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  #16 (permalink)  
Old 12-09-2003, 01:51 AM
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Quote:
Originally posted by TurboS10
For the average 2000-6000 RPM street bruiser, I like big pipe and header tubes without going overboard. I believe you get the most usuable power through the range. I will point out again, a simple swap without retuning is not a good indicator.
Chris
But as you are saying there is an overboard where you will gain no more power, and may lose low end (on a street bruiser 1000-3000) which with street gearing will make the car a dog on the low end. Right?

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  #17 (permalink)  
Old 12-09-2003, 02:06 AM
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I don't know too much about exhaust.
But I did know when I had an exhaust problem.
I put on a new intake and my truck started overheating at high rpm's.
After a few checks I determined that my factory exhaust manifolds were the problem.
I put on a set of hooker comp. headers... long tube style... I can't remember what that tube diameter size was... but the collecter was 2 1/2 inches...
I couldn't afford a total exaust (pipes, mufflers, hangers, etc.) on it as I had spent most of my money on the intake and carb. So I just got 2 1/4 reducers for it and welded my old glasspacks (18 inches long, 2 1/4 inch internal diameter) right to the reducer. My exhaust is basically nothing but headers with glasspacks hanging off of the ends. no pipes. at all.
This is on a 350 in a 72 chevy truck.
not only does this setup sound great... I have actually popped my tire burning out with this setup and have noticed GAINS in torque all the way from stopsign to redline.
I don't know how or why it works... I just know that it does and since it does I've left it alone for over a year now.
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  #18 (permalink)  
Old 12-09-2003, 02:58 AM
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Basicly from what I've read and seen you need to match your exast size and length to the motor out put for best results.
You need to have a suficient scavenge speed coming out of each header pipe into the colector in order for each cylinder to help suck the exhast out of the next one. To big of any pipe and you have no speed. To small and the exhast is restricted.
4 banger racing bikes use this perfectly. Thats why racing bikes only have one muffler/baffle and not 4. Or even 4 open headers.
Thats also why 2 stroke engines respond so well to a simple exhast change. They tune the size and shape of the expantion chamber.
There are other ways(atmospheric reverberation) and reasons to tune an exhast and intake to your motor but its late and takes too long to explain right now.
But wasn't there a factory musle car that used this perfectly to acheive 115% of the incoming air/fuel mix in each cylinder instead of the normal some what less than 100%. Though it only happened at a single small RPM range.
In the 50's Mercedes Benz even worked out a way for your gas peddle to change the exhast leagth to match the RPM. Thearby trying to make the best power band over the longest range. It worked but the exhast leaked or got stuck.
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  #19 (permalink)  
Old 12-09-2003, 09:02 AM
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Dubz, there is always an overboard, or at least a point of diminshing returns. Since I have not seen any dyno test, I will just say that my opinion is that there will be some losses in the low end if pipe size in increased, but I do not believe it will be significant enough to justify a high restriction exhaust. While larger tubing may hurt 3000 and below, it will help on top. Really depends on the combo. IMO, top end is more usable since you are not having to fight tire spin. Roasting the tires does not make a car fast, ya know.

Smlblks10, from what I have read the scavenging is most benifitial in the header tubes and right after. It seems it is more benifitial when the EG is very hot and at high velocity, but this is just what I have gathered from reading so dont hold me to it. I really dont think there would be any performance gained from running a larger pipe from the muffler back. When the exhaust gas slows in the muffler it is going to loose some heat and really should need less volume in the pipe since it will be at a lower volume.

Another point dealing with tuning and backpressure. When running open headers, an extension tube should be used to extend the header collector. You can use paint to check the point where the EG gets cool enough to stop burning the paint. At this point, you can chop the pipe off. This is suppose to be the ulitmate tune on open exhaust. I would venture a guess that this is due to the added back pressure with the added pipe length. Same paint trick can be used to find the best crossover pipe location on duals.
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  #20 (permalink)  
Old 12-09-2003, 09:40 AM
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This has been covered in great detail on this board. I did a quick search and came up with the following links. In summary, there is no 'best' header/exhaust system for every application. Engine design is a compromise - a header that works best in normal driving would be a bad choice for a high RPM racing engine. Peak torque is highly dependent on header length. The longer the primary header pipe, the lower the peak torque RPM. Detroit's compromise is to use a header log that has no primary pipes so doesn't have as high a torque as it could have but also doesn't 'peakie' torque characteristics.

http://hotrodders.com/showthread.php...hlight=exhaust

http://hotrodders.com/showthread.php...ghlight=header

http://hotrodders.com/showthread.php...ghlight=header
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  #21 (permalink)  
Old 12-09-2003, 10:43 AM
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Some interesting info Willys. Basically, looks like backpressure is bad all around.....hmmmm sounds familiar. What the question comes down to is do you want to optimize headers and exhaust for lower or higher end velocity. Low end velocity can be achieved with the smaller tubes. High end they become restrictive and cause backpressure problems.

I have to disagree a little to some of the other threads. There are alot of aftermarket headers that achieve very close to equal tube length. The afterburner headers actually attempt to also add a longer tube for what they explain to be an uneven pulse since each bank does fire at an even interval. Take 18436572, the most widley memorized order on the planet. If you look, you can see how equal length would not give an equal scavenging pulse for all cylinders since 84 fire together, then one cylinder later 6, and two cylinders later 2. Properly designed uneven length could help achieve pulses that correspond to the uneven firing order.

Just some food for thought. Shoot it full of holes guys

Chris
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  #22 (permalink)  
Old 12-09-2003, 11:40 AM
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we should know better by now than to talk in terms of absolutes when it comes to motors.

Bigger carb is not always better.

Bigger cam is not always better

Bigger valves and runners are not always better...

and bigger exhaust is not always better either.

IMO the more you build a motor up, the more exact you have to be about your parts due to cam overlap. My professor explained it this way in my auto-restoration class:

Bigger cams move columns of air and fuel through your engine (a reason you should always match your intake to your cam...), a bigger cam moves a bigger column, thats why it moves the RPM range of the motor up, because the air has to be moving that much faster to get that full column into the motor. A well designed exhaust system (which on flat crankshafts can be tuned ex. Italian sports cars use this method) works with the cam using a scavenging effect. The gas expansion from one header tube creates a pulling or low pressure area on the next opening valve and actually sucks the exhaust out of the port and because of the overlap (both intake and exhaust valves open at once) that much intake air is sucked into the chamber as well. What a great system eh?

Furthermore, on these "tuned" flat crank engines, a perfect muffler (exactly 20x the size of the combustion chamber) is used and creates a sound pulse that travels up the exhaust, back to the header and helps create a low pressure area that pulls the gasses out of the chamber. The "tuning" involves positioning all parts of the exhaust perfectly so that the sound pulse hits the port at the desired instant. That must be why these Italian cars cost so much! LOL

K
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  #23 (permalink)  
Old 12-09-2003, 11:54 AM
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weight matters, so does driving in traffic

Vehicle weight plays a big part also. Optimum performance for pipe size varies with weight of the vehicle, as it does with torque converters.

The idea is to expel as must of the gas as possible while still retaining back pressure for low end torque and the scavenging effect of from valve timing overlap. Hence, cam spec come into play when choosing the best pipe size.

The hotter the gasses, the faster they go: its physics.

So if you have coated headers, the gasses stay hotter. If you wrap them => even hotter.

Small primaries stepped out to *slightly* larger pipes create a "restirctor plate" type effect that keeps the gasses from going back up the tube, as *easily*.

Ridiculously HUGE pipes that cool off easily, keep a low pressure environment that acts like a solid wall to push through. The gasses are NOT HOT ENOUGH TO GET OUT OF THE PIPES.

Race cars are different. They spend most of there lives above 5000 rpm.

The same goes for Fart boxes on Turbo, they only have power in the high range, but they will NEVER need dual 3" exhaust on a <2 liter engine.
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  #24 (permalink)  
Old 12-09-2003, 12:20 PM
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Yes, I have heard all of the stories on how back-pressure is desirable in certain situations but none make sense to my pea-brain. Also, the organ pipe theory used in header primary-pipe design ignores effects pipe interference. Assuming the collector size is reasonably larger than the primary tubes, each tube acts independently from its neighbors. The main advantage gained in equal length, independent primary header tubes is from the strong negative pressure pulse that is reflected from the tube end when the strong positive pressure pulse form the exhaust valve reaches the collector. Other pulses from other header tubes are of much smaller magnitude in the tube of interest and can be ignored. Thus tuning length is very easy to determine once you have an estimate of the speed of sound in the hot gasses. A useful equation is

L = 120V/rpm

For
L = pipe length, less port length in head, in inches

and

V = velocity of sound in hot gasses. Values of 1300ft/sec to 1700 ft/sec are common.

Using V = 1700ft/sec the equations simplifies to

L = 204,000/rpm.

There have been various permutations on this basic design like tri-Y headers, stepped tubing size, etc. Each takes advantage of modifying the pressure pulse arrival time at the instant the exhaust valve closes to achieve a scavenging/ higher volumetric efficiency/ more torque result. The good is that you can achieve a very significant torque increase at the design rpm. The bad is that you likely will also achieve less torque at other RPMs.

There are other design theories like the Helmholtz resonator which are useful in designing systems with more than one degree of freedom than a single pipe/cylinder, i.e., Tri-Y.

Test show that smaller tube diameter has no effect on tuning speed but the smaller the tube, the stronger the signal. Conversely, the smaller the tube the more bad pressure loss there is. Tubing bends, within reason, have no effect on header performance.
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  #25 (permalink)  
Old 12-09-2003, 12:54 PM
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Re: weight matters, so does driving in traffic

Quote:
Originally posted by 2DoorCaprice

The same goes for Fart boxes on Turbo, they only have power in the high range, but they will NEVER need dual 3" exhaust on a <2 liter engine.
I am curious of the fart boxes you speak of. New term to me Are you talking intake here? Or, large pipe after the turbo to take the "twist" out so the flow can become linear? If the later, this is a very viable setup. General rule of thumb is put as large a downpipe as will fit on a turbo. Should also be as open as possible. I have read on a turbo engine you can loose 10-20 percent by using a chambered muffler(flowmaster) over straight(dynomax) through. Same for turbo mufflers.

Chris
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Old 12-09-2003, 02:07 PM
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so if there is no performance gain by using bigger tailpipes than exhaust pipes (headers-mufflers), is there any performance LOSS? i mean pretty much everybody can agree that bigger diameter pipes make your motor sound better and lots of people prefer the look of big pipes coming out the back (me included) so is there any reason why it is NOT a good idea to use, say, 2 1/4" pipes all the way back to the mufflers and then use maybe 2 1/2" pipes for the tailpipes for the look and sound?? it should just flow very similar to a system thats 2 1/4" all the way through since that is the smallest pipe being used, correct??
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Old 12-09-2003, 02:33 PM
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i cannot for the life of me get the second page of posts on this to come up and i've tried 3 times so if i step on any toes please forgive me but smlblks10 you are right on the money and that is exactly what you will read 99 times out of 100. that is what my muffler shop told me to do and that is whats on my truck, which is build for LOW END TORQUE and they are THE shop around here to go to. i have 2" collectors from rams horns going into 2 and 1/4 pipe for 3 or four feet then 2 1/2 inch out thru the muff to the rear end.

turbo s10 is no liar by any stretch of anyones imagination and i have never seen him slip up before and it wasnt much if you just go by your first post the only thing you said wrong was that big pipes wont kill low end power and they most certainly do. my only proof i will offer up to you is last years popular hotroddings engine master challege, the winner, joe sherman won using 1 3/4 headers and he said afterwards, because of the close victory that he could have done better with 1 5/8's headers. but mind you they were limited to only go to like 6500 rpm with their motors

also, if there wasnt' some proof (via dynos) then heddman nor flowmaster would have enen come up with the ides of stepped headers. i know turbo you are up to your neck in turbos, and that may be a different world, i know they are putting 4 and 5 inch exhausts on turbo's mainly the deisel trucks, but for us sbc cats w/o the AFR heads, we should maintain some degree of backpressure via a normal sized exhaust.
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  #28 (permalink)  
Old 12-09-2003, 04:10 PM
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Quote:
Originally posted by bullheimer

turbo s10 is no liar by any stretch of anyones imagination and i have never seen him slip up before and it wasnt much if you just go by your first post the only thing you said wrong was that big pipes wont kill low end power and they most certainly do. my only proof i will offer up to you is last years popular hotroddings engine master challege, the winner, joe sherman won using 1 3/4 headers and he said afterwards, because of the close victory that he could have done better with 1 5/8's headers. but mind you they were limited to only go to like 6500 rpm with their motors

also, if there wasnt' some proof (via dynos) then heddman nor flowmaster would have enen come up with the ides of stepped headers. i know turbo you are up to your neck in turbos, and that may be a different world, i know they are putting 4 and 5 inch exhausts on turbo's mainly the deisel trucks, but for us sbc cats w/o the AFR heads, we should maintain some degree of backpressure via a normal sized exhaust.
Curious why Sherman did not use 1.625 in the first place. Probably trying to edge out the competition in the HP area. If an proven engine builder like that cant get the parts right at first, then the rest of us are hopeless.

The turbo stuff is certainly different. That is a different thread. If you read the other post, willys has some information showing that backpressure is always bad. Basically what you are seeing is that with larger pipes velocity is all but gone in the lower rpm range. It is not really backpressure, which is what begins to happen as the engine revs up. So yes, smaller pipes will produce more low end. It is my belief that the added power up top will net more power under the torque curve and result in a faster vehicle.

Later,

Chris
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Old 12-09-2003, 06:40 PM
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HEADER DESIGN VS. EXHAUST PIPE SIZE

Just about any engine I've ever dyno'd (and I would like to stress here that the proper jetting changes were made) will make more power with open headers.
I want to disspell the myth that "backpressure" is a good thing.
Header tuning, and header pipe size, and design can become very involved, and although the exhaust pipe and muffler are a part of the entire intake/exhaust tract, if the pipes and mufflers offer low restriction, then the header design and pipe size is going to dictate exhaust scavenging. I've tested alot of so called "equal length" headers and really found no power to be found vs. a header with a similar pipe size, but that had "unequal" length pipes.
If an engine makes more power with open exhaust, then it's going to make more with bigger exhaust pipes vs. smaller pipes, for the most part.
I really see no big advantage to "bellmouthing" your exhaust pipes except they might sound nice like the great big exhaust tips you can buy and weld on the ends.
Exhaust tuning is a science, but on a streeter V-8, I'd want the lowest restriction pipes I could use, coupled with the properly sized header pipes and collector. IE, 1-5/8 for mild small blocks, 1-3/4 for hotter small blocks and then larger for very, very hot , high horsepower applications and fat block engines.

I would like to add that extremely high RPM, high horsepower racing engines are very sensitive to header selection, and tube size. You can lose 50-75 horsepower in the wink of an eye with the wrong set of pipes on a 650-700 horsepower engine. Street engines and even hot street engines are more forgiving.
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  #30 (permalink)  
Old 12-09-2003, 06:49 PM
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open headers are too loud for street use to me which is why I welded the glasspacks right to the reducers...
seems like I am getting the flow of open headers without all the noise
todd
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