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ROAD RACE 360 HEADER SIZE

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gmachinedart1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gmachinedart1 Quote  Post ReplyReply Direct Link To This Post Topic: ROAD RACE 360 HEADER SIZE
    Posted: May/31/2017 at 3:36pm
Hi everyone.......I need help with choosing header primary size for my 73 javelin that will be set up for recreational road course use. The engine will be a 10.6:1 360 with ported edelbrocks with 2.08 intakes flowing 300cfm and . I dont have the engine dynoed yet , so not sure what the power numbers will be. Camshaft is roughly 600 lift and 260duration @50. I think rpm range will be 3800-7000. Im looking at the thorley headers ....1-3/4 or 1-7/8. I dont have a ton of experience with amc 360's regarding what they tend to like . I run a 1-7/8 header with my ported w2 small block mopar  , which it seems to like. Any input would be helpful . 

Thank you
Justin
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Hurst390 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurst390 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/01/2017 at 7:44am
I built 1-3/4-1-7/8 headers with a merge collector for my Dirt track AMC...It really like them.

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amcenthusiast View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcenthusiast Quote  Post ReplyReply Direct Link To This Post Posted: Jun/01/2017 at 12:54pm
Woah there willbur! -your question is filled with too many other dubious assumptions!

The potential is there but it takes a lot of know-how to build any '66-'91 AMV8 to turn 7000 rpm dependably; it's not merely a matter of buying bolt on parts!

(the question puts the apple cart before the ox)

But to satisfy your header tube size question with a direct answer:

Take an approximately ten inch long piece of relatively thin wire and shape it/bend it to match the exhaust port shape exactly & snip off the ends where the wire overlaps.

Unfold the wire, to make it straight again, and measure the length.

That measurement is the circumference which tells the diameter of the correct tube size that should be used.


Link to XRV8 Race Parts website: http://amcramblermarlin.1colony.com/favorite_links.html
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304-dude View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Jun/01/2017 at 1:38pm
Here is a link on engineering exhaust systems. A bit techy but if you can get passed the calculation standpoints, there is a bit of interesting stuff to take in account when having to design or pick which type of header you need.

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

Sounds like you are having a dual purpose role with the car. Mine will be done that way as well.

For my build I plan on a stepped flange with side exhaust. May incorporate T/A cross over to allow better torque while keeping a wide curve. This article dose not explain the odd even exhaust crossover to even out pulses within the header exchange into the collector for scavenging.

It is a bit complex and requires room to fit. It is trick and makes you v8 sound like a mad v6. But it is not required, just noted for now.
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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gmachinedart1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gmachinedart1 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/01/2017 at 2:27pm
Understood on the 7000rpm and making it live....just curious if any guys had any dyno comparisons with header tube size on a similar build.....Yes it will be somewhat of a dual purpose deal.....Thanks for the info so far.

Justin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Jun/01/2017 at 8:49pm
On a 600 hp 409 the 1 7/8 were about 15 more hp than the 1 3/4, but the 1 3/4 made a distinctly better mid range torque curve. For road race use it might make sense for the smaller header. For drag race use where mid-range is irrelevant use the bigger header.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RTTComanche17 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2017 at 3:31am
gmachinedart1,

See the last paragraph for an opinion based short answer to your question. Feel free to skip all the jargon in the middle if you are not interested. 

Unfortunately, this is an extremely complex question and there is very little info given. The link from 304-dude and the tip from amcenthusiast will get you close and that may be all that can happen here. I'll breakdown some points below greatly simplifying some of the theory and issues presented above. 


Disclaimer: I am not a professional, but i do know enough to make an idiot of myself. Feel free to correct me if I am incorrect, I will gladly accept more knowledge at the expense of my pride. 
None of the following is because I think you don't know this, but for posterity's sake I figured i'd post enough for you or for someone who finds this post to be able to at least know what terms to search for if you want to dive deeper. This is a very deep rabbit hole full of convoluted engineering theory if you choose to follow it. But now at least people know where the entrance to said rabbit hole is....


First, as amcenthusiast pointed out, 7000 rpm is no cake walk. You really have to know what you are doing. Not saying you don't, but it's difficult to say when we only talk a few sentences at a time on the forum.

Second, a bit of simplified theory. Holding engine size constant (and assuming you are at least in the ball park on primary size), a smaller primary will give better low-end and mid-range power and torque, a larger primary will give better top end numbers. The whole goal is to get mass (exhaust gas) out of the engine. Velocity and flow are the ways to do that. For a given engine speed, a smaller primary will have a higher exhaust velocity (same mass of air thru a smaller area) and that velocity can be thought of as an inertial force. Meaning the exhaust gas will have more momentum behind it to help evacuate the cylinder. So a smaller primary helps when engine speeds (and exhaust volume) are lower since they keep velocities higher. As engine speeds get higher, this smaller primary may not have the area needed to support the needed flow to evacuate the exhaust gasses effectively and will pose a restriction (like trying to breathe thru a straw) and hurt top end power. Conversely, a larger primary will pose less restriction at higher rpm, giving better flow and better top end power, but will be a bit lackluster in the low and mid rpm due to the exhaust gas velocity being lower since the primary is larger. This will kill that "inertial momentum" that helps to clear the cylinder of exhaust gas. 

Third, any change in the cross sectional area of the exhaust path causes pressure waves (good or bad ones). I'd recommend the article in 304-dude's post as a more in depth primer to much more nasty math. But basically, any time there is a change in the cross sectional area (port size to primary size, stepped header, primary into collector, etc), there is a corresponding pressure wave (positive or negative depending on if the size change is bigger or smaller). This pressure wave can either help or hurt power at specific rpm points and those with really big brains and wallets will 'tune' their exhaust using this theory to help increase power at specific rpm points they deem most useful to the type of racing they do. 

Fourth, Ken_Parkman is the only one to offer a data point (so far). And he gained 15 peak hp by switching to a 1 7/8 primary but lost some in the mid-range because of it. His motor was also 50 cubic inches larger than yours (1 7/8" may be too big for a 360). And we have no idea if the exhaust was the limiting factor. Meaning he could have gained more if his intake was completely optimized, or it could have hurt him if it was the reverse. It's the engine combination that determines what mods are most useful. Also, 15 hp is also only a 2.5% difference at the peak. 
For a road racer, you need to look at average power in the rpm band your motor will be operating at on the road course. 15 extra hp from 6500 to 7000 will not help if you give up 10 hp from 4000-6500. Think of average power (area under the hp curve) over the rpm range you will see on track as a better measure than peak. 

Basically it all boils down to how much effort you want to put into this. Yes you can hire someone to model you engine and tell you their best guess on primary size and where to put the steps. Yes you can spend $$ on dyno time testing multiple headers. However, since you are building a double duty car and not a class specific race car, i'd get the most convenient exhaust i could afford. Meaning I'd look for a quality header that was easy (as can be) to install and remove, that cleared all steering and frame and suspension components, and just drive the car. Unless you are a professional race car driver, that few percent power increase (or decrease) will likely not be the deciding factor in your lap times. Your experience, suspension set up, tire characteristics, skill, and time on track will be much larger influencers of your lap time.

Hope this helps. Not trying to put you down. Just trying to get info out there for anyone. 


TLDR (too long, didn't read):
It's complicated. I'd look for a quality header in the 1 3/4" range and spend my time driving the car and being on the track. If you are not a professional race car driver, time on track will be much more beneficial to lap time than a couple % increase in power. 
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gmachinedart1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gmachinedart1 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2017 at 9:21am
Thanks for all the response and info......1-3/4 it will be for now. I believe the thorleys should be a decent header for my 73 javelin. 

Thank you again

Justin

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343sharpstick View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 343sharpstick Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2017 at 10:34am
The only long-tube off-the-shelf headers with decent ground clearance that I've found are the Dougs D103. Those even took a bit of work to get them to fit. I cut off the 3-bolt flange and put on a ball-flange on, then had 'em jet hot coated.

If a guy had some fab skills and dyno time, I would guess a 4-2-1 would have the best combination of mid-range power and packaging for ground clearance. Getting that right would be a big undertaking.
The Penske Javelin's had the mother of all 4-2-1 headers, but the cars didn't exactly have stock floor pans



Edited by 343sharpstick - Jun/02/2017 at 12:57pm
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White70JavelinSST View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote White70JavelinSST Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2017 at 1:11pm
There's a guy who fabs stainless steel headers for AMC applications.I do not remember the name.
By the time you jet hot coat regular steel headers, you might have as much invested in them as if you bought stainless steel headers. I've always wondered about that.
The ss headers that are or used to be on ebay has not had a product review done for them that I have seen, we know they are chinese, we just don't know how badly they fit or if the material is cheap crap that a magnet sticks to or somewhere closer to inconel.  Ha ha fat chance...

70 Javelin SST, second owner, purchased 1972
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