I am running a 351w with headers. The motor is bone stock out of a 96 F250. I am running a t-5 so I am not making big power and maybe torque than I should. Is 2" exhaust big enough or should I be running 2 1/4. I will be running flowmaster mufflers with cutouts. I am looking to go as small as I can without producing backpressure.

 

Personally I would go with 2 1/2 inch. That way you still have a decent amount of back pressure, and if I am not mistaken 2 1/2 inch exhaust is good for up to 300 or so hp without restricting flow too much. I have a 289 and I run 2 1/2 and it runs good. It is up to you, but I would not go under 2 1/4. oh and a certain amount of back pressure is neccesary or your hp numbers drop.

 

I think you mean "columnar flow", which is a utilization of the inertia of the moving gasses, which is what equal-length headers use to increase power. This is why cutting off the tailpipes actually reduces power, for example. Back pressure is not desirable, at all.

The stock 428SCJ and BOSS 302 used 2.25 inch exhaust, for example. But then, so did the 351. Oddly, the 67 GT500 428 8V engine used 2" exhaust, because that's what they had, from the 390GT.

 

well then why don't we all have a party and put 6 " exhaust pipes on our cars to reduce backpressure. Cars come from the factory with restrictive exhaust. That is why you put on headers, and larger exhaust system. You reduce the amount of back pressure, you are not trying to get rid of back pressure all together. You need a certain amount of back pressure in your exhaust if your car is n/a. too much back pressure and performance is reduced. too little back pressure and performance is reduced. The only time you really do not have to worry about back pressure as much is when you are running a forced induction system.

 

I would go 2 1/2" this way you are good to go with any future mods you may want to do down the road. I had 2 1/4" and went to 2 1/2", should have gone 2 1/2" from the get go instead of spending extra money. Live & learn.

 

The columnar flow is exactly what I am looking for. It is where the exhaust is small enough so the exhaust get the most velocity heading out creating a vacuum pulling the next cylinders exhaust out. Too big of exhaust pipe will lessen the velocity thus reduce the pull for the next cylinder. I guess, I am looking for the smallest pipe size I can go without making any back pressure.

 

if your using cast ex manifolds then go 2 1/2, if using headers run 2 1/4 if using a stock 351w with no major mods, you can run 2 1/2 with headers as well but the 2 1/4's will help low end on a stock 351w with headers. Dont forget an H pipe.

 

yea I second that. 2.5" will give you a small amount of room to grow as well if you really want it that way.

Im running a warmed over 289 with 2.25. Im at the point now 325bhp I think Im loosing power from it... Once the new motor is done, exhaust upgrades will be in the near future for my car.

 

My sons and I have installed 2 1/2" stuff in the last 2 projects, both 351/408 engines, the first about 500HP or so the second about 450 HP.

My next project of similar power will be of 2" pipe. This is for ease of installation and for a "more traditional" look that I will be trying to achieve.

Besides, I think 2" is "big enough" for all practical reasons.. (could be wrong about that.... )

 

I'm going to put a 2.25 inch head pipe on my '65 because I need to get the hot exhaust away from my engine or else I'll keep cracking manifolds