Technical discussion: Exhaust manifolds on turbo cars
02-12-2005, 10:27 AM
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An interesting subject. One I have been looking into a fair bit recently, as nobody builds a turbo manifold for my choice of engine and car. I have found a fair bit of information. Some of it may prove usefull enough to include here.
The exhaust manifold plays a key role in all performance aspects of the turbo system. The turbo manifold has many and varied duties to perform. Direct responsibilities include support for the turbo, guidance of exhaust gases to the turbine, keeping exhaust gas pressure pulses moving along intact at a steady pace, and trying hard not to let any heat escape through the manifold walls. To accomplish these chores while glowing cherry red, trying to remain straight, not developing cracks, and hanging in there for year after year is not exactly an assignment for boys. An exhaust manifold leads a very hard life.
The application, whether competition or high-performance street, will strongly influence material selection, design style, and method of manufacture. Any maximum-effort turbo system will be configured around a tube-style, fabricated manifold. One-off designs, for cost reasons alone, must also be fabricated. A cast manifold is the obvious choice when large numbers are to be made.
Heat retention. Clearly, performance of the turbine is in part determined by the temperature of the exhaust gasses. It is reasonable, then, to expend some effort toward getting the gas from the combustion chamber to the turbine with the least possible temperature loss. This is fundamentally true, although the strength of the materials at elevated temperatures must sometimes be considered and some form of cooling provided. The thermal conductivity of a material is a measure of that material's ability to conduct heat. Since the objective here is to keep the heat inside the manifold, it is reasonable to try to use a material with the poorest ability to transfer heat.
Material selection:
Stainless steel. Stainless offers an interesting combination of properties. It is low in thermal conductivity, which is certainly desirable. Stainless grade 304 is an excellent choice. Easily welded, crack resistant, and relatively easy to work with. All stainless materials have a very high coefficient of thermal expansion; thus, the design, style, and fit of the manifold must account for this unusual property. For example a stainless header flange drilled perfectly for 8mm diameter, and using 8mm diameter bolts on the cylinder head, will shear half the bolts on the first warm up cycle. Larger than normal bolt holes are therefore necessary. Stainless steel enjoys long term corrosion resistance. Because od this and it's low heat transfer capability, stainless steel deserves strong consideration as the material choice for high performance manifolds.
Cast Iron, Iron alloys offer a designer many options. While not exactly putty in a designers hands, iron alloys do have the ability to be moulded into complex shapes. The limitsl lie with the ability of the pattern makers. The casting process is the only viable way to make an exhaust manifold with a wide variety if section shapes and wall thicknesses. An experienced or thoughtfull designer can take advantage of this characteristic to produce a low-surface-area, thin walled, smooth, constant-section-passageway manifold.
A wide variety of Iron alloys exist, but perhaps the most usefull for exhaust manifold design is the alloy called 'Ductile Iron'. Ductile Irons's characteristics range from good crack resistance and high temperature shape stability to free machining, all with relatively high basic strength.
Mild Steel, Although mild steel has no particular characteristics that make it an ideal choice of exhaust manifold materials, it does, indeed, do almost everything well. This material is an inexpensive, easy to machine and weld, and readily available in a wide variety of sizes and shapes. Perhaps its poorest characteristic is corrosion resistance. This can be helped significantly by chrome plating. Ask for industrial-quality plating, which is many times thicker than decoratice chrome. Perhaps better than chrome are some modern ceramic coatings.
Thermal Characteristics,
The wall thickness of a particular material will strongly influence the heat transfer. in that the thicker the material, the faster heat will travel through it. This seems contrary to logic at first thought, but consider how fast heat would be drawn out of a high-conductivity, infinitely thick aluminum manifold, as opposed to a very thin piece of stainless surrounded by a nice insulator like air! Heat transfer is directly proportional to surface area. It is therefore reasonable to give considerable thought to keeping the surface area of an exhaust manifold to an absoloute minimum. Clearly, the less surface area, the less heat loss. Reducing the amount of ambient air flowing aroung the exhaust manifold and turbocharger will further reduce heat loss from the system. It is generally not feasable to directly wrap the exhaust manifold with an insulating material, as the manifold material will overheat at some point to the point of structural failure. This is especially true with Mild steel.
A further effect on heat transfer out of the exhaust manifold is heat distribution inside the manifold. Hot spots inside the manifold should be avoided, because these can quickly pump a lot of heat out. They are created by sharply angled intersections or by too many exhaust gas pulses through one segment of the manifold. Keep in mind that the temperature difference between the inside and outside of the manifold is the force that pushes heat through the manifold.
Reversal,
Reversal of the exhaust gas flow back into the combustion chamber during valve overlap is called reversion. Creating an aerodynamic barrier that reduces the reverse flow yet does not impede outward-flowing gasses can pay dividends in the future.
Style on manifolding,
In general, much greater freedom exists in the choice of manifold styles when the manifold is fabricated. These choices range from the simple 'log style', to the equal-length, multiple-tube, individual-runner style. A great amount of research has been done on the performance benefits of various manifold styles. Most of this research, plus the tremendous efforts put into the recent era of turbocharged Grand Prix cars, strongly indicates that the best manifolding is a multiple-tube, individual-runner style.
Tubing sizes,
Almost all applications of a turbo are to an existing engine. Therefore, the choice of tube sizes will usually be dictated by the port size and the size of the turbine inlet on the turbocharger. Where a clear-cut choice does not exist, it is usually best to select the smaller of the sizes available, thus increasing exhaust gas velocity.
The strength of the manifold will be controlled largely by the wall thickness of the materials. In a fabricated manifold where wall thickess drops below 2.3mm it may be necessary to support the turbo by a brace or a small truss assembly. The thermal expansion of the manifold will tend to move the turbo around as the heat cycles go up and down. Thus, a mount must have some degree of flexibility while supporting the weight of the turbo. Mandrel bend tubes are available in a wide variety of sizes to meet the needs of the custom manifold maker. These are generally high-quality items that can be fabricated into any bundle of snakes style turbo manifold your imagination can cunjure up!
Wastegate Integration,
Early in the planning of the exhaust manifold design, consideration must be given to the location of, and bleed-of to, the wastegate. The principles in integrating the wastegate into the system are that bleed-off to the wastegate must occur after all exhaust pulses headed for the turbo have been combined into one tube, and flow path streamlined
Thermal expansion,
Changes in the shape of a manifold as it's temperature rises from ambient to operating must be considered during layout. Heat-induced warpage can cause severe problems with constant exhaust gas leaks. Warpage is caused by unequal temperature distrubution through the material of the manifold. As an example, the header flange will not reach the same temperature as a segment of the tubing or collectors; therefore, it will not change the length qas much. These varying changes in lenght will induce warpage if they are not accounted for in the design. Each port flange, for example, should be seperate from the others.
Thermal expansion chracteristics will require attention to bolt hole sizes, particularly at the cylinder head. Tight, close-tolerance bolt patterns can actually cause fastener failure by placing the fasteners under severe binding when a manifold reaches maximum operating temperature. The solution to this problem is enlarging the bolt hole size as it gets further from the centre of the manifold. This is essential when stainless steel is the material used. On long engines, like six-cylinder inline, the designer should consider using two manifolds, with interconnections from a tubular flexi joint. This type of slip joint is common on aircraft and large industrial engines.
Steve
The exhaust manifold plays a key role in all performance aspects of the turbo system. The turbo manifold has many and varied duties to perform. Direct responsibilities include support for the turbo, guidance of exhaust gases to the turbine, keeping exhaust gas pressure pulses moving along intact at a steady pace, and trying hard not to let any heat escape through the manifold walls. To accomplish these chores while glowing cherry red, trying to remain straight, not developing cracks, and hanging in there for year after year is not exactly an assignment for boys. An exhaust manifold leads a very hard life.
The application, whether competition or high-performance street, will strongly influence material selection, design style, and method of manufacture. Any maximum-effort turbo system will be configured around a tube-style, fabricated manifold. One-off designs, for cost reasons alone, must also be fabricated. A cast manifold is the obvious choice when large numbers are to be made.
Heat retention. Clearly, performance of the turbine is in part determined by the temperature of the exhaust gasses. It is reasonable, then, to expend some effort toward getting the gas from the combustion chamber to the turbine with the least possible temperature loss. This is fundamentally true, although the strength of the materials at elevated temperatures must sometimes be considered and some form of cooling provided. The thermal conductivity of a material is a measure of that material's ability to conduct heat. Since the objective here is to keep the heat inside the manifold, it is reasonable to try to use a material with the poorest ability to transfer heat.
Material selection:
Stainless steel. Stainless offers an interesting combination of properties. It is low in thermal conductivity, which is certainly desirable. Stainless grade 304 is an excellent choice. Easily welded, crack resistant, and relatively easy to work with. All stainless materials have a very high coefficient of thermal expansion; thus, the design, style, and fit of the manifold must account for this unusual property. For example a stainless header flange drilled perfectly for 8mm diameter, and using 8mm diameter bolts on the cylinder head, will shear half the bolts on the first warm up cycle. Larger than normal bolt holes are therefore necessary. Stainless steel enjoys long term corrosion resistance. Because od this and it's low heat transfer capability, stainless steel deserves strong consideration as the material choice for high performance manifolds.
Cast Iron, Iron alloys offer a designer many options. While not exactly putty in a designers hands, iron alloys do have the ability to be moulded into complex shapes. The limitsl lie with the ability of the pattern makers. The casting process is the only viable way to make an exhaust manifold with a wide variety if section shapes and wall thicknesses. An experienced or thoughtfull designer can take advantage of this characteristic to produce a low-surface-area, thin walled, smooth, constant-section-passageway manifold.
A wide variety of Iron alloys exist, but perhaps the most usefull for exhaust manifold design is the alloy called 'Ductile Iron'. Ductile Irons's characteristics range from good crack resistance and high temperature shape stability to free machining, all with relatively high basic strength.
Mild Steel, Although mild steel has no particular characteristics that make it an ideal choice of exhaust manifold materials, it does, indeed, do almost everything well. This material is an inexpensive, easy to machine and weld, and readily available in a wide variety of sizes and shapes. Perhaps its poorest characteristic is corrosion resistance. This can be helped significantly by chrome plating. Ask for industrial-quality plating, which is many times thicker than decoratice chrome. Perhaps better than chrome are some modern ceramic coatings.
Thermal Characteristics,
The wall thickness of a particular material will strongly influence the heat transfer. in that the thicker the material, the faster heat will travel through it. This seems contrary to logic at first thought, but consider how fast heat would be drawn out of a high-conductivity, infinitely thick aluminum manifold, as opposed to a very thin piece of stainless surrounded by a nice insulator like air! Heat transfer is directly proportional to surface area. It is therefore reasonable to give considerable thought to keeping the surface area of an exhaust manifold to an absoloute minimum. Clearly, the less surface area, the less heat loss. Reducing the amount of ambient air flowing aroung the exhaust manifold and turbocharger will further reduce heat loss from the system. It is generally not feasable to directly wrap the exhaust manifold with an insulating material, as the manifold material will overheat at some point to the point of structural failure. This is especially true with Mild steel.
A further effect on heat transfer out of the exhaust manifold is heat distribution inside the manifold. Hot spots inside the manifold should be avoided, because these can quickly pump a lot of heat out. They are created by sharply angled intersections or by too many exhaust gas pulses through one segment of the manifold. Keep in mind that the temperature difference between the inside and outside of the manifold is the force that pushes heat through the manifold.
Reversal,
Reversal of the exhaust gas flow back into the combustion chamber during valve overlap is called reversion. Creating an aerodynamic barrier that reduces the reverse flow yet does not impede outward-flowing gasses can pay dividends in the future.
Style on manifolding,
In general, much greater freedom exists in the choice of manifold styles when the manifold is fabricated. These choices range from the simple 'log style', to the equal-length, multiple-tube, individual-runner style. A great amount of research has been done on the performance benefits of various manifold styles. Most of this research, plus the tremendous efforts put into the recent era of turbocharged Grand Prix cars, strongly indicates that the best manifolding is a multiple-tube, individual-runner style.
Tubing sizes,
Almost all applications of a turbo are to an existing engine. Therefore, the choice of tube sizes will usually be dictated by the port size and the size of the turbine inlet on the turbocharger. Where a clear-cut choice does not exist, it is usually best to select the smaller of the sizes available, thus increasing exhaust gas velocity.
The strength of the manifold will be controlled largely by the wall thickness of the materials. In a fabricated manifold where wall thickess drops below 2.3mm it may be necessary to support the turbo by a brace or a small truss assembly. The thermal expansion of the manifold will tend to move the turbo around as the heat cycles go up and down. Thus, a mount must have some degree of flexibility while supporting the weight of the turbo. Mandrel bend tubes are available in a wide variety of sizes to meet the needs of the custom manifold maker. These are generally high-quality items that can be fabricated into any bundle of snakes style turbo manifold your imagination can cunjure up!
Wastegate Integration,
Early in the planning of the exhaust manifold design, consideration must be given to the location of, and bleed-of to, the wastegate. The principles in integrating the wastegate into the system are that bleed-off to the wastegate must occur after all exhaust pulses headed for the turbo have been combined into one tube, and flow path streamlined
Thermal expansion,
Changes in the shape of a manifold as it's temperature rises from ambient to operating must be considered during layout. Heat-induced warpage can cause severe problems with constant exhaust gas leaks. Warpage is caused by unequal temperature distrubution through the material of the manifold. As an example, the header flange will not reach the same temperature as a segment of the tubing or collectors; therefore, it will not change the length qas much. These varying changes in lenght will induce warpage if they are not accounted for in the design. Each port flange, for example, should be seperate from the others.
Thermal expansion chracteristics will require attention to bolt hole sizes, particularly at the cylinder head. Tight, close-tolerance bolt patterns can actually cause fastener failure by placing the fasteners under severe binding when a manifold reaches maximum operating temperature. The solution to this problem is enlarging the bolt hole size as it gets further from the centre of the manifold. This is essential when stainless steel is the material used. On long engines, like six-cylinder inline, the designer should consider using two manifolds, with interconnections from a tubular flexi joint. This type of slip joint is common on aircraft and large industrial engines.
Steve
02-12-2005, 11:16 AM
#83
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There are so many variables that it must be virtually impossible to quantify the difference between tubular manifolds.
Anyone care to elaborate on the benefits of a true-split pulse manifold like the one below?
Phil
Anyone care to elaborate on the benefits of a true-split pulse manifold like the one below?
Phil
02-12-2005, 11:21 AM
#84
Scort its amazing you managed to paste in such an enormous amount of information about manifolds and yet still it didnt actually touch on the subject the thread is about 
The phrase "Black art" springs to mind!
The phrase "Black art" springs to mind!
02-12-2005, 11:59 AM
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Originally Posted by chip-3door
Scort its amazing you managed to paste in such an enormous amount of information about manifolds and yet still it didnt actually touch on the subject the thread is about
Steve
02-12-2005, 12:16 PM
#87
Testing the future
Originally Posted by Philip
There are so many variables that it must be virtually impossible to quantify the difference between tubular manifolds.
you just need someone with a lot of time and money to change the manifold several times to measure any differences.i'm kind of waiting for mike to say why the one he is selling is any good, and if it's been compared on a dyno to other types like the one that you have shown, and what the results were.
until i see some evidence of back to back measurements of various designs, i will remain sceptical about any differences.
02-12-2005, 12:44 PM
#88
Nick,
The design of our manfold has been tested on various dynos and the wastegate set up is as good as it gets. On a GT30 turbo, with the wastegate fully open, the turbo only makes single figure psi. This shows that the wastegate is capable of controlling the flow PERFECTLY (many designs do NOT).
I have yet to see a better manifold for the money.
Also for anyone who is interested, we are currently designing and producing a tubular manifold to replace the standard 2wd one to give better response and spool up. This will be available in both the usual stainless-steel we use, as well as inconnel (for ALS). Back to back engine dyno figures will be published as and when it is all done
.
However, none of this helps my arguement, as I am sure I am right and the other guy is sure he is right.
The design of our manfold has been tested on various dynos and the wastegate set up is as good as it gets. On a GT30 turbo, with the wastegate fully open, the turbo only makes single figure psi. This shows that the wastegate is capable of controlling the flow PERFECTLY (many designs do NOT).
I have yet to see a better manifold for the money.
Also for anyone who is interested, we are currently designing and producing a tubular manifold to replace the standard 2wd one to give better response and spool up. This will be available in both the usual stainless-steel we use, as well as inconnel (for ALS). Back to back engine dyno figures will be published as and when it is all done
.However, none of this helps my arguement, as I am sure I am right and the other guy is sure he is right
.
02-12-2005, 12:59 PM
#91
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Originally Posted by Mike Rainbird
Also for anyone who is interested, we are currently designing and producing a tubular manifold to replace the standard 2wd one to give better response and spool up. This will be available in both the usual stainless-steel we use, as well as inconnel (for ALS). Back to back engine dyno figures will be published as and when it is all done
IMO as good as I am sure it is going to be - the price has to be right
02-12-2005, 12:59 PM
#92
Originally Posted by Mike Rainbird
Chip,
Okay, I have told Tomas what I want, he is getting a few designs made and I am arranging for Harvey to have a play on his dyno. I'm an ideas man - my hands ALWAYS stay clean 
.
Okay, I have told Tomas what I want, he is getting a few designs made and I am arranging for Harvey to have a play on his dyno
. I'm an ideas man - my hands ALWAYS stay clean .Or did it say "shiney, lots of power"
Seriously though, takes people like yourself to get the people who do know what they are doing into gear so fair play to you.
Ive never designed an exhaust manifold either and wouldnt know much about how to either, so im only kidding with you!
02-12-2005, 01:12 PM
#94
Originally Posted by Doug Stirling
Originally Posted by Mike Rainbird
Also for anyone who is interested, we are currently designing and producing a tubular manifold to replace the standard 2wd one to give better response and spool up. This will be available in both the usual stainless-steel we use, as well as inconnel (for ALS). Back to back engine dyno figures will be published as and when it is all done
IMO as good as I am sure it is going to be - the price has to be right
Im not sure why though to be perfectly honest.
Im having one made at the moment for my engine that is of a good standard, and its not much over a third of that price!
02-12-2005, 01:17 PM
#95
DEYTUKURJERBS
Originally Posted by chip-3door
Im pretty sure the last one was something like about 1200 quid when i looked on his site.
Im not sure why though to be perfectly honest.
Im having one made at the moment for my engine that is of a good standard, and its not much over a third of that price!
Im not sure why though to be perfectly honest.
Im having one made at the moment for my engine that is of a good standard, and its not much over a third of that price!
Lets do some pics of mine...
02-12-2005, 01:29 PM
#97
Originally Posted by chip-3door
Im pretty sure the last one was something like about 1200 quid when i looked on his site.
Im not sure why though to be perfectly honest.
Im having one made at the moment for my engine that is of a good standard, and its not much over a third of that price!
Im not sure why though to be perfectly honest.
Im having one made at the moment for my engine that is of a good standard, and its not much over a third of that price!
02-12-2005, 01:36 PM
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Originally Posted by sbd16v
Originally Posted by chip-3door
Im pretty sure the last one was something like about 1200 quid when i looked on his site.
Im not sure why though to be perfectly honest.
Im having one made at the moment for my engine that is of a good standard, and its not much over a third of that price!
Im not sure why though to be perfectly honest.
Im having one made at the moment for my engine that is of a good standard, and its not much over a third of that price!
02-12-2005, 01:45 PM
#102
Originally Posted by chip-3door
PON, its designed and made to order by a Dutch guy I know.
He does an inlet for the same engine too which is quite cool.
He does an inlet for the same engine too which is quite cool.
cannot wait for mine
02-12-2005, 01:55 PM
#104
Chip,
The manifolds I sell take 40 hours to produce and also have to be shipped to this country, hence the cost (which is Ł1100).
I can provide a cheaper one that is electro-plated instead of hand polished that is Ł200 cheaper (see pictures below):
The manifolds I sell take 40 hours to produce and also have to be shipped to this country, hence the cost (which is Ł1100
).I can provide a cheaper one that is electro-plated instead of hand polished that is Ł200 cheaper (see pictures below):
02-12-2005, 01:55 PM
#105
Originally Posted by Itsmeagain
Anyone seen an equal length inlet manifold done in the same style as an exhaust?
I have once, wonder if i can find the pics...
I have once, wonder if i can find the pics...
02-12-2005, 01:57 PM
#106
Originally Posted by Itsmeagain
Anyone seen an equal length inlet manifold done in the same style as an exhaust?
I have once, wonder if i can find the pics...
I have once, wonder if i can find the pics...
the standard vauxhall one i belive has 4 equal lenght inlet tracks but its a cast manifold so you cannot see it so well seams 450bhp+ is possible out of a standard one with and enlarged top had thou
02-12-2005, 01:57 PM
#107
Originally Posted by Mike Rainbird
Chip,
The manifolds I sell take 40 hours to produce and also have to be shipped to this country, hence the cost (which is Ł1100).
I can provide a cheaper one that is electro-plated instead of hand polished that is Ł200 cheaper (see pictures below):
The manifolds I sell take 40 hours to produce and also have to be shipped to this country, hence the cost (which is Ł1100
).I can provide a cheaper one that is electro-plated instead of hand polished that is Ł200 cheaper (see pictures below):
I think someone is spinning you a line mike!
either that or they arent very well tooled up!
02-12-2005, 01:58 PM
#108
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Originally Posted by Itsmeagain
Anyone seen an equal length inlet manifold done in the same style as an exhaust?
I have once, wonder if i can find the pics...
I have once, wonder if i can find the pics...
02-12-2005, 02:00 PM
#109
DEYTUKURJERBS
Well ive seen a 4cyl turbo car with a big tubular inlet manifold with single throttle that looks just like your usual 4 branch tubular exhaust manifold.
Looked cool, wether it was pointless or not im not sure.
Looked cool, wether it was pointless or not im not sure.
02-12-2005, 02:03 PM
#110
Chip,
That's not the one that takes 40 man hours (that's on the first page). The one above is the one that is Ł900.
The one that I am doing to replace a standard 2wd manifold, should be another Ł200 cheaper again, due to not have an external waste-gate and hopefully longer mandrel bent (which is still labour intensive) non-welded tubes.
That's not the one that takes 40 man hours (that's on the first page). The one above is the one that is Ł900.
The one that I am doing to replace a standard 2wd manifold, should be another Ł200 cheaper again, due to not have an external waste-gate and hopefully longer mandrel bent (which is still labour intensive) non-welded tubes
.
02-12-2005, 02:07 PM
#112
Ranj,
Read a few posts up, it's currently being made in stainless-steel as we speak to test on the dyno. When this is done, my own one will be made in inconnel to the final design. Is pointless wasting the inconnel material until the design is settled on.
Read a few posts up, it's currently being made in stainless-steel as we speak to test on the dyno. When this is done, my own one will be made in inconnel to the final design. Is pointless wasting the inconnel material until the design is settled on.
05-12-2005, 08:25 AM
#116
PassionFord Post Troll
Originally Posted by chip-3door
beach, turbos are different
It was my only understanding of what difference it made to my bike, how can things be cahnged with turbo manifolds to alter where the power is made ??
05-12-2005, 05:38 PM
#118
PassionFord Post Whore!!
is that wot your seen on a dyno?
wot positionin will it put the turbo, compared to a 2wd mani, as space is more limited on mine to a normal cossie.
