428 cobra jet exhaust manifolds vs gt390

[v8Dave]

Hello everyone.
I have a question I am asking on behalf of a elderly friend who owns a 68 gt390 coupe.
He has recently had the engine rebuilt and has stroked it to a 445 fe with trick flow heads and I believe blue thunder 4v aluminium intake manifold and has dressed it to look factory stock however with some upgrades like mentioned above plus aluminium water pump etc.
Now he has a pair of gt390 exhaust manifolds I know he wants to keep with exhaust manifolds and not switch to headers.
He asked me recently could he fit a pair of 428 cobra jet exhaust manifolds in place of his gt390 manifolds and if he switched what would be the hp/to over the gt390 manifolds?
I believe there have been tests comparing the 428 cobra jet manifolds vs different brand headers and just wondered if anyone on the forum here with all the extensive knowledge would know if and what the gain would be swapping  exhaust manifolds.
I believe I am correct in thinking the 428 cobra jet exhaust manifolds would fit in the tight engine bay of the 67/68 mustangs as the 428 cobra jet was released mid year and offered as a option in the 68 mustang.
Also I know there's the early 428 cobra jet exhaust manifolds that use a spacer on the passenger side manifold and the later 70 design that did away with the spacer on the passenger side. Am I right in saying he could use the 70 design cobra jet manifolds without the spacer to make things simple (as I believe he has converted his Carb to electric choke) and just then get a 70 mustang h pipe to bolt up to it and then fabricate his own exhaust system from there back out to the valance.
Many thanks for your time and any help or information  anyone can give is much appreciated.
Have a nice day.

[Rory428]

Jay Browns excellent book "The Great FE Intake Comparo" has a section covering numerous different factory FE exhaust manifolds, and several sets of headers. The tests were performed on Jays engine dyno, with various levels of HP engines, and the factory 390 "GT" style exhaust manifolds were real dogs, basically no better than the more common low performance "log style" exhaust manifolds as used on full sized cars and pickup trucks. The 428 Cobra Jet exhaust manifolds are certainly better performing than the 390 GT versions, but still lag behind most headers. As far as I am concerned, hanging a pair of the GT exhaust manifolds on the great TFS heads, will eliminate much of the gains of the TFS heads. So if he is dead set on not using a decent set of headers, the CJ exhaust manifolds will be a better choice that the 390 GT pieces, but he will still be losing a fair bit of power. And yes, the 428 CJ exhaust manifolds should fit a 67/68 Mustang just fine. The 1969 exh. manifolds are much more common, as there were many more 428 Cobra Jet engines factory installed by Ford in 1969, than both 1968 and 1970, combined.

[427Fastback]

I have done it and the answer do they interchange is not really. There are a couple of end bolts that are half a hole out.It was no big deal I just threaded plugs in and redrilled and tapped new holes (C8AE-H heads 14 bolt). But i did it on my bench/drill press. This was over 35 years ago and I forget the details. Get some gaskets and lay them on top of each other.

The spacer does not have a butterfly in it..

[galaxiex]

I had a set of 69 CJ exhaust manifolds (with the spacer) on my 68 Mustang Fastback.
They fit just fine.
The manifolds were from my 69 Cyclone GT R code.
Sadly that car got scrapped long ago...

The Mustang is also no longer in my possession...
Sold it long ago...

[My427stang]

All the info is there but just my 2 cents

- 390 GT manifolds will not bolt to the Trick Flows without modifying the head
- 428 CJ will bolt up
- 428 CJ likely a smidge better than the horrible GT, but leaving a ton of power on the table from headers. True for any head, but especially for a high flowing head and intake
- If this is remaining concourse-style, I also think he will need to change to a 428CJ exhaust (H-pipe back) and it should be larger diameter and shaped to match the manifolds for either year

[GerryP]

The CJ manifolds will fit the heads and the chassis just fine.  The Trick Flow heads use the CJ 16-bolt exhaust flange.  You will need a new H pipe for the CJ manifolds as the CJ manifolds exit in a different location.  Don't get wrapped around the axle on the question of power loss when compared to headers.  Yes.  There is some power loss but does it really matter in his particular driving situation?  On my '67 Fairlane, I switched from Hooker Super Comps to repro CJ manifolds and know I made the right choice for me.

[427Fastback]

Just reread it. I missed the trick flow head part...

[v8Dave]

Hello everyone.
Thanks for the information and time to reply everyone.
Rory428 I apologise I thought the great fe book by Jay browns  compared the difference between 428 cobra jet manifolds(some of the best shock tower car manifolds) to headers I didn't realise it compared other standard exhaust manifolds too. I must of forgotten as I do have that book somewhere...
Thanks for confirming the fitment of the exhaust manifolds, I believe he wants to get a set of the 1970 428 cobra jet exhaust manifolds that don't use the spacer/exhaust valve chock set up as he has a electronic choke conversion on his carburetor and is not using a heat style choke and by doing away with the spacer as the 70 version of manifolds  do is just simplifying the installation and eliminating one less hassle or area to worry about ever leaking or degrading with the extra exhaust donuts  and spacer used I guess.
I also believe he will purchase a 1970 mustang h pipe to fit the 1970 exhaust manifolds but it will be a 2 1/2 diameter h pipe with a 2 and 1/2 inch exhaust running all the way out back where as I believe the 68 gt500kr and later 69/70mach 1 used factory 2 1/4 h pipe with 2 1/4 exhaust system all the way out to the back.
So it will be a slight improvement in that area with a 1/4 inch more from front to back on the entire exhaust system than the factory performance cars of the era(68gt500kr ect)
I also read somewhere that the reproduction 428 cobra jet exhaust manifolds flow abit better due to better casting then the originals, not sure if that's true
Thanks everyone for all the help and advice it's much appreciated I will pass it on.
Have a nice day
Dave