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Posted: Wed Dec 20, 2006 7:53 pm 
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whats this with " its gotta have back pressure"
I've had a few cars that had zero back pressure, measured at 5000rpm, and had no untoward problems, no burnt valves, reasonable fuel economy, wasn't over the top loud, droned at 100kph though.
If it can breathe in, its gotta breathe out, eh?

 

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Posted: Wed Dec 20, 2006 9:23 pm 
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Agreed with above, never seen any evidence to show backpressure gives any kind of benefit. Seen plenty of evidence to show the contrary.
After the extractors, bigger is better. Its an urban myth as far as im concerned.

 

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Posted: Wed Dec 20, 2006 9:30 pm 
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My 2 cents:

I had replacement headers, hi flow cat and a 2 1/2" stainless steel exhaust system fitted to my XR6 and it made around 138kw at the rears with that set-up. Not too shabby IMO.

Corey Munday (FM member) has a 3" single on his AU VCT XR6 and that thing f**k moves.

Would be nice to see the difference (on a dyno) between a 2 1/2" a 3" and a twin 2 1/4" or 2 1/2" on a 4.0 litre I6. Would make for interesting science.
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Posted: Wed Dec 20, 2006 9:51 pm 
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Sturmovik wrote:
After the extractors, bigger is better. Its an urban myth as far as im concerned.


There is plenty of theories either way. For example

Quote:
We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.


Im of the feeling that exhaust are just as much an art as science (unless you have the budget of a race team to turn it into a science). If you go small, their are issues. If you go big, there are another set of issues. 2.5 to 3" pipework is the ballpark range for the "right" size for a 4.0. Why do I say this - look at the size of pipe on the cars that win dyno comps and drag days.

Having said that, i am yet to see a falcon 6 with a 3" exhaust outperform a similar engine with a 2.5. Hell even my EF makes 95mph through the traps with a budget brand 2.5" press bent exhaust. I think once you have removed the restriction of a factory system you have to be very lucky or very smart to find something that will outperform an off the shelf redback system by a noticable amount.

 

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Posted: Wed Dec 20, 2006 9:57 pm 
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If you're going twins, 2 1/4" is more then enough.

 

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Posted: Thu Dec 21, 2006 1:57 pm 
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can you get off the shelf 3inch exhaust for el i6
how much $
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Posted: Thu Dec 21, 2006 4:39 pm 
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stockstandard wrote:
Sturmovik wrote:
After the extractors, bigger is better. Its an urban myth as far as im concerned.


There is plenty of theories either way. For example

Quote:
We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.


Im of the feeling that exhaust are just as much an art as science (unless you have the budget of a race team to turn it into a science). If you go small, their are issues. If you go big, there are another set of issues. 2.5 to 3" pipework is the ballpark range for the "right" size for a 4.0. Why do I say this - look at the size of pipe on the cars that win dyno comps and drag days.

Having said that, i am yet to see a falcon 6 with a 3" exhaust outperform a similar engine with a 2.5. Hell even my EF makes 95mph through the traps with a budget brand 2.5" press bent exhaust. I think once you have removed the restriction of a factory system you have to be very lucky or very smart to find something that will outperform an off the shelf redback system by a noticable amount.


Plenty of evidence to show unbolting the exhaust at the headers gives a measurable hp gain on any car. Hence why on drag days and dyno comps they run open headers. (or a screamer pipe off the back of a turbo)

 

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Posted: Thu Dec 21, 2006 7:40 pm 
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Sturmovik wrote:
Plenty of evidence to show unbolting the exhaust at the headers gives a measurable hp gain on any car. Hence why on drag days and dyno comps they run open headers. (or a screamer pipe off the back of a turbo)


lets all run open headers then :roll:

 

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Posted: Thu Dec 21, 2006 8:27 pm 
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Sturmovik wrote:
Plenty of evidence to show unbolting the exhaust at the headers gives a measurable hp gain on any car. Hence why on drag days and dyno comps they run open headers. (or a screamer pipe off the back of a turbo)


stop for a minute & think...Its not a Russian 2500hp Sturmovik iL2 you're piloting.
They've done test over & over to prove that going any bigger then 2 1/2" ain't gonna give you any gains for any A/N engine. a matter of fact, you loose power. If there where 3" off the self sport system, don't you think that every one would be having one?

Turbos..... how much power & compression pressure do they make compare to a A/N?


:?:

 

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Posted: Thu Dec 21, 2006 8:30 pm 
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Just getting off fords for a second, a 3.7lt (225cid) slant six Valiant engine, a factory performance exhaust (made by Hooker), is a set of extractors that are split into twin 2 1/2" outlets. they are pullin 10 sec 1/4's, no gas, no turbo or superchargers & full steel bodies, this is in the USA. But anyway for the ford 4.0l, i found that a hiflow cat that has twin 2 1/4" straight out of it down to a large capicity( internal volume), then 2 1/2" tailpipe works very well. and a set of the Pacemaker extractors(the good ones) makes them go very well.
If you were racing at speedway, twin 2 1/2" would be the go.

 

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Posted: Fri Dec 22, 2006 12:23 am 
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Sturmovik wrote:
stockstandard wrote:
Sturmovik wrote:
After the extractors, bigger is better. Its an urban myth as far as im concerned.


There is plenty of theories either way. For example

Quote:
We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.


Im of the feeling that exhaust are just as much an art as science (unless you have the budget of a race team to turn it into a science). If you go small, their are issues. If you go big, there are another set of issues. 2.5 to 3" pipework is the ballpark range for the "right" size for a 4.0. Why do I say this - look at the size of pipe on the cars that win dyno comps and drag days.

Having said that, i am yet to see a falcon 6 with a 3" exhaust outperform a similar engine with a 2.5. Hell even my EF makes 95mph through the traps with a budget brand 2.5" press bent exhaust. I think once you have removed the restriction of a factory system you have to be very lucky or very smart to find something that will outperform an off the shelf redback system by a noticable amount.


Plenty of evidence to show unbolting the exhaust at the headers gives a measurable hp gain on any car. Hence why on drag days and dyno comps they run open headers. (or a screamer pipe off the back of a turbo)

your both right, your just not repecting each others views.
the quote above, states that the loss of performance is achieved by the gases slowing through the large diameter, and length of the exhaust.
therefore, max performance would be gained by having a shorter exhaust, of bigger diameter.
this would cause less backpressure because of the large diameter, but the pipe is too short to allow the gasses to condense.
therefore, for a full length exhaust on a NA 6, 3in is to big and allows the gas to cool too much.
by unbolting from the extractors/headers/screamerpipe your reducing the length the gas remains in the exhaust, so performance is also gained.

the only thing i dont actuly understand is that if the quote is all true, it pritty much states that by increasing the pipe diameter, your INCREASING backpressure, no decreasing it...
anyone wanna shed some light?
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Posted: Fri Dec 22, 2006 4:26 am 
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Slick wrote:
Sturmovik wrote:
Plenty of evidence to show unbolting the exhaust at the headers gives a measurable hp gain on any car. Hence why on drag days and dyno comps they run open headers. (or a screamer pipe off the back of a turbo)


stop for a minute & think...Its not a Russian 2500hp Sturmovik iL2 you're piloting.
They've done test over & over to prove that going any bigger then 2 1/2" ain't gonna give you any gains for any A/N engine. a matter of fact, you loose power. If there where 3" off the self sport system, don't you think that every one would be having one?

Turbos..... how much power & compression pressure do they make compare to a A/N?


:?:


Whos done these tests over and over? Ive never seen a back to back dyno run of someone loosing power from a good quality larger exhaust. Plenty of dynos showing the opposite, ive also seen dynos of people gaining very little increase.
Also an off the shelf 3in system is available for Falcons. 2.5 is only popular because of cost and clearance issues.

The only time I have seen a larger exhaust loose power was due to the design of it; s**t welds, s**t bends and poor pipe alignment.

Do you realise the EBII XR6 did run a dual exhaust? Now why would Ford do that if there was no benefit. It was subsequently dropped due to cost.
Theres also plenty of late model factory 3L V6s getting around with dual exhausts all the way to the end.

 

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Last edited by Sturmovik on Fri Dec 22, 2006 4:27 am, edited 1 time in total.

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Posted: Fri Dec 22, 2006 4:26 am 
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your right^^

by having a larger diametre exhaust the gases cool down alot more than a smaller diametre pipe... which means that the motor isn't running freely... cooler air is denser... which requires more "force" to be shifted through the pipe... which in turn decrease's the amount to pressure the motor has to push the exhaust gas's out at.

which also makes the statement of taking the exhaust off and dumping the gas's at the extractors a more effecient way of expelling hot gas's due to it has to only travel 1/4 of the way that it would normally have to. increaseing HP becuase the motor doesn't have to force the gas as far... but the reason why we don't do that on everyday cars is because of the noise the f**k would make...

mandrel bent exhaust also come in handy due to the airflow is even the whole way through the exhaust with no "choke" points at the bends...

it is a hard one to explain but i hope that i have helped...

Cheers

Josh
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Posted: Sat Dec 23, 2006 11:46 am 
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I was looking at putting either a 3" or twin 2.5" on my AU about 2 years ago, after talking to the guy who supplied and fitted it I opted for a single 2.5".

He basically said that the 2.5" would give me optimal performance (power - noise - fuel economy) over the other options. After having it on for 2 years I have to say it sounds very nice, not too loud but loud enough to turn heads and it doesn't drone on the freeway, there was also a significantly noticeable lift in power over the factory system.

IMO the single 2.5" is a good choice for an AU with no extractors or hi-flow cat, maybe go larger with the extractors and hi-flow cat. The other thing to note is that the guy who fitted it had the opportunity to make more money with the 3" or twin 2.5" systems and if he wasn't serious about the benefits he could have easily talked me into a system that was more expensive and had no significant performance gains, I guess he knew what he was talking about.

My 2c.

Cheers!

 

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Posted: Fri Dec 29, 2006 5:47 pm 
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omg, for those who think back pressure does nothing, drive your car with nothing behind the cat, then with an exhaust, omg it will drive with an exhaust coz it will have back pressure.
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