Outer Limits 4x4 Board

Ok, this is for the brainy ones.....

Can someone please explain to me all about BACK PRESSURE?

When fluid flows through lines, hoses or pipes, pressure is lost in overcoming friction, turbulent flow and obstructions to flow.

The down stream pressure drop/loss from the point of interest is called back pressure.

Was talkin about exhaust back pressure....... Sorry, will refine the question next time....

look i will add my view. Before i start


THIS IS THE MOST CONTESTED TOPIC IN THE MODERN WORLD. YOU HAVE STARTED ARGUEMENTS!

i think it is a load of . i said it.

a very poor term.

depends how you look at it

BHP:

does f1 have lots of back pressure, how about gp bikes. no they don't.

Yet every muffler man says you NEED backpressure. i have been told not to put 21/2 side pipes and free flowing boxes on my last 350. i would lose backpressure and bhp. what a crock.

how could free flowing boxes reduce hp . when you open your throttle your hp increases because along with other things, your engines overall flow restriction decreases as your throttle butterfly is no longer reducing gas flow. this is the same at the other end with freer boxes, although due to the pressures involved less important.

however there is a syphon effect generated by the correctly sized-for-application exhaust tubing. as the gas is expanding it rushes to the end of the tube to freedom. the correctly designed tube will aid this, too big reduce it as the gas is content and not rushed to free itself of energy - pressure. this is why the expansion chamber works on a twostroke motocrosser for example - suck 'supercharging" the intake and besides more fully charging the cylinder it removes spent gasses more effectively.

this is the effect that many people refer to as backpressure. yet it is not a restriction like a small pipe or box like many think rather a gas syphon effect.

this is my story i am sticking to it

cheers

bru

What are you particularly looking for info on?

Back pressure is bad for power- it's an old wives tale that you need it. You want the exhaust gases to flow fast with as little restriction as possible, they will then help scanvenge burnt gases from the combustion chamber. Too large an exhaust will slow the exhaust gases and not help scavenging- "back pressure" is not what's required.

Have a read of the tech section on this site for a better explanation:
http://www.magnaflow.com/07techtips/faq/question10.htm

Um.... I wasnt meaning to start an argument. I was just seeing how many switched on people are out there...

I'm prob going to get hammered now for askin the question.. but hey, got sum people thinkin and about another 10 or so lookin at the thread and not sayin a word.

Lost Bundy wrote: I was just seeing how many switched on people are out there...

I'm prob going to get hammered now for askin the question.. but hey, got sum people thinkin and about another 10 or so lookin at the thread and not sayin a word.

Are you expecting people to help you out with future questions after that effort?

Not realy worried about the help thing. Sorry, I do know how to do things myself.

It comes from the "old skool" terminology.....
Obviously is to do with the pressure in the exhaust but relates more to the actual pressure at the back of the exhaust valve.

Basically the rule was like this ....
The more back pressure at the exhaust valve the higher the engine torque and the lower the engine horsepower. The total opposite occours when the pressure is reversed ... IE: the lower the back pressure, the lower the torque and the horsepower goes up !
This is old skool style motors like carby type engines from henrys first plord up untill the mid 70's when materials, fuels, design etc etc all had rather radical changes specifically in the world of formula 1 racing.

F1 engines were drastically redesigned as were drag motors to get that extra HP to the wheels ...... many other forms of motor racing soon followed.
We have seen things like baffled intake manifolds, de programed puter chips, etc etc which are all attempts to quieten down new cars. A point in example is M/cycles, many bikes that come out of japan simply wont pass the tough EPA noise levels without the things I listed being fitted, one particular bike was converted to shaft drive because the chain pushed that level past the limit !!

The problem is that pasenger cars needed to be quiet which meant stuffy exhaust system & mufflers. Obviously car manafactures cant sell "loud" cars so they kept the back pressure up as part of the quieting process, a sort of built in muffler if you like.

The std familly car engine is designed to run a certain amount of back pressure and if you remove it completly then the risks are things like burnt valves, cracked valves, burnt valve faces on the head, detination of pistons, bulged cylinders, etc etc ...... this is of course in EXTREAM cases where there is zero back pressure..... like on a drag car!
With such low back pressure levels one would have to increase the fuel rate that enters the engine in order to overcome the "lean burn" that low back pressure creates.

Back Pressure also helps with ecconomy too, it "stabilises" the flow of gases through the motor, makes it a smoother flow if you like. This helps by the smoothing out the "pulse" of air that rushes out when the exhaust valve opens and in doing so the new unburnt gas doesnt get sucked out with the exhaust gases. It stops the syphoning ( sp?) effect so to speak

We have seen things like baffled intake manifolds, de programed puter chips, etc etc which are all attempts to quieten down new cars. A point in example is M/cycles, many bikes that come out of japan simply wont pass the tough EPA noise levels without the things I listed being fitted, one particular bike was converted to shaft drive because the chain pushed that level past the limit !!

Kingy

V8Patrol wrote:We have seen things like baffled intake manifolds, de programed puter chips, etc etc which are all attempts to quieten down new cars. A point in example is M/cycles, many bikes that come out of japan simply wont pass the tough EPA noise levels without the things I listed being fitted, one particular bike was converted to shaft drive because the chain pushed that level past the limit !!

Kingy

U said that bit twice

Ok, so your saying that some back pressure is needed while some say its now.
In saying that, Do u think that running twin 2/1/2 inch zorst pipes on a more or less stock 308 would be too much?
Am running std manifolds at this stage.....

screwy

I dont think anyone is majorly against "backpressure" more the terminology. Apart from the extreme end where damage results from leaning. If an exhaust was too big wouldnt there be an actual reduction in flow thus canceling the benefits of the draw through effect? I'm sure we touched on this in flow characteristics of building AC systems at uni. I read in another thread that someone in here was a fluid mechanics engineer or some such thing, couldnt they explain it?


John

P.S I dont think just asking questions for the sake of starting a debate is really fair. You said "I do know how to do things myself". If this is so and you want to start an informative thread why dont you start with putting your oppinion up first.

Screwy_ScrewBall wrote:

V8Patrol wrote:We have seen things like baffled intake manifolds, de programed puter chips, etc etc which are all attempts to quieten down new cars. A point in example is M/cycles, many bikes that come out of japan simply wont pass the tough EPA noise levels without the things I listed being fitted, one particular bike was converted to shaft drive because the chain pushed that level past the limit !!

Kingy

U said that bit twice

Ok, so your saying that some back pressure is needed while some say its now.
In saying that, Do u think that running twin 2/1/2 inch zorst pipes on a more or less stock 308 would be too much?
Am running std manifolds at this stage.....

screwy

The design of the manifold will be the limiting factor. If it is only 2.25" diameter or so you already have a severe flow resriction at a critical point in the exhaust system. Think of it like this - hot, volumous gasses being pushed down a narrow pipe before being progressively cooled in the exhaust system. For an example of the effect try blowing as hard as you can through a straw, now cut the straw in half and try again, and repeat as often as you like until you pass out. The effort required really doesn't get much easier the shorter the straw gets (in effect like having no pipe after the manifold) The practical outcome is that going to a larger diameter pipe further down the exhaust system will only make small differences in power/torque

Manifolds do create a reasonable amount of back pressure, this is why so many people fit headers/extractors when searching for extra horsepower, that combined with larger diameter exhaust tube and more "open" mufflers, twin systems, etc all decrease back pressure.

The only true zero back pressure setup would be a straight out exhaust port for a minimul distance.
I know I said above that drag cars run a zero backpressure and that wasnt exactly right, they do have a very small amount. The pipes we see comming off of the "Funny Cars" and Rail Cars" is gives a very low pressure reading, its the 90degree bend that gives a regulated control of backpressure that these engines need. You must ofcourse take into account that they are 6500 HP engines and run totally differant fuels to the stock familly sedan...... they also only last a few minutes of running before requiring a full rebuild, many topfuelers will actually replace pistons,crank,& heads between runs !

On another track the turbo diesel engine really doesnt need that much of an exhaust, the turbo does a fair amount of the noise reduction that the EPA require and it also gives back pressure at the same time, the problem here is that we have to by law have the exhaust fumes exiting from the behind the rear most opening door for safety reasons, realistically the exhaust pipe is just plumbing that takes the fumes out where they have to exit.
This is why many members on OL with turbo diesel engines go for that 3" exhaust on their 4bys..... the muffler thay fit just needs to quieten down that "crack" from the zorst.

A well designed zorst system will see gains in power, ecconomy, engine life, a poor system sees the opposite. I have seen the local "boys" and their 2litre screamers actually drop top speed because of an over opened up zorst system...... you have to remember that the manafactures designed their engines to have some degree of back pressure.

There are many differant variants involved in a good system and one has to consider all of them to get a 'good flow" thats not detrimental to the motor.
Things like ........... is it a ....
inline motor ( 4 cyl & 6 cyl typicaly ) where the exhaust ports are all on the same side.
A "V' engine ( V6 , V8, V10, V12 ) where the ports exit from bothe sides of the engine.
a rotary engine ..... whole new kettle of fish !

Then we have to look at the fuel we run .....
The petrol we get here in Australia is crap compared to what Americans use ..... you would not belive the differance between it !
Even the diesel we have is of a low std compared to other countries.
Methanol fuels are cheap to produce but the volume a motor consumes to do the same miles is huge in comparison to running on petrol. Sure you get a 30% horsepower increase but you use 4 times as much, not to mention the mods a petrol motor needs to run methanol correctly !
LPG is one of the better fuels we have the chioces of and again it has differant requirements both on the intake side aswell as the exhaust side.
Nitro......... very few can afford this mothers milk let alone the engine !

So with all that to consider you can begin to imagine the exhaust differance requirements between a non turboed V10 diesel motor and a rotary motor running on nitromethane......

Engine mods also come into consideration......
add a supercharger and a differant back pressure is required,
Up the compression ratio from 6-8 to 1 ( familly sedan) to 10 - 11 to 1 and start lookin at straight LPG or Methanol + a new back pressure requirement.
grab a 500 cubic inch alloy block, stuff some $20,000 heads on it, add a blower, plus an injection setup and fuel it with nitro and all ya need is 3' long extractor pipe with a bend half way along it per port !

Its the age old question ........

How Fast Can You Afford ?

Kingy

I didn't understand any of this "back pressure" stuff until I had it explained thus:

With the standard setup ex-factory the exhaust creates a set amount of "back pressure," for which the rest of the engine is tuned.

Once this "back pressure" is reduced (by whatever means) gases leave the cylinder more easily, including more of the incoming air-fuel mixture - effectively the valve overlap has been increased. Most car buffs understand that increasing valve overlap helps with top-end power (engine breathes better - more air-fuel mixture in, more burnt gases out) at the expense of bottom end torque (don't understand that bit , other than more unburnt fuel out the exhaust upsets the O2 sensor, so the actual mixture in the chamber is no longer optimum. NFI how it affects carby fed engine...)

So sticking a larger exhaust on an engine and doing nothing else isn't necessarily a good thing. Stick on a larger exhaust and re-tune the engine to suit should be a good thing!

That's MY story, and I'M sticking to it!

Cheers,

Scott

NJ SWB wrote: ...increasing valve overlap helps with top-end power (engine breathes better - more air-fuel mixture in, more burnt gases out) at the expense of bottom end torque (don't understand that bit , other than more unburnt fuel out the exhaust upsets the O2 sensor, so the actual mixture in the chamber is no longer optimum. NFI how it affects carby fed engine...)

increasing cam DURATION shifs the powerband into a higher rpm range. a side effect of increased duration is increased overlap. with increased cam duration, the valves spend more time open. the corollary of that is, obviously, the valves spend less time CLOSED. the time with the valves closed is tthe only time the engine can make compression and power. at low rpms, the burning mixture is robbed of some of its potential to do work against the piston by the exhaust valve opening "too" early, so some low torque is lost. as rpms increase, the ability of the cammed engine to breathe more mix in and more exhaust out overcomes the disadvantage of the shorter efective compression and power stokes of the engine.

GET BIGGER ZORST
GET SUPERCHARGER
RETUNE ECU

DRIVE IT

very very interesting in deed......
Might have to do some research.

Still think im gonna do the twin 2.5 inch zorst on the 308 on std manifolds.
If i put extractors on and the twin 2.5 id say id lose too much back pressure, but with the restriction at the start, ive got what i need.
Doing this in my mind is the same as running extractors on a thin exuast.....

screwy

Screwy_ScrewBall wrote:very very interesting in deed......
Might have to do some research.

Still think im gonna do the twin 2.5 inch zorst on the 308 on std manifolds.
If i put extractors on and the twin 2.5 id say id lose too much back pressure, but with the restriction at the start, ive got what i need.
Doing this in my mind is the same as running extractors on a thin exuast.....

screwy

Not really the way to go in my opinion.
The hotter the exhaust gasses the more volume they take up so really you want the exhaust to be freest flowing where the gass is hottest ie. near the motor.
Paul.

aaahhh i see,
prob is i cant really fit extractors in there as the MQ has little room down the sides. Id have to get em custom made and i cant afford that....

screwy

The Bible has what ya looking for ......


D.I.Y. HEADER KITS

NUMBER DESCRIPTION PRICE
EXT.KIT B/B CHEV BIG BLOCK HEADER KIT $320
EXT.KIT S/B CHEV SMALL BLOCK HEADER KIT $320
EXT.KIT S/B CHRYSLER 318-360 HEADER KIT $320
EXT.KIT S/B CHRYSLER 383-440 HEADER KIT $320
EXT.KIT WIN FORD WINDSOR HEADER KIT $320
EXT.KIT CLEV FORD CLEVELAND HEADER KIT $320
EXT.KIT B/B FORD 429/460 HEADER KIT $320
ALL KITS CONSIST OF 2 X HEADER PLATES & BOLTS, 10 X MANDREL BENDS (180° & 90°), 2 X 4-1 COLLECTORS, 4 X 3 BOLT FLANGES, 2 X REDUCER CONES & GASKETS.

see .........
http://www.castleauto.com.au/headers.htm

check the other links in the V8 conversions section .....

Screwy_ScrewBall wrote:very very interesting in deed......
Might have to do some research.

Still think im gonna do the twin 2.5 inch zorst on the 308 on std manifolds.
If i put extractors on and the twin 2.5 id say id lose too much back pressure, but with the restriction at the start, ive got what i need.
Doing this in my mind is the same as running extractors on a thin exuast.....

screwy

that is funny as.

i dont think it works like that. the engine pushes out 'pulses' of exhaust.
the extractors are made so that the momentum of the pulse draws the next cylinders pulse out with it.

if you run the std manifolds. you wouldnt get that pulse extraction as the runners are different lengths. and they all just crash into each other.

my mate had twin 2.25s on his 350. you could hear him coming a mile away. he used to set car alarms off.

i dont think backpressure is a good term for it. but an exhaust needs momentum, thats why 6 inch pipes on a N/A mini wouldnt be a good thing.

Kingy,

I dont have a Small Block Chev mate, remember...


383 FJ45,

So u reckon id be far better off with extractors and my twin 2.5's?

screwy

PAD

high n mighty wrote:PAD

NICE WORK
PAD PAD PAD

We might get in trouble 4 this

Lost Bundy wrote:Was talkin about exhaust back pressure....... Sorry, will refine the question next time....

I guessed you was talking about exhaust back pressure. My post answered your question. Exhaust gas is a fluid and what I said applies equally to exhaust systems.

bru21 wrote:look i will add my view. Before i start


THIS IS THE MOST CONTESTED TOPIC IN THE MODERN WORLD. YOU HAVE STARTED ARGUEMENTS!

i think it is a load of . i said it.

a very poor term.

depends how you look at it

BHP:

does f1 have lots of back pressure, how about gp bikes. no they don't.

Yet every muffler man says you NEED backpressure. i have been told not to put 21/2 side pipes and free flowing boxes on my last 350. i would lose backpressure and bhp. what a crock.

how could free flowing boxes reduce hp . when you open your throttle your hp increases because along with other things, your engines overall flow restriction decreases as your throttle butterfly is no longer reducing gas flow. this is the same at the other end with freer boxes, although due to the pressures involved less important.

however there is a syphon effect generated by the correctly sized-for-application exhaust tubing. as the gas is expanding it rushes to the end of the tube to freedom. the correctly designed tube will aid this, too big reduce it as the gas is content and not rushed to free itself of energy - pressure. this is why the expansion chamber works on a twostroke motocrosser for example - suck 'supercharging" the intake and besides more fully charging the cylinder it removes spent gasses more effectively.

this is the effect that many people refer to as backpressure. yet it is not a restriction like a small pipe or box like many think rather a gas syphon effect.

this is my story i am sticking to it

cheers

bru

Gas syphon effect

i think it is a load of . You said it.

a very poor term.

depends how you look at it

Sorry, couldn't resist turning your own words back on you there.

Any syphon effect is negligible. If it wasn't then you would be in trouble if the exit end of the exhaust was higher than the exhaust valves.

And you explanation of how 2 stroke expansion chambers work is way off the mark. Designing/tuning these exhaust systems is all about the physics of pressure waves. The same physics that apply to organ pipes and other musical wind instruments. I could explain further but it is off topic.

ORSM45 wrote:

Screwy_ScrewBall wrote:very very interesting in deed......
Might have to do some research.

Still think im gonna do the twin 2.5 inch zorst on the 308 on std manifolds.
If i put extractors on and the twin 2.5 id say id lose too much back pressure, but with the restriction at the start, ive got what i need.
Doing this in my mind is the same as running extractors on a thin exuast.....

screwy

that is funny as.

i dont think it works like that. the engine pushes out 'pulses' of exhaust.
the extractors are made so that the momentum of the pulse draws the next cylinders pulse out with it.

if you run the std manifolds. you wouldnt get that pulse extraction as the runners are different lengths. and they all just crash into each other.

my mate had twin 2.25s on his 350. you could hear him coming a mile away. he used to set car alarms off.

i dont think backpressure is a good term for it. but an exhaust needs momentum, thats why 6 inch pipes on a N/A mini wouldnt be a good thing.

Unfortunately the extractors available to us plebs off the shelves are not tuned as you describe- their main aim is to be bigger diameter, shinier and make more noise than stock, which they achieve well.
The "pulse extraction" you mention is a pretty good way of expressing the idea of tuning an exhaust system's natural frequency to that of the desired peak hp rpm. Such tuned systems are found on racing cars, european supercars etc. They are expensive and specific in application, and $500 pacemakers simply do not do the same thing. It's sad, and a lot of people won't like me for saying it.

why the dig?

not to mention, an illegal dig.

bru21 wrote:look i will add my view. Before i start


THIS IS THE MOST CONTESTED TOPIC IN THE MODERN WORLD. YOU HAVE STARTED ARGUEMENTS!

i think it is a load of . i said it.

a very poor term.

depends how you look at it

BHP:

does f1 have lots of back pressure, how about gp bikes. no they don't.

Yet every muffler man says you NEED backpressure. i have been told not to put 21/2 side pipes and free flowing boxes on my last 350. i would lose backpressure and bhp. what a crock.

how could free flowing boxes reduce hp . when you open your throttle your hp increases because along with other things, your engines overall flow restriction decreases as your throttle butterfly is no longer reducing gas flow. this is the same at the other end with freer boxes, although due to the pressures involved less important.

however there is a syphon effect generated by the correctly sized-for-application exhaust tubing. as the gas is expanding it rushes to the end of the tube to freedom. the correctly designed tube will aid this, too big reduce it as the gas is content and not rushed to free itself of energy - pressure. this is why the expansion chamber works on a twostroke motocrosser for example - suck 'supercharging" the intake and besides more fully charging the cylinder it removes spent gasses more effectively.

this is the effect that many people refer to as backpressure. yet it is not a restriction like a small pipe or box like many think rather a gas syphon effect.

this is my story i am sticking to it

cheers

bru

Agreed.

Tuned length extractors/turbo manifolds etc in my eyes are not worth the huge amount of extra money, when every .01 or .1 second counts in a drag car etc then yes but on the average street car or even 10 second street car to me its not worth the extra coin.

The jury is still out on Exhausts haha.