Will I loose backpressure with the addition of a Flowmaster 70 muffler or full cat-back? same question goes for the gibson model. please explain backpressure to me while your at it

-bigdawg

 

Backpressure is the amount of pressure in the exhaust system while you are driving. You will loose backpressure which is a good thing, your EGT's will be lower, you will get better MPG and have more power because the engine is not fighting its self.

 

Have a friend that has a 1999 F350 4x4 with psd it has a whistle while driving. He has been told that it is because of his stright pipes not giving him enough back pressure and the turbo is causing the noise. Is this true and or any ideas what it is?

 

Turbo's by nature whistle. It's more noticeable with straight pipes. If he put a tymar or other open element air filter on it it would become even louder.

 

Thanks RRMike
So this whistle is not going to hurt or damage anything?

 

Not my words but it's what I would say if I felt like typing a whole lot tonight.

I *hate* the word backpressure. Hates it, my precious.

"Backpressure" is an illusion. A figment. A fiction. An imaginary item
like that mythical monster of mechanics - centrifical force.

I get peeved about backpressure, becuase when most people say
"backpressure" what they really mean is "flow capacity", and if they
knew they really meant "flow capacity", they'd have a better
undestanding about what goes on in their exhaust system.

So, lets talk about "flow capacity" in a "backpressure" context, which
means: let's talk about exhaust systems.

Ever tried running while wearing a gas mask? If you haven't, don't. It
sucks. Oh sure, it starts out fine enough (except for that perhipheral
vision thing) but before long, your body starts requiring more air than
the mask filter is willing to flow. The natural reaction is to breathe
even harder, but no matter how hard you try and pull in, you never get
enough. It feels a little like drowning (I guess) except that you are,
in fact, getting enough air to survive - but it's a really nasty
feeling.

Well, that gas mask is a flow limited system, much like your average
exhaust. It flows a certain amount of gas at a certain pressure, and
then that's it. No more. If you want to try and cram more mass into it,
you can (gasses compress after all) but the pressure in the system is
going to rise at the end getting filled, and then slowly drop as you
move downstream.

OK, get a straw, pinch one end down so that it's almost closed, and try
to blow a lungfull of air through it as fast as you can. Tough, isn't
it? Gotta blow real hard eh? It's almost like it was pressing back at
you, right? Backpressure, right?

Well, that's what the muffler companies call backpressure, but that's
not backpressure, not the way I define it. That's your lungs
encountering the flow limit of our system, raising the system pressure
to match the internal pressure in your lungs, and then maintaining that
pressure while it bleeds off.

Now get a pipe, like a 1/2" garden hose, and try again. Boom! All at
once, in a burst. Reduced backpressure? No, increased flow capacity! In
fact, that ol' garden hose has a higher flow capacity than your lungs,
which is exactly what we want for our exhaust system. Aha!

So what is backpressure then?

Take the same hose, attach a balloon on the end. Breath out, and the
balloon inflates. When you can't breath out any more, the pressure in
the balloon and in your lungs has equaled out.

Now have your wife sit in the balloon. Lungs just filled up again,
right? Backpressure. (More correctly - flow reversal) Something that
happens all the time on naturally asperated engines, and never on a
properly designed turbo engine, under boost.

I wish, I wish, I wish that muffler and exhaust system manufacturers
would tell you the flow capacities of their systems instead of talking
about backpressure. I know how much air I need to flow, I know how much
fuel I need to flow, I should be able to select an exhaust system by the
amount of exhaust I need to flow. *sigh* Oh well.

Summing up: Unless your exhaust is flowing backwards, you've got no
backpressure. You may have a flow capacity problem, that you can solve
by installing an exhaust system with a higher flow capacity, but that
ain't backpressure.

Dennis Grant

 

Not if it's the turbo. It's a little lower pitched whistling howl I would say. Less high pitched than a vacuum leak for example. Kinda like a jet engine sound but without the roar. Does it get louder as the throttle is increased ie. climbing a hill? On mine I roll it up to about 16-1800 rpm's real easy then just mash it and the increase in exhaust gas when you floor it causes the turbo to spool up and howl like a mashed cat. I love it.

There could be things wrong with the motor like vacuum hoses or intercooler tubes for example but your mention of the straight piped exhaust makes me think it's just more turbo noise you're hearing. If you can hear it from the exhaust pipe then it is almost certainly the turbo you're hearing. I would find someone who's familliar with turbo diesels and have them listen to it if you suspect a problem but I don't think you have one.

 

Very good Monsta!

Fitter

 

So then, why do we have discussions surrounding the loss of torque as "backpressure" decreases? There is another thread that I'm following here regarding hollowing out cats (I know, I know) where the argument against it is that backpressure and therefore low-end torque is reduced. By the way, that thread and my question pertain to NA gas engines.

I'm not trying to argue; I'm really curious.

 

Okay...here goes.

The exhaust flowing through your pipes is not a continuous stream, it's
a series of gas pulses. These pulses have mass, they have inertia, and
they have velocity.

Let's examine something else with lots of mass, inertia, and velocity -
a semi trailer truck. Ever been pulled over on the side of a road when a
semi goes by? If it's moving fast enough, the low pressure area behind
it can almost suck you back onto the road.

OK, now pretend you're standing in a primary tube as an exhaust pulse
goes by you. Same thing - the pulse tends to grab you and carry you down
the pipe, but if it doesn't, then low pressure area behind it might suck
you in behind it. The faster the pulse, the bigger this low pressure
area, just like the semi - except that as the pulse seals off the pipe
(think of a semi in a one lane tunnel) it has a _very_ low pressure
area.

Here's where we get tricky. Whenever the pipe changes diameter, you get
a reflection wave where part of the pulse bounces back up the pipe. And
thanks to resonance and some other gas dynamics, you get reflections of
the low pressure waves as well, sort of an "anti-wave" moving back UP
the exhaust.

Honest.

If an anti-wave peak arrives at the exhaust valve just as it opens, then
the pressure just past the valve is lower than normal, and the reduced
pressure allows more of the exhaust to leave the chamber. This is
"scavenging", and it makes more power. In some cases, it makes a LOT
more power.

Now if we want to try and make sure that one of these anti-waves is
always outside the exhaust port, we have a few changes we can make.
Firstly, we can alter the resonance of the pipe by varying its length.
This works very well, but for a limited RPM range. This is "tuning" the
exhaust.

The other option is to make the anti-waves bigger, by increasing the
velocity of the pulses. Velocity is related to pressure, so the best way
to increase velocity is to reduce the volume (diameter) of the pipes.
The smaller the pipes get, the better the scavenging. (you can do it by
insulating the pipes as well, but there's a practical limit to this)
However, by reducing the pipe diameter, we also reduce total flow
capacity, so at some point further reduction is counter productive. Like
everything else, there's a compromise.
Bigger primaries trade low end torque for high end power. Race engines
need the top end, so they typically have big pipes. Street engines, even
hot ones, need the low end more than the high end, and so get more out
of smaller pipes.

Note that this does NOT apply to turbos under boost, nor does it apply
to the pipe(s) after the header collector. Once you've merged the
primaries, the bigger and freer flowing you get, the better.

Does that help at all?

 

Sure does; thanks a lot. That's the sort of answer I was looking for. So, unless I completely missed the point, increasing flow at the cat (past the y-pipe), whether by hollowing out or by replacing with a high-flow unit, will not harm low-end torque. However, rebuilding a 2-1/4" y-pipe with 3" pipes would.

 

Good post Monsta! If anyone here is familliar with 2 stroke engines and tuned pipes they use a similar principle. The pipe is set up to create a ressonance wave at a particular rpm that in effect bounces back and hits the exhaust port at the time it's uncovered to prevent excessive fuel/air from escaping and being wasted. That's why different pipes will give a dirt bike for instance more low end or top end depending on which brand or type of pipe it is. It's all in the tuning.

 

you guys are great, thanks Monsta now if i can only read my school work like i can truck stuff hahahahaha.

 

With all that being said why would anyone want to mod their exhaust? A normal street driven truck will have the torque band in the right place for optimal performance in every day drivivg..
Yes you might gain some ponies wide open but not too many of us drives that way...I have heard of guy's saying that some of the low end is gone but that I really can't tell cause of the noise increase.. Thats why exhaust makers torque headers have small pipes to keep the flow down at low rpm's....Its my opinion that any increases in performance for a 5.4 6.8 sd are not worth the effort, Unless you want a little sound.

 

I agree with 99f350sd, and also vote Monsta as the king of the cut and paste!!!

I fell a sleep half way through the first post....