Some clarity on exhaust sizing

Most of my modding has been on GMs. Like most people I tend to slightly oversize exhausts; at least from the cats back or headers back. My Impala SS has dual 2.5". I had a 73 Impala with a 454 that had dual 2.5". I know many Camaro guys use a single 3"

I now need to do a complete exhaust on my 02 F150. At least manifolds and cat-back. The guys on Ford forums are telling me that dual 2.5" will cause it to run very lean and trip a code. They're even saying that a single 2.5" is too big and could cause problems. So I modified my goals and I'm thinking dual 2". That will still be plenty for the 5.4L and 250-ish hp, but I'm curious why they're being so conservative.

Thoughts on what you would do? And are Ford ECMs really that inflexible? I don't want to have to do a custom tune just because I'm putting a bigger exhaust on.

If they're really having any such problems it must be due to some kind of ECM stupidity. In terms of exhaust flow a dual 2.5 is quite reasonable for that engine.

With that engine and amount of power, you need dual 2.25" or a single 3". Anything less is too small and will cost power.

Also, there's no way in hell that'll make it throw lean codes. It should be able to adjust at least +/- 15% based on O2 sensor feedback before it starts complaining. Exhaust changes alone, especially with stock manifolds, won't get anywhere near that. I'm going with "the guys on the Ford forums are idiots" for this one. I'd be really surprised if that small increase in flow pushes it over the edge in terms of air through the MAF too. That would mean they cut it way, way too tight in the factory tuning and barely left room for varying operating conditions and production variation.

It just seems like one of the standard mods they do is a custom tune. Not really against that, just strange that they're having those problems with a factory tune.

On a GM, you could take the exhaust OFF after the cats and not get a CEL.

rslifkin wrote: With that engine and amount of power, you need dual 2.25" or a single 3". Anything less is too small and will cost power. Also, there's no way in hell that'll make it throw lean codes. It should be able to adjust at least +/- 15% based on O2 sensor feedback before it starts complaining. Exhaust changes alone, especially with stock manifolds, won't get anywhere near that. I'm going with "the guys on the Ford forums are idiots" for this one. I'd be really surprised if that small increase in flow pushes it over the edge in terms of air through the MAF too. That would mean they cut it way, way too tight in the factory tuning and barely left room for varying operating conditions and production variation.

That's what I was thinking. The factory uses a single 2.25" or as I like to call it, a drinking straw

what i dont understand is that on the f250 5.4's, which are no different then f150 5.4's and are at the same power rating, have 3 inch exhaust from the Y-pipe to the tail pipe. maybe the factory tune for the f250 is set to accommodate that.

Curious, if you still run a 2.25" cat, does it even matter what size the pipe is after that? Sure, it may change the note of the exhaust, but will it really change the flow?

RealMiniParker wrote: Curious, if you still run a 2.25" cat, does it even matter what size the pipe is after that? Sure, it may change the note of the exhaust, but will it really change the flow?

Nope, there's no advantage to using pipe that allows more flow than the most restrictive part of the system.

For example the downpipe outlet on my 4AGE has a proprietary size somewhere between 2.3~2.4in (I think it was 2.38?). I put a 2.5" exhaust on it, but if the rest of the pipe were the same 2.3whatever size it would make no flow difference. To get really technical, it might even perform better because it would eliminate the small step-up in exhaust diameter, which reduces exhaust velocity.

What ever you do, make sure you take out the y pipe, thats the most restrictive part on those. Single 3in or dual 2.25 would be fine. 2.5 if you want to spend more money for no gain.

No, quad 1.5" would equal a single 3". Dual 2.25" is similar to a single 3" in flow.

rslifkin wrote: No, quad 1.5" would equal a single 3". Dual 2.25" is similar to a single 3" in flow.

Your right, i got mixed up. I edited that part out of my previous post.

By my calculations, 1.5" will only flow 1/6th of a 3" at the same static pressure drop. 2.25" is just less than half of what a 3" will do at the same S.P.D.

GameboyRMH wrote:

RealMiniParker wrote: Curious, if you still run a 2.25" cat, does it even matter what size the pipe is after that? Sure, it may change the note of the exhaust, but will it really change the flow?

Nope, there's no advantage to using pipe that allows more flow than the most restrictive part of the system.

For practical purposes I agree, however there is something to be said for friction over long lengths. It won't matter much for me because I will likely dump them out the side in front of the axle. But a 10' length of 2.5" pipe will flow less than a 2' length of 2.5". It has little real effect in length with automobile applications. The other thing to consider (which supports your theory of bigger isn't better) is that the exhaust gasses cool and therefore reduce volume. My Impala SS came with 2" duals from the factory that necked down to 1-7/8 after the resonator; the theory being that the smaller pipe had little effect after that much cooling.

Splitting hairs.

Dual 2.25" it is then.

Now if I could only find a decent set of manifolds. The factory ones crack, Dorman aftermarket manifolds crack and warp, and I'm not keen on opening the big wallet for shorty headers.

Have you looked much into the ebay headers? Dads got a 5.4 hes had for a decade or more and he combats the "ponys leaving the stable" after 300k miles by adding more go fast parts. Headers are the next step for him but i dont know if people had good luck with them not cracking like a lot of ebay headers like to do.

There's always the snail shaped exhaust modifications...

It seems to me that the Cats would be the restriction in the exhaust system so what ever you put down stream shouldn't effect things that much unless going way to big. On a 5.4L a single 3 inch if OK, a dual 2.25 or 2.5 is too. But really on a truck which if driven like a truck should rarely will see the high side of 4K rpm. So a single 2.5 would work too.

Dual 2.5" pipes work on everything with a V8.

rslifkin wrote: There's always the snail shaped exhaust modifications...

As long as its behind a diesel... which has also crossed my mind.

I dunno, but I'm guessing the exhaust isn't the only thing they're changing, and the combos may be what's messing up their AFR measurement.