2.25 or 2.5 inch exhaust.

[Vinyl66]

I had a 2" system with cherry bombs on a 289. I ran a best time of 12.83 with nothing more than a solid cam, 650 Holley and headers. The car ran within a tenth with open headers as it did with the full exhaust.

If the engine was a little lean to start with having the full exhaust on then the gains would not be huge going to open. Did you jet up the carb once you went to open exhaust to take advantage of the better flow? Were you using good long tube headers with 3" collectors? Did you use the appropriate length header collector extensions?

Anything short of a full on rece motor does not need 2.5".

Wow! There are tons of people that make huge gains with street cars going to a 2.5" and even 3" exhaust.

Most of the power gains in a 2.5" system are in your head and not real. Just my 2 cents

You're on a roll. Do you understand that low backpressure is a good thing? Backpressure is definitely good for one thing: lowering power. After the header collectors your exhaust should be as free flowing as possible to make the most power. This does not mean 2" exhaust with crush bends behind a good running V8.

 

[Wicked65]

I had a 2" system with cherry bombs on a 289. I ran a best time of 12.83 with nothing more than a solid cam, 650 Holley and headers. The car ran within a tenth with open headers as it did with the full exhaust. Anything short of a full on rece motor does not need 2.5". This is the same as the guy who has a stock 289 with a 480 cfm autolite, bolts on a holley 600 and says wow what a different's that made! And it actually lost power and slowed down. Most of the power gains in a 2.5" system are in your head and not real. Just my 2 cents

Wow, thats some really bad info.

 

[tx65coupe]

What about an 800hp 351w?

There are exceptions, but 99 percent of our cars are nothing like that.

 

[tx65coupe]

tx65coupe,
You said:

"Oh. Its too bad it doesn't have an H pipe instead." Please explain. Is an "H" pipe better than an "X" pipe and why? From what I can tell it seems a topic of spirited debate with only minimal differences. I could be wrong. But if you click on the exhaust link near the top of the page, scroll down to the setup for your car and click the part number, it will link you to the Magnaflow site with an expandable pic that shows all the parts that appear to be quite well made with many good reviews throughout the internet.
Gene

I know the differences are minimal, but I can't stand how X pipes sound. I like how true duals or H pipe duals sound. The X just sounds too exotic.

 

[BiminiBlueLX]

2.5" would definitely be better considering the engine work you've done. It's usually the best route to go.

 

[tx65coupe]

Undoubetedly 2.5". Do not crutch a V8 with any less.

Are you saying that even a bone stock 289 2V needs 2.5 inch exhaust?

 

[tx65coupe]

There's gains on even my motor with 3" duals at its little 347 cu inch. 2 1/2 would be too small.

That magnaflow chart seems pretty good to me if you go by HP of the engine. I'm sure most 2 1/2 is more than sufficient on this site.

I just don't see how 3 inch duals is necessary on a small block V8. I don't think that 2.5 inch duals are too small for a 347.

 

[Vinyl66]

Are you saying that even a bone stock 289 2V needs 2.5 inch exhaust?

If you want to keep a 289 engine bone stock it doesn't need anything if you want it to remain slow But yes it will help it by replacing the 2" kinked full exhaust with 2.5" mandrel bent pipes. We do these modifications for performance. If you do not want performance and want to remain slow then keep your 2" pipes.

 

[Vinyl66]

I just don't see how 3 inch duals is necessary on a small block V8. I don't think that 2.5 inch duals are too small for a 347.

Do you seriously think his 347 would run as good as it does with 2.5" pipes? He runs in the 9's at near 135 mph! His engine regularly sees well over 8 grand. Do you know how much air that little 347 passes through it? Well it's easily big block territory. 3" pipes are certainly needed on his car.

 

[brianj5600]

I just don't see how 3 inch duals is necessary on a small block V8. I don't think that 2.5 inch duals are too small for a 347.

His motor makes over 600hp. The chart in the link above does not even go that high. Blown65, did you dyno your motor with your exhaust?

 

[65FBE2]

I would like to see you prove it on a chassie dyno. 2" compared to 2.5" 5-10hp if your lucky. I did it at the track and I know what happened. This was some 20 odd years ago but still valid. Not saying this holds true for a 400 plus hp engine combo but thats not what he has. You knuckle heads all have him with a 347 AFR motor with a big cam and are making arguments about how much better even 3" would be. His combination does not need more than 2.25 and anything bigger is a wast of money and much harder to fit under a car designed for 2" Not to memntion the sound. This is a classic Mustang not a Civic

 

[Wicked65]

First off, rarely do open headers give you more power, unless your making REALLy big horsepower. Which one this board, is maybe a handful of people. Your engine does need a little bit of backpressure from my experience. For instance, on my supercharged 87 5.0, i hade stock headers, 2.5in h-pipe and flowmasters with turndowns. I had a incident where my car caught a railroad track, and took my mufflers off.. Anyway, long story short, my car felt like it lost 40hp without mufflers. 2.5in is a good size up until you get between 500-600hp from everything i have read or been told. Then you really should step up to 3in. The OP said he had ported heads and high compression. Why on earth would you not run better exhaust?

 

[blown65]

His motor makes over 600hp. The chart in the link above does not even go that high. Blown65, did you dyno your motor with your exhaust?

no, I actually had to use their little dyno street rod headers. 1 5/8" primaries and way way too long. The rest is up for the task, since they run big blocks so the main pipes and mufflers are 3 1/2 or 4" I believe. Probably would of picked up a little hp if I could of used the shorter 1 3/4 headers I run.

I run dual 3" with the x-pipe and get tons of complements on how it sounds. Ive had a few different exh systems on the car, and this is by far the best sounding on my car. Too loud for a street daily driver deal, but for the few times I drive it here and there on the street I love it.

Engine guy wants me to put 1 7/8" headers on it with 3" but I just dont wanna mess with headers again.

BTW, 351 at 800hp would be 3 1/2", 3" would be too small IMO.

 

[blown65]

I just don't see how 3 inch duals is necessary on a small block V8. I don't think that 2.5 inch duals are too small for a 347.

So in your line of thinking, a Nascar motor at 358" should just run 2 1/2 inch? Or does that 11 more cubic inches need 3" now? What size do you think they run. LOL

 

[jonfor]

So in your line of thinking, a Nascar motor at 358" should just run 2 1/2 inch? Or does that 11 more cubic inches need 3" now? What size do you think they run. LOL

Well seems how they cruise down the road at a constant 8Krpm’s and most of us here don’t, I think your analogy is poor at best.

after owning many V8’s over the years and few small blocks, I would say that pipe diameter is more a matter of what the car is being used for and less about displacement. Most street cars make their usable power at lower rpm’s and smaller pipes aren’t a performance issue. I realize this is a stupid blanket statement (it should be right at home inside this thread), but in general small pipes hurt a race engine and big pipes hurt a street engine. Personally, I like bigger pipes for their acoustical purposes.

 

[Wicked65]

Heres my setup. Hooker super Comp Headers with 1 7/8 primarys with 3in collectors, 3in custom x with 3 in magnaflow. i have video, but id like to get a better video for public. Blown65 has heard it though.

BTW, my fuel line is moved now. That was installed long before the exhaust was during the build. 10AN to the front, 8AN return.

 

[brianj5600]

Well seems how they cruise down the road at a constant 8Krpm’s and most of us here don’t, I think your analogy is poor at best.

after owning many V8’s over the years and few small blocks, I would say that pipe diameter is more a matter of what the car is being used for and less about displacement. Most street cars make their usable power at lower rpm’s and smaller pipes aren’t a performance issue. I realize this is a stupid blanket statement (it should be right at home inside this thread), but in general small pipes hurt a race engine and big pipes hurt a street engine. Personally, I like bigger pipes for their acoustical purposes.

If you look at the original poster's combo, you will see it was built for performance. Why would you put an exhaust system for putting around on a motor that makes power over 6000rpm? Most people size their exhaust for the most it will need to flow, not what it flows most of the time.

BTW, 351 at 800hp would be 3 1/2", 3" would be too small IMO.

I agree, but 3" is about as big as I want on the street. I have been thinking of making a 5" downpipe for serious track outings. Probably exit in front of the front tire.

 

[Vinyl66]

I see no reason to go with less than 2.5" pipes. They are not too loud, they do not cost more, they fit under our Mustangs perfectly all the way out the back, and they will flow fine for most pretty well built engines. 3" pipes are much louder and that is a good reason not to go with them if you want to keep your street car somewhat docile. But higher hp cars such as some good running 347's on here can definitely benefit from 3" exhaust.

Our 4 stroke engines do not need backpressure. Backpressure in the exhaust will put more pressure on the pistons causing the engine to waste hp. After appropriately sized headers and collectors, the freer flowing the exhaust the better.

 

[Max Power]

These arguments can go on for days. All I am gonna say is that if you aren't getting mandrel bent pipes, a 2.5" system is actually closer to 2" because of the level of crush in the bends.

I am running 2.5" mandrel on a 450hp 393. It's good enough.

 

[horseballz]

Update/FYI,
My new Magnaflow #15816:

Magnaflow 15816 - 1967-1970 Mustang 2.5" w/Tru-X Exhaust

just arrived for my 68 Coupe and I must say the stuff is gorgeous. And HUGE!Nice welds and bends and overall well made. I ordered it Monday and got it today, Thursday. If it fits as nice as it looks, it will be the best value I've been able to find from months of research. By the way, they only have 2 of this number left for 67-70 cars and I'll bet dollars to donuts that when they have more, the price will go up. And remember, the price listed on that link is more than we as Stangnetters can get it for! You snooze you lose.
Just my $.02,
Gene