I know this is off the original subject but i just wanted to add to the rim size's, years ago i rang the RTA and asked them what i have to do to put on larger tyres on my 4X4 to keep it legal i was told that as long as i put some flares on the guards it would be legal, in the area that i live in you see a LOT of 4X4's with very big tyres and rims.
On my Hatch i have 14x8 with 245x50 & 15x10 with 265x50 and have been pulled over plenty of times were they will check out for tyre wear etc etc and never say anything about the width, luck? i don't know or maybe they think it's STD?? either way it's worth it or at least it has been so far.
just my 2 cents worth.
As for the original subject i have dumped the pipes on the Hatch before the diff but i will be getting some pipes made up to go under the diff at some stage.
dumping exhaust at the diff
Started by
makka
, May 11 2006 04:52 PM
61 replies to this topic
#52
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Posted 19 May 2006 - 10:44 PM
So even though they only had 14x6 from the factory, the tyre placard says 14x7?Hi Guys.
A 13 x 6 rim was available on non-A9X Toranas anyway, check the tyre placard, in the glovebox.
An A9X has a unique tyre placard that lists the 14 x 7 rims.
Dr Terry.
Can any A9X owner confirm or deny this?
#53
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Posted 21 May 2006 - 07:07 AM
TORANR AMORE,
There is no time when back pressure is an advantage!!!
The ultimate exhaust system has high gas velocity and zero back pressure which is why you get a hp increase when dumping the exhaust straight from a set of headers.
With any system it is important to reduce back pressure to a minimum while maintaining gas velocity to produce the greatest scavenging effect. It is a balancing act between the two and every system is a compromise. Too small a pipe and scavenging is great but back pressure restricts power, too large a pipe and back pressure is low but you loose the scavenge effect.
With a stock small block reving to 5500rpm you need a small pipe to keep the velocity up. That 7500rpm big block needs big pipes to perform but that costs it low down torque due to the low velocity.
With a turbo car the scavenge pulses are chopped up by the turbo so the aim is too reduce back pressue to the minimum. Thats why a 2.6l Skyline GTR is quite happy on the street with a 3" or 4" exhaust
Dumping an exhaust before the diff reduces the length of the system, reducing backpressure without affecting the scavenging.
There is no time when back pressure is an advantage!!!
The ultimate exhaust system has high gas velocity and zero back pressure which is why you get a hp increase when dumping the exhaust straight from a set of headers.
With any system it is important to reduce back pressure to a minimum while maintaining gas velocity to produce the greatest scavenging effect. It is a balancing act between the two and every system is a compromise. Too small a pipe and scavenging is great but back pressure restricts power, too large a pipe and back pressure is low but you loose the scavenge effect.
With a stock small block reving to 5500rpm you need a small pipe to keep the velocity up. That 7500rpm big block needs big pipes to perform but that costs it low down torque due to the low velocity.
With a turbo car the scavenge pulses are chopped up by the turbo so the aim is too reduce back pressue to the minimum. Thats why a 2.6l Skyline GTR is quite happy on the street with a 3" or 4" exhaust
Dumping an exhaust before the diff reduces the length of the system, reducing backpressure without affecting the scavenging.
#54
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Posted 21 May 2006 - 08:35 AM
Hi Guys.
In my last post I must have confused eveybody with that typo.
The tyre placard in an A9X reads 14 x6 rims.
Sorry about that.
Dr Terry.
In my last post I must have confused eveybody with that typo.
The tyre placard in an A9X reads 14 x6 rims.
Sorry about that.
Dr Terry.
#55
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Posted 21 May 2006 - 11:15 AM
I think you'll find "back pressure" is caused by baffling in the muffler, and to some VERY MINOR extent, curves in the exhaust pipes.TORANR AMORE,
There is no time when back pressure is an advantage!!!
The ultimate exhaust system has high gas velocity and zero back pressure which is why you get a hp increase when dumping the exhaust straight from a set of headers.
With any system it is important to reduce back pressure to a minimum while maintaining gas velocity to produce the greatest scavenging effect. It is a balancing act between the two and every system is a compromise. Too small a pipe and scavenging is great but back pressure restricts power, too large a pipe and back pressure is low but you loose the scavenge effect.
With a stock small block reving to 5500rpm you need a small pipe to keep the velocity up. That 7500rpm big block needs big pipes to perform but that costs it low down torque due to the low velocity.
With a turbo car the scavenge pulses are chopped up by the turbo so the aim is too reduce back pressue to the minimum. Thats why a 2.6l Skyline GTR is quite happy on the street with a 3" or 4" exhaust
Dumping an exhaust before the diff reduces the length of the system, reducing backpressure without affecting the scavenging.
Whereby a muffler's baffles are designed through destructive interference in the muffler itself to reduce noise by design as opposite sound waves are likely to colide, canceling each other out, but this also increases the force engines are required to push the gas through the exhaust system, tehreby decreasing horse power.
I'm sorry, but it's basic physics of waves and kinetic energy.
In this the waves are gases, as the exhaust comes down the extractors and meet up they are all going in the same general direction and therefore their energy combines and push forward (since they don't join perfectly but instead on an angle there will be SOME collision causing gases to want to go backwards, but the effect is minimal and unrestrictive (hence why the design of extractors are far superior to standard headers).
Tuned length extractors etc. all play on this principle.
Everytime there is a curve in the exhaust pipe, gases collide with this like waves in a pool, SOME of this energy is diverted backwards, but because it is not a perpendicular "wall" most of it is diverted in the direction (or 'glances' off) the pipe is bending (It will always travel in the path of least resistance).
Technically, an exhaust going under the diff has fewer curves and therefore has less restrictive flow, even still this effect of curves in the pipes is minimal compared to the baffling in a muffler.
For want of a better word, I'm saying your post is wrong by saying that dumping at the diff reduces back pressure significantly enough to notice any decrease or increase in performance.
Especially if your exhaust went under the diff to begin with, if you dumped before (or removed basically) your mufflers you would increase flow exponentially.
It's an asthetic sound thing and that's about it.
Alternatively you could think of it as putting a hose down different tubes, If you stick two pieces together such that they create a "Y" and the water meets and comes out the single end, the energy of the waves of water combines as one with minimal collision (and thus some "back pressure").
If you had a pipe with a perfect right angle in it, water would hit the vertical wall and want to go backwards - water behind it would push it back into the wall and it would follow the path of LEAST RESISTANCE - in this case where the water isn't constantly coming from (i.e. it would go downstream).
Same shit for exhaust gases, just replace the idea instead of water.
Bit long winded but without hand gestures or sketches to explain it's hard =]
I think maybe I found my forum title "Physics expert - but hopeless at explaining"
Edited by Loki, 21 May 2006 - 11:28 AM.
#56
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Posted 21 May 2006 - 11:33 AM
Finally though, scavenging is is having a low pressure reflective wave of energy at the exhaust valve when the intake and outlet's are open, so the negative or low pressure creates induces intake into the cylinder, which is good.
And this is why you need to have the correct diameter exhausts (the big long winded post before was on lengths of exhaust/bends/muffler and how it effects "back pressure" as opposed to actual diamter size which is different again).
Hope that's not tooooo confusing.
[edit] Or maybe my title should be Forum Medic... I dunno *Shrugs*
And this is why you need to have the correct diameter exhausts (the big long winded post before was on lengths of exhaust/bends/muffler and how it effects "back pressure" as opposed to actual diamter size which is different again).
Hope that's not tooooo confusing.
[edit] Or maybe my title should be Forum Medic... I dunno *Shrugs*
Edited by Loki, 21 May 2006 - 11:45 AM.
#57
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Posted 21 May 2006 - 12:00 PM
Thanks for the clarification. I was scratching my head after reading that. Now I just need to get the splinters out from underneath my fingernails.Hi Guys.
In my last post I must have confused eveybody with that typo.
The tyre placard in an A9X reads 14 x6 rims.
Sorry about that.
Dr Terry.
#58
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Posted 21 May 2006 - 02:51 PM
Hey Loki,
Its an interesting subject allright... You have to think of it two ways at one time, the flow of the exhaust gas and the action of the exhaust pulses on this flow. As you said the back pressure is created by the mufflers, bends, surface friction, etc so leaving out the bends over the axle and a couple of meters of pipe must have some effect... You would need a better butt dyno than the one I am equiped with to measure the improvement though. Anyway a decent system will produce as much or more power than open headers...
I see what you are saying about the Y pipe but don't forget the exhaust gasses are traveling in pulses, 4 per engine rev or two per side per rev. The trick is to use the correct size pipe to maximise the scavenging effect in the headers and merge pipes created by these pulses.
Think of a slinky spring you have laid on the floor. The coils in front of a pulse traveling down the spring are wide apart, in the pulse itself they are close together and directly behind the pulse they are even wider than in front. Relate that to the pressure in an exhaust pulse and you have low pressure in front, high pressure in the pulse and even less pressure behind it.
Taking your Y pipe, a pulse comes bolting down one pipe and thru the Y. A small pressure wave bounces back up each pipe and the bulk of the pulse travels on down the pipe. This leaves a low pressure area behind it to recieve the pulse that is barreling down the other pipe. This way each pipe is assisting the pulse in the other pipe through the merge.
This is where the gains are to be had in both headers and merge pipes. The headers primary pipe length and diameter time the scavenge pulse in the cylinder, and the gas velocity and the collector design determines its strength. In a merge pipe the velocity determines the scavenge effect and , from there on, the exhaust pulse has access to the other pipe doubling the volume it can flow through.
Once you get as far down the system as the axle the gas has cooled considerably and the pulse strength has been weakened by its travels and the mufflers so you can reduce the pipe diameter. Probably without creating any more restriction than you would with the bends to go over it. Even a short section under the axle may help with clearance problems. Oval tube is another option...
Its an interesting subject allright... You have to think of it two ways at one time, the flow of the exhaust gas and the action of the exhaust pulses on this flow. As you said the back pressure is created by the mufflers, bends, surface friction, etc so leaving out the bends over the axle and a couple of meters of pipe must have some effect... You would need a better butt dyno than the one I am equiped with to measure the improvement though. Anyway a decent system will produce as much or more power than open headers...
I see what you are saying about the Y pipe but don't forget the exhaust gasses are traveling in pulses, 4 per engine rev or two per side per rev. The trick is to use the correct size pipe to maximise the scavenging effect in the headers and merge pipes created by these pulses.
Think of a slinky spring you have laid on the floor. The coils in front of a pulse traveling down the spring are wide apart, in the pulse itself they are close together and directly behind the pulse they are even wider than in front. Relate that to the pressure in an exhaust pulse and you have low pressure in front, high pressure in the pulse and even less pressure behind it.
Taking your Y pipe, a pulse comes bolting down one pipe and thru the Y. A small pressure wave bounces back up each pipe and the bulk of the pulse travels on down the pipe. This leaves a low pressure area behind it to recieve the pulse that is barreling down the other pipe. This way each pipe is assisting the pulse in the other pipe through the merge.
This is where the gains are to be had in both headers and merge pipes. The headers primary pipe length and diameter time the scavenge pulse in the cylinder, and the gas velocity and the collector design determines its strength. In a merge pipe the velocity determines the scavenge effect and , from there on, the exhaust pulse has access to the other pipe doubling the volume it can flow through.
Once you get as far down the system as the axle the gas has cooled considerably and the pulse strength has been weakened by its travels and the mufflers so you can reduce the pipe diameter. Probably without creating any more restriction than you would with the bends to go over it. Even a short section under the axle may help with clearance problems. Oval tube is another option...
#59
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Posted 21 May 2006 - 06:29 PM
Hey Loki and Tobes, it sounds like you have both given all of this a lot of thought. Not really following much of it too well myself though.Think of a slinky spring you have laid on the floor. The coils in front of a pulse traveling down the spring are wide apart, in the pulse itself they are close together and directly behind the pulse they are even wider than in front. Relate that to the pressure in an exhaust pulse and you have low pressure in front, high pressure in the pulse and even less pressure behind it.
Tobes: Im having trouble picturing the coils at the front of the pulse on a slinky being wider? Are we talking longitudinal or tranverse wave on the slinky?
Loki: What sort of velocities are we talking about in the pipe, arent they producing a wave due to the frequency of the pulses and reflections you talk about in the muffler? Would the exhaust gases benefit from being in a pipe exactly one wavelength long?
and arent the sound waves moving at another speed entirely, ie the speed of the sound in hot exhaust gas which is? 400m/s?
#60
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Posted 21 May 2006 - 08:54 PM
Yeah I pretty much agree with you Tobes with what you say there.
Pipe diameter has a large effect and so does the length of the pipe and all the points you mentioned.
All I'm saying is that removing the bit of piping from the muffler back to the rear of the car - the effects (whether positive or negative) would be minimal (as that's not really the main contributing factor to back pressure, scavenging and so on) which you have also basically agreed on in that post
The sound waves are different to the gases produced, but they are both still affected by the pipes and the mufflers.
Pipe diameter has a large effect and so does the length of the pipe and all the points you mentioned.
All I'm saying is that removing the bit of piping from the muffler back to the rear of the car - the effects (whether positive or negative) would be minimal (as that's not really the main contributing factor to back pressure, scavenging and so on) which you have also basically agreed on in that post
The sound waves are different to the gases produced, but they are both still affected by the pipes and the mufflers.
#61
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Posted 21 May 2006 - 11:06 PM
I would expect a straight large pipe to be able to push out more volume of gas faster and easier than a U shaped curved one (or a thinner pipe).
#62
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Posted 22 May 2006 - 02:02 AM
Check out this for a good explaination of header theory... Also take a look at the 'Resources' page for more...
http://headerdesign....tras/design.asp
And this one for some more theory and some superb products. The merge collectors are almost works of art...
http://www.burnsstai...ory/theory.html
Happy reading and I'll dig out some more if you are keen...?
http://headerdesign....tras/design.asp
And this one for some more theory and some superb products. The merge collectors are almost works of art...
http://www.burnsstai...ory/theory.html
Happy reading and I'll dig out some more if you are keen...?
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