would 3" pipe be to much for a stock-ish 390FE, the reason I ask is I have 2 40 series flowmaster 3" in 3" out

 

If you dont have the inhale and exhale side warmed up, 3" won't do much good. However, it wont do much harm either so if you already have the mufflers, run them.

 

that's along the lines I was thinking, the guy I got them from ran them on a stock 350 chevy so I figured they'd be fine on a 390

 

Could always get a reducer on in/out and run whatever size you want. Are you running headers? If stock exhaust manifolds 3 inch pipe is way overkill. All it will do is cost about 30% more for the tubing.

 

I drive a 77, F150, 400 ci, Hooked 33” primary headers, with true dual 2.5”s and Flowmaster 40 series. The pipes had 1 each 60 degree collapsed bend and exited in front of the rear wheels. It was as loud as an open pipe Harley. Sounded cool, but the cabin drone at 55 and above was nerve racking. I ended up adding an 18” resonator to each pipe near the headers to kill the drone.
Quick and Dirty Exhaust System Math
Easy Way To Estimate: Your intake system needs to flow 1.5 CFM per engine horsepower, and your exhaust system needs to flow 2.2 CFM per engine horsepower.
Good Way To Estimate: Take engine RPM x engine displacement, then divide by two. This is the intake volume. Use this same volume of air for the exhaust system, but then correct for thermal expansion (you need to know exhaust temps to figure things out).
Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:
Pipe Diameter (inches) Pipe Area (in2) Total CFM (est.) Max HP Per Pipe Max HP For A Dual Pipe System
1 1/2 1.48 171 78 155
1 5/8 1.77 203 92 185
1 3/4 2.07 239 108 217
2 2.76 318 144 289
2 1/4 3.55 408 185 371
2 1/2 4.43 509 232 463
2 3/4 5.41 622 283 566
3 6.49 747 339 679
3 1/4 7.67 882 401 802
3 1/2 8.95 1029 468 935
NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge steel.
The table above is probably over-estimating pipe size, but you can see that a 400 hp vehicle with a dual exhaust system only needs 2 1/4 – 2 1/2 inch pipes. Anything larger is overkill.
How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos

 

On my parents '78 we put 3" single on a stock 400. The '89 crewcab also has 3" single on a 5.8W. My brother put 3" single on his '92 F150 with a 5.8W. His '79 351M runs 3" dual (that was how he bought it). A friend put 3" on a 300 in a bullnose. You will gain some because it is freer flowing. 3" single would probably be all you need, but you can run dual if you want.

When you start looking into exhaust sizes people always talk about back pressure. "I lost power because I lost back pressure." What is actually happening when you upsize your exhaust it you are changing the tune. This will change how the exhaust pulses scavenge the exhaust gasses from the cylinder. Larger exhaust is more typically found on engines that work at higher RPM, so if you do a lot of low end towing you can hurt your power with too big an exhaust. My Tacoma is an example of this. I put a header, high flow cat, and built a full 2.5" for a 2.7l. I lost the scavenging at lower RPM so now it is a little bit of a dog from a stop, but once it starts spinning it will rev very quickly. That would be a less detrimental on a V8, especially if you just run a single 3".

Do what you like though, it is your truck. Just have fun doing it. In my experience, Flowmaster = Dronemaster. I like Magnaflow.

 

Backpressure is always, always bad. What you want is an exhaust large enough to not have back pressure, but small enough to retain velocity. Exhaust velocity helps scavenging, which is essentially pulling the exhaust out instead of the piston having to push it.

http://www.hotrod.com/how-to/engine/...pes-work-best/

Also see this thread on dual exhaust:

https://www.ford-trucks.com/forums/s....php?t=1419231

 

What i'm thinking i will do is run 2.5 all the way to the flowmasters then leave the 3" dumps, i know they drone i went on an hour trip with a buddy in his 72 c10 with dual 40s going 75mph the whole way, but i got these for 10 bucks each so im gonna use them, if the get to loud im gonna put a dynomax bullet in front of them it takes the drone off.

 

Big block other make, pickup length systems 2.5inch vs 3inch. Dyno results. Now just remember what the man at the dyno controls says, you have to think of the whole system from carb to exhaust tip to get the full benefits of the larger exhaust.

 

Thanks for the extra information, one of my racing buddies told me that if i want to run 2.5" pipe i could have the pipe swelled to put the 3" flowmasters on it, i think that is what i'm going to do.

 

As mentioned in post #4 a 2.5 to 3 inch adapter will work on both sides. Available at any parts store..

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