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My 66 Convertible is a stock 289/2 bbl AT. I just replaced the original exhaust manifolds with HiPo ones, and changed from single to dual exhaust for a better sound. Any guess as to how much extra HP this will give me? I know it's nothing drastic but just curious.
 

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Did you enlarge the exhaust ports in the heads and grind out the Thermactor bumps if your heads have them? That's where the real kick in the pants comes from.
 

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Did you enlarge the exhaust ports in the heads and grind out the Thermactor bumps if your heads have them? That's where the real kick in the pants comes from.
No I didn't.
 

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Intake and exhaust systems need to be matched for the best performance. without a free flowing intake your gains will be in the 5-10 HP. range. You can more than double that with the addition of most any performance 4 barrel intake.

but you’ve taken a good 1st step, which is how every journey starts.

Z
 

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My friend put a 4bbl and HiPo manifolds with dual exhaust on his stock 1967 C-Code and the difference was noticable. But it's small because it still has tiny heads and cam.
 

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The addition of HiPo manifolds and dual exhaust won't add anything because you still have the restrictive exhaust ports in the heads. It's the same thing as connecting a 2" fire hose to a standard water spigot. You won't get any more water through the 2" fire hose than you will get through a 5/8" garden hose.
 

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Sounds about where I was 20 years ago.
If what you have is good enough...then great.

If you're looking for more power you next decision would it be better to build off your current engine or start fresh. That will depend on your budget and the condition of your current engine.

Aftermarket heads, a good cam, a good intake manifold, and a 4 barrel carburetor will significantly increase power. If you go that route on a 289 you will also likely need to replace your pistons to save your compression ratio and get enough valve relief for the larger valves in aftermarket heads. When I went that route on my C-code...I did it all at one time. Everything has to work together.

Of course...after that the vintage C4 went south in a matter of a few weeks. You add another 100 HP in front of a 50 some year old transmission...it isn't going to like it.

Go back to "If what you have is good enough...then great." Because you've started down an expensive road. Not saying it's not a really fun road...just sayin.

Phil
 

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The addition of HiPo manifolds and dual exhaust won't add anything because you still have the restrictive exhaust ports in the heads. It's the same thing as connecting a 2" fire hose to a standard water spigot. You won't get any more water through the 2" fire hose than you will get through a 5/8" garden hose.
Close, but not exactly. The sudden expansion to the larger diameter results in a low pressure area that increases flow velocity, basically helps suck the exhaust out. Naturally, there is a limit, but the better flowing exhaust helps edge towards that limit.
 

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Discussion Starter #10
Thanks for the comments and information! I started down this road initially just wanting to clean up the old, rusty exhaust manifolds. In talking to a couple of trusted Mustang friends and mechanic, they suggested just replacing them with the HiPo ones in case I ever decided to upgrade the intake/carb. And since I was changing the exhaust manifolds, I decided to "upgrade" to a dual exhaust mainly just to make the car sound a little better. But again, that set me up in case I decide to do something on the front end down the road.
 

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No I didn't.
Well, mark this for a future project. Years ago I took my own advice, and while I had one of the heads off for a minor repair, I took the opportunity to remove both heads, and do this:
Port-Matching

Even though I already had a 289HP engine, with the HP manifolds, it felt like I'd upgraded the cam. It also ran smoother, the shift lever was markedly smoother at all rpm ranges.

If I had to guess, I'd say the port-matching was good for 15-25 hp, on top of the 5-10 hp the manifolds provide. My only comparison was when I removed long-tube headers to put the HP manifolds on the engine. I probably lost 10 hp at the top end (where my engine spends less than 1% of its time) and gained bottom end smoothness, quiet, and way less heat under the hood.
 

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A nice exhaust note sounds better for sure,.....
 

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The 4bbl/hipo manifolds/dual exhaust is a great set of mods to do to a stock 2bbl. It adds value on all but maybe the most pristine Fastback/Convertible. It will not hurt drivability (assuming the carb is properly tuned) and should give you power and grunt you can feel.

As I said, my friend did it on his 1967 C-Code and has no regrets. He's now transferring all of that to the new p-head 306 he just bought with 1.7 rockers and a 5.0 roller cam. Didn't have to re-buy anything and altogether that will be a nice kick in the pants.
 
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