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Question. Will 2.5" exhaust fit up to my stock 289 manifolds? Looking to purchase a full dual exhaust kit. From stock 289 manifolds through the rear valance . (not below). All hardware clamps etc. Looking at NPD but not sure on parts listed are compatible . ie, H-pipe to flow master set ups. Complete assembly may help.
Thoughts ? Thanks
 

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Stock exhaust is 2” so you will need an h pipe that converts the 2” manifolds to 2 1/2” pipes. I am not sure why you would want to do this. Unless you have a really big engines out don’t need a 2 1/2” exhaust. Even the 428 SCJ didn’t have an exhaust that big. You’ll get no gains from it because the manifolds are the restriction. So anything larger than 2” is a waste of money.

The only reason I would consider a 2 1/2” exhaust is if you have plans to add a 347 or larger motor in the future.
 
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Question. Will 2.5" exhaust fit up to my stock 289 manifolds? Looking to purchase a full dual exhaust kit. From stock 289 manifolds through the rear valance . (not below). All hardware clamps etc. Looking at NPD but not sure on parts listed are compatible . ie, H-pipe to flow master set ups. Complete assembly may help.
Thoughts ? Thanks
Complete waste of time doing anything 2.5" that's connected to the factory stock restrictive cast iron manifolds.
Lets say you decide to put headers on it cuz you're dying to have 2.5" exhaust...... then you need the head work and
engine to support that large an exhaust system. Unlikely you would ever have that much engine in a street-driven
vintage Mustang. (not a 289/302 Windsor anyway)
 
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Imagine if you will, that you're using a wood burning stove. You have a nice 6" pipe coming out of the stove, and a 6" chimney. However, you need to make a bend, so you duct-tape some stainless-steel shower hose in between.


Your exhaust manifolds are that shower hose.


Realistically, a nice little hot street 302 might need 2 1/4 exhaust for max power. An aftermarket block 302, putting down 500 horsepower might need 2.5" Realistically, 2" exhaust would be great for anything you'd care to do with a stock 289 or 302, on dual exhaust. To get the most of that, you would need some good headers. The smaller tubes allow for much easier fitment, and save some weight too.
 

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Question. Will 2.5" exhaust fit up to my stock 289 manifolds? Looking to purchase a full dual exhaust kit. From stock 289 manifolds through the rear valance . (not below). All hardware clamps etc. Looking at NPD but not sure on parts listed are compatible . ie, H-pipe to flow master set ups. Complete assembly may help.
Thoughts ? Thanks
What are you trying to accomplish? A certain sound? I'm successfully running a few ponies, 300+, on my 331 through a 2" Arvinode exhaust that is definitely rowdy sounding when you get on it and a crowd pleaser at every autocross. I probably should upgrade to a larger diameter exhaust but it's not a priority.

 
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I have 2.5 exhaust installed by PO with series 40 flowmasters. It sounds awesome, and runs well. I was burning clutch cables with long tube headers and ended up solving the issue by installing stock log manifolds with these adapters I found on ebay. They are magnaflows.
 

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I got a 2-1/2" kit for my '66 289 years ago because that was the only Magnaflow kit Summit sold for my car (and I think it still is), and I didn't know any better at the time. In addition to the comments above, I'll add that the big tubes cause issues in the rear. I finally had to use a reducer and a 2" pipe on the driver side to get it away from the fuel tank.
 

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In addition to being unnecessary there will be fitment issues. I have never bought a prebent exhaust system that didnt need to be modified to work.

Put a set of hipo manifolds and a oem style 2in system, will sound great, look great, deliver more power and be much easier to install.

Also if you ever need to drop the tranny that 2.5in system will have to come out first.
 

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I have a very healthy 333 stroker, TW alum. heads hearty cam, TRI-Ys, etc. I'm running 2.5 exhaust. If I had to do it all over, I'd dummy down to 2.25. The 2.5 system creates fitting issues, a rattle here and there etc. Don't do it!

if you anticipate more engine in the future, then, maybe the 2.25, with a set of ceramic coated Tri-Ys, a sold 500 (600 max) CFM carb, have your Distributor recurved, and a dual plane intake. This is a reasonable step up the performance ladder.
 

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Have you ever installed a complete exhaust system? If so, then you know what you're getting into. Cool.

However, if you've never installed a complete exhaust system, stand by for heavy rolls. It is like stabbing yourself in the eye.

I took my '70 Mach 1 down to bare metal and rebuilt it doing almost everything myself; welding in new metal, body and paint work, rebuilding the engine and suspension and installing the interior. I didn't even consider installing my own exhaust system. I took the car to a great, local exhaust shop who did an amazing job. They have all the benders, flare tools and chops to do it easily. Likely, I would have committed seppuku before I ever got an exhaust kit assembled, in place and working well.

Just something to consider.
 

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I had a 351w in my 70.
Engine was mostly stock with long tube headers and 2.5 exhaust.
No issues whatsoever.
But I didn’t have an H pipe.

I’m now switching to a 302 and plan to use the same exhaust pipes.
H pipe isn’t a necessity. No noticeable performance gains from an H pipe. But it does help balance out the sound.
 

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I had a 351w in my 70.
Engine was mostly stock with long tube headers and 2.5 exhaust.
No issues whatsoever.
But I didn’t have an H pipe.

I’m now switching to a 302 and plan to use the same exhaust pipes.
H pipe isn’t a necessity. No noticeable performance gains from an H pipe. But it does help balance out the sound.
IIRC, According to a Hot Rod dyno test years ago, the crossover adds 5-15 hp, depending on other factors. That's why Ford, who never spent 2¢ on a car unless they had a very good reason, used H pipes on virtually all their dual exhaust cars back in the day.

Interestingly, HR also tested deleting the tailpipes, a fad at the time. Turns out it reduced hp.

The closer the crossover is to the engine, the better it works. Fuelie 'Vettes actually had the crosstube wrapped under the engine.

 

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In addition to being unnecessary there will be fitment issues. I have never bought a prebent exhaust system that didnt need to be modified to work.

Put a set of hipo manifolds and a oem style 2in system, will sound great, look great, deliver more power and be much easier to install.

Also if you ever need to drop the tranny that 2.5in system will have to come out first.

This is what I did. Bought a kit from CJ's. Everything fit well and I was pleasantly surprised by the throaty sound.
 

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Very true- The design of the system is more important than diameter. The Arvinode system proved that. While loud under load/acceleration, it was pretty calm considering it has no mufflers.
 

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I guess I got lucky:

1.) I have been running long tube headers for years with a T-5 and have never melted a clutch cable
2.) I am running the Magnaflow 2 1/2" system. Thanks to the Hedman Elite Hedders with the ball and socket connection the system did not need to be modified. With standard 3 bolt flanges the front tubes would have had to be modified for the angle to work.
3.) Zero issues clearing the gas tank
4.) Sounds awesome

My only complaint is that it hangs a bit low vs my old system so it can scrape on large speed bumps

All that said, 2 1/2" pipes on a small block with stock manifolds is totally pointless.
 
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