That chart is complete BS. You are not going to get that kind of power out of a stock 460 at the wheels with just a tune-up, headers and exhaust.

 

backpressure, plain and simple, is for 2 stroke engines only! Has been that way forever and hasn't changed. a few feet of anything is all thats needed to scavenge. air leaks close to the exhaust port is the worst thing possible, worst than the whole rest of the system, "period", barring a crushed pipe. This backpressure "needed" thing is like i stated earlier "petty" unless you have a snomobile or weedeater engine under the hood. if your engine needs backpressure to pull or simply drive off idle without bogging then you have a very sick engine with internal or carb problems. if someone here can explain clearly "why" a good well tuned engine needs it, without any other possible problems causing the so called "bottem end blues", please let everybody know.

 

If back pressure were not neccesary, then why would you lose torque when going from a full exhaust to open headers. Yes this is a fact, i know from personal expierence.

 

truckin4life, don't take this wrong but my "junior user" status here is because of my recent enrollment. I'm 53 and built my first engine when i was only 15 and not legal to drive yet. Cars and trucks then were REAL machines, not half computer. I have never owned a vehicle that hasn't benefited from a free flowing exhaust. I know there is a "slight" drop with "open headers" but, who the hell actually runs them onthe street? Add a tight and larger system the rest of the way out the back and there is always an improvement. I have never had an engine lose enough power for me to feel it. Now, i also, have not run anything other than a complete system on the street. Anything not routed out from underneath is bad for the brain, literally. How much power is actually lost off idle and what is it preventing. If it's a drag race issue then get a manual tranny or a high stall converter. If you can't pull your boat out of the water then something else is really wrong or you need a boat pulling engine, it's not the large exhaust pipes. I just wish someone could actually explain why they lose so much and for nearly 40 yrs i had only felt improvements. If its a new and improved supercomputer controlled fly by wire vehicle it's beyond me because i still don't understand why a gas cap not clicked the "correct" number of time can set of a disaster light, or why an itty bitty sensor can cost as much as a complete engine rebuild kit, but that's another issue i won't, or basically i "can't" get involved in. I'm still here to learn and not too old yet to do it.

 

Im not trying to start any kind of argument or insult your intellegen in anyway shape or form, sorry if thats how i came off.
I have run open headers on the road, long story, I had dual exhaust with glass packs exiting right before rear axle, that all got ripped out at the header flanges, i noticed an IMEDIIEATE loss in torque, as my truck was quite a bit slower on acceleration, it was like that for about 6 weeks, then i welded up a new 2.5" straight pipe exhaust and noticed an IMMEDIATE improvement in torque. This is just my personal expierencece. I have rebuilt motors in my time too, while probably not nearly as many as you have, i have rebuilt my fair share. I honestly do belive that a SMALL amount of back pressure is good, but as we all know to much is bad, why is it good, i honestly couldnt tell ya, just something i belive in. With a gasser motor i wouldnt really see the need for anything bigger than a 3" pipe. 3.5" at the max.
This is speaking from my personal expierences, not what ive heard, but from what i myself have seen and felt.

I deff agree if you cant pull a boat out of the water, theres something wrong, and it aint your exhaust ( unless its clogged and full of water ) I do agree the biggest thing toworry about is exhaust leaks, as those can be real killers.

Later,
Don

 

To answer the original question I think the only thing 3" cats would do is lighten your wallet. 2.5" is just fine.

On my truck I have headers running into 2.5" pipe into mufflers then 2.25" tailpipes. I think I actually lost a little bit of power going to headers instead of manifolds...enough to notice anyway...not enough to run out and find a good set of manifolds. As far as the size of pipes and what is good I personally think depends on the motor, stock/not stock, and a few other things. If you are running a single exhaust 3" might benefit you really well. Most duals I would say 2.5" max if mainly stock. 2.25" is good enough in a lot of cases. These are my opinions given the few years I've been around custom exhaust. Opinions may vary.

 

Don, no, you didn't come off bad at all, i was worried i would (lol). I know (as i stated) that there is a slight loss with open headers ( dumb move for the street) and also if you have a damaged or unequal set. You are right as to pointing out anything larger than 3" as being a waste of money basically. Any change in exhast flow must be compensated at the intake end of it also! See where i'm goin'? To just slap on headers and large pipes and expect the job to be done there is totally wrong. Now ya gotta go back and rejet or at least readjust your carb. And if all ya got is a 2bbl you may need to go to 4 to really see a seat of the pants difference. Large exhaust only for people who like it or will take advantage of the mid to high rpm range. Don't put them on a puney, bone stock, small cube, low torque engine to begin with. Equal length and leak free is the ticket from then on. I've also built many Harley Davidsons and if one pipe was not identical in size and length, there was a definate increase in roughness and vibration. Each cylendar could not run the same with one common carb. Thanks for the reply Don.

 

You just hit on one of the main reasons people think they screwed something up when going to a higher flowing exhaust system. Especially when dealing with a carb, the air/fuel settings that were perfect with a stock system are now null and void. Which is probably the reason Truckin4life saw a loss of power when his exhaust fell off. Major change in exhaust flow = running very lean. If already running lean to begin with, now dangerously lean. This equals a power loss. Have I mentioned backpressure anywhere in this post?

Im most cases (except mabye Truckin4life) the change in low end, IF ANY is not noticable.

To all of you that claim you noticed some huge loss of low end from just an exhaust change hire your butts out to the race car builders, because your butt-dyno is far more sensitive than anything they have.

I have said it already once in this thread - a gain of mid range torque will always seem like the engine is suddenly weaker before reaching the mid range. REGARDLESS of whether there is really a loss or not. The engine pulls better at a different point than before, and your brain thinks all kinds of things are different.

My point?
A better flowing, low backpressure exhaust system will not cause....
torque loss
burned valves
babies to cry
the world to stop turning
people to laugh at you
your milk to spoil

Go ahead and claim you lost all torque and the engine could no longer get a moped moving all you want, a dyno doesnt lie.

The ENGINE makes torque, believe it or not, and an exhaust system will not have half the effect so many people on this site claim.