[TurboS10]

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:boxing:

Chris

 

[killerformula]

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

 

[2DoorCaprice]

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.

 

[willys36@aol.com]

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.

 

[TurboS10]

Re: weight matters, so does driving in traffic

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

 

[smlblcks10]

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??

 

[bullheimer]

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.

 

[TurboS10]

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

 

[NAIRB]

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.

 

[toddtheodd]

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

 

[Dubz]

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.

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).

 

[ShamrockCustoms]

for anyone seeking some dyno results.......



http://www.boyleworks.com/ta400/psp/exhaust2.html

 

[TurboS10]

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

 

[adryan16]

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...

 

[4 Jaw Chuck]

Re: 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.

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?!

 

[bullheimer]

[/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.ain:

 

[4 Jaw Chuck]

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.

 

[smlblcks10]

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.

 

[willys36@aol.com]

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.

 

[TurboS10]

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. ain:

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