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Discussion Starter · #1 ·
I have been reading up on exhaust pulse and exhaust header building in general.

One thing that keeps nagging at me is that the primaries are all the same diameter on 180 degree headers that I keep finding. As you can imagine most of them are custom. Yet, they all seem to have the same diameter primaries throughout.
It seems that a smaller diameter tubing would be used on the longer primaries to keep the pulse correct and avoid having the air become dense over/under/behind the block.

In my case this applies to 180 degree headers. But the idea of different diameter primaries could also apply to inline 8's or a 702 setup where the distance between the pulses is great enough that a smaller diameter tubing in these areas would be beneficial. But, once again any kind of header I see is set up with the same diameter primary.

Is there a reason why no one (it seems) is using different diameter primaries?
 

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what do you mean,I dont quite get it?
My primaries start at 1 7/8 " diameter then step up to 2 ". Or are you talking a tapered pipe like what is used on a 2 stroke engine?expansion chamber
 

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I have been reading up on exhaust pulse and exhaust header building in general.

One thing that keeps nagging at me is that the primaries are all the same diameter on 180 degree headers that I keep finding. As you can imagine most of them are custom. Yet, they all seem to have the same diameter primaries throughout.
It seems that a smaller diameter tubing would be used on the longer primaries to keep the pulse correct and avoid having the air become dense over/under/behind the block.

In my case this applies to 180 degree headers. But the idea of different diameter primaries could also apply to inline 8's or a 702 setup where the distance between the pulses is great enough that a smaller diameter tubing in these areas would be beneficial. But, once again any kind of header I see is set up with the same diameter primary.

Is there a reason why no one (it seems) is using different diameter primaries?
Surprisingly dyno testing has shown that pulse timing within the header is not so sensitive to individual pipe length as equations and theory would lead you to think. The result of this is that headers tend to have their pipes adjusted first to fit the chassis which results in slightly different lengths of tube cylinder to cylinder with no apparent deficiency in performance.

That is not to say that shorty headers will perform the same as long tube headers, it does say that getting the tubes within a couple, three inches of each other's length is sufficient.

Bogie
 

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Surprisingly dyno testing has shown that pulse timing within the header is not so sensitive to individual pipe length as equations and theory would lead you to think. The result of this is that headers tend to have their pipes adjusted first to fit the chassis which results in slightly different lengths of tube cylinder to cylinder with no apparent deficiency in performance.

That is not to say that shorty headers will perform the same as long tube headers, it does say that getting the tubes within a couple, three inches of each other's length is sufficient.

Bogie
related to this discussion, have any good reading on collector design/impact?
 

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related to this discussion, have any good reading on collector design/impact?
I find that collectors are a cut and try ordeal. Sometimes you get really good results with adding a foot to a foot and a half to the short collector on the header. Remember when headers first became popular bolt on’s in the 1960s they all had long to longer collectors. Then they became shorter and shorter, sometimes this leaves me wondering if that was done for engine or fiscal performance. Generally they do enhance the top end almost ipso-facto but getting under the torque peak RPMs they can be unpredictable.

I know David Vizard likes long collectors and merge collectors which are a whole new adventure. Both concepts hold places of honor in his new books. My experience says like carb spacers you need to try them out and see what you get.

At least plain old collector extensions aren't so expensive that only Cup teams have the money to play with them rather you can too. But merge collectors are pretty costly to where the average guy just can't buy a couple, three sets to see if or which work for him.

Bogie
 

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Rather than extensions I was talking about the internal and external shape and transitions. I've seen the center of the 4 primaries with flat and "pointed centers" and like you said, the merge collectors start to play with several variables.
 

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Rather than extensions I was talking about the internal and external shape and transitions. I've seen the center of the 4 primaries with flat and "pointed centers" and like you said, the merge collectors start to play with several variables.
If you want to see something wild, look into the early Mickey Thompson "Scavenger" headers and how the primary pipes are in the collector. :D
 

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I remember the Ramchargers painting the collectors with white paint and then cutting them off at the point where the paint stopped blistering. Voodoo?
And what happens when you change the paint , or the ignition timing or the fuel or the airflow under the car etc. etc.

Temp does not transmit the same as the gas pulses, which I'm sure you of all people know.
 

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I remember the Ramchargers painting the collectors with white paint and then cutting them off at the point where the paint stopped blistering. Voodoo?
I never found this to work.....maybe it did back then with whatever cheap spraybomb lacquer was available back then......but it don't work with modern day lacquer or enamel.
 

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Discussion Starter · #14 ·
Short or long header in order to make number 1 primary header match number 7 you need to lengthen the header.

Typically this is not an issue but in longer engines such as a inline 8 or a twin 6 v12 702 the engine is long enough to begin with. Lets take the 702 because it is a good example of the potential for better scavenging and power.

The twin 12 has a firing order of 1-4-9-8-5-2-11-10-3-6-7-12. breaking that down to sides you have

4 8 2 10 6 12

1 9 5 11 3 7

In stock form the twin 6 uses 4 manifolds. What this means is that once number 1 fires that gas charge has to wait until number 5 fires. This is because number 3 is so far back any scavenging that could possibly take place is killed by the diameter of the pipe.

Now if you were to use typical headers linking the 6 cylinders together on each side of the block you need to wait till 2 fires. But for that to happen the pulse needs to be pulled from whatever cross over you have back to number one meaning the scavenging of number 1 is once again killed.

That hot air is pulled back into the cylinder and leads to heat. The exhaust that should be leaving hot now becomes more dense.

On the topic of equal length falls back to lengthening the headers and you simply can not do this. The alternative is to have number 1 being a smaller diameter then number 3. You are making the primaries equal. You are not doing it with length but with the area that the exhaust travels. This leads to the collector being closer to the block which allows number 2 to pull some of the exhaust off number 1.

The 4 manifold approach turns out to be a good system over full length headers because while you are waiting on 3 the rear manifold output could merge with the first moving the scavenging effect closer to number 1 then with full length headers where the scavenging effect takes place at the collector.

The same thing could be said for a inline 8 running full length headers to a lesser effect. By using a smaller diameter pipe in number 1 you are matching the area of number 8 over the longer distance.

Would this be more restrictive?
It should not be because the scavenging effect is increased and the air that would become more dense is getting to the collector leading to gains.



Now lets flip the smaller diameter thing and try for better packaging. Lets say you are currently have your engine set up to run long headers. But for whatever reason running long headers is not a option. You could keep your number 1 primary 1 3/4 for example but increase the diameter on the next fired cylinder on the header so that that primary is actually shorter in length but is the same in area.








Now, I never said I know anything about exhaust science. I am just interested in it right now and this simple question of why people don't run different diameters keeps running through my head.
 

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exhaust tuning.

I don't remember the year , but the SAE journal (Society of Automotive Engineers), in the late 40's or early 50's had a very long review of the Jaguar racing engines and the Exhaust and Intake tuning that resulted in 105 % volumetric efficiency. For maximum HP the intake and exhaust have to work together.In the Early 60's a guy had his tri-power Ford 390 Fe on the chassis dyno at the college engineering lab, It was late in the afternoon on the west side and with the sun shinning in the windows we could see the "cloud" about a foot above the carbs,
some of the race car drivers could see the cloud and that's when they started using long fuel injection stacks to "capture" what they thought was fuel bouncing back up the intake.
 

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Short or long header in order to make number 1 primary header match number 7 you need to lengthen the header.

Typically this is not an issue but in longer engines such as a inline 8 or a twin 6 v12 702 the engine is long enough to begin with. Lets take the 702 because it is a good example of the potential for better scavenging and power.

The twin 12 has a firing order of 1-4-9-8-5-2-11-10-3-6-7-12. breaking that down to sides you have

4 8 2 10 6 12

1 9 5 11 3 7

In stock form the twin 6 uses 4 manifolds. What this means is that once number 1 fires that gas charge has to wait until number 5 fires. This is because number 3 is so far back any scavenging that could possibly take place is killed by the diameter of the pipe.

Now if you were to use typical headers linking the 6 cylinders together on each side of the block you need to wait till 2 fires. But for that to happen the pulse needs to be pulled from whatever cross over you have back to number one meaning the scavenging of number 1 is once again killed.

That hot air is pulled back into the cylinder and leads to heat. The exhaust that should be leaving hot now becomes more dense.

On the topic of equal length falls back to lengthening the headers and you simply can not do this. The alternative is to have number 1 being a smaller diameter then number 3. You are making the primaries equal. You are not doing it with length but with the area that the exhaust travels. This leads to the collector being closer to the block which allows number 2 to pull some of the exhaust off number 1.

The 4 manifold approach turns out to be a good system over full length headers because while you are waiting on 3 the rear manifold output could merge with the first moving the scavenging effect closer to number 1 then with full length headers where the scavenging effect takes place at the collector.

The same thing could be said for a inline 8 running full length headers to a lesser effect. By using a smaller diameter pipe in number 1 you are matching the area of number 8 over the longer distance.

Would this be more restrictive?
It should not be because the scavenging effect is increased and the air that would become more dense is getting to the collector leading to gains.



Now lets flip the smaller diameter thing and try for better packaging. Lets say you are currently have your engine set up to run long headers. But for whatever reason running long headers is not a option. You could keep your number 1 primary 1 3/4 for example but increase the diameter on the next fired cylinder on the header so that that primary is actually shorter in length but is the same in area.








Now, I never said I know anything about exhaust science. I am just interested in it right now and this simple question of why people don't run different diameters keeps running through my head.
over thinking it.

this V-12 is set up like a paired set of inline 6's, which is set up like two inline 3's- meaning 4 sets of headers can scavenge quite well.

pairing 1,3,5 7,9,11 2,4,6 and 8,10,12 IS a correct header design, and you could easily turn those manifolds into headers with collectors. the pulses between these cylinders is NO LONGER than what an inline 3 cylinder would see and they are EVENLY SPACED (unlike a sbc). This isn't a compromise because of packaging (engine length), but rather the correct way to do it. Instead of a pulse every 180 degrees (like you see with 180 degree headers on a sbc) you would see a pulse every 240 degrees.
 

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Discussion Starter · #17 ·
I did not go into it but I did admit that the 4 exhaust was
s a better system on that 12. On the packaging topic I was receding to a v8. I missed throwing that in. What I am looking at is having a 1 3/4 on the number 1 but say 2" on number 7 with 7being shorter to give better packaging while still giving the effect of a longer 1 3/4 primary.
 

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Discussion Starter · #19 ·
I'm talking about having the collectiors parallel. With the rear cylinders which would. Allow a single pipe to pass under the firewall while still giving long header charestics.
 
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