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  #31 (permalink)  
Old 12-09-2003, 06:42 PM
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So to recap then....

backpressure is not good no matter what...

smaller tubes will make more torque DOWN low
bigger pipes will make more horsepower up top (may even make more power under the curve but we weren't talking about that it was power down low)

The common thought that backpressure or the greater restriction of smaller tubes creates the low-end torque is untrue, it is in fact related to exhaust velocity of the smaller tubes that creates the low down torque.

There is such a thing as using TOO big of pipes on a motor (for streeting) but it will not be as horrible for power loss as using too small of pipes for the motor. As in everything threre is a balance which provides the best option (for a given setup)

we also have 2 schools of thought, one that is the rodders that street their cars most of the time, and others that sacrifice streetability for performance at the track. One set perfers power on the low end, one perfers power up top, although this shouldn't matter for an issue of "Will big pipes kill power on the low end" it appears to.

Quote:
Originally posted by TurboS10
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.
would this not depend on the paint that was used as well?? i had heard of using tape, but thought the same thing, some won't burn at the same temp as others. Would it not be as exact a science on a street motor to make a difference as on a race motor (which is what i would think this is more for).

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  #32 (permalink)  
Old 12-09-2003, 06:59 PM
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for anyone seeking some dyno results.......



http://www.boyleworks.com/ta400/psp/exhaust2.html
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  #33 (permalink)  
Old 12-09-2003, 09:49 PM
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I like the summation Dubz

I am sure the paint will vary, but I always use a cheapo low temp rattle can. It is usually very evident where the burn stops. Just take it for a spin and a WOT run and you will see.

NIARB, thank you for varifying all of this bickering with some hard facts. It is great to have someone on the board with actual dyno experience and not that of a magazine.

Later,

Chris
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  #34 (permalink)  
Old 12-09-2003, 10:39 PM
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I got wondering about the paint as well. I wondered if doing the paint thing with 3-4 different types/brands of paint would give you different results, and that maybe the average length would give your best results.

The whole crossover tube aiding in scavenging is obviously dependent on exhaust temp. Is there an optimum temp for this to occur, assuming that you're at an optimum air/fuel ratio(ie not too rich, not too lean) to begin with? If so, couldn't you just use one of those laser-pointer style digital temp gauges to check tube temps to determine what the best place was for the crossover tube to most benefit from the scavenging effects that the crossover proposes? (also assuming that there was little to no temp difference between the tube and the exhaust temp)

Just wondering...
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  #35 (permalink)  
Old 12-09-2003, 11:29 PM
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Re: HEADER DESIGN VS. EXHAUST PIPE SIZE

Quote:
Originally posted by NAIRB
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.


Here, here Nairb!

Ever wonder why they never do two or three dynos runs back to back to check their numbers for consistency? Because many times the dyno gains they are claiming can be eliminated by the tolerance or accuracy of the measuring equipment. Not many dynos around that can repeat within 5%. +- 10 Hp is not uncommon on a 300Hp engine. I like to say if you don't like the measuring instruments results, repeat the testing until you get a reading you like! Maybe in the morning when the air is cool and the engine has cooled down overnight, pick up 20HP!?

Simple physics dictate that a positive valved engine will gain nothing from fancy exhaust tuning if there is virtually no valve overlap (ie. stock engine or cam duration). Flow increases (ie larger pipe) will aid in power production (up to a point) but fooling around with Tri-Y headers with steps and bellmouthed diffusers and ceramic coated flapper valves is a waste of time and money for low overlap cammed engines. You are moving a fluid and reflected waves mean squat when the valve openings don't overlap...where is the reflected wave supposed to go? Wait around and sneak in there when the valve opens next?

Now if you want to talk about long duration cams with significant overlaps (ie. over 280 duration or so) then you start being able to manipulate a reflected wave to increase flow by timing it's reflection at the port exit. Now you know why they use log manifolds from the factory...size is all that matters and fancy tube pipes that flow exactly the same as a comparible log type make virtually no difference in engine output with cam profiles that have no overlap.

Note: The reason why expansion chambers like those used on two stroke engines are never used on a four stroke is because you don't want the reflected wave to ever be strong enough to stagnate flow at the valve...thats the valves job. Basically an expansion pipe would negate your valves function and interfere with flow at certain resonant frequencies. It has been tried.

The ideal 4 stroke exhaust system would have adjustable length primary tubes, tapered in profile from engine to tailpipe coupled to a variable volume collector/reflector constructed in such a way that gas cooling would not cause a reduction in flow speed along the tubes length. Tube diameter would have to adjust constantly too so that higher rpm would not increase the average gas speed through the tubing. Tuning the pipe by size determines when the pipe system achieves peak flow efficiency, it is also affected by temperature and length. If you don't have that...open headers will give you the best readings.

Best practice for low overlap engines to broaden the torque curve under the HP limit is to run larger exhaust tubing close to the engine and go down a size after the muffler to maintain gas speed after the cooling effect of the muffler.

As a matter of fact that is exactly how stock exhaust systems are made. Amazing huh?!
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  #36 (permalink)  
Old 12-10-2003, 12:02 AM
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Quote:
Originally posted by Dubz
[/B
The common thought that backpressure or the greater restriction of smaller tubes creates the low-end torque is untrue, it is in fact related to exhaust velocity of the smaller tubes that creates the low down torque.
[/B]
is this quote up there?
that's what i shoulda said but didn't.

well, fine, if nairb and 4JC are gonna gang up on me then i guess i better just shut the hell up.
you are gonna have me crawling under my car now checking to see if the pipes get smaller at the end. i have just never heard that before (didnt turbos10 say the same thing?) so i am having a hard time believing it. i read that whole "sultan of stainless" article on making exhausts in phr, and other than the spray paint trick being the best place to put a cross over, must have been concentrating too hard on squeezing one out.
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  #37 (permalink)  
Old 12-10-2003, 01:14 AM
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Sorry if it was harsh, just trying to write down my experience in as simple a form as possible. Most of the stuff floating around today is put out by the manufacturers and involves more than a little bias. I read a great book called "Two Stroke Tuners Handbook" many years ago. If you have a copy I would be interested in buying it, very good book explained in laymens terms. Covers 4 stroke in relation to 2 stroke very well and explains flow theory so you can understand it. Got two boys who need to learn it as well.

Two Stroke Tuners Handbook.
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  #38 (permalink)  
Old 12-10-2003, 01:27 AM
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so its better to go from bigger pipes from the headers-muffler area and smaller tailpipes??? is there any performance difference in doing it the other way around?? i just thought if there is no performance DECREASE with using bigger tailpipes than actual exhaust pipes to get the bigger look and sound then why not do just that??? for example, what would be the performance differences between, say, a system with 2 1/4 duals all the way through, one with 2 1/4 duals to the mufflers then 2 1/2 tailpipes, or a full 2 1/2 dual system???? basically all im wanting to know is if i can use 2 1/2" tailpies on my current full 2 1/4 system just for the bigger sound and look but NOT have any decrease in performance.
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  #39 (permalink)  
Old 12-10-2003, 08:06 AM
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4Jaw - I did my senior project @ Fresno State on designing intake and exhaust manifolds for a Briggs & Stratton 5hp one lunger. Spent most of a semester on the dyno comparing port pressure to hp readings and cutting and welding conduit. For my 3000rpm peak power design point, organ pipe theory gave me a consistent +5% power boost with an the exhaust header. That boost corresponded to port pressure at valve closing - the lower the pressure, the higher the power. This engine has little or no valve overlap.

The intake design was much less dramatic, only a 1% increase but that was due to B&S's 'easy start' cam grind - they keep the intake valve open slightly for several degrees on the compression stroke to lessen the pull on the starter chord.

The organ pipe equation gave me optimal pipe length to within an inch or so. I finally attributed the error to inaccuracy in measurement of the speed of sound in the exhaust gas.
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  #40 (permalink)  
Old 12-10-2003, 08:14 AM
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Quote:
Originally posted by bullheimer

you are gonna have me crawling under my car now checking to see if the pipes get smaller at the end. i have just never heard that before (didnt turbos10 say the same thing?) so i am having a hard time believing it.
I think I started off with the "large pipe will not kill low end torque" statement, which was not totally accurate. But, NIARB and Chuck have said better what I was trying to explain. Alot of my problem is not being able to explain why I know something. It seems in my learning process I kindof skip over the details and remember the conculusion. Not good when trying to dispell widely accepted misconceptions....

Chris
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  #41 (permalink)  
Old 12-10-2003, 09:18 AM
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Quote from 4Jaw

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As a matter of fact that is exactly how stock exhaust systems are made. Amazing huh?

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Quote from Troy-Curt

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Most OEM stuff comes with smaller tailpipe than the header pipe.

I have always used the same size all the way , with good results.

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I wonder how many millions the manufactures spent to figure this out.
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  #42 (permalink)  
Old 12-10-2003, 11:07 AM
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OEM systems are designed to have as flat a torque curve as possible so mom & pop won't experience any disturbing power surges. Power is a secondary consideration to quietness and smoothness. Goal is to eliminate any indication that the car even has an engine. The more expensive the car, the closer they come to this ideal. Thus a header log and small exhaust system is desirable since that guarantees no 'tuning' effects form the exhaust system.

There are exceptions like sports cars, pickups and muscle cars which compromise quiet and smooth for more power, thus bigger exhausts and sometimes even factory 'tuned' headers.
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  #43 (permalink)  
Old 12-10-2003, 12:16 PM
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Another point of oem's is that they are looking for ease of manufactor and fitting. Large exhaust pipe and tubular headers are most always harder to fit and make. Cast manifolds take much less time and money to make than tubular headers. They are also more durable and stand the test of time much better unless we start talking heavy stainless tuning or something at which time costs really go up. Then there is the whole sealing aspect of headers that I would not think oem's would want to touch.

Chris
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  #44 (permalink)  
Old 12-10-2003, 12:58 PM
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can anybody take a stab at my post afew posts back?? sorry about the impatience haha, im just really curious. thanks
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  #45 (permalink)  
Old 12-10-2003, 02:32 PM
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I dont think you will see a real difference one way or the other. If you read through everything on the post, I think you will gather that further back is less crucial than up front. 2.5 inch pipes should not give you any problems IMO, but you will not know untill you try it.

Chris
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