Backpressure

ponysarepretty · Nov 4, 2005

Whenever I say something about having straight pipes, someone always mentions something about back pressure and not having enough low end torque. WIth that in mind, whenever someone mentions something about getting a catback, people always denounce it as being stupid and useless compared to an O/R X pipe.

Now which restricts more? The 4 cats or the 2 mufflers? I mean if you got a O/R X wouldn't you lose more backpressure than on straight pipes or resonators?

todd4566 · Nov 4, 2005

The stock H-pipe is a greater restriction that the factory cat-back.

Figure this:
Buy an off-road X/H pipe for $100+, get +/- 10hp (peak).
Buy an aftermarket cat-back for $300+, get 5hp.

Cat-back's are more of a cosmetic mod IMO.. unless your making serious HP, in which case a stock cat-back could be considered the bottle-neck when combined with upgraded headers & mid pipe.

sgarlic · Nov 4, 2005

ponysarepretty said:
Whenever I say something about having straight pipes, someone always mentions something about back pressure and not having enough low end torque. WIth that in mind, whenever someone mentions something about getting a catback, people always denounce it as being stupid and useless compared to an O/R X pipe.

Now which restricts more? The 4 cats or the 2 mufflers? I mean if you got a O/R X wouldn't you lose more backpressure than on straight pipes or resonators?

Who denounced getting a catback? That's practically everybody here's first mod besides the silencer removal. Catbacks are about sound.. they are more or less an aid in getting your exhaust to sound how you want it. They aren't designed to give you tons of extra horse power. You gain a few, due to the bigger diameter pipe, and some due to the "high flow" mufflers.

You don't feel a difference with a catback in power, but you do with sound.. I ran my foxbody with no mufflers for a long time, and although it sounded fine, the low end power loss was agonizing.

ponysarepretty · Nov 4, 2005

Denounced the catback as a means of gaining horsepower I mean.

"You don't feel a difference with a catback in power, but you do with sound.. I ran my foxbody with no mufflers for a long time, and although it sounded fine, the low end power loss was agonizing."

That is exactly what I mean... you say that the catback makes no difference but then go back and say that the no mufflers isn't aweful? Am I missing something here? And isn't something like the SLP LM just straight pipes?

DogVomit · Nov 4, 2005

Forget that garbage...

Anything that's spent. needs to be expelled as quickly as possible, with the least resistance... PERIOD!

If someone is looking for a certain hum or rumble in the sound of thier exhaust, they're not looking for power.

Dragsters wouldn't run straight pipes if it didn't reduce BACKPRESSURE...., nor would guys look for X pipes, etc.

In quick and cool, burn rappidly, exhaust without pressure.

That's it.

ponysarepretty · Nov 4, 2005

No... I want to hear the logic...

If the Cats are the major obstructions, doesn't an X pipe rob A LOT more low end power than having no mufflers?

squares98 · Nov 4, 2005

Different Engines require different amounts of backpressure. If you were running open 2.25" piping you might see a gain, but since you are doing 2.5" you do not have enough backpressure for an engine of your horsepower level to function optimally. Adding exhaust to a 2.5" pipe will create enough backpressure to put your engine in an optimal air flow range, creating more low end power

DogVomit · Nov 4, 2005

Huh?

Once the exhaust valves are done with it, there's nothing left, aside of expusion.

Are we talking about stock, with cats and sensors?

What's this back pressure garbage about? Is there a sensor on anyone's tailpipes to say that it's releasing the spent gas too quick?

sgarlic · Nov 4, 2005

ponysarepretty said:
Denounced the catback as a means of gaining horsepower I mean.

"You don't feel a difference with a catback in power, but you do with sound.. I ran my foxbody with no mufflers for a long time, and although it sounded fine, the low end power loss was agonizing."

That is exactly what I mean... you say that the catback makes no difference but then go back and say that the no mufflers isn't aweful? Am I missing something here? And isn't something like the SLP LM just straight pipes?

I don't understand how that statement confused you. I said you don't gain much horsepower from adding a catback (an aftermarket catback versus your stock one,) but you gain some better sound.

Then, I said I had straight pipes on my 5.0, and although it sounded fine I lost lots of backpressure, so it felt like I lost low end torque.

So, to clear up whatever is confusing you: The catback MAKES a difference in sound, but not much in gaining hp/torque over the stock catback. The SOUND of no mufflers is good, but results in a loss of power.

DogVomit said:
Huh?

Once the exhaust valves are done with it, there's nothing left, aside of expusion.

Are we talking about stock, with cats and sensors?

What's this back pressure garbage about? Is there a sensor on anyone's tailpipes to say that it's releasing the spent gas too quick?

Unbolt your midpipe, and take a run with open exhaust manifolds at the dragstrip, then bolt them back up, and take another run, then come back and tell me no back pressure is good. No backpressure on a top fuel dragster, may be fine, but it isn't fine on a street driven vehicle.

DogVomit · Nov 4, 2005

sgarlic said:
I don't understand how that statement confused you. I said you don't gain much horsepower from adding a catback (an aftermarket catback versus your stock one,) but you gain some better sound.

Then, I said I had straight pipes on my 5.0, and although it sounded fine I lost lots of backpressure, so it felt like I lost low end torque.

So, to clear up whatever is confusing you: The catback MAKES a difference in sound, but not much in gaining hp/torque over the stock catback. The SOUND of no mufflers is good, but results in a loss of power.

Unbolt your midpipe, and take a run with open exhaust manifolds at the dragstrip, then bolt them back up, and take another run, then come back and tell me no back pressure is good. No backpressure on a top fuel dragster, may be fine, but it isn't fine on a street driven vehicle.

I'm slightly confused.
Are we talking about an internal combustion engine, or where it's driven?

Okay, If I take my lawnmower, or moped (so we share the same visual) and I tell that 2 stroke/cycle engine to increase back-pressure, I should then expect higher output.

So if I cover your A$$ cheeks with a tight woven cloth, then tell you to take a dump, or merely a fart, will your motor require more energy for the PUSH???

Get it?

PUSH requires energy.

Free flowing like a "silent and deadly" requires the system to do what it does... release, without denial.

My uncles tombstone reads.... "Fart free, where ever you be. Withholding this fart, was the death of me.

So, let it blow out your tailpipe.

ponysarepretty · Nov 5, 2005

sgarlic said:
I don't understand how that statement confused you. I said you don't gain much horsepower from adding a catback (an aftermarket catback versus your stock one,) but you gain some better sound.

Then, I said I had straight pipes on my 5.0, and although it sounded fine I lost lots of backpressure, so it felt like I lost low end torque.

So, to clear up whatever is confusing you: The catback MAKES a difference in sound, but not much in gaining hp/torque over the stock catback. The SOUND of no mufflers is good, but results in a loss of power.

What about an O/R X pipe, aren't the cats the MAJOR obstruction. If so then you would lose MORE backpressure by taking the cats off than having straight pipes in lieu of mufflers? Then isn't everyone who has an O/R X basically killing their low end torque?

ponysarepretty · Nov 5, 2005

I want an answer please

CanadaStang · Nov 5, 2005

In a nut shell:
Lots of back pressure = low end torque, less Hp in the higher rpm range.
No back pressure = less low end torque, more Hp higher in the rpm range.

Some of the newer Honda Sportbikes use a remote flapper valve in the exhaust that closes to provide more back pressure for low end torque but then opens at 6000rpm to provide less restriction in the exhaust thus a strong top end as well.

ponysarepretty · Nov 5, 2005

CanadaStang said:
In a nut shell:
Lots of back pressure = low end torque, less Hp in the higher rpm range.
No back pressure = less low end torque, more Hp higher in the rpm range.

Some of the newer Honda Sportbikes use a remote flapper valve in the exhaust that closes to provide more back pressure for low end torque but then opens at 6000rpm to provide less restriction in the exhaust thus a strong top end as well.

Yeah lots of high end cars have that, I have nothing against some backpressure. But people who say i lost too much torque by having no mufflers don't say that x pipes lose torque. yet people say the H pipe is a greater restriction (therefore a greater source of backpressure) than the mufflers.

CanadaStang · Nov 5, 2005

ponysarepretty said:
Whenever I say something about having straight pipes, someone always mentions something about back pressure and not having enough low end torque. WIth that in mind, whenever someone mentions something about getting a catback, people always denounce it as being stupid and useless compared to an O/R X pipe.

Now which restricts more? The 4 cats or the 2 mufflers? I mean if you got a O/R X wouldn't you lose more backpressure than on straight pipes or resonators?

Depending on the mufflers of course, but I would think they would provide more back pressure than just running 4 cats and straight pipes, but running just the cats may make it sound crappy at high rpms, and with no X, also less scavanging abilty.

CanadaStang · Nov 5, 2005

ponysarepretty said:
Yeah lots of high end cars have that, I have nothing against some backpressure. But people who say i lost too much torque by having no mufflers don't say that x pipes lose torque. yet people say the H pipe is a greater restriction (therefore a greater source of backpressure) than the mufflers.

It would be good to run both systems on a dyno to find out for sure. I just have straight pipes after the cats on my 91' but the 5.0's have lots of low end torque. I never tried it on the 4.6 though, so I really can't say for sure.

ponysarepretty · Nov 5, 2005

high rpm is just a deafening roar... sounds good

CanadaStang · Nov 5, 2005

ponysarepretty said:
high rpm is just a deafening roar... sounds good

When your ahead it doesn't matter what it sounds like

timeless2 · Nov 5, 2005

The z06 Vette's exhaust combines the best of both worlds in regards to backpressure, low-end torque, and top-end HP.

Much steeper price tag, mind you.

ponysarepretty · Nov 5, 2005

4u2nv said:
The z06 Vette's exhaust combines the best of both worlds in regards to backpressure, low-end torque, and top-end HP.

Much steeper price tag, mind you.

Porsches and Ferraris have had this for quite a while, its more to pass noise emissions than for any backpressure purposes IMO.

Backpressure

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