Am I wrong for thinking that using a 3" exhaust is a waste? I've read the whole backpressure thing and that makes sense to me, but, my question is (all things being equal) that if the beginning connection(from the manifold) is only 2 1/2 inches, then expanding it to three is just as useless as SI/DO exhaust setup.

This is not meant disrespectfully towards anybody's setup, I'm new at this "MOD" game and am trying to figure out my next best move. I thought I would be happy with K&N FIPK + Troyer Performance 1715 programmer, but I WANT MORE.

I'm saving for either a turbo/supercharger setup, but in the meantime I want to upgrade the exhaust. Word on the street is dual with a crosspipe and high flow cats will ultimately be in my best interest(might even be right now, depending on who you ask). But the whole 2 1/2" pipe Q boggles me, should I also be thinking about headers, too?????

Thanks for everybody's help(as usual)
Scott


YES!!! I FINALLY FIGURED OUT HOW TO PUT MY TRUCKS PIC IN WITHOUT A LINK!!! AND IT'S 2 A.M. (always thought I did my best work this late)

 

Focus on the "single" 2.5" pipe exiting the Y-intersection in the factory setup. Flow runs from the engine to the rear. At this single 2.5" point, you will never flow anything more rearward. So, yes, 3" behind this is a waste of time exactly like fake duals. If you have enough power to need a single 3" exhaust, you have to increase the Y-intersection exit to 3". This is done by chopping out the Y-intersection and replacing it with a 2.5" DI/3" SI aftermarket Y-pipe. There are a bunch of posts about this mod in the archives if you need part numbers and photos.

 

That's just not so.

Restriction of pipe is a function of diameter(cross sectional area) and length. A few inches of 2.5" pipe from the Y to the expansion of the 3" catback is not a flow restriction to worry about. The fact that you're getting rid of 10-11 feet of restrictive crimped 2.5" exhaust pipe and a crap muffler makes a big difference. And it's all you'll need for a stock motor. I had a look at the Y-pipe on my 2000 5.4 Expy, and it looks like a fine design. It's as good as anything out there short of a Nascar merge collector.

A 5.4 truck should always run a 3" catback. It's probably too much for a 4.6 though.

Dual 2.5" pipes are TOO MUCH exhaust for a 5.4. The only time you should run dual 2.5" is:
1) You have ported heads and cam
2) You have a SC
Otherwise you will stall the exhaust flow at low RPM causing a huge loss of bottom end torque.

I had a Dodge 318 Magnum. I put a 3" hiflow cat on along with a 3" catback. I lost a bunch a bottom end. I went back to the stock cat and custom single 2.5" system and got all my torque back. Later when I did ported heads and cam I put the 3" cat and exhaust back on and still picked up low end torque. Then I dropped a 360ci crate motor in it and went to a custom dual 2.5" side exit system and STILL picked up more low end torque. Because I kept the exhaust sized properly with the motor.

 

3", but not dual 3", right? If dual 2 1/2" is too much, then dual 3" is, too. I always thought duals were better, and they look better, but screw the look, what's better for low end and overall HP?

Thanks for the input.
Scott

 

Show us a legitimate physics equation that proves your hypothesis that 2)2.5" pipes flowing into 1)3" pipe will not have a flow restriction if you put 1)2.5" pipe at the transition. Since "worry about" is vague, include how the 3" pipe is utilized. At the 2.5" transition segment, please include the loss at pipe entry and loss at gradual enlargement equations. [quote] "Restriction of pipe is a function of diameter(cross sectional area) and length"...Since this is not all inclusive, include the additional parameters involved in the flow characteristics for your example.

 

[QUOTE]Originally posted by max mitchell
Show us a legitimate physics equation that proves your hypothesis that 2)2.5" pipes flowing into 1)3" pipe will not have a flow restriction if you put 1)2.5" pipe at the transition. Since "worry about" is vague, include how the 3" pipe is utilized. At the 2.5" transition segment, please include the loss at pipe entry and loss at gradual enlargement equations.

Quote:

"Restriction of pipe is a function of diameter(cross sectional area) and length"...Since this is not all inclusive, include the additional parameters involved in the flow characteristics for your example.

h = f(L/D)V^2/2g Darcy-Weisbach Friction Loss Equation

where :
f = Moody friction factor
L = length
D= diameter
V= gas velocity f/s = cfs/area
g = gravity 32ft/sec^2

Lets compare 6 inches of 3" vs 2.5"

using a standard factor of .02 for gases
and cfm of 600 like an old Holley carb = 10cfs.

(6" of 2.5" pipe) .02*(.5)/.208*(10/.034)^2/2/32 = 65ft head loss

(6" of 3" pipe) .02*(.5)/.250*(10/.049)^2/2/32 = 26ft head loss

Now the rest of a 3" catback at say 10ft of pipe
.02*(10)/.250*(10/.049)^2/2/32 = 520ft head loss (ideal no bends)

So there you go friend. That's as in depth as I'm willing to go.

 

Oh definitely. NO to 3" duals. The only guys needing that are 600+ rwhp '03/'04 Cobras.

The best is a single 3" on 5.4 and a single 2.5" on a 4.6 unless
1) you have ported heads and cam
2) forced induction - SC or turbo

 

That equation doesn't answer the question in my example. Now that you're level of understanding is clear, I'll just suggest blowing through a straw, squeezing it in the middle, and analyzing the airflow.

 

I got a better idea. Why don't you explain to the number one exhaust fabricator for NASCAR why you can't neck down the exhaust flow path for a few inches.

http://www.burnsstainless.com/MergeC...ollectors.html

http://www.burnsstainless.com/Crosso...rossovers.html

 

But you just said above that its OK and not to worry about it.

So is it OK to have a few inches of 2.5 or is it not OK to neck it down for a few inches? You just contradicted yourself.

Max is correct. This has been dyno proven on our trucks by companies like Gibson, Borla and Magnaflow. That is why their systems are 2.5" into the muffler. Some do get to 3" after the muffler but that is for sound and appearance.

 

It is OK and don't worry about it. And Burns Stainless and all the Winston Cup teams say so too.



It is OK. And no I have not contradicted myself. You're just not reading correctly.

I don't believe you're correct on that. I've seen Borlas system and it's 3" in and 3" out. Just remember this, it would take 4ft of 2.5" pipe to equal the restriction of 10ft of 3" at 600cfm. So even a foot of 2.5" premuffler is less flow resistance than the rest of the 3" pipe. Having 3" pipe vs 2.5" pipe over LONG LENGTHS makes a BIG DIFFERENCE. Just remember that part.

 

From your initial statement, it was obvious that you weren't capable of answering your original premise; and you still haven't. Describing the factory Y-intersection as "as good as anything out there short of a Nascar merge collector" is indicative of your knowledge. After years of discussion on this topic, even the supporting vendors now recommend and sell this upgrade. Since you didn't bother searching the PAGES of USEFUL information on the poor flow characteristics of the factory Y-intersection, here's a picture of the inside of the factory Y-pipe and the typical replacements:

http://www.f150online.com/galleries/...w.cfm?num=2504

Your single friction loss formula is a very small piece of the puzzle. If anyone has the intelligence and interest, I will add this link of additional relevant information for them to draw their own conclusions. Here lies the reason why you can't answer the original question. It's impossible because you have no way to postulate the parameters of the multiple formulas if your life depended on it.

http://www.rit.edu/~rfaite/courses/t.../pipe/pipe.htm

Your Nascar references really don't help you. If anything, they prove my point. All exhaust engineers use restriction to tune their systems. Those pictured are used in pairs to adjust backpressure and torque curves. Proving your point in that manner would require you to post a link of a Nascar exhaust system that runs ONE 3" pipe with a poorly engineered 2.5" restriction in the middle of it. If it is appropriate for a vehicle to have a SINGLE 2.5" exhaust system, the SIZE of the factory F-150 Y-intersection is fine. The design and flow characteristics of it are inexcusable at best. Yes, a 3" cat-back will reduce the restrictions in the factory press-bent <2.5" tailpipe where the multiple axle clearance bends reside. Yes, the typical high flow muffler will reduce restriction compared to the factory unit. If it is appropriate for a vehicle to have a 3" exhaust system, no competent engineer would add an additional 2.5" restriction in the middle of it. Since you haven't added one shred of relevant information to this topic and have only shown meaningless interest in a pissing contest, I will no longer respond to this thread. I won't see any responses you make anywhere on this board because you're now on my ignore list. Besides the marginal humor, you've proven nothing but your lack of education and impulse control. I feel that I tried to keep this discussion courteous, but it's obvious to me that I forgot more about physics than you ever knew.

 

MedVader, you are wrong and Magnaflow, Borla, Gibson, Spintech, etc are right.

If Borla has a 3" input how does it connect to Ford 2.5" pipes? Cat back systems are designed to connect to stock pipes from the cat and replace everything back from there, hence the name catback. Most F-150 cat backs are 2.5" on the cat side (muffler input side). As I said some do increase to 3" on the muffler output side.

Again Max is correct and you are wrong.

Scott, buy a Magnaflow catback system and you will have an excellent exhaust.

Backpressure is a misnomer, it is velocity we are concerned with for proper scavenging of exhaust gases. More on this topic can be found by using the search function.

 

You havn't added anything at all to this thread. I suggest you keep me on ignore and keep the useless misinformation to yourself.

I havn't cut open a factory Y pipe, so if the inside is crap, and from the photos, it is, then yes by all means a good Y will help. And it still won't matter much if it's a 2.5" or 3" collector, given 600cfm of flow. But a 3" catbackwill make a huge difference over a 2.5" catback. Pretty easy to follow so far?

Sorry my easy to understand flow equation threw you. But math does make common sense seem stupid sometimes.

You still can't understand what the restriction in the merge collector does. It's not about tuning backpressure, son. It's about MAGNIFYING exhaust pulses like a lens for pressure waves to increase pulse scavenging of the other cylinders. And it augments a well designed longtube header, not the mass produced crap most people are talking about buying. But it is a restriction point nonetheless that is expanded back up to typically 3.5- 4" pipe all the way back.

And dream on about the physics. Remember, Engineering Tech degrees arn't real physics.

 

Ever heard of an expansion joint? I was just looking at the Borla system. It sure looks like 3" in and 3" out, but maybe the picture's misleading. And what's your point anyway? That the 3" pipe is just for sound? BULL. And I just proved it with some very easy to follow math. Even if the first 2 ft of the catback is 2.5", a post muffler 6 ft long 3" pipe will still significantly reduce restriction and increase flow on 5.4 mod motor.

And I agree, buy the Magnaflow system. I just did.