Pennock's Fiero Forum
  Technical Discussion & Questions - Archive
  Headers 101

T H I S   I S   A N   A R C H I V E D   T O P I C
  

Email This Page to Someone! | Printable Version


Headers 101 by FieroGTguy
Started on: 10-26-2003 02:27 AM
Replies: 8
Last post by: Doug Chase on 10-28-2003 04:20 PM
FieroGTguy
Member
Posts: 3084
From: Noblesville, IN
Registered: Mar 2001


Feedback score:    (12)
Leave feedback





Total ratings: 101
Rate this member

Report this Post10-26-2003 02:27 AM Click Here to See the Profile for FieroGTguyClick Here to visit FieroGTguy's HomePageClick Here to Email FieroGTguySend a Private Message to FieroGTguyDirect Link to This Post
I ran into this write-up at ssheaders.com, and I found it to be very informative. Just passing it out in hopes that you guys will find the same, or form an interesting discussion on the topic.

You have probably heard words like: back pressure, scavenging, tuned length, merged collector, rotational firing order, compatible combination and many others that meant something, but how they relate to a header may be a little vague. This article should give you a basic understanding of how a header works, what the terminology means, and how it plays a part in the header's performance gains.

The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance. Just the opposite is true. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.

The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s. If too small a diameter is used the engine will pull hard at low R.P.M.s but at some point in the higher R.P.M.s the tube will not be able to flow as much air as the engine is pumping out, and the engine will "sign off" early, not reaching its potential peak R.P.M. This situation would require going one size larger in tube diameter.

The second consideration is the proper tube length. The length directly controls the power band in the R.P.M. range. Longer tube lengths pull the torque down to a lower R.P.M. range. Shorter tubes move the power band up into a higher R.P.M. range. Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26" long. Engines that are torquers and Red Line at 5,500 R.P.M.s would need a tube length of 36". This is what is meant by the term "Tuned Length". The tube length is tuned to make the engine operate at a desired R.P.M. range.

The third consideration is the collector outlet diameter and extension length. This is where major differences occur between four cylinder engines and V-8 engines. The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. (Example: 1 3/4" primary tubes need a 2" collector outlet diameter.) The rule of thumb here is two tube sizes. This keeps the velocity fast to increase scavenging, especially at lower R.P.M.s. Going to a larger outlet diameter will hurt the midrange and low R.P.M. torque.

The amount of straight in the collector extension can move the engines torque up or down in the R.P.M. range. Longer extension length will pull the torque down into the midrange.

Engines that "Red Line" at 10,000 R.P.M. would only need 2" of straight between the collector and the megaphone. This is just enough length to straighten out the air flow before it enters the megaphone. This creates an orifice action that enhances exhaust velocity.

In the case of V-8 firing order, the five pulses fire alternately back and forth from left to right collector, giving the ideal 180 degree firing cycle. Then it fires two in succession into the left collector, then two in succession into the right collector. If the proper collector outlet diameter is being used (two sizes larger than primaries) the two pulses in succession load up the collector with more air than it can flow. This results in a very strong midrange torque, but causes the engine to "sign off" early, not reaching its potential peek R.P.M. The improper firing order on a V-8 engine results in the need to use large diameter collectors so the engine will perform well at high R.P.M.s. Unfortunately the large diameter collectors cause a tremendous drop in air velocity, resulting in less scavenging through the entire R.P.M. range.

Often cams are used with extended valve timing to help the exhaust cycling. This results in valve timing overlap (Intake and Exhaust valves both open at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this happens, exhaust actually backs up into the cylinder causing intake air to be pushed back out the intake. This reversion causes "Standoff" (fuel blowing out of the Intake) at low R.P.M.s. This whole improper cycling has resulted in a number of "Cure Alls" to help stop this reversion and standoff.

The plentum intake was created to stop the fuel "Standoff". Then came "Anti Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the header, extended collector lengths and even plentums in the exhaust tubes.

In this chain of events beginning with improper firing order, a series of cures has developed, each one causing a new problem.

The optimum cure to this whole problem is to correct the exhaust firing cycle. The two cylinders that fire in succession into each collector have to be separated. This can be done partially by a "Tri-Y" header, where the four primary tubes from each bank merge into two secondary tubes (separating the two pulses firing in succession) and finally collect into a single collector. This type of header helps, but the two pulses are still coming back together at the collector.

The second optimum cure is to cross the two center tubes from each bank, across the engine running them into the collector on the opposite side. This makes the firing cycle in each collector 180 degrees apart, the same as a four cylinder engine. Once this firing order is achieved, the small collector outlet diameter can be used and the "High Velocity Scavenging" at low R.P.M.s cures the reversion problems and eliminates the need for extreme cam duration.

This sounds so easy, you are probably asking why wasn't this done from the start?

If you have ever seen a set of 180 degree headers you would understand.

On today's cars, with space virtually nonexistent, crossing four tubes either under the oil pan or around the front or rear of the engine presents major problems. On racing applications where it is possible, there is still the problem of keeping the tube length down to a reasonable 32" long. If that's not enough challenge, then try to arrange the tubes into each collector so they fire in a "Rotational Firing" pattern. Then you have, what has been called "A Bundle of Snakes".

Arranging the tubes to fire rotationally adds to the scavenging capabilities. The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more suction on that tube than it would on a tube on the opposite side of the collector.

The next problem is "Turbulence" in the collector. When four round tubes are grouped together in a square pattern, so a collector can be attached, you notice a gapping hole in the center of the four tubes. The standard method in manufacturing headers is to cap this hole off with a square plate. This plate in the center of the four tubes creates dead air space, or turbulence, disrupting the high velocity in the collector. This problem is solved by using a "Merge Collector". This collector is formed from four tubes, cut at approximately an 8 degree angle on two sides. When the tubes are all fitted together they form a collector with a "Pyramid" in the center. This has eliminated the need for the square plate and has taken up some of the volume inside the collector, speeding up the air velocity.

Other methods of curing this problem are: fabricating a pyramid out of sheet metal and welding it over the hole between the tubes, or squaring the tubes on two sides so they fit together forming a "+" weld in the center eliminating the hole all together.

You can see that there are a great many factors that go into making a good header. When the header, intake system, and cam timing are all designed to operate to their maximum in the same R.P.M. range, then you have a "Compatible Combination". This combination can be tuned to deliver maximum power at any desired R.P.M. range.

These are some of the "Basics" you need to know about building a good high performance header. There are many other adjustments that can be made to fine tune a header, but this should give you a basic understanding of how all the components work together.

Hope you enjoyed,
Greg

IP: Logged
PFF
System Bot
vortecfiero
Member
Posts: 996
From: Toronto Area, Canada
Registered: Feb 2002


Feedback score: (1)
Leave feedback





Total ratings: 57
Rate this member

Report this Post10-26-2003 09:04 AM Click Here to See the Profile for vortecfieroClick Here to visit vortecfiero's HomePageClick Here to Email vortecfieroSend a Private Message to vortecfieroDirect Link to This Post
Hedders by Ed pretty much follows the same thinking...
and it works...
a really good example of this is the early GT40 and the
"bundle'o'snakes" tuned exhaust manifolds

the only thing ya left out is the collector effect/scavenging effect
can't happen if there is an exhaust system attached to the collecor
in effect it lengthens the collector and adds the dreaded back pressure

what hedders will do, with an exhaust system attached, is freeup some
restricted velocity [martha stewart voice]and that is a good thing[/martha stewart voice]

------------------
84 Fiero Turbo Vortec 4300 Phantom GT
L35 block, Syclone Intake and ECM T04B H3 Turbo
www.cardomain.com/id/vortecfiero
Murphy's Constant Matter will be damaged in direct proportion to its value
Murphy's Law of Thermodynamics Things get worse under pressure.
Arthur C. Clarke "Any significantly advanced technology is indistinguishable from magic"

IP: Logged
FieRoy
No longer registered
Report this Post10-26-2003 10:12 AM Click Here to See the Profile for FieRoyClick Here to Email FieRoySend a Private Message to FieRoyDirect Link to This Post
hello,

thanks very much for this text! It will be a great help, by designing and making a new type of equal-length (!) header, for the fiero. (im doing so, at the moment.)

FieRoy

IP: Logged
Beene
Member
Posts: 225
From: Calgary, Alberta, Canada
Registered: Apr 2003


Feedback score: N/A
Leave feedback

Rate this member

Report this Post10-27-2003 12:11 AM Click Here to See the Profile for BeeneClick Here to Email BeeneSend a Private Message to BeeneDirect Link to This Post
I heard something interesting about inducing scavenging in your exaust system. Start by paintint your whole exaust system, then run it from idle to WOT, shut the car off and coast to the side. Look at the exaust and find "hot spot" where the paint bubbles. Cut off the exaust about 1" before this and when the exaust leaves the car this will cause the vacume effect that draws in the next fuel charge. I haven't tried this yet but sounds interesting to me
IP: Logged
THE BEAST
Member
Posts: 1177
From: PORT SAINT LUCIE,FLORIDA,USA
Registered: Dec 2000


Feedback score: N/A
Leave feedback

Rate this member

Report this Post10-27-2003 04:55 PM Click Here to See the Profile for THE BEASTClick Here to Email THE BEASTSend a Private Message to THE BEASTDirect Link to This Post
Beene,
If you don't mind could you explain yourself, please?

"Cut off the exhaust about 1" before this and when the exhaust leaves the car this will cause the vacuum effect that draws in the next fuel charge"

I don't get this part? After cutting off one inch before the "hot spot" you weld it together or not? And how? This will cause a vacuum effect?

Anyone?

IP: Logged
Will
Member
Posts: 13417
From: Where you least expect me
Registered: Jun 2000


Feedback score: (1)
Leave feedback





Total ratings: 234
Rate this member

Report this Post10-28-2003 10:49 AM Click Here to See the Profile for WillClick Here to Email WillSend a Private Message to WillDirect Link to This Post
I read everything I could get from Headers by Ed... and still have questions. They say on their website that they'll soon be posting information about 180 degree headers and Tri-Y headers, but nothing is forthcoming on the website. The information in their header design packet about Tri-Y headers is worthless, and the packet says absolutely nothing about 180 degree headers.

------------------
'87 Fiero GT: Northstar, Getrag, TGP wheels, rear sway bar, rod end links, bushings, etc.
'90 Pontiac 6000 SE AWD: Leaking ABS unit fixed, load levelling rear suspension fixed, still slow

IP: Logged
THE BEAST
Member
Posts: 1177
From: PORT SAINT LUCIE,FLORIDA,USA
Registered: Dec 2000


Feedback score: N/A
Leave feedback

Rate this member

Report this Post10-28-2003 11:22 AM Click Here to See the Profile for THE BEASTClick Here to Email THE BEASTSend a Private Message to THE BEASTDirect Link to This Post
 
quote
Originally posted by Beene:
Cut off the exaust about 1" before this and when the exaust leaves the car this will cause the vacume effect

How?

IP: Logged
DPWood
Member
Posts: 540
From: Aylmer, Ont. Canada
Registered: May 2002


Feedback score: N/A
Leave feedback

Rate this member

Report this Post10-28-2003 12:27 PM Click Here to See the Profile for DPWoodClick Here to visit DPWood's HomePageClick Here to Email DPWoodSend a Private Message to DPWoodDirect Link to This Post
Good info!!

There's also a write up in the Dec. Car Craft on the same subject. Although, it didn't have quite as much detail. One thing it did give was a quick formula for matching the headers to the objective.

Peak torque RPM = Primary pipe area X 88,200 / displacement of one cylinder.

With this calculation it works out as follows for a 350.
Pipe calculated torque peak
1 5/8 4173
1 3/4 4415
1 7/8 5100

Not as accurate as a computer set up one of the header companies would use but it gives you a ballpark figure for planning.

David

------------------
His Fiero: 1984 2M4 Coupe

Her Fiero: 1984 2M4 Convertible

IP: Logged
Doug Chase
Member
Posts: 1487
From: Seattle area, Washington State, USA
Registered: Sep 2001


Feedback score: N/A
Leave feedback





Total ratings: 88
Rate this member

Report this Post10-28-2003 04:20 PM Click Here to See the Profile for Doug ChaseSend a Private Message to Doug ChaseDirect Link to This Post
 
quote
Originally posted by THE BEAST:

How?

I'd probably use a hacksaw.

Doug

IP: Logged



All times are ET (US)

T H I S   I S   A N   A R C H I V E D   T O P I C
  

Contact Us | Back To Main Page

Advertizing on PFF | Fiero Parts Vendors
PFF Merchandise | Fiero Gallery | Ogre's Cave
Real-Time Chat | Fiero Related Auctions on eBay



Copyright (c) 1999, C. Pennock