chemistry insert(youtube video available). My low end and midrange suffered, but top end benefited, by about 8 hp. Now, after the dyno tune was done, I notice I got my low end back but not midrange. Manufactures tune their motors to with backpressure, and to just get rid of it with no means of tuning to adjust for it, you gas mileage will drop and performance will suffer. Everyone has read tons of articles all saying different points, this subject will always be a back n forth argument, lol.
Thanks NC for the article again.
Later
 

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Nice article for sure, it is funny though, I have read for years and been taught and shown in performance shops to always size the piping correctly to make sure it has some backpressure. The funny thing is, my camaro's old setup was long tube headers, off road y-pipe, straight pipe out the back with a car chemistry insert(youtube video available). My low end and midrange suffered, but top end benefited, by about 8 hp. Now, after the dyno tune was done, I notice I got my low end back but not midrange. Manufactures tune their motors to with backpressure, and to just get rid of it with no means of tuning to adjust for it, you gas mileage will drop and performance will suffer. Everyone has read tons of articles all saying different points, this subject will always be a back n forth argument, lol.
Thanks NC for the article again.
Later
Quoted for truth.

Everything I have read over the years contradicts the Op's allegations. In fact in the automotive design industry, there have been several studies done on the modern internal combustion engine. Several of those did lab tests that definitively prove that each engine requires some back pressure to achieve optimum horse power and toque figures.
Still required reading Is this set of Papers dating back to 1973. The information contained within is still valid.
Please understand, That if Absolute Zero back pressure were in fact the goal. It would be immediately Implemented on the race track. To this day High performance Drag racing engines run Tuned headers to optimize engine output. Do you honestly think those people would waste their time with that if they did not need to? Today's drag engines develop as much as 7,000 horse power, and they still take the time to Tune the exhaust headers.

Does that make any sense ?

You must also realize that drag engines are designed to run at a specific RPM setting. The low and mid range in are not done with Engine speed. It's done with an adjustable variable clutch assembly.

Still all the road racing engines to this day have Exhaust systems tuned to provide the highest engine output at all RPM ranges. They are designed around creating the ultimate back pressure range for that specific engine setup, under the race conditions. Never Zero back pressure.

Dom
 

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the exhaust headers are tuned for scavanging NOT back pressure. and if you dont have a header or ex manifold yes you can and will pull fresh air into the cylinders from the ex. just like my bug that has an o2 sensor for tining the two big two barrells on top of each cylinder head. the o2 sensor would not work at low rpm due to a short ex system(about 2 feet after the well built merge collector) it was pulling up fresh air & cooling off the o2 sensor, so after much research & talking with the "experts) I got one of the expensive heated o2 sensors. problem solved. on the race car it had 2 1/2" primarys going into a 5" or 6" collector, how much backpressure do you think that size tube has? the colector is pretty much just for holding the tubes togeather when it's that size. it's not the back pressuer that you want & need it is the scavanging & volicity.
some of the facyory racing units thay need a biger tube that hurts low end due to loss of volicity had butterflys in the collector to combat this problem & help scavanging by increasing the exhaust gass speed at the colector by reducing the size of the tube with the butterflys thus increasing ex gass speed(volicity) so as to help evaquate the other pipes witch do the same .and it works !!!that is why tube sizing and how long a tube is importaint as is the weight &use of vehicle. but with that said you can have the very best tuned ex header,cat,muffler,ressonator and put a potato in the end then you got squat. a restriction is a restriction unless it is used for a reason other than being shiney,like those slip in baffels, they are restricktors.put a good muffler on it and let it flow. and those baffels are part of the reason for having a way to big exhaust pipe on these small moters(2. 1/2"). size it right & put on a good mulffer and make more tq,hp,mpg. done
 

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Mark Scavenging Is done By the velocity of exiting gas from one cylinder creating a negative pressure In the next cylinder in order exhaust port. However your premise is sound. I also agree completely with your analysis of proper sizing in a street car.

I found I was in Error. I find that This proves that I misjudged the output of the drag racing engine. They in fact produce an astounding 8,000 Horse power! Not the meager 7,000 horse power I reported! :grin:

Dom
 

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nice geraph !!! notice the 2.25 pipe has all it's ducks in a row? and the 3" pipe is fixen to loose some of it's ducks or have a pile up? thats power lost,mpg lost,pulling torque lost. just like in life try to keep all your ducks in a row:razz:

and yes dom it is the velosity & a negitive pressure wave from the ventirie action going on in the collector. it works with air ,with water,sand blaster. and even when a wife is sucking the life out of you from her nagging(or so I have herd but not experanced):razz: I hope everybody had a great & safe weekend !!!
 

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I prefer to use a really big turbo as my engine backpressure. It decreases highway drone too.:evil:
 

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the only one I ever worked on was a small turbo being fed by the bigist fricken turbo I ever seen.120 psi on a drag truck ,he said it would do 160 psi but we couldent get the hoses to stay ,it broke a clamp & had one more run for lissenceing.so I showed him how to use a defrent type of clamp. 8.20 in the 1/4 mile in a diesel truck not making all the boost !!!!at bristol dragway up in the mountains with out a bunch of air any way. I also did machine work on turbo engines for outlaw street class.one was the first one to go under 4.30 in the 1/8 mile.or might of been 4.20 dont remember too long ago (joey martin of joey martin racecars)he builds some wicked cars,promod,outlaw street etc.and he can drive with the best of them or better.
 

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When I was racing (many years ago, bugeye sprite, backmarker) I built my own header/exhaust system based on a formula that measured cam lift, duration, etc and gave the length and diameter of the pipes, length and pitch of the megaphones at the end of the pipes and a whole lot of other stuff. I'm not a great welder but I cobbled up a system that dramatically improved the HP between 6 and 8000 rpm. The carbs were highly modified SUs; you could probably shave with the butterfly valve. the point is in road racing I never got below 6000 rpm so you could design it to run like a bat over that speed. Low end power wasn't much but it would really scream at 8k. A street engine can be made to do what ever you want but trying to keep it revved in traffic isn't fun. I drove it on the street after I stopped racing and it was a mess in traffic; fouled plugs, overheating, getting the fisheye from the police etc. Flat tourque curve is what I want in a street car
se
 

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bug eyed sprite,su's mga,mgb's,miget,tr6,spit fire,alfaromao, straight out of high school I was europein auto mechanic,but first I had to find the bonnet:shock: dang that was along time ago.I did that for a living and built and raced musell cars for fun .no not the ones you dig up and boil :razz: sorry couldent resist , it was so easy, un like those british &english cars.what were they thinking??? only kids can and should work on those things,you need to tie your self into a knot to do much of any thing at all,not recomended for any one over 26.( I think thats when our bodys start telling us HEY STUPID THAT HURTS!!!
 

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But you know, on a fine spring day almost nothing beats a sportscar cruising down the road, top down, exhaust burbling, and a pretty girl next to you. Ah youth!
se
 
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