What size exhaust- 351c 4v 400hp

My question is in two parts. First, what size exhaust should I use? Second, will the exhaust flow only as well as the smallest pipe (or muffler) will allow?

My car is a '73 Mach-1 with 10:1, a cam, Air Gap intake and Super Comp headers. The original (when I bought it) 3 inch exhaust was all hacked up. When I had the exhaust done, I had it done with Magnaflow mufflers but they did 2 1/4" instead if 2 1/2 " like I had wanted. Then, they only did turn downs because they said the fuel line was too close to the drivers side (exhaust exit).

I still want (I think) 2 1/2" dual exhaust. But if I do a 2 1/2" exhaust but only use. 2 1/4" tail pipes, wouldn't that basically make the whole exhaust flow like a 2 1/4" system? Thanks..

Mike

Although I don't have data to back it up there seems to be some general agreement "out there" that the tail pipes can be smaller than the exhaust system before the mufflers without significant losses because the exhaust is so much cooler (denser) by the time it gets back there. I also think that if you run an x pipe you have a little more freedom after the x pipe. For 400 FWHP I'd try to run a 2 1/2" system.

I'm thinking 2 1/2" exhaust, FTW!

I picked up a nice 2 1/2" stainless X-pipe Pypes system for mine from Summit Racing. Everything you need from the collectors to the tips for $440, but they have a bolt-in steel system (with mufflers) for $200.

They also have a 2 1/4" system that you could shelf the mufflers (to keep your Magnaflows) for about the same price.

Most systems will be for headers that exit straight back, but the Hookers point slightly in toward the center. Having a muffler shop bend the lead pipe slightly should get you there, though. I have standard Hooker Comps, but I think the Super Comps are close.

Here's mine - bolted in, took about an hour (on a lift at the Auto Hobby Shop... probably take a bit longer jacked up from the floor).







Hope this helps!

Although I don't have data to back it up there seems to be some general agreement "out there" that the tail pipes can be smaller than the exhaust system before the mufflers without significant losses because the exhaust is so much cooler (denser) by the time it gets back there.

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this is incorrect

I also think that if you run an x pipe you have a little more freedom after the x pipe. For 400 FWHP I'd try to run a 2 1/2" system.

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2 1/4" would be the smallest and 2 1/2" would be ideal mufflers should be straight thru.

I still want (I think) 2 1/2" dual exhaust. But if I do a 2 1/2" exhaust but only use. 2 1/4" tail pipes, wouldn't that basically make the whole exhaust flow like a 2 1/4" system? Thanks..

Mike

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yes . . also use free flow mufflers . . magnaflow super quiets if you want them quiet in the cab.

I'm thinking 2 1/2" exhaust, FTW!

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nice car and no leaks!

It's only close if your exhaust guy sucks. Also, tweak the sender outlet and line to make it tight to the tank.

Sounds great everyone. Thank you. I also appreciate the pics..

IMO, you're kinda caught between sizes.

I think either will work just fine on a street car. If you were after the last .01 of a second in the quarter, then maybe the 2.5" would be the best choice.

There is no right or wrong answer.

On my '73 Torino, the Walker factory replacement 351C dual exhaust did have smaller diameter tailpipes than the H-pipe was. I do not know what size they were, it was verrrrry long ago. If I had to guess, they were 2.25 and 2" tails.

Purely a guess, though.

I just upgraded from 2 1/4" to 2 1/2". No drama. Actually more room as it's bent up properly.

I just upgraded from 2 1/4" to 2 1/2". No drama. Actually more room as it's bent up properly.

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Did you notice much as far a performance goes?

I also have Hooker Super Comp headers and those run parallel to the drivetrain unlike the Hooker Comp headers that angle inward. I used the PYPES SFM04S 2-1/2" system too with 3" rolled slant tips from Flowmaster. Chose the Street Pro mufflers as I wanted the interior sound levels to be low. The slant tip is not as pronounced as the original but I think that they still look good. I had no clearance issues by the tank. Attached a video that Hyena429 took, you can get an idea of the exhaust tone.

Did you notice much as far a performance goes?

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Honestly, I can't say. The car sounds so different and perceptions don't always translate into performance. Seems like it has a tad more low end torque actually, but again that's the perception.

My engine has a lot of shortcomings in its build so I upgraded the exhaust now, as I didn't want it to be a restriction when the new engine goes in.

I have a set of Super Comps that i'm planning to use on the vert.

That PYPES SFM04S 2-1/2" system sounds great. Any drone?

While they don't post the math behind the contents of the chart, I assume there is a factual basis, other than wanting to sell exhaust systems, for the content. Chuck

http://www.magnaflow.com/07techtips/faq/question10.asp

I also have Hooker Super Comp headers and those run parallel to the drivetrain unlike the Hooker Comp headers that angle inward.

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That's good to know info, J! ::thumb::

Don, I have the same Street Pro mufflers on mine - basically the same system, actually. I can't speak to any kind of 'drone' yet since it's not quite on the road, but you'll hear it in-person soon.

I also think that "too close to the fuel line" is something of a misnomer. If you have a ratty exhaust along with a questionably leaky fuel system, sure you'll have problems. Exhaust tail pipes don't get all that incredibly hot on 'normal' street cars, after all - the 1/8" difference in airspace between 2 1/4" and 2 1/2" (if they run in exactly the same place) won't be enough to make a difference. If it's that bad, have them make a little heat shield or something similar - if they can't figure it out, I'd suggest finding a new exhaust shop.

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just find a place that sells heat barrier tubing/sleeves and slip around a 3 - 4" long piece of it over the fuel line . . they use it on spark plug wires also.

http://www.thermotec.com/products/14040-ignition-plug-wire-sleeving.html

http://thermotec.shptron.com/p/1-2-x-3-black-heat-sleeve

Pypes 2.5" all the way. Great fit.

Bought 15* mandrel bent pieces to mate with the Hookers.