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180 Degree Header Design

  1. Apr 29, 2009 #1
    Let me begin by saying a I did not complete ME degree so please take it easy on me.

    With that said, here is my dilemma. I want to make some 180 degree headers for my car. 1965 Mustang road race car if anyone cares. I’m having a hard time deciding which cylinders are the 180 degree pairs.

    Motor: Small Block Ford 302 5.0 HO
    Firing Order: 1-3-7-2-6-5-4-8
    Cylinder Configuration:
    Front of Motor
    5 1
    6 2
    7 3
    8 4

    Someone told me, and this seems to be the accepted idea by many, that if I were to take the first 4 cylinders in the firing order and place them on top of the next 4 cylinders, those would be my pairs. Which would result in pairs:
    #1 w/ #6
    #3 w/ #5
    #7 w/ #4
    #2 w/ #8

    I was thinking that a complete cycle of the firing order would be 2 turns of the crankshaft: 720 degrees. Each cylinder would then fire 90 degrees apart:

    1-3-7-2-6-5-4-8 with 90 degrees in between each cylinder. So I would say the 180 degree pairs then would be:
    #1 w/ #7
    #3 w/ #2
    #6 w/ #4
    #5 w/ #8

    Which of these 180 degree pairs would seem correct to you?

    I have many more questions that I will post later, but I would like to establish the 180 degree pairs first.

    Thanks in advance for any input you guys may offer.

  2. jcsd
  3. Apr 30, 2009 #2

    Ranger Mike

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    couple of questions..are you racing in SCCA?
    we ran 180 degree headers and it was a mess. to get the 180 degree working correctly the primary tubes had to be routed over the valve covers and both secondary dumped out the right side of the car. my question about sanctioning bodies is because I do not think there is enough room under the stock mustang hood to run this configuration...our super late model stock car had a Howe Chassis and the engine was set super low in the chassis..
    if you want.I have some header tricks to amp up the effectiveness on " stock" type header packages..
    Last edited: Apr 30, 2009
  4. Apr 30, 2009 #3

    Ranger Mike

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    If you dont know what 180 degree headers are, instead of all 4 of one side going to a collector, the cylinders that are 180 degrees apart in their firing are paired together, maximizing scavenging, the bad part is its a nightmare to plumb it.While one is on the compression stroke, the other is on the exhaust stroke so that theres a constant flow, helping to smooth out the pulses.
    180 degree headers pair up the cylinders that are 180 degrees apart. You still have two collectors, but some of the pipes for each collector come from both sides of the motor. Common in dirt track modified cars. Plumbing is a pain because half the tubes have to cross over the motor somewhere. The modifieds are tube framed and plastic or sheet metal bodied so have more freedom for pipe routing.

    To do a 180 degree header setup on a standard V8 (90 degree crank) you will have to have 2 pipes from each side of the engine cross over to the other side. It will sound like a Ferrari engine too. Why? The Ferrari engines use flat plane cranks so they essentially run like two four cylinders in the same engine which means if you connect all of the cylinders of bank to each other and do the same on the other side they'll automatically be separated by 180 degrees as a flat plane V8 fires L-R-L-R-L-R-L-R (that's left and right) (basically, you're getting even pulses of the exhaust through the exhaust system).

    The exhaust pulses of a V8 with its firing order of 1-8-7-3-6-5-4-2 are like so:

    R-L-R-R - L-R-L-L (R=right, L=left, these are the banks)

    Notice two cylinders from the same bank fire right after the other, this is why the cylinders must crossover to have 180 degree separation.

    This is the cylinder location:

    F/R 1-3-5-7 back/right/passenger
    F/L 2-4-6-8 back/left/driver

    An easy way to see how to separate them by 180 degrees is to put the first 4 firing over the last four like so:


    1 and 6 connect, 8 and 5 connect, 7 and 4 connect, and finally 3 and 2 connect. This gives you the 180 degree ignition separation. Still confused? Here's a visual of how it works. Say the V8 had a distributor, it would hit the contacts of each cylinder like this:

    see attached jpeg below

    Notice the points that are exactly 180 degrees apart from firing... this gets the connecting cylinders listed above.
    Last edited: Jul 24, 2009
  5. Apr 30, 2009 #4
    Hello Mike,
    To deal with the clearance/room issue I was thinking of stealing an idea from tri-y headers. I was planning on merging the two tubes that stay and merging the two tubes that cross similar to what is done on tri-y headers. I would go from 1 5/8" primaries, merged into a 2" pipe which would then cross over under the motor or trans depending on where there is room and that 2" pipe would then merge with the 2" pipe already present from the two tubes already there. It would involve a total of 6 y-pipes.

    My issue with determining 180 degree pairs with the method you mentioned is that a complete cycle of the firing order is really 2 complete turns of the crank = 720 degrees but only 1 turn of the cam = 360 degrees. I went to an old ME professor yesterday but he was out of his office unfortunately.

    Since the crank has to turn twice for all cylinders to fire, wouldn’t each cylinder then fire 90 degrees apart from each other? So then every other cylinder in the firing order would fire 180 degrees apart from the previous one.

    Is the measurement 180 degree separation in the crank or the cam?

    My issue with determining 180 degree pairs with the method you mentioned is that a complete cycle of the firing order is really 2 complete turns of the crank = 720 degrees but only 1 turn of the cam = 360 degrees. I went to an old ME professor yesterday but he was out of his office unfortunately.
  6. Apr 30, 2009 #5

    Ranger Mike

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    each cylinder is firing 90 degrees apart..that is why you got 8 cylinders firing 90 deg for 720 degrees..the idea of 180 hds is to have the slug lighting off in series with the next cyl to fire so you got one constant pulse of exhaust scavenging out the spent fuel/air mix..minor hp improvement but is pretty efficient

    for all around convenience and being able t work on the engine..I would go with TRI -Y headers..180 degree cross overs are expensive and a major pain to cobble up..ifin you are not a pro welder and tube bender..chances are the small HP gain of the 180 degree hdrs will be negated by restrictions in your hand built system..you need gentle radius bends to keep up exhaust velocity..I would go with stainless steel tri y headers ..this ssteel keeps the ex temp up and helps the HP...
    plus commercially available from many sources..the 180s are trick but..you know as well as I that wrenching on the car at the track is a fact of life..and the easier it is to get to the valve covers, intake etc.. the better
    but hey..at last u r racing and turning right too!
    get a checker!
  7. May 1, 2009 #6
    I looked the tri-y headers over but it seemed to me that they pair the wrong cylinders together for my motor.

    With my cylinder configuration, I figured I would need the first and last cylinder of one side merged and the two middle cylinders of that same side merged. The tri-y headers merge first and second, then third and fourth.

    http://www.mustangsunlimited.com/ProdImages/652804-C.jpg [Broken]

    I was thinking of going off the tri-y design and making my own by modifying my existing headers and merging the cylinders needed, then only having one crossover tube from each side. 1 5/8" primaries merged to a 2" tube that then would merege to a final size 2.5" tube.

    Sorry for the crappy MS Paint rendering.

    Attached Files:

    • 180.JPG
      File size:
      12.6 KB
    Last edited by a moderator: May 4, 2017
  8. May 1, 2009 #7

    Ranger Mike

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    Cross over tube is a tube connecting the collectors or exhaust pipe right after the collectors. it smooths out engine pulses.

    Tri y headers are good for mid range HP and torque..if you are twisting the mill to hi end more than hanging out on the mid range ...go with 4 in 1 headers that are top end oriented....if you run a dual plane intake manifold and dual exhaust w/o cross over tube the engine thinks it has two 4 cylinder engines tied to the crank
    ifin you have an open plenum intake and cross over tube you have smoother running engine as pulses are smoothed out..most road course tracks we run is 4 to 7000 rpm situation..we have Hewland Trans axle so gear changes are super easy..I don't know your gearing but it is one huge consideration..ifin you are dual purpose and running the Stang on the street too
    go with 4 -1 headers and cross over tube and single plane ( open plenum ) intake..

    IMO ..I been there on the cobble up a header drill..no way dude..I would buy a good set of headers from Doug Thorley, Hooker, Jere Stahl..you want min headaches at install max durability...you don't need a pin hole popping up at race event and melting a spark plug wire..it happened to us..you got 1200 degrees F at the head/header and up to 400 deg F at exit...so with stainless steel ..it will give you more HP than the 180 degree design your are thinking about..but hey..its yer racer..go for it..
    Last edited: May 1, 2009
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