Folks,
Regarding an N/A car - I understand that you can have an exhaust system that is too big, and an exhaust system that is too small.
Too big = no back pressure
Too small = restriction.
Can someone please explain to me how the perfect size is calculated?
I've also heard some comments about Torque and Horsepower being affected by the back pressure and restriction ratios. What that all about?
And also, how do people determine what extractors setup is the correct one for their application? i.e. how is a 4 to 1 better than a 4 to 2 to 1 system - or vise versa?
How is the length of the extractors determined before they all connect up into the one exhaust system?
Thanks in advance,
Greg
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Exhaust Explanation
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Exhaust Explanation
DMA Founding Member #1 - Now Retired
Go have a look at the tech articles on this site: www.magnaflow.com
The idea is to get the gases flowing fast so they help pull out the burnt gases from the combustion chamber. Too little a size will impede the flow; too large will slow it down. "Backpressure" is a bad term to use- as you don't want more back pressure, you want the best flow.
4-1 extractors give best gains at high revs, whereas 4-2-1 give more midrange and low rev torque improvements (ie are best for a 4wd application)
If you change your exhaust system to a larger size you most likely will lose torque at lower revs (the torque curve will change and the torque peak move higher in the rev range), but you will gain a higher peak power output at higher revs.
It's all a trade off and depends on your driving style too as to what suits you best.
The idea is to get the gases flowing fast so they help pull out the burnt gases from the combustion chamber. Too little a size will impede the flow; too large will slow it down. "Backpressure" is a bad term to use- as you don't want more back pressure, you want the best flow.
4-1 extractors give best gains at high revs, whereas 4-2-1 give more midrange and low rev torque improvements (ie are best for a 4wd application)
If you change your exhaust system to a larger size you most likely will lose torque at lower revs (the torque curve will change and the torque peak move higher in the rev range), but you will gain a higher peak power output at higher revs.
It's all a trade off and depends on your driving style too as to what suits you best.
David
I read an article this week (but can't remember the URL - if I get time I might be able to track it down again) which explained that reducing the back pressure has the same effect as increasing the overlap on the cam timing. At low revs more of the incoming charge flows out the exhaust pipe, which (amongst other things) can trick the EGO sensor into thinking the mixture is too rich. Effectively, bottom end performance is sacrificed for top end.
I believe 4-1 (also called "tuned length") extractors are typically tuned to work optimally at a given engine speed, to optimise the scavenging effect between runners. 4-2-1 (also called "interference" !?!) are less "peaky" and better suited to general driving. The tuned length design must also take into account the header diameter. I don't see why 4-1 extractors couldn't be tuned to operate at a lower speed rather than a higher speed.
Cheers,
Scott
I believe 4-1 (also called "tuned length") extractors are typically tuned to work optimally at a given engine speed, to optimise the scavenging effect between runners. 4-2-1 (also called "interference" !?!) are less "peaky" and better suited to general driving. The tuned length design must also take into account the header diameter. I don't see why 4-1 extractors couldn't be tuned to operate at a lower speed rather than a higher speed.
Cheers,
Scott
dunno technically but my experiences.....
GQ 4.2 n/a diesel with interference extractors...
changed from 2 1/4 to 2 1 /2 and torque and power peaks moved up the rev range about 400 revs.
used to cruise in fifth at 100 kmh but then had to cruise at 110 to avoid using fourth so often.
GU 4.2td stock then changed to 3" mandrel from the back of turbo...
went up toowoomba range 1 gear higher.
turbo spools up much faster.
turbo used to cut in at 1800 revs and now does the same at 1250 revs.
S.O.P. testing expert
GQ 4.2 n/a diesel with interference extractors...
changed from 2 1/4 to 2 1 /2 and torque and power peaks moved up the rev range about 400 revs.
used to cruise in fifth at 100 kmh but then had to cruise at 110 to avoid using fourth so often.
GU 4.2td stock then changed to 3" mandrel from the back of turbo...
went up toowoomba range 1 gear higher.
turbo spools up much faster.
turbo used to cut in at 1800 revs and now does the same at 1250 revs.
S.O.P. testing expert
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Exhausts are extremely complex fluid flow systems, given all the bends, the restrictions (muffler, cat, etc) and the non-uniform flow near the engine output (at least at lower revs).
To work out what is best for each particular car you would have to model it in a program like fluent - this is what car and truck manufacturers do if they care about performance. Few aftermarket exhausts are designed this was though - except for racing applications.
But the basic principles are to keep the pipes as straight as possible and minimise bends and restrictions. Rapid changes in direction or pipe size can cause turbulent recirculation zones that don't help flow out of the exhaust.
Turbo charged cars want as little back pressure as possible of course so the turbo spins more freely.
To work out what is best for each particular car you would have to model it in a program like fluent - this is what car and truck manufacturers do if they care about performance. Few aftermarket exhausts are designed this was though - except for racing applications.
But the basic principles are to keep the pipes as straight as possible and minimise bends and restrictions. Rapid changes in direction or pipe size can cause turbulent recirculation zones that don't help flow out of the exhaust.
Turbo charged cars want as little back pressure as possible of course so the turbo spins more freely.
_____________________________________________________________
RUFF wrote:Beally STFU Your becoming a real PITA.
Yes, turbos are a totally different kettle of fish- with them you just put the biggest exhaust you can fit! The idea is to get the biggest pressure drop across the exhaust wheel of the turbo so it spins up quicker to give more boost.
I had 4-2-1 extractors fitted to my Feroza with the standard cat and rear section- immediately noticed a big improvement in low rev torque and midrange (was very happy!) Then, being greedy, decided to get the rear section after the cat done in 2" pipe (slightly larger than standard)- now I've lost most of that low rev torque I gained, but have an engine that likes to rev it's ring out...... To me it was better with the stock rear section.
I had 4-2-1 extractors fitted to my Feroza with the standard cat and rear section- immediately noticed a big improvement in low rev torque and midrange (was very happy!) Then, being greedy, decided to get the rear section after the cat done in 2" pipe (slightly larger than standard)- now I've lost most of that low rev torque I gained, but have an engine that likes to rev it's ring out...... To me it was better with the stock rear section.
David
do it once and do it right!
www.flowmastermufflers.com
everything else is just a compromise
plus nothing sound as good
www.flowmastermufflers.com
everything else is just a compromise
plus nothing sound as good
big red wrote:GU 4.2td stock then changed to 3" mandrel from the back of turbo...
went up toowoomba range 1 gear higher.
turbo spools up much faster.
turbo used to cut in at 1800 revs and now does the same at 1250 revs.
Big Red
Are you sure you didn't make any changes other than the exhaust? I can accept (now
) that exhaust modifications can allow the turbo to make boost sooner, but that much sooner? You didn't make adjustments to the fuel pump?
Scott
Complicated stuff -- Lots of equations - But If were talking just for wheeling Don't fit anything that will interfere with idle and above scavenging , Big diameter- long primarys will cause off idle loss of tourqe
Sure it may give you a bit more at mid to top end , But all that means crap when you bring the flame down to below idle on some obsticle then
try to relight it with a delicate right foot before she goes out .
JMO
Larry.
Sure it may give you a bit more at mid to top end , But all that means crap when you bring the flame down to below idle on some obsticle then
try to relight it with a delicate right foot before she goes out .
JMO
Larry.
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