Std Pistons and Rods - Power/Torque limit ??
16 May 2002, 03:25 PM
#32
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Interesting. I was also always led to believe revs broke rods.
Seen a few vauxhall XE/LET ( 16v & Turbo ) engines where the end cap bolts have snapped causing alot of damage, to prevent this ARP etc bolts are used that are stronger. Never seen an ARP'd bottom end go.
I would agree with Mark and err on the side of caution, do the rods if its coming apart, if you aren't stripping it be sensible and it should survive ( like cosie converts car ? ).
Seen a few vauxhall XE/LET ( 16v & Turbo ) engines where the end cap bolts have snapped causing alot of damage, to prevent this ARP etc bolts are used that are stronger. Never seen an ARP'd bottom end go.
I would agree with Mark and err on the side of caution, do the rods if its coming apart, if you aren't stripping it be sensible and it should survive ( like cosie converts car ? ).
16 May 2002, 04:04 PM
#33
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Torque is what will bend rods not bhp and revs will break or snap rods.
Increased boost and airflow will mean you get greater in-cylinder pressures during combustion. The pressure rise from combustion is always a factor of your starting pressure therefore more in-cylinder pressure before combustion = more pressure produced during/after combustion hence more torque and power.
As a result you will be subjecting the rod to a higher load under compression as it is coming back up the cylinder and around TDC when combustion pressures are at their highest. This is more likely to bend the rods than running standard boost.
Increasing the rev limit will cause a greater stretching load on the rods during the exhaust stroke due to the increased inertia and therefore is more likely to stretch and snap a rod.
Andy
[Edited to add
James,
Your theory about the maximum interial forces is correct if the piston is moving around in free space. Seeing as how the piston is actually performing work by compressing the fuel/air mixture does this change the inertial loads during the compression stroke and therefore mean that any increased torque will have a greater effect on the rod rather than if the rod was spinning freely?]
[Edited by EvoRSX - 5/16/2002 4:13:02 PM]
Increased boost and airflow will mean you get greater in-cylinder pressures during combustion. The pressure rise from combustion is always a factor of your starting pressure therefore more in-cylinder pressure before combustion = more pressure produced during/after combustion hence more torque and power.
As a result you will be subjecting the rod to a higher load under compression as it is coming back up the cylinder and around TDC when combustion pressures are at their highest. This is more likely to bend the rods than running standard boost.
Increasing the rev limit will cause a greater stretching load on the rods during the exhaust stroke due to the increased inertia and therefore is more likely to stretch and snap a rod.
Andy
[Edited to add
James,
Your theory about the maximum interial forces is correct if the piston is moving around in free space. Seeing as how the piston is actually performing work by compressing the fuel/air mixture does this change the inertial loads during the compression stroke and therefore mean that any increased torque will have a greater effect on the rod rather than if the rod was spinning freely?]
[Edited by EvoRSX - 5/16/2002 4:13:02 PM]
16 May 2002, 06:42 PM
#34
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So then people. What are the best rods we are supposed to be using?? UK, STi what age?? What are the differences? Who makes the best steel items ( at realistic prices ). What do the WRC boys use? etc etc...We all sem to agree that its gonna happen, but what is out there to stop it? ( and the big end problem too 
)
)
16 May 2002, 10:09 PM
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This is where you can find the answer. I didn't understand it. Maybe somebody can.
http://www.seas.upenn.edu/~meam100/h...s/dynamics.pdf
cheers
http://www.seas.upenn.edu/~meam100/h...s/dynamics.pdf
cheers
16 May 2002, 11:01 PM
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Thanks Guys
Found the source of the 310lb-ft 'explosion limit' quote, it wasn't too scientific or conclusive so I'll boldly disregard it
Onward and upward with the torque and BHP whilst observing the design rpm limit.
Andy
Found the source of the 310lb-ft 'explosion limit' quote, it wasn't too scientific or conclusive so I'll boldly disregard it
Onward and upward with the torque and BHP
whilst observing the design rpm limit.Andy
04 June 2002, 05:40 PM
#38
Hi Guys,
The issue of how much load a std. con rod can take before "letting go" is an interesting one.
IMHO I do not believe that anyone could make the statement "a std. con rod is safe up to xxxlbft" as there are too many variables in production engines to be certain of this. All you can do is “destruction test” several engines take an average and then say “Well they’re certainly not safe above xxx”
An engine that is "out of the box" will have con rods fitted that are capable of dealing with the factories intended power figures, and have something left in reserve. Which is where the reliability comes from.
The problem with increasing the output of the engine is that you then start encroaching into that reserve, so reliability suffers. Unfortunately, because of production tolerances, it would be difficult to say that "Subaru con rods break at xxx". As each rod is slightly different. Otherwise all 4 rods would break at the same time!
As you will appreciate, when you mass-produce components they will not all be the same size & weight. These differences, albeit tiny, are magnified many times over at the point the engine is at its peak torque/revs. So the limits of an item such as a con rod will vary. This problem is only made worse when squeezing more power from an engine.
I am sure that most of us are aware that two, supposedly identical "off the shelf" engines can feel different in the way they drive and the amount of power that they produce. This is mainly due to two factors… Most production engines are built from the components that just happened to be "the next one in the box". Although certain items such as Pistons & Bearings do have different grades & sizes, they still have tolerances. So nothing is "perfect" it's just "acceptable". I will say though, that anyone who has run in a Subaru will know that they use tighter tolerances than most mass producers, (with the obvious exception of the No.4 piston in 98my engines
) ) which is why it takes approx. 7-8000 miles to properly run one in! The way that a new performance engine is initially fired up & then subsequently “Run-In” is very important. This is why a computer controlled dyno is ideal for this job, it has the ability to monitor all of the engines sensors, temperatures & perimeters, whilst carrying out a set program of bedding & then initial running in, followed by power testing. This is the reason that manufactures such as Ferrari, Rolls Royce & Jaguar pay particular attention to the way their engines are built. They also run their engines in before they are fitted into the vehicle. This goes a long way to ensure that all of their power plants have very similar power outputs & are silky smooth (any engine that isn’t 100% goes back for reworking). This is obviously very time consuming & expensive, but the benefit of this is that failures are extremely rare & their engines seem to go forever! It also allows certain tuning company’s to substantially increase power levels without having to strip the engine to fit stronger internals.
We have been running GpN. Impreza's for many years now, and although we do build road & recce car engines "in house", our rally engines are built by Prodrive. This is because it takes a lot of knowledge & specialized equipment to build a powerful engine that is going to stay reliable, especially in the harsh environment in which we use ours.
Because of the rules in GpN. these units are built using off the shelf, std. components although it has to be said that the engine is blueprinted and built to exacting standards. This is to say that each new component (piston, con rod, bearings etc) is meticulously weighed & checked for tolerance down to the last gram/thou, and then compared to a bin full of similar components until a perfectly matching set can be found. This in turn enables us to push the limits whilst still retaining reliability. A con rod fitted in this way would be able to put up with a higher engine output whilst still retaining a very good level of reliability. This is due to it being in "tune" with all the other internals, so they end up working in perfect harmony (well, as near to perfect as is humanly possible).
Interestingly, after each rebuild on our No.1 rally engine, it consistently produces almost identical power/torque figures to the last build, and we have never had a failure (not even a big end!!)
The most important factor in all of this is, how competent the technician is and how well the components have been "matched" during the build process. An engine built from parts that have been measured for minimal tolerance + equal weight & balance will produce more power/torque & will be an inherently stronger unit, capable of withstanding higher boost & torque.
A con rod failure is induced by a combination of Peak Torque, RPM & Mileage. Although, the killer on an engine that has been moded is high rpm’s (It just can't put up with the extra internal forces at high speed and stretches itself until it breaks!). A good international rally spec GpN. engine running on a high octane control fuel (102ron) & fitted with a 32mm restrictor is capable of running at 1.75bar boost (25psi) which in turn produces circa 390lbft. of torque @ around 3,500 rpm. BHP is relatively low @ 280 due to the dreaded restrictor. These engines have an upper rev limit reduced to just under 7,500 rpm, as the integrity & reliability of the rods becomes questionable if pushed any further whilst coping with this sort of torque output.
For an engine running in this spec, it is highly recommended that a rebuild be carried out after every 1000km’s of competition use. This is mainly done to change the rods, but whilst she is in bits, a crank and bearings are fitted. In certain championships the rules are not so stringent, so the engine can be fitted with the much stronger GpA. rods. These engines will then allow a rev limit of over 8000rpm with no probs & no requirement to strip every 1000km’s. It is however very important that the engine oil is changed after each "leg" i.e. at the end of each days rallying. This is due to the fact that when you run engine oil to the extremes that we do, it looses some of its properties as it cools back down. This is also true for cars that take part in track days!
Well that’s me done!… What started out as a short reply has turned into a bit of an essay… Sorry Guys
Phil
The issue of how much load a std. con rod can take before "letting go" is an interesting one.
IMHO I do not believe that anyone could make the statement "a std. con rod is safe up to xxxlbft" as there are too many variables in production engines to be certain of this. All you can do is “destruction test” several engines take an average and then say “Well they’re certainly not safe above xxx”
An engine that is "out of the box" will have con rods fitted that are capable of dealing with the factories intended power figures, and have something left in reserve. Which is where the reliability comes from.
The problem with increasing the output of the engine is that you then start encroaching into that reserve, so reliability suffers. Unfortunately, because of production tolerances, it would be difficult to say that "Subaru con rods break at xxx". As each rod is slightly different. Otherwise all 4 rods would break at the same time!
As you will appreciate, when you mass-produce components they will not all be the same size & weight. These differences, albeit tiny, are magnified many times over at the point the engine is at its peak torque/revs. So the limits of an item such as a con rod will vary. This problem is only made worse when squeezing more power from an engine.
I am sure that most of us are aware that two, supposedly identical "off the shelf" engines can feel different in the way they drive and the amount of power that they produce. This is mainly due to two factors… Most production engines are built from the components that just happened to be "the next one in the box". Although certain items such as Pistons & Bearings do have different grades & sizes, they still have tolerances. So nothing is "perfect" it's just "acceptable". I will say though, that anyone who has run in a Subaru will know that they use tighter tolerances than most mass producers, (with the obvious exception of the No.4 piston in 98my engines
) ) which is why it takes approx. 7-8000 miles to properly run one in! The way that a new performance engine is initially fired up & then subsequently “Run-In” is very important. This is why a computer controlled dyno is ideal for this job, it has the ability to monitor all of the engines sensors, temperatures & perimeters, whilst carrying out a set program of bedding & then initial running in, followed by power testing. This is the reason that manufactures such as Ferrari, Rolls Royce & Jaguar pay particular attention to the way their engines are built. They also run their engines in before they are fitted into the vehicle. This goes a long way to ensure that all of their power plants have very similar power outputs & are silky smooth (any engine that isn’t 100% goes back for reworking). This is obviously very time consuming & expensive, but the benefit of this is that failures are extremely rare & their engines seem to go forever! It also allows certain tuning company’s to substantially increase power levels without having to strip the engine to fit stronger internals.We have been running GpN. Impreza's for many years now, and although we do build road & recce car engines "in house", our rally engines are built by Prodrive. This is because it takes a lot of knowledge & specialized equipment to build a powerful engine that is going to stay reliable, especially in the harsh environment in which we use ours.
Because of the rules in GpN. these units are built using off the shelf, std. components although it has to be said that the engine is blueprinted and built to exacting standards. This is to say that each new component (piston, con rod, bearings etc) is meticulously weighed & checked for tolerance down to the last gram/thou, and then compared to a bin full of similar components until a perfectly matching set can be found. This in turn enables us to push the limits whilst still retaining reliability. A con rod fitted in this way would be able to put up with a higher engine output whilst still retaining a very good level of reliability. This is due to it being in "tune" with all the other internals, so they end up working in perfect harmony (well, as near to perfect as is humanly possible).
Interestingly, after each rebuild on our No.1 rally engine, it consistently produces almost identical power/torque figures to the last build, and we have never had a failure (not even a big end!!)
The most important factor in all of this is, how competent the technician is and how well the components have been "matched" during the build process. An engine built from parts that have been measured for minimal tolerance + equal weight & balance will produce more power/torque & will be an inherently stronger unit, capable of withstanding higher boost & torque.
A con rod failure is induced by a combination of Peak Torque, RPM & Mileage. Although, the killer on an engine that has been moded is high rpm’s (It just can't put up with the extra internal forces at high speed and stretches itself until it breaks!). A good international rally spec GpN. engine running on a high octane control fuel (102ron) & fitted with a 32mm restrictor is capable of running at 1.75bar boost (25psi) which in turn produces circa 390lbft. of torque @ around 3,500 rpm. BHP is relatively low @ 280 due to the dreaded restrictor. These engines have an upper rev limit reduced to just under 7,500 rpm, as the integrity & reliability of the rods becomes questionable if pushed any further whilst coping with this sort of torque output.
For an engine running in this spec, it is highly recommended that a rebuild be carried out after every 1000km’s of competition use. This is mainly done to change the rods, but whilst she is in bits, a crank and bearings are fitted. In certain championships the rules are not so stringent, so the engine can be fitted with the much stronger GpA. rods. These engines will then allow a rev limit of over 8000rpm with no probs & no requirement to strip every 1000km’s. It is however very important that the engine oil is changed after each "leg" i.e. at the end of each days rallying. This is due to the fact that when you run engine oil to the extremes that we do, it looses some of its properties as it cools back down. This is also true for cars that take part in track days!
Well that’s me done!… What started out as a short reply has turned into a bit of an essay… Sorry Guys
Phil
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