Exhaust Manifolds
07 October 2001, 06:18 PM
#1
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Can someone please educate me a bit on the subject of the Impreza manifold.
- How do equal length manifolds improve performance above and beyond the standard item?
- Why do Subaru continue with what is apparently a les than optimised design?
- What are the downsides of equal length manifolds?
- How do equal length manifolds improve performance above and beyond the standard item?
- Why do Subaru continue with what is apparently a les than optimised design?
- What are the downsides of equal length manifolds?
07 October 2001, 11:57 PM
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You wouldn't really notice much difference on a road car (for the money spent), but in a racing environment you need everything balanced so parts of the engine are not doing more than others etc.
They are cheaper to manufacter, almost all car manufacters build something to "DO" the job intended nothing more nothing less. It wouldn't be much fun if you purchased a car and couldn't do anything to it.
There are really no down sides just price.
They are cheaper to manufacter, almost all car manufacters build something to "DO" the job intended nothing more nothing less. It wouldn't be much fun if you purchased a car and couldn't do anything to it.
There are really no down sides just price.
08 October 2001, 10:06 AM
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Fitted HKS ex. manifold 4 days ago (lookslike a work of art).
Power band moved adout 1500 rpm higher along with max power and torque. Lost very litle torque below 2500 rpm.
Adjustment in driving the car, that means that you have to work the box a bit more but it is much faster as it revs and revs.
Noise is like a beefed up Alfa 33!
Power for the money spent not good BUT it lowers exhaust gas temps dramatically and its an item that you cannot further upgrade!. (and Pat recommends it so there must be something to it!)
Power band moved adout 1500 rpm higher along with max power and torque. Lost very litle torque below 2500 rpm.
Adjustment in driving the car, that means that you have to work the box a bit more but it is much faster as it revs and revs.
Noise is like a beefed up Alfa 33!
Power for the money spent not good BUT it lowers exhaust gas temps dramatically and its an item that you cannot further upgrade!. (and Pat recommends it so there must be something to it!)
08 October 2001, 04:38 PM
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Submannz,
what absolute bollocks!
Dont go sprouting crap as people will believe you and it is wrong to mislead. Do what I do, ask someone who knows then claim that you know it yourself, but at least get it right.
First of all equal length headers are expensive, but not because they are equal length, but simply because they take a lot of effort to produce and you have to pay for the man hours as well as the r and d.
Every volume of air has a resonance frequency, likewhen you blow a cross a bottle of beer, if you change the volume of liquid you change the note.
If you drive a column of vibrating air at its resonace frequency, you can store a lot of energy in the gas and also remove a lot of stored energy.
Compare it with pushing a kid on a swing, if you push at the right time you can get them to swing very high, if you push against them when they come towards you and pull them back as they swing away, you will be removing energy from the swing.
With equal length headers all the tubes have the same resonance point (length dependent) sothere is one rpm point in which the column of gas will resonate much more, this will equate to higher pressure driving the turbo as high power high amplitude pressure waves oscillate up and down the air column.
In essence you get a high and sharp torque peak, which is good for race engines, as their gear ratios are set out to keep the engine on the boil and using this peak to a performance advantage.
On the road it makes for a less drivable car as you have to work it more to extract the benefits of a peaky torque curve.
whereas unequal length headers such as standard give four smaller peaks each one corresponding to the resonance frequency of the length of each exhaust port pipe up to the turbo. The four peaks are each lower and so make for a flatter more usable but ultimately lower torque curve. Hence better for standard road cars!
If equal you can still choose the length of the headers so that the sharp spike comes in high or low down, but either side you lose out compared with non equal length when not in the torque spike.
Tubular headers can be made equal or non equal. Equal will also lose the warble and sound more like the wrc, but non equal tubular headers are better than standard cast headers in that they are less restictive but can still give a nice flat torque curve whilst retaining the flat four noise and increasing volumetrc efficiency.
They are not standard because they are time consuming and expensive compared with cast headers.
[This message has been edited by Adam M (edited 08 October 2001).]
what absolute bollocks!
Dont go sprouting crap as people will believe you and it is wrong to mislead. Do what I do, ask someone who knows then claim that you know it yourself, but at least get it right.
First of all equal length headers are expensive, but not because they are equal length, but simply because they take a lot of effort to produce and you have to pay for the man hours as well as the r and d.
Every volume of air has a resonance frequency, likewhen you blow a cross a bottle of beer, if you change the volume of liquid you change the note.
If you drive a column of vibrating air at its resonace frequency, you can store a lot of energy in the gas and also remove a lot of stored energy.
Compare it with pushing a kid on a swing, if you push at the right time you can get them to swing very high, if you push against them when they come towards you and pull them back as they swing away, you will be removing energy from the swing.
With equal length headers all the tubes have the same resonance point (length dependent) sothere is one rpm point in which the column of gas will resonate much more, this will equate to higher pressure driving the turbo as high power high amplitude pressure waves oscillate up and down the air column.
In essence you get a high and sharp torque peak, which is good for race engines, as their gear ratios are set out to keep the engine on the boil and using this peak to a performance advantage.
On the road it makes for a less drivable car as you have to work it more to extract the benefits of a peaky torque curve.
whereas unequal length headers such as standard give four smaller peaks each one corresponding to the resonance frequency of the length of each exhaust port pipe up to the turbo. The four peaks are each lower and so make for a flatter more usable but ultimately lower torque curve. Hence better for standard road cars!
If equal you can still choose the length of the headers so that the sharp spike comes in high or low down, but either side you lose out compared with non equal length when not in the torque spike.
Tubular headers can be made equal or non equal. Equal will also lose the warble and sound more like the wrc, but non equal tubular headers are better than standard cast headers in that they are less restictive but can still give a nice flat torque curve whilst retaining the flat four noise and increasing volumetrc efficiency.
They are not standard because they are time consuming and expensive compared with cast headers.
[This message has been edited by Adam M (edited 08 October 2001).]
08 October 2001, 05:28 PM
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Guys,
Thanks, the guidance is much appreciated.
Adam - Mega thanks, this seem to tie in with my rudimentary knowledge of physics. Has anyone manufactured non-equal tubular headers, and perhaps more interestingly has anyone put them onto an Impreza and done a before and after test (I'm sure someone told me that these were available in Japan)?
Thanks, the guidance is much appreciated.
Adam - Mega thanks, this seem to tie in with my rudimentary knowledge of physics. Has anyone manufactured non-equal tubular headers, and perhaps more interestingly has anyone put them onto an Impreza and done a before and after test (I'm sure someone told me that these were available in Japan)?
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08 October 2001, 08:40 PM
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Well I am not sure what you are referring to becuase you just said pretty much exactly what I said? just in more detail.
The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.
[This message has been edited by submannz (edited 08 October 2001).]
The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.
[This message has been edited by submannz (edited 08 October 2001).]
08 October 2001, 09:02 PM
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Sorry the site wouldn't let me post this all as one post.
These primary tubes create a suctioning effect, called scavenging, which helps draw exhaust gases out of the cylinder.
This scavenging effect works in conjunction with the engine's own natural vacuum to actually help pull in the fresh fuel charge. To do this with maximum efficiency, the length and diameter of the primary tubes must correspond to many factors, including engine size, vehicle weight, gearing and expected rpm range.
These primary tubes create a suctioning effect, called scavenging, which helps draw exhaust gases out of the cylinder.
This scavenging effect works in conjunction with the engine's own natural vacuum to actually help pull in the fresh fuel charge. To do this with maximum efficiency, the length and diameter of the primary tubes must correspond to many factors, including engine size, vehicle weight, gearing and expected rpm range.
09 October 2001, 01:22 PM
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As it so happens, normally aspirated cars benefit much more from equal length headers as the scavenging suchs out the exhaust as described by submannz.
The whole point being achieving a volumetric efficiency of greater than 100%, this is imprtnat more in normally aspirated cars as the exhaust and inlet valve overlap so the scavenging helps to suck in air.
With forced induction engines, the valve overlap is non existant at full power (overlap is good for economy and emissions which go out the window).
Forced induction engines have a volumetric efficiency of greater than 100% due to the pressure in the intake manifold.
The important area for svacnging is after the turbo ie downpipe and wastegate, as these contributr to the pressure in the intake manifold much more directly.
If anyone has learned anything here, its me. I asked pat about thsi last night and he highlighted that most gains are generally in normally aspirated engines.
FYI, my new headers are tubular and unequal length and retain the boxer warble. They are made by MRT, and dont come cheap.
The whole point being achieving a volumetric efficiency of greater than 100%, this is imprtnat more in normally aspirated cars as the exhaust and inlet valve overlap so the scavenging helps to suck in air.
With forced induction engines, the valve overlap is non existant at full power (overlap is good for economy and emissions which go out the window).
Forced induction engines have a volumetric efficiency of greater than 100% due to the pressure in the intake manifold.
The important area for svacnging is after the turbo ie downpipe and wastegate, as these contributr to the pressure in the intake manifold much more directly.
If anyone has learned anything here, its me. I asked pat about thsi last night and he highlighted that most gains are generally in normally aspirated engines.
FYI, my new headers are tubular and unequal length and retain the boxer warble. They are made by MRT, and dont come cheap.
09 October 2001, 06:10 PM
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All equal length manifolds are not equal.
What you get from them depends on what you buy. Pipe lengths & diameters make a huge difference to where the power and torque arrives.
I have a Power Engineering one and am entirely happy. I have not lost power at any point in the rev range, but have gained huge amounts low down.
See my pictures & dyno runs at
What you get from them depends on what you buy. Pipe lengths & diameters make a huge difference to where the power and torque arrives.
I have a Power Engineering one and am entirely happy. I have not lost power at any point in the rev range, but have gained huge amounts low down.
See my pictures & dyno runs at
09 October 2001, 07:21 PM
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Stephen,
I was hoping you were going to join in here as It was probably your dyno results that first got me interested in this.
Adam - had a look at the MRT site (v. useful!) and they don't have a lot of detail (or a price!) on their replacement manifold but seem to suggest it is an equal length type.
I was hoping you were going to join in here as It was probably your dyno results that first got me interested in this.
Adam - had a look at the MRT site (v. useful!) and they don't have a lot of detail (or a price!) on their replacement manifold but seem to suggest it is an equal length type.
09 October 2001, 08:53 PM
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I was referring to the power gained from just the manifold change not the entire system(s).
My header are not equal length but tuned length, the header pipes expand a shrink to the same shape as the flame pulse from the engine. This happens all the way to the single collector.
[This message has been edited by submannz (edited 09 October 2001).]
My header are not equal length but tuned length, the header pipes expand a shrink to the same shape as the flame pulse from the engine. This happens all the way to the single collector.
[This message has been edited by submannz (edited 09 October 2001).]
10 October 2001, 12:34 AM
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The mrt headers may well be advertised as equal length but they are far from it.
Having seen them in the flesh, you can measure how far off equal they are.
They are so unequal that the car retains its boxer warble.
ANd they work very well indeed.
The guy before me was forced to add 25% more fuel to certain zones at near the top of his map.
Having seen them in the flesh, you can measure how far off equal they are.
They are so unequal that the car retains its boxer warble.
ANd they work very well indeed.
The guy before me was forced to add 25% more fuel to certain zones at near the top of his map.
10 October 2001, 01:53 PM
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Looking back at Stephen Done's Dynomite results he has done three separate runs.
Run 1 - Standard set-up (243 bhp @ 6460 rpm 220 lb/ft @ 3570 rpm)
Run 3 - Power Engineering T75 downpipe, centre section with cat and back box (252 bhp @ 6460 rpm 230 lb/ft @ 3060 rpm)
Run 3 - Also with PE Power Manifold (257 bhp @ 5940 rpm 261 lb/ft @ 3150 rpm).
The manifold seems to be contributing 30 lb/ft additional torque right across the rev mid-range range. The BHP figures are equally impressive and only start to tail off after 5.5K.
Run 1 - Standard set-up (243 bhp @ 6460 rpm 220 lb/ft @ 3570 rpm)
Run 3 - Power Engineering T75 downpipe, centre section with cat and back box (252 bhp @ 6460 rpm 230 lb/ft @ 3060 rpm)
Run 3 - Also with PE Power Manifold (257 bhp @ 5940 rpm 261 lb/ft @ 3150 rpm).
The manifold seems to be contributing 30 lb/ft additional torque right across the rev mid-range range. The BHP figures are equally impressive and only start to tail off after 5.5K.
10 October 2001, 02:47 PM
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As someone with PPP i went in similar ways:
1. Manex d/p (no cats centre PD bacbox)
2. HKS manifold + up pipe
no induction mods yet
Stage 1: very good gains linearity across rev range
Stage 2: Peak torque increased but also moved to the higher end of revs but a lot ov torque everywhere testament of which is ease that the engine hits high revs with no ease off of power.
My worry is fuelling at high revs although the PD/sti ecu is said to add more fuel than std ecu highup (and i use SUL+booster)
1. Manex d/p (no cats centre PD bacbox)
2. HKS manifold + up pipe
no induction mods yet
Stage 1: very good gains linearity across rev range
Stage 2: Peak torque increased but also moved to the higher end of revs but a lot ov torque everywhere testament of which is ease that the engine hits high revs with no ease off of power.
My worry is fuelling at high revs although the PD/sti ecu is said to add more fuel than std ecu highup (and i use SUL+booster)
10 October 2001, 02:57 PM
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Adam
Bob had to add more fuel to run a certain company's downpipe, even though it was costing him nearly 30bhp. Adding fuel is no indication.
He also trested a manifold that cost 100lb of torque. Looked pretty though
Bob had to add more fuel to run a certain company's downpipe, even though it was costing him nearly 30bhp. Adding fuel is no indication.
He also trested a manifold that cost 100lb of torque. Looked pretty though
10 October 2001, 04:29 PM
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Pete,
Did it cost him 30bhp at the point where he added the fuel? or are you just quoting the peak bhp?
I have to ask one thing. If fuel is not indicative of more power, then why would we need bigger injectors when we increase boost and volumetric efficiency?
Perhaps I have misunderstood something here. If so, can you please explain?
Did it cost him 30bhp at the point where he added the fuel? or are you just quoting the peak bhp?
I have to ask one thing. If fuel is not indicative of more power, then why would we need bigger injectors when we increase boost and volumetric efficiency?
Perhaps I have misunderstood something here. If so, can you please explain?
10 October 2001, 08:35 PM
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Yes, looking at the pictures of the so called equal length extractors. I can't work out how they can advertise that, seems to me to be false advertising. Just need to look at the picture and you can see that the right bank is far shorter than the left.
10 October 2001, 10:22 PM
#23
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To clarify what Pete said, I needed to add fuel to the map in order to maintain the afr I was using, I lost 30 bhp peak power comparitively, I lost 80 bhp and 100 ftlbs torque at 4k rpm due to the whole curve moving to the right significantly (measured comparitively on the road), I gained in only a very narrow rev band but that is expected dependant on the design, there are plenty of tech articles around that explain what and how equal and non-equal manifolds do, the common theme is that equal length must be tuned in diameter and length to suit the car and the requirements, they must also use gas tight jointing, they were originally designed to complement open exhausted racers with very short open pipes. (see Adams post above)
When I reverted to flowed and ported standard headers I ended up having to adjust the fuel map everywhere (upwards again) to get the same afr. During all this I had also swapped turbo's several times which confused things a bit, I am just completing a re-evaluation of the downpipe combinations I had previously tried plus a third option I had not previously tried. This experiment has been limited as my injectors were maxed from 5500 upwards. That problem is now resolved. It was also interesting to find that the turbo wastegate was blowing open set at 0.8 bar (standard is 0.4 ish), that has also now been resolved.
Its possible to get decent figures from almost any system on the rollers as the car can be suitably loaded to bring up the boost, on the road the load impedance limits this and so that is where I did my comparative testing, not on the rollers. Looking at the curves posted in this thread its clear that the torque curve has shifted to the right, not a problem as driving style can be adjusted but ... that means turbo spool up has been slugged somewhat due to the change in exhaust gas flow at the lower end of the rev range. This is the fundemental issue, gas velocity thro' the turbine is what makes boost, if the velocity decreases then spool up rate reduces. If the header design is not good then you end up losing everywhere.
Horses for courses at the end of the day and a great deal to do with personal choice, I prefer the ported and flowed standard header route as it give the best all round result imho. Also used by tuners of monster cars in Oz. I love a good crisp part throttle response (read decent turbo spool rate) and as I do not plan on driving my car above 5500 rpm for 90 % of the time my setup suits.
When I reverted to flowed and ported standard headers I ended up having to adjust the fuel map everywhere (upwards again) to get the same afr. During all this I had also swapped turbo's several times which confused things a bit, I am just completing a re-evaluation of the downpipe combinations I had previously tried plus a third option I had not previously tried. This experiment has been limited as my injectors were maxed from 5500 upwards. That problem is now resolved. It was also interesting to find that the turbo wastegate was blowing open set at 0.8 bar (standard is 0.4 ish), that has also now been resolved.
Its possible to get decent figures from almost any system on the rollers as the car can be suitably loaded to bring up the boost, on the road the load impedance limits this and so that is where I did my comparative testing, not on the rollers. Looking at the curves posted in this thread its clear that the torque curve has shifted to the right, not a problem as driving style can be adjusted but ... that means turbo spool up has been slugged somewhat due to the change in exhaust gas flow at the lower end of the rev range. This is the fundemental issue, gas velocity thro' the turbine is what makes boost, if the velocity decreases then spool up rate reduces. If the header design is not good then you end up losing everywhere.
Horses for courses at the end of the day and a great deal to do with personal choice, I prefer the ported and flowed standard header route as it give the best all round result imho. Also used by tuners of monster cars in Oz. I love a good crisp part throttle response (read decent turbo spool rate) and as I do not plan on driving my car above 5500 rpm for 90 % of the time my setup suits.
11 October 2001, 12:31 AM
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I am still alittle confused on this fuelling though.
Bob can you possibly clear it up for me.
Pete mentioned something about scavenging so can you explain how a better exhaust can require less fuel?
From purely a physical chemistry point of view more fuel - more energy.
I thought power was determined by air flow rate.
SO if you add more fuel to the map and teh lambda reading is the same as it was before your change, then you must have a higher air flow rate.
Higher air flow rate means more air through the engine and hence more power.
Some one please explain where I have gone wrong, I am racking my brains and getting tied up in knots.
Bob? Pete? please help, I hate being confused.
Bob can you possibly clear it up for me.
Pete mentioned something about scavenging so can you explain how a better exhaust can require less fuel?
From purely a physical chemistry point of view more fuel - more energy.
I thought power was determined by air flow rate.
SO if you add more fuel to the map and teh lambda reading is the same as it was before your change, then you must have a higher air flow rate.
Higher air flow rate means more air through the engine and hence more power.
Some one please explain where I have gone wrong, I am racking my brains and getting tied up in knots.
Bob? Pete? please help, I hate being confused.
11 October 2001, 09:57 AM
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From submannz:
>I was referring to the power gained from
>just the manifold change not the entire
>system(s).
>
I have a run that purely shows manifold change.
Interestingly, all the talk so far has been about shifting the torque band to the right. Mine shifted left significantly, not right. Does anyone else have a PE power manifold with a before and after dyno run ? Perhaps this is a characteristic of the style of manifold that I have. Or are we saying it only shows up on the road, and that the dyno runs are a little deceptive.
Cheers
Steve
>I was referring to the power gained from
>just the manifold change not the entire
>system(s).
>
I have a run that purely shows manifold change.
Interestingly, all the talk so far has been about shifting the torque band to the right. Mine shifted left significantly, not right. Does anyone else have a PE power manifold with a before and after dyno run ? Perhaps this is a characteristic of the style of manifold that I have. Or are we saying it only shows up on the road, and that the dyno runs are a little deceptive.
Cheers
Steve
11 October 2001, 08:16 PM
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Don't forget:
A huge amount of the engine's horsepower is lost in restrictions through induction and exhaust system.
Thus a free flow system can render back some of the lost power.
A physical chemistry point of view Alex as we always talk fore gains, never for loss in power.
And as you know sometimes the theory is completely different from the practice...
Or, just my opinion, don't shoot me!
Cheers
JIM
A huge amount of the engine's horsepower is lost in restrictions through induction and exhaust system.
Thus a free flow system can render back some of the lost power.
A physical chemistry point of view Alex as we always talk fore gains, never for loss in power.
And as you know sometimes the theory is completely different from the practice...
Or, just my opinion, don't shoot me!
Cheers
JIM
11 October 2001, 08:38 PM
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Originally posted by StephenDone:
"Interestingly, all the talk so far has been about shifting the torque band to the right. Mine shifted left significantly, not right. Does anyone else have a PE power manifold with a before and after dyno run ? Perhaps this is a characteristic of the style of manifold that I have. Or are we saying it only shows up on the road, and that the dyno runs are a little deceptive."
Originally posted by Bob Rawle:
"Its possible to get decent figures from almost any system on the rollers as the car can be suitably loaded to bring up the boost, on the road the load impedance limits this and so that is where I did my comparative testing, not on the rollers. Looking at the curves posted in this thread its clear that the torque curve has shifted to the right, not a problem as driving style can be adjusted but ... that means turbo spool up has been slugged somewhat due to the change in exhaust gas flow at the lower end of the rev range. This is the fundamental issue, gas velocity thro' the turbine is what makes boost, if the velocity decreases then spool up rate reduces. If the header design is not good then you end up losing everywhere."
Stephen what about the driveability of your car before and after the exhaust modifications?
Is the power on the rollers demonstrable on the road?
JIM
"Interestingly, all the talk so far has been about shifting the torque band to the right. Mine shifted left significantly, not right. Does anyone else have a PE power manifold with a before and after dyno run ? Perhaps this is a characteristic of the style of manifold that I have. Or are we saying it only shows up on the road, and that the dyno runs are a little deceptive."
Originally posted by Bob Rawle:
"Its possible to get decent figures from almost any system on the rollers as the car can be suitably loaded to bring up the boost, on the road the load impedance limits this and so that is where I did my comparative testing, not on the rollers. Looking at the curves posted in this thread its clear that the torque curve has shifted to the right, not a problem as driving style can be adjusted but ... that means turbo spool up has been slugged somewhat due to the change in exhaust gas flow at the lower end of the rev range. This is the fundamental issue, gas velocity thro' the turbine is what makes boost, if the velocity decreases then spool up rate reduces. If the header design is not good then you end up losing everywhere."
Stephen what about the driveability of your car before and after the exhaust modifications?
Is the power on the rollers demonstrable on the road?
JIM
12 October 2001, 12:55 PM
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I lost the 'wall' of acceleration that I got at 3000ish RPM, since the power was now coming in earlier and more gradually. Whilst this 'feels' slower, it isn't in practise. When the load is higher e.g. 5th on a motorway, this extra torque at low revs means you don't have to change down a gear at times. Also very useful when you're coming up to a junction - too fast to drop into first, but too slow to get any acceleration in second. Since I have much more low down, I don't need to mash my sychromesh to do the necessary.
Unfortunately, my induction mods have messed up my bottom end, hence my posting about lambda sensors & rich/lean running - go on Bob, help me out here ! I have lost all the bottom end power that I gained from the manifold, though throttle response is much perkier, and the top end is great.
Steve
Unfortunately, my induction mods have messed up my bottom end, hence my posting about lambda sensors & rich/lean running - go on Bob, help me out here ! I have lost all the bottom end power that I gained from the manifold, though throttle response is much perkier, and the top end is great.
Steve
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