Well. Perhaps the thumbs down was the wrong smilie to use. Unfortunetly, that's one of the disadvantages of the internet. 10% of human communication is words. 90% is body language/tone of voice. I'll better choose my method of implication next time.

 

Shesh Adam, whats with the fng's always wanting to start fights with you?

Anyway, without any kind of backpressure or "restriction", you will loose a whole lot of your low end power. Now that would work fine for a race or drag car, since your going to be living in the higher rpms most of the time, but for a street car in which you plan to mostly live in the lower rpms, you need some backpressure. Not a whole ton, but you need some.

If you really want to go as free flowing as you can, get a 2.5~2.75" exhaust made, with a high flow cat, and two resonators. That should deepen up and slightly lower the sound, give you enough just enough back pressure that your lower end doesn't go to hell, and pretty much offer very little restriction on the high end.

 

I guess...

 

its cause there wanting to get there post counts up so adam i hate you lets have a post war j/k btw get on aim adam heh

 

IMO NJHK was completely correct in his description I read that entire link on the CAT and not one thing in it says he was misinforming anyone. Hell i understood his version a hell of a lot better than that Link. Simplifying thing is good because not everyone on here is a genius. and that IMO

 

Thank You.

Atleast someone understood my purpose.

 

LOL Fine

 

Now you have blind horses following you, willing to drink the water....

Along with another one who thinks backpressure is good for torque.

What's next? You gonna sell someone the Golden Gate bridge?

 

Oh, ok. And I would really like to hear you explain how having no backpressure at all will improve your low end power. Go on, explain it.

You Sir are a troll who needs to quit trying to pick fights with people here. How about you put up some money and become an actual paying member if you wanna keep ragging on people. Because otherwise, all you are is a Troll. Now where is my ignore button.

 

I posted about the myths of backpressure on www.***************

I'm under the same username there as I am here. Look it up if you must.

And by all means, if you feel the need to be blindly lead by myths such as backpressure being good, then please, add me to your "Ignore" list. It won't effect me either way.

 

Why not? I could use the money...

 

I did. It was loud and annoying. Changed it ASAP, false knock, rattles, not good.

Infinityzen: Backpressure is for two-strokes. This isn't 1950, you have a boosted, injected motor with modern electronics. You do NOT want backpressure. Oversized exhaust can cause a reduction in exhaust gas VELOCITY which isn't ideal for horsepower, but within reason, will never reduce low-end torque as a result. It just doesn't happen.

 

Sorry, your right Spinner. Backpressure is not the right word, since we are talking about velocity and thermo dynamics. So what everyone actually wants is an exhaust that an exhaust that is tuned to provide a fairly constant flow velocity capable of handling their engines flow rate from idle to WOT with full boost, that has to take into account the varibles of exhaust temperture throughout that full rpm range.

As of right now, I don't believe any company has managed to design an exhaust that will cover that full range tuned to constantly provide max flow through all the differences of flow rate and temp. Pretty much you ether have guess'ta'ments (including almost all those one off custom fab exhaust and several of the major aftermarket ones) or exhaust tuned for a certain rpm range, ie near idle, mid range, or WOT.

Those guess'ta'ment pipes there is no telling how they will flow until you hook them up. However, I can pretty much guess that those big 3" full pipes aren't getting enough flow rate at the lower rpms, thus causing rapid expansion and cooling of the exhaust gasses and "Backpressure" resulting in loss of low end power.

With a range tuned exhaust, you are going to suffer some loss of velocity outside their range, ether due to cooling, expanding, or restriction. However, a WOT/high rpm tuned exhaust will handle all the flow rate of the engine at WOT (maybe not as much as a 3", but being able to handle more flow then the engine produces doesn't gain you anything but overhead for future mods) with the same or better velocity (I'd guess better) and far less loss on the low end because it doesn't allow as much expansion and cooling of the exhaust gasses at lower flow rates (idle or part throttle). This is the type of exhaust you would want in a drag or race car if you went with a full exhaust instead of a cutout.

The mid range tuned exhaust would work better for a street going sports car. You would still loose a little low end (though not as much) and your high end would start to drop a little sooner, but it is nicely balanced and doesn't suffer any of the larger tradeoffs.

The low/idle range tuned exhaust would be fairly rare except if your going for the best possible gas milage out of a vehicle you can get. Most cars that are not performance cars, if their exhaust is tuned at all, would be a mix of low and mid range rpm flow rate.

 

The long and short of it is, you want an exhaust that flows enough to support your engine.

A good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.) David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss

So, in theory, assuming the muffler flows as much as the pipe, we have (at the flywheel and with 12% drivetrain loss)

A 2.75” single system is good for a 310hp engine with zero loss… (272.8 WHP)
A 3” Single system is good for a 370hp engine with zero loss… (325.6 WHP)
A 3.5” Single system is good for a 503hp engine with zero loss… (442 WHP)
A 4” Single system is good for a 657hp engine with zero loss… (578 WHP)

Now these numbers are assuming that everything else is set up perfectly. The muffler must flow as much as the open pipe or more to get zero loss at the listed hp levels. He also states that using a muffler with a larger inlet/outlet diameter than your exhaust pipe is a great way to get more out of a smaller diameter system since the muffler flow will be able to match the straight pipe flow. Now there are many other things to consider when designing an exhaust system. Lets consider the flipside.

The other side of the equation: scavenging. This is the name of the effect a proper exhaust system should induce. Basically, your exhaust is flowing at a high enough velocity that it creates a vacuum at the exhaust manifold. You want this because, that small amount of vacuum helps facilitate pulling the spent exhaust gases out of the cylinder when your exhaust valve opens on that particular stroke of the camshaft. Specifically tuned length header tubes/collecters can help amplify this effect. Oversized exhausts will lessen/nullify this effect, and that is why they MAY lose power compared to a slightly smaller size, expecially at lower rpms where total flow and velocity is low(er). Remember, you only need enough flow to support the amount of air in/out of your motor with zero or negligible loss due to backpressure. Any more then that, and you lose your scavenging effect. Hope this clears a bit up.


I have two mufflers and one resonator on my 3" exhaust. Each of them is roughly 4-4.5" OD and all are 3" striaght through. Dont forget muffler choice is critical to maintain flow (and velocity).

 

Thank you, Spinner.

It's about time I ran into someone else who actually studies and researches this type of thing and tries to remove the myth about backpressure, instead of blindly repeating something they read once on the internet, whether or not they know it's true.

I believe you're correct on the time frame when this backpressure myth originated. I don't think it pertained so much to two-stroke engines though, although two stoke engines do benefit.

The carburated engines of the early years started getting an automatic choke installed which eliminated the need for a manual choke cable. It used the exhaust as a heat source through a crossover port in the intake manifold that ran right under the carb. That's basically how the carb knew the engine was warm enough to open the choke.

Engineers also realized the heat helped fuel atomization which made the engine more efficient. That's also about the time EGR came along as well as the EVAP system.... blah blah blah

Anyway, there was a valve installed on one of the exhaust pipes which was controlled by manifold vacuum. When vacuum was high, the valve would close, restricting exhaust flow on one side (on "V" style engines. A valve in the actual exhaust manifold was used on inline engines).

The restriction would help slow the exhaust flow and heat the intake manifold. This restriction was referred to as "backpressure".

Good fuel atomization will help increase efficiency, which normally translates into increased torque in the lower and upper RPMs. But keep in mind, the valve used was vacuum controlled. Once you accelerated, or stabbed the throttle wide open, manifold vacuum would drop and the exhaust would be allowed to flow freely. A lot of people (including myself) didn't know or realize this, and would blindly remove the valve thinking they were increasing the performance.

E.G., my 1983 Chevy C10, Scottsdale has the exact valve I'm talking about. A couple of years ago I unhooked it because the vacuum line was old and dry rotted, then I just plugged that vacuum port with a screw. Ever since then it takes a very long time for the engine to become warm enough to release and open the choke plate. And because of that, the engine is very hesitant off the line. I stumbles and coughs, giving the feeling of a lack of low RPM torque.

AAMOF, after it does finally open I can take short trips around town and it's fine. But if I go out on the highway for a while then back to in-town driving, the choke plate closes again because the increased exhaust flow, and lack of that valve slowing it down, causes the intake manifold to cool down. The high-idle will come back and it'll start to hestitate and stumble off idle.

This is also what a lot of other people would experience. They'd take it to a repair shop, the mechanic would hook the valve back up and the engine would run fine again. I'm sure you can figure out the rest.

 

wow lol i just got on to read this and let me clear up the confusion... this has nothing to do with the cat, i was only talking bout the muffler its self. i couldnt get to a computer so i hade my friend post this sorry if it confused you guys. but yes i was only talking about the muffler. and i've decided not to do it. my dealership has a totaled ss/sc with a magnaflow cat back, and a hand full of other mods that are in good shape that i'm trying to get ahold of. thank you for the happy new year! i hope you hade a good new year also Dragon007.. and thanks for the help guys! sorry agaiN!