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Discussion Starter #1
As I'm getting closer and closer finally I'm thinking ahead for the exhaust.

I did do a search and found a post where Scotty said 500hp was the fine line between 2.5"-3" and sure enough that's about where I planned to end up with this build.

Streetable 521
10:1 engine
Iron heads with 2.25”/1.74” Ford Racing valves built by Paul K.
Hydrualic cam specs approx .550/580 from Parkland Machine that Paul ordered for me to match the direction I wanted to go in with vehicle data.
Roller Rockers
Performer RPM intake
And either the Fitech Meanstreet 800 or the Holley 850 I just picked up.
Headers with 1 3/4 primaries and 3" collector
C6 Trans that I feel I just over paid getting rebuilt (Slaps forehead)
9" Rear with a 3.50 Locker in it.

I DON'T plan on going over the rear axle so I don't have to worry about too much bend restriction I don't think. I was planning on dumping out the side in front of the rear tires. Leaning towards buying a stainless kit and welding it in myself and just running a H-pipe. I'm not too worried about it sounding loud or drone but I kinda want something different then Flowmaster since that is what is on 90% of the classics in my area because it's the go-to muffler for the local custom shop.

I'm leaning towards 3" mainly right now because if I ever get the inclination to throw more cam or cubes (Because I'm dumb like that and figure who needs a retirement anyways?) I will have room to grow.

Advice, experience and critique is appreciated. Thanks all!
 

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last two BBF cars I did...wouldn't think of anything less than 3 1/2,back to the axle.Both were 521's,maybe a little hotter than your deal.Both were street cars. This came with the expected wicked sound that was cool at first but it didn't take too long for the noise to start wearing on me,to the point of rarely driving the cars. Tried the in front of the wheel thing with big oval tubing which was worse.Finally went from 3 1/2 collectors to 2 1/2 pipes to the bumper.Used an X pipe and some off brand cheap turbo mufflers and fittings from Summit. It's still loud enough but enjoyable to drive without ear plugs. I never noticed one bit of performance loss that was always assumed would happen with the smaller pipes.
 

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As I noted in my post via my personal experience on my 560+ hp 11 to 1 460 inch powered 1973 LTD...

The addition of the 3" mandrel bent exhaust significantly improved the top 2000 rpm of my power band.
Peak torque numbers were not hurt BUT. Throttle response and part throttle torque softened up a bit.

At 521 inches you will be fine imo.


SJ / Parkland auto machine
used to be RHP

.
 

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Discussion Starter #4
As I noted in my post via my personal experience on my 560+ hp 11 to 1 460 inch powered 1973 LTD...

The addition of the 3" mandrel bent exhaust significantly improved the top 2000 rpm of my power band.
Peak torque numbers were not hurt BUT. Throttle response and part throttle torque softened up a bit.

At 521 inches you will be fine imo.


SJ / Parkland auto machine
used to be RHP

.
Just how much low end would you say you lost? I'm in a 79 F100 so I'm thinking I'm going to have traction woes to begin with, lol
 

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Reread my post. Peak numbers not so much.
Part THROTTLE RESPONSE and torque are softer.

Chassis dyno run after 3" mandrel bent exhaust with magnaflows.

456 NET RWHP




SJ
used 2b RHP


.
 

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Discussion Starter #8
Oh, I thought you were referring to the post from 2011 I read the other day. I got confused.
 

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I'm almost thinking with the truck a little softer hit off the line might not be a bad thing either. Thanks again!
At WOT from a standing start isn't where I noted the difference. I have a 3450 stall converter based on a 12" OEM core.
Part throttle, tip in, throttled torque were softer.


SJ
used 2b RHP
 

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I'm starting to do my exhaust now on my 72 F250 4x4 w/460 @ 425hp. Going with L&L mid length headers, 1-3/4 primaries. 2.5" from collectors to a single, dual inlet/single outlet muffler. 3" tail pipe, dump under bed to reflect off of ground. Muffler is a Jones max flow (magna flow know off) straight thru, welded SS. I think this system will flow enough for my application, which is a big heavy truck with a motor built for lower rpm torque.

Exhaust gas velocity is inversely proportional to area.
3.0d" tube -> 1.50r^2 * pi = 7 in^2
2.5d" tube -> 1.25r^2 * pi = 4.9 in^2
3" is 42% more area than 2.5".

My 04 Cobra makes 600 hp/700 tq with stock manifolds and 2.5". Most cobra guys don't go 3" until 900-1000 hp. Different applications have different needs.
 
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