headers / exhaust manifolds?

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   my very original 390 70 amx has an old rusty set of headers that I want to replace.  is there that much of a performance / horsepower difference between them?  the motor was rebuilt 30 over years ago, don't know if that matters.  any opinions or recommendations would be helpful.    thanks, john

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johnsue79 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Sep/15/2014 Location: new jersey Status: Offline Points: 194	Post Options Post Reply Quote johnsue79 Report Post Thanks(0) Quote Reply Topic: headers / exhaust manifolds? Posted: Oct/21/2014 at 5:59am
	my very original 390 70 amx has an old rusty set of headers that I want to replace. is there that much of a performance / horsepower difference between them? the motor was rebuilt 30 over years ago, don't know if that matters. any opinions or recommendations would be helpful. thanks, john

6768rogues Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/03/2007 Location: United States Status: Offline Points: 6241	Post Options Post Reply Quote 6768rogues Report Post Thanks(0) Quote Reply Posted: Oct/21/2014 at 12:33pm
	If you are using the engine for street duty, there is not a large difference no matter which headers or manifolds you use. If you modified the engine and run high RPMs at the track, headers that take advantage of your modifications will enhance performance. I would use manifolds. I have two cars with headers and I have had leaks, cracks at welds, low collectors bottoming out, difficulty servicing starters, etc. I will never use headers on a street car again.
	Content intended for mature audiences. If you experience nausea or diarrhea, stop reading and seek medical attention. Located usually near Rochester, NY and sometimes central FL.

johnsue79 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Sep/15/2014 Location: new jersey Status: Offline Points: 194	Post Options Post Reply Quote johnsue79 Report Post Thanks(0) Quote Reply Posted: Oct/21/2014 at 1:58pm
	motor is basically stock. I was leaning toward stock manifolds.

H60 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Apprentice Joined: Aug/09/2014 Location: Wister,Ok Status: Offline Points: 162	Post Options Post Reply Quote H60 Report Post Thanks(0) Quote Reply Posted: Oct/21/2014 at 4:40pm
	I spent half a day grinding and flap sanding a pair of manifolds I took off a jeep 401. I filled the air holes then just smoothed the whole manifold painted with high temp stainless colored paint. Been on a 68 390 for about 6-7 years now. Looks like cast iron shorties.

bcsideshow Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: May/19/2010 Location: Kenosha Status: Offline Points: 1228	Post Options Post Reply Quote bcsideshow Report Post Thanks(0) Quote Reply Posted: Oct/21/2014 at 6:30pm
	I agree with 6768Rouges. You can upgrade to the free flow manifolds instead of the log style. You can also port the free flows to get a little better flow out of them. After that you have a nice compromise. Better fit than headers and better flow than your stock manifolds. Assumes you are going to need some exhaust pipe work going to manifolds.
	1969 SCRambler "B" 1970 AMX 390 Auto 1968 AMX 390 Auto 1970 Rebel Machine Project 1969 AMX 390 4SPD Project 1969 AMX 390 Auto Project 1969 Javelin 390 project 1997 Cherokee lifted 1998 TJ Lifted

Red20 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jan/29/2013 Location: Oceanside CA Status: Offline Points: 1556	Post Options Post Reply Quote Red20 Report Post Thanks(0) Quote Reply Posted: Oct/21/2014 at 11:54pm
	I've fought with headers for years, cutting the flanges, trying different gaskets, patching the rusted through holes over time, etc. I'm upgrading to fuel injection and am seriously concerned with leaks considering the O2 sensor, so I'm looking to go with manifolds myself, now. That said, Patriot Exhaust, part of Pertronix is making a set of headers that look to dump out right at the stock location. I may just give them a shot and continue my battle with headers. I'm a glutton for punishment. http://www.summitracing.com/parts/pte-h8600/overview/year/1969/make/american-motors/model/javelin
	1969 Javelin SST "Screamin' Banana" - Totaled Feb 2018 1973 Hornet Sportabout X "yellajack" - 360/727/TracLoc DAILY DRIVER

johnsue79 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Sep/15/2014 Location: new jersey Status: Offline Points: 194	Post Options Post Reply Quote johnsue79 Report Post Thanks(0) Quote Reply Posted: Oct/22/2014 at 5:55am
	I checked out the patriot exhaust headers, they certainly look like an interesting option. the free flows look like a good option as well. what years & applications of the free flows would fit 70 390 amx.

ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Oct/22/2014 at 10:27pm
	And now for the other side. I have had a set of headers on my car for 25 years and am quite happy with them. The upside is that they have better mid range power and better top end power, plus they sound good. My car (1970 Javelin) is at stock ride height and while I do OCCASSIONALLY scrape a header, usually when going too fast, they have not caused real issues that way. I have not tended to cook starters nor do they interfere with my steering or anything else. I do not have trouble with leaks and the only ones I have had were because when they are installed you need to re-torque them once or twice before they take a set. Information that I have seen indicates that there will be about a 15 horse and a 20 pound increase in torque going to a set of free flows or shorty headers over stock logs. You can add about 10 horse and a similar amount of torque to that for a set of proper full length headers. Only you can decide what that means in your world. Several members of my club have made noises about changing back to headers after using free flows for years. They had all joined the "I hate headers" brigade years ago for the same reasons you give. The reason they are reconsidering is that they have seen my headers and now the ones from Control Freak that were bought by another club member. The power is noticeable! The key to the whole header equation is that you MUST buy good ones and that costs money! Mine are old reproduction Thorleys from a limited run years ago. They have thick pipes and 7/16" flanges and they do not warp, leak or crack. The CF ones are the same with nearly the same thickness of metal. Both sets are well designed with proper clearances etc. Here is the rub. Mine cost $400 in 1989. The CF ones are $600-800 bucks, depending on the finish. If you want nice headers that don't give you nightmares you need to spend a bit of money. The choice is yours! Chris

kingsX Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Nut Joined: Jan/20/2011 Location: Holland, MI Status: Offline Points: 343	Post Options Post Reply Quote kingsX Report Post Thanks(0) Quote Reply Posted: Oct/23/2014 at 5:58am
	I lowered my car quite a bit and switched to Eddy shorties a few years ago due to the long tubes scraping the ground. The power loss was very noticeable. I've never had free flows or log style manifolds but going from long tubes to shorties was a big compromise for me. BUT now i dont scrape unless i try going over a speed bump. I'd really like to go back to full length headers but dont want to raise my car back to stock height.
	73 AMX Javelin PC

farna Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19692	Post Options Post Reply Quote farna Report Post Thanks(0) Quote Reply Posted: Oct/23/2014 at 7:55am
	The Patriots seem to be a copy of the Edelbrock shorties. Wonder if they bought the jigs from Eddy or just made up a copy.. or engineered a similar looking set from scratch?
	Frank Swygert