rx7speed

over at a mustang forum I started a little debate on backpressure


figured some of you guys have your own opinions of it so feel free to post



just please keep it respectfull\



http://forums.stangnet.com/showthrea...8&page=1&pp=40

unknown_host

iTrader: (1)

When you have an exhaust that is too big for a motor, the exhaust doesnt have the velocity at low rpm like it would with a smaller exhaust system. Whereas before at lower rpms the exhaust velocity would help pull fuel and air into the cylinder during overlap while the exhaust valve is still open (not necessarily as true on wider LSA's), the lack of exhaust velocity seen with bigger exhaust pipe would sort of cancel out this effect, and therefore cause a loss in low end. This is my thinking at least.

flyordie

Hey what ya teachin' those guys how to make more power for. Let whoever keep running those jumbo pipes if they choose to.

rx7speed

I am an import guy


so see I can go to both camps if I want

but no I believe share the wealth

even if it is to your rival car manufac still give them the help they want or help show them


that way everyone is to gain

No4NJunk

Here is a post I dug up from way back. We talked alot about the physics closed systems and how density plays on flow.

click here

Longer91Cam

So will a 3in cat back on a stock LO3 hurt performance or will it not matter since it is only 3in???

flyordie

I always knew backpressure helps bottom an mid, just from having different set ups for one car and changing them and reading and the foot drop tests. Plus you open much more up with cam swaps and head work,then intake then cooling..then ya need more tires and...I'll shut up now. unknown_host nailed it.Thats the best way I've heard it put into words.

rx7speed

um he said loss of velocity not backpressure

but if you always knew backpressure helps your knew wrong

flyordie

When i went from 3" duals to 2 1/2" duals after crushing a pipe I had an increase in power, very noticeable. Both sytems mandrel bent with delta flows and scavenger turndowns. A sytem that has less flow creates more velocity right. Is'nt the gain in velocity a result from added backpressure?

rx7speed

if your trying to run and I stand in front of you pushing back (backpressure) are you going to run faster?

unknown_host

iTrader: (1)

I think he was trying to state that back pressure is often a side effect of exhaust velocity.

flyordie

I was trying to say volocity is a result of backpressure. rx7speed, you stopped making sence (not that you made a whole lot to begin with). If volocity on the exaust side helps to suck in more air/fuel mix on the intake side and you have the right sytem to exit the air properly to match and compliment your setup it's all cherry. makes your engine that much more efficient equaling more power. your running theory does'nt explain jack $hit in this situation.

rx7speed

lol

like your theory has explained anything either


all you have said so far is backpressure speeds up the exhuast



but here since I am half asleep and prolly going to rant here I go



to talk about my last analogy

backpressure.
what do you define it as?
if you take the literal meaning of it
it sounds as though there is a pressure causing something to go back to where it came from

so lets say you are running along at full steam
you are now the exhuast pulse
I stand in your way (I am backpressure , kinda like what a cat, muffler, bends or something else would do) and start resisting you

you are now going to slow down
and now instead of putting your energy into running like you wanted to you instead have to WASTE energy to push me along (overcomming backpressure)

and you say that didn't make sence?

same with the exhuast pulse
something gets in it's way causing backpressure it is going to slow down


now the common analogy
well if I put my finger over a hose the water comes out faster

your right it does

but I ask what does that have to do with an exhuast system?


very little

water comming out of the hose is under static pressure all points of the hose are constantly full of water taking up it's volume

an exhuast system on the other hand is NOT static
you have a series of low/high pressure pulses

so that analogy is junk



now on to further describe backpressure

first one thing that helps speed up the exhuast and also helps create more of a suction event is delta P
Delta P= delta pressure
delta pressure = pressure differential

now what happens when you put a low pressure zone next to a high pressure zone?
the high pressure will try to move to the low pressure zone
think of an airplane.

under the wing is a zone of high pressure
low pressure is above the wing
so guess what the high pressure under the wing tries to move to the low pressure (which is how lift is created)
well the faster you go the more of a difference there is and the more lift you get because of that differnce

so now you ask "well what in the blue blazes does that have to do with my exhuast system and backpressure"

well sonny oh boy I am going to answer that

what is your cylinder full of after combustion?

MASSIVE AMOUNTS OF PRESSURE
so what is one way to increase the rate of excavation of exhuast gasses? make for more of a pressure differential right?

so how do you do that?

reduce the pressure inside the exhuast

but we came into a dilema here now haven't we?

backpressure while generally is considered a form of resistance to flow it also generally has a way to be mesured
that is in PSI

so if you want the greatest pressure differential how is it you are going to have a low pressure zone in the exhuast and still have backpressure?

those two don't go together



now what is it that added velocity is to work?

we all seem to agree that that velocity is needed but why?

it just doesn't sit there and do nothing

but alas we come into another problem wit hbackpressure

that velocity creates a vacuume at the valve to help make a greater delta p

see what is happening is as that exhuast pulse leaves the cylinder and heads towords the exhuast tip the volume behind that pulse is getting larger yet nothing else is filling that volume.
so what happens when volume goes up and yet the mass inside that volume stays the same?

pressure goes down

what happens when pressure goes down in the exhuast?
it pulls the exhuast out of the cylinder faster

this once again goes against that backpressure is good


so now you ask then why is it when I went from 3" down to 2 1/2 inch duals I gained more power and how is it velicoty goes up if it isn't due to backpressure


ok with a 3" pipe vs that 2 1/2 inch pipe you have more volume per inch of pipe length right?

well what happens when you have a pump pushing air?

if you take the same pump to two different volumes the smaller volume will fill faster correct?

well same thing goes with the exhuast

you have all that cross sectional area to fill up making the exhuast in turn slow down which in a round about way is like having extra backpressure ( I say round about because messurable pressure might not really go up but pressure at the valve is up)

and now we have a compound effect here

that velocity goes down in the exhuast

which is going to make the exhuast pulses more or less run into one another

it is going to create less of a vacuume which is even further going to slow exhuast gasses down.... remember folks... Delta P


and to top it all off now since the exhuast pulse isn't being sucked out the motor is going to WASTE energy pushing it out



and now some other things that cause backpressure

crush bends,catalitic converters, mufflers,bends

now how do all these things slow down the exhuast?
well think they are a restriction
what does a restriction do?
slow things down
what is that going to do?
create more backpressure
and I think by now we have talked about what is causes



now lets look at some other problems with backpressure


more heat in the exhuast system
reversion
reversion is NASTY

basicly it is when the exhuast gasses get back into the cylnder

well they have been burnt once. their energy has already been used... so you think they are going to burn again?
so this is going to create wasted space taht can be used for FRESH air/fuel

also if you know anything about physics you know heat means more pressure
pressure means more power


well since that stuff is wasting space the cylinder has less energy availible to make heat


another problem is the burnt gas will suck heat out of the already burning mixture so it's effect is two fold



and you ask how does backpressure create reversion

remember that delta P thing again?
please if not go back up my *** is too lazy to type it again


but the greater difference in delta P the faster the equilization of the pressure zones

well if you have high pressure in your exhuast mixed with low velocity (remember what causes that low velocity) now the cylinder is moving from TDC to BDC it is creating that vacuume that was already talked about and so you have high pressure in the exhuast and low pressure in the cylinder which can make it to where the cylinder cna pull in exhuast gas now

big no no








and after this you say I make no sence?
your the one saying that backpressure is good

unknown_host

iTrader: (1)

There are too many variables in this equation to make an analogy as simple as "if you are running, i am holding you back". Make an analogy involving the exhaust exiting, exhaust scavenging other exhaust, exhaust velocity and exhaust scavenging air and fuel through cam overlap and then we can talk .

unknown_host

iTrader: (1)

There are too many variables in this equation to make an analogy as simple as "if you are running, i am holding you back". Make an analogy involving the exhaust exiting, exhaust scavenging other exhaust, exhaust velocity and exhaust scavenging air and fuel through cam overlap and then we can talk .

rx7speed

well granted it was simplified

but you do get the idea correct?

unknown_host

iTrader: (1)

A better analogy would be that runner speed is somewhat important, but more important is helping to pull more runners along behind you. If a track is 3 lanes wide, the runners wont be in a line and wont take advantage of drafting the runner in front of them. If a track is 1 lane wide, they can all take advantage of less wind resistance because they have to run single file. That would be my analogy for smaller, higher velocity pipe.

Now take that same example, but a larger amount of runners (more radical/bigger cube engine) and try to run them all on the same 1 lane track that the lesser number of runners ran well on, and they wont do as well because they will all be crammed onto a little track. Expand the track out to 3 lanes, and suddenly they can run at speed again....

This is my analogy as to why an infinitely large exhaust is inefficient, but at the same time a small exhaust can be inefficient too. If you want low end torque, back pressure will come into play because you are going to want an exhaust system that is most efficient at low rpm. Back pressure is not the cause of the increased torque, but is a result of an exhaust system that is efficient at low rpm and inefficient in the upper rpms.

25THRSS

You are confusing backpressure with velocity. Backpressure will always hurt power no matter what. Velocity on the other hand helps aid in the effeciency of an exaust system kind of like your analogy about the runners. Velocity and backpressure are two very different things and people commonly confuse them with one another.

flyordie

I wasted a new set of t/a 265's yesterday. brought em up to maybe 3 thousand. The first time i just did it to hook better then the second time i punched and wrecked em. the did'nt stick good.soooooo smokey.

flyordie

my couzin sara is only 15 now.her bros. just run stangs,Her rx looks the same. turboed now. I was trying ta **** you off don't take it personal or anything.Not that i would'nt run you and not win or anything either.

rx7speed

not taking it personal

just more or less returning your rash of crap

see notice this little guy


he generally means giving you a hard time



you little boy don't know jack and yet act like your all high and mighty... STFU *****


see


notice how it gives it a friendly tone


well ok maybe

but I hope you get the idea

just messing around man

joshwilson3

.

rx7speed

hey you can't do that
only I can bring post back from the dead guys :-D

joshwilson3

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